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linux/Documentation/filesystems/proc.txt
Shaohua Li cf8496ea80 proc: show MADV_FREE pages info in smaps 
Show MADV_FREE pages info of each vma in smaps.  The interface is for
diganose or monitoring purpose, userspace could use it to understand
what happens in the application.  Since userspace could dirty MADV_FREE
pages without notice from kernel, this interface is the only place we
can get accurate accounting info about MADV_FREE pages.

[mhocko@kernel.org: update Documentation/filesystems/proc.txt]
Link: http://lkml.kernel.org/r/89efde633559de1ec07444f2ef0f4963a97a2ce8.1487965799.git.shli@fb.com
Signed-off-by: Shaohua Li <shli@fb.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Minchan Kim <minchan@kernel.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Hillf Danton <hillf.zj@alibaba-inc.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2017-05-03 15:52:08 -07:00

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------------------------------------------------------------------------------
T H E /proc F I L E S Y S T E M
------------------------------------------------------------------------------
/proc/sys Terrehon Bowden <terrehon@pacbell.net> October 7 1999
Bodo Bauer <bb@ricochet.net>
2.4.x update Jorge Nerin <comandante@zaralinux.com> November 14 2000
move /proc/sys Shen Feng <shen@cn.fujitsu.com> April 1 2009
------------------------------------------------------------------------------
Version 1.3 Kernel version 2.2.12
Kernel version 2.4.0-test11-pre4
------------------------------------------------------------------------------
fixes/update part 1.1 Stefani Seibold <stefani@seibold.net> June 9 2009
Table of Contents
-----------------
0 Preface
0.1 Introduction/Credits
0.2 Legal Stuff
1 Collecting System Information
1.1 Process-Specific Subdirectories
1.2 Kernel data
1.3 IDE devices in /proc/ide
1.4 Networking info in /proc/net
1.5 SCSI info
1.6 Parallel port info in /proc/parport
1.7 TTY info in /proc/tty
1.8 Miscellaneous kernel statistics in /proc/stat
1.9 Ext4 file system parameters
2 Modifying System Parameters
3 Per-Process Parameters
3.1 /proc/<pid>/oom_adj & /proc/<pid>/oom_score_adj - Adjust the oom-killer
score
3.2 /proc/<pid>/oom_score - Display current oom-killer score
3.3 /proc/<pid>/io - Display the IO accounting fields
3.4 /proc/<pid>/coredump_filter - Core dump filtering settings
3.5 /proc/<pid>/mountinfo - Information about mounts
3.6 /proc/<pid>/comm & /proc/<pid>/task/<tid>/comm
3.7 /proc/<pid>/task/<tid>/children - Information about task children
3.8 /proc/<pid>/fdinfo/<fd> - Information about opened file
3.9 /proc/<pid>/map_files - Information about memory mapped files
3.10 /proc/<pid>/timerslack_ns - Task timerslack value
3.11 /proc/<pid>/patch_state - Livepatch patch operation state
4 Configuring procfs
4.1 Mount options
------------------------------------------------------------------------------
Preface
------------------------------------------------------------------------------
0.1 Introduction/Credits
------------------------
This documentation is part of a soon (or so we hope) to be released book on
the SuSE Linux distribution. As there is no complete documentation for the
/proc file system and we've used many freely available sources to write these
chapters, it seems only fair to give the work back to the Linux community.
This work is based on the 2.2.* kernel version and the upcoming 2.4.*. I'm
afraid it's still far from complete, but we hope it will be useful. As far as
we know, it is the first 'all-in-one' document about the /proc file system. It
is focused on the Intel x86 hardware, so if you are looking for PPC, ARM,
SPARC, AXP, etc., features, you probably won't find what you are looking for.
It also only covers IPv4 networking, not IPv6 nor other protocols - sorry. But
additions and patches are welcome and will be added to this document if you
mail them to Bodo.
We'd like to thank Alan Cox, Rik van Riel, and Alexey Kuznetsov and a lot of
other people for help compiling this documentation. We'd also like to extend a
special thank you to Andi Kleen for documentation, which we relied on heavily
to create this document, as well as the additional information he provided.
Thanks to everybody else who contributed source or docs to the Linux kernel
and helped create a great piece of software... :)
If you have any comments, corrections or additions, please don't hesitate to
contact Bodo Bauer at bb@ricochet.net. We'll be happy to add them to this
document.
The latest version of this document is available online at
http://tldp.org/LDP/Linux-Filesystem-Hierarchy/html/proc.html
If the above direction does not works for you, you could try the kernel
mailing list at linux-kernel@vger.kernel.org and/or try to reach me at
comandante@zaralinux.com.
0.2 Legal Stuff
---------------
We don't guarantee the correctness of this document, and if you come to us
complaining about how you screwed up your system because of incorrect
documentation, we won't feel responsible...
------------------------------------------------------------------------------
CHAPTER 1: COLLECTING SYSTEM INFORMATION
------------------------------------------------------------------------------
------------------------------------------------------------------------------
In This Chapter
------------------------------------------------------------------------------
* Investigating the properties of the pseudo file system /proc and its
ability to provide information on the running Linux system
* Examining /proc's structure
* Uncovering various information about the kernel and the processes running
on the system
------------------------------------------------------------------------------
The proc file system acts as an interface to internal data structures in the
kernel. It can be used to obtain information about the system and to change
certain kernel parameters at runtime (sysctl).
First, we'll take a look at the read-only parts of /proc. In Chapter 2, we
show you how you can use /proc/sys to change settings.
1.1 Process-Specific Subdirectories
-----------------------------------
The directory /proc contains (among other things) one subdirectory for each
process running on the system, which is named after the process ID (PID).
The link self points to the process reading the file system. Each process
subdirectory has the entries listed in Table 1-1.
Table 1-1: Process specific entries in /proc
..............................................................................
File Content
clear_refs Clears page referenced bits shown in smaps output
cmdline Command line arguments
cpu Current and last cpu in which it was executed (2.4)(smp)
cwd Link to the current working directory
environ Values of environment variables
exe Link to the executable of this process
fd Directory, which contains all file descriptors
maps Memory maps to executables and library files (2.4)
mem Memory held by this process
root Link to the root directory of this process
stat Process status
statm Process memory status information
status Process status in human readable form
wchan Present with CONFIG_KALLSYMS=y: it shows the kernel function
symbol the task is blocked in - or "0" if not blocked.
pagemap Page table
stack Report full stack trace, enable via CONFIG_STACKTRACE
smaps an extension based on maps, showing the memory consumption of
each mapping and flags associated with it
numa_maps an extension based on maps, showing the memory locality and
binding policy as well as mem usage (in pages) of each mapping.
..............................................................................
For example, to get the status information of a process, all you have to do is
read the file /proc/PID/status:
>cat /proc/self/status
Name: cat
State: R (running)
Tgid: 5452
Pid: 5452
PPid: 743
TracerPid: 0 (2.4)
Uid: 501 501 501 501
Gid: 100 100 100 100
FDSize: 256
Groups: 100 14 16
VmPeak: 5004 kB
VmSize: 5004 kB
VmLck: 0 kB
VmHWM: 476 kB
VmRSS: 476 kB
RssAnon: 352 kB
RssFile: 120 kB
RssShmem: 4 kB
VmData: 156 kB
VmStk: 88 kB
VmExe: 68 kB
VmLib: 1412 kB
VmPTE: 20 kb
VmSwap: 0 kB
HugetlbPages: 0 kB
Threads: 1
SigQ: 0/28578
SigPnd: 0000000000000000
ShdPnd: 0000000000000000
SigBlk: 0000000000000000
SigIgn: 0000000000000000
SigCgt: 0000000000000000
CapInh: 00000000fffffeff
CapPrm: 0000000000000000
CapEff: 0000000000000000
CapBnd: ffffffffffffffff
NoNewPrivs: 0
Seccomp: 0
voluntary_ctxt_switches: 0
nonvoluntary_ctxt_switches: 1
This shows you nearly the same information you would get if you viewed it with
the ps command. In fact, ps uses the proc file system to obtain its
information. But you get a more detailed view of the process by reading the
file /proc/PID/status. It fields are described in table 1-2.
The statm file contains more detailed information about the process
memory usage. Its seven fields are explained in Table 1-3. The stat file
contains details information about the process itself. Its fields are
explained in Table 1-4.
(for SMP CONFIG users)
For making accounting scalable, RSS related information are handled in an
asynchronous manner and the value may not be very precise. To see a precise
snapshot of a moment, you can see /proc/<pid>/smaps file and scan page table.
It's slow but very precise.
Table 1-2: Contents of the status files (as of 4.8)
..............................................................................
Field Content
Name filename of the executable
Umask file mode creation mask
State state (R is running, S is sleeping, D is sleeping
in an uninterruptible wait, Z is zombie,
T is traced or stopped)
Tgid thread group ID
Ngid NUMA group ID (0 if none)
Pid process id
PPid process id of the parent process
TracerPid PID of process tracing this process (0 if not)
Uid Real, effective, saved set, and file system UIDs
Gid Real, effective, saved set, and file system GIDs
FDSize number of file descriptor slots currently allocated
Groups supplementary group list
NStgid descendant namespace thread group ID hierarchy
NSpid descendant namespace process ID hierarchy
NSpgid descendant namespace process group ID hierarchy
NSsid descendant namespace session ID hierarchy
VmPeak peak virtual memory size
VmSize total program size
VmLck locked memory size
VmPin pinned memory size
VmHWM peak resident set size ("high water mark")
VmRSS size of memory portions. It contains the three
following parts (VmRSS = RssAnon + RssFile + RssShmem)
RssAnon size of resident anonymous memory
RssFile size of resident file mappings
RssShmem size of resident shmem memory (includes SysV shm,
mapping of tmpfs and shared anonymous mappings)
VmData size of private data segments
VmStk size of stack segments
VmExe size of text segment
VmLib size of shared library code
VmPTE size of page table entries
VmPMD size of second level page tables
VmSwap amount of swap used by anonymous private data
(shmem swap usage is not included)
HugetlbPages size of hugetlb memory portions
Threads number of threads
SigQ number of signals queued/max. number for queue
SigPnd bitmap of pending signals for the thread
ShdPnd bitmap of shared pending signals for the process
SigBlk bitmap of blocked signals
SigIgn bitmap of ignored signals
SigCgt bitmap of caught signals
CapInh bitmap of inheritable capabilities
CapPrm bitmap of permitted capabilities
CapEff bitmap of effective capabilities
CapBnd bitmap of capabilities bounding set
NoNewPrivs no_new_privs, like prctl(PR_GET_NO_NEW_PRIV, ...)
Seccomp seccomp mode, like prctl(PR_GET_SECCOMP, ...)
Cpus_allowed mask of CPUs on which this process may run
Cpus_allowed_list Same as previous, but in "list format"
Mems_allowed mask of memory nodes allowed to this process
Mems_allowed_list Same as previous, but in "list format"
voluntary_ctxt_switches number of voluntary context switches
nonvoluntary_ctxt_switches number of non voluntary context switches
..............................................................................
Table 1-3: Contents of the statm files (as of 2.6.8-rc3)
..............................................................................
Field Content
size total program size (pages) (same as VmSize in status)
resident size of memory portions (pages) (same as VmRSS in status)
shared number of pages that are shared (i.e. backed by a file, same
as RssFile+RssShmem in status)
trs number of pages that are 'code' (not including libs; broken,
includes data segment)
lrs number of pages of library (always 0 on 2.6)
drs number of pages of data/stack (including libs; broken,
includes library text)
dt number of dirty pages (always 0 on 2.6)
..............................................................................
Table 1-4: Contents of the stat files (as of 2.6.30-rc7)
..............................................................................
Field Content
pid process id
tcomm filename of the executable
state state (R is running, S is sleeping, D is sleeping in an
uninterruptible wait, Z is zombie, T is traced or stopped)
ppid process id of the parent process
pgrp pgrp of the process
sid session id
tty_nr tty the process uses
tty_pgrp pgrp of the tty
flags task flags
min_flt number of minor faults
cmin_flt number of minor faults with child's
maj_flt number of major faults
cmaj_flt number of major faults with child's
utime user mode jiffies
stime kernel mode jiffies
cutime user mode jiffies with child's
cstime kernel mode jiffies with child's
priority priority level
nice nice level
num_threads number of threads
it_real_value (obsolete, always 0)
start_time time the process started after system boot
vsize virtual memory size
rss resident set memory size
rsslim current limit in bytes on the rss
start_code address above which program text can run
end_code address below which program text can run
start_stack address of the start of the main process stack
esp current value of ESP
eip current value of EIP
pending bitmap of pending signals
blocked bitmap of blocked signals
sigign bitmap of ignored signals
sigcatch bitmap of caught signals
0 (place holder, used to be the wchan address, use /proc/PID/wchan instead)
0 (place holder)
0 (place holder)
exit_signal signal to send to parent thread on exit
task_cpu which CPU the task is scheduled on
rt_priority realtime priority
policy scheduling policy (man sched_setscheduler)
blkio_ticks time spent waiting for block IO
gtime guest time of the task in jiffies
cgtime guest time of the task children in jiffies
start_data address above which program data+bss is placed
end_data address below which program data+bss is placed
start_brk address above which program heap can be expanded with brk()
arg_start address above which program command line is placed
arg_end address below which program command line is placed
env_start address above which program environment is placed
env_end address below which program environment is placed
exit_code the thread's exit_code in the form reported by the waitpid system call
..............................................................................
The /proc/PID/maps file containing the currently mapped memory regions and
their access permissions.
The format is:
address perms offset dev inode pathname
08048000-08049000 r-xp 00000000 03:00 8312 /opt/test
08049000-0804a000 rw-p 00001000 03:00 8312 /opt/test
0804a000-0806b000 rw-p 00000000 00:00 0 [heap]
a7cb1000-a7cb2000 ---p 00000000 00:00 0
a7cb2000-a7eb2000 rw-p 00000000 00:00 0
a7eb2000-a7eb3000 ---p 00000000 00:00 0
a7eb3000-a7ed5000 rw-p 00000000 00:00 0
a7ed5000-a8008000 r-xp 00000000 03:00 4222 /lib/libc.so.6
a8008000-a800a000 r--p 00133000 03:00 4222 /lib/libc.so.6
a800a000-a800b000 rw-p 00135000 03:00 4222 /lib/libc.so.6
a800b000-a800e000 rw-p 00000000 00:00 0
a800e000-a8022000 r-xp 00000000 03:00 14462 /lib/libpthread.so.0
a8022000-a8023000 r--p 00013000 03:00 14462 /lib/libpthread.so.0
a8023000-a8024000 rw-p 00014000 03:00 14462 /lib/libpthread.so.0
a8024000-a8027000 rw-p 00000000 00:00 0
a8027000-a8043000 r-xp 00000000 03:00 8317 /lib/ld-linux.so.2
a8043000-a8044000 r--p 0001b000 03:00 8317 /lib/ld-linux.so.2
a8044000-a8045000 rw-p 0001c000 03:00 8317 /lib/ld-linux.so.2
aff35000-aff4a000 rw-p 00000000 00:00 0 [stack]
ffffe000-fffff000 r-xp 00000000 00:00 0 [vdso]
where "address" is the address space in the process that it occupies, "perms"
is a set of permissions:
r = read
w = write
x = execute
s = shared
p = private (copy on write)
"offset" is the offset into the mapping, "dev" is the device (major:minor), and
"inode" is the inode on that device. 0 indicates that no inode is associated
with the memory region, as the case would be with BSS (uninitialized data).
The "pathname" shows the name associated file for this mapping. If the mapping
is not associated with a file:
[heap] = the heap of the program
[stack] = the stack of the main process
[vdso] = the "virtual dynamic shared object",
the kernel system call handler
or if empty, the mapping is anonymous.
The /proc/PID/smaps is an extension based on maps, showing the memory
consumption for each of the process's mappings. For each of mappings there
is a series of lines such as the following:
08048000-080bc000 r-xp 00000000 03:02 13130 /bin/bash
Size: 1084 kB
Rss: 892 kB
Pss: 374 kB
Shared_Clean: 892 kB
Shared_Dirty: 0 kB
Private_Clean: 0 kB
Private_Dirty: 0 kB
Referenced: 892 kB
Anonymous: 0 kB
LazyFree: 0 kB
AnonHugePages: 0 kB
ShmemPmdMapped: 0 kB
Shared_Hugetlb: 0 kB
Private_Hugetlb: 0 kB
Swap: 0 kB
SwapPss: 0 kB
KernelPageSize: 4 kB
MMUPageSize: 4 kB
Locked: 0 kB
VmFlags: rd ex mr mw me dw
the first of these lines shows the same information as is displayed for the
mapping in /proc/PID/maps. The remaining lines show the size of the mapping
(size), the amount of the mapping that is currently resident in RAM (RSS), the
process' proportional share of this mapping (PSS), the number of clean and
dirty private pages in the mapping.
The "proportional set size" (PSS) of a process is the count of pages it has
in memory, where each page is divided by the number of processes sharing it.
So if a process has 1000 pages all to itself, and 1000 shared with one other
process, its PSS will be 1500.
Note that even a page which is part of a MAP_SHARED mapping, but has only
a single pte mapped, i.e. is currently used by only one process, is accounted
as private and not as shared.
"Referenced" indicates the amount of memory currently marked as referenced or
accessed.
"Anonymous" shows the amount of memory that does not belong to any file. Even
a mapping associated with a file may contain anonymous pages: when MAP_PRIVATE
and a page is modified, the file page is replaced by a private anonymous copy.
"LazyFree" shows the amount of memory which is marked by madvise(MADV_FREE).
The memory isn't freed immediately with madvise(). It's freed in memory
pressure if the memory is clean. Please note that the printed value might
be lower than the real value due to optimizations used in the current
implementation. If this is not desirable please file a bug report.
"AnonHugePages" shows the ammount of memory backed by transparent hugepage.
"ShmemPmdMapped" shows the ammount of shared (shmem/tmpfs) memory backed by
huge pages.
"Shared_Hugetlb" and "Private_Hugetlb" show the ammounts of memory backed by
hugetlbfs page which is *not* counted in "RSS" or "PSS" field for historical
reasons. And these are not included in {Shared,Private}_{Clean,Dirty} field.
"Swap" shows how much would-be-anonymous memory is also used, but out on swap.
For shmem mappings, "Swap" includes also the size of the mapped (and not
replaced by copy-on-write) part of the underlying shmem object out on swap.
"SwapPss" shows proportional swap share of this mapping. Unlike "Swap", this
does not take into account swapped out page of underlying shmem objects.
"Locked" indicates whether the mapping is locked in memory or not.
"VmFlags" field deserves a separate description. This member represents the kernel
flags associated with the particular virtual memory area in two letter encoded
manner. The codes are the following:
rd - readable
wr - writeable
ex - executable
sh - shared
mr - may read
mw - may write
me - may execute
ms - may share
gd - stack segment growns down
pf - pure PFN range
dw - disabled write to the mapped file
lo - pages are locked in memory
io - memory mapped I/O area
sr - sequential read advise provided
rr - random read advise provided
dc - do not copy area on fork
de - do not expand area on remapping
ac - area is accountable
nr - swap space is not reserved for the area
ht - area uses huge tlb pages
ar - architecture specific flag
dd - do not include area into core dump
sd - soft-dirty flag
mm - mixed map area
hg - huge page advise flag
nh - no-huge page advise flag
mg - mergable advise flag
Note that there is no guarantee that every flag and associated mnemonic will
be present in all further kernel releases. Things get changed, the flags may
be vanished or the reverse -- new added.
This file is only present if the CONFIG_MMU kernel configuration option is
enabled.
Note: reading /proc/PID/maps or /proc/PID/smaps is inherently racy (consistent
output can be achieved only in the single read call).
This typically manifests when doing partial reads of these files while the
memory map is being modified. Despite the races, we do provide the following
guarantees:
1) The mapped addresses never go backwards, which implies no two
regions will ever overlap.
2) If there is something at a given vaddr during the entirety of the
life of the smaps/maps walk, there will be some output for it.
The /proc/PID/clear_refs is used to reset the PG_Referenced and ACCESSED/YOUNG
bits on both physical and virtual pages associated with a process, and the
soft-dirty bit on pte (see Documentation/vm/soft-dirty.txt for details).
To clear the bits for all the pages associated with the process
> echo 1 > /proc/PID/clear_refs
To clear the bits for the anonymous pages associated with the process
> echo 2 > /proc/PID/clear_refs
To clear the bits for the file mapped pages associated with the process
> echo 3 > /proc/PID/clear_refs
To clear the soft-dirty bit
> echo 4 > /proc/PID/clear_refs
To reset the peak resident set size ("high water mark") to the process's
current value:
> echo 5 > /proc/PID/clear_refs
Any other value written to /proc/PID/clear_refs will have no effect.
The /proc/pid/pagemap gives the PFN, which can be used to find the pageflags
using /proc/kpageflags and number of times a page is mapped using
/proc/kpagecount. For detailed explanation, see Documentation/vm/pagemap.txt.
The /proc/pid/numa_maps is an extension based on maps, showing the memory
locality and binding policy, as well as the memory usage (in pages) of
each mapping. The output follows a general format where mapping details get
summarized separated by blank spaces, one mapping per each file line:
address policy mapping details
00400000 default file=/usr/local/bin/app mapped=1 active=0 N3=1 kernelpagesize_kB=4
00600000 default file=/usr/local/bin/app anon=1 dirty=1 N3=1 kernelpagesize_kB=4
3206000000 default file=/lib64/ld-2.12.so mapped=26 mapmax=6 N0=24 N3=2 kernelpagesize_kB=4
320621f000 default file=/lib64/ld-2.12.so anon=1 dirty=1 N3=1 kernelpagesize_kB=4
3206220000 default file=/lib64/ld-2.12.so anon=1 dirty=1 N3=1 kernelpagesize_kB=4
3206221000 default anon=1 dirty=1 N3=1 kernelpagesize_kB=4
3206800000 default file=/lib64/libc-2.12.so mapped=59 mapmax=21 active=55 N0=41 N3=18 kernelpagesize_kB=4
320698b000 default file=/lib64/libc-2.12.so
3206b8a000 default file=/lib64/libc-2.12.so anon=2 dirty=2 N3=2 kernelpagesize_kB=4
3206b8e000 default file=/lib64/libc-2.12.so anon=1 dirty=1 N3=1 kernelpagesize_kB=4
3206b8f000 default anon=3 dirty=3 active=1 N3=3 kernelpagesize_kB=4
7f4dc10a2000 default anon=3 dirty=3 N3=3 kernelpagesize_kB=4
7f4dc10b4000 default anon=2 dirty=2 active=1 N3=2 kernelpagesize_kB=4
7f4dc1200000 default file=/anon_hugepage\040(deleted) huge anon=1 dirty=1 N3=1 kernelpagesize_kB=2048
7fff335f0000 default stack anon=3 dirty=3 N3=3 kernelpagesize_kB=4
7fff3369d000 default mapped=1 mapmax=35 active=0 N3=1 kernelpagesize_kB=4
Where:
"address" is the starting address for the mapping;
"policy" reports the NUMA memory policy set for the mapping (see vm/numa_memory_policy.txt);
"mapping details" summarizes mapping data such as mapping type, page usage counters,
node locality page counters (N0 == node0, N1 == node1, ...) and the kernel page
size, in KB, that is backing the mapping up.
1.2 Kernel data
---------------
Similar to the process entries, the kernel data files give information about
the running kernel. The files used to obtain this information are contained in
/proc and are listed in Table 1-5. Not all of these will be present in your
system. It depends on the kernel configuration and the loaded modules, which
files are there, and which are missing.
Table 1-5: Kernel info in /proc
..............................................................................
File Content
apm Advanced power management info
buddyinfo Kernel memory allocator information (see text) (2.5)
bus Directory containing bus specific information
cmdline Kernel command line
cpuinfo Info about the CPU
devices Available devices (block and character)
dma Used DMS channels
filesystems Supported filesystems
driver Various drivers grouped here, currently rtc (2.4)
execdomains Execdomains, related to security (2.4)
fb Frame Buffer devices (2.4)
fs File system parameters, currently nfs/exports (2.4)
ide Directory containing info about the IDE subsystem
interrupts Interrupt usage
iomem Memory map (2.4)
ioports I/O port usage
irq Masks for irq to cpu affinity (2.4)(smp?)
isapnp ISA PnP (Plug&Play) Info (2.4)
kcore Kernel core image (can be ELF or A.OUT(deprecated in 2.4))
kmsg Kernel messages
ksyms Kernel symbol table
loadavg Load average of last 1, 5 & 15 minutes
locks Kernel locks
meminfo Memory info
misc Miscellaneous
modules List of loaded modules
mounts Mounted filesystems
net Networking info (see text)
pagetypeinfo Additional page allocator information (see text) (2.5)
partitions Table of partitions known to the system
pci Deprecated info of PCI bus (new way -> /proc/bus/pci/,
decoupled by lspci (2.4)
rtc Real time clock
scsi SCSI info (see text)
slabinfo Slab pool info
softirqs softirq usage
stat Overall statistics
swaps Swap space utilization
sys See chapter 2
sysvipc Info of SysVIPC Resources (msg, sem, shm) (2.4)
tty Info of tty drivers
uptime Wall clock since boot, combined idle time of all cpus
version Kernel version
video bttv info of video resources (2.4)
vmallocinfo Show vmalloced areas
..............................................................................
You can, for example, check which interrupts are currently in use and what
they are used for by looking in the file /proc/interrupts:
> cat /proc/interrupts
CPU0
0: 8728810 XT-PIC timer
1: 895 XT-PIC keyboard
2: 0 XT-PIC cascade
3: 531695 XT-PIC aha152x
4: 2014133 XT-PIC serial
5: 44401 XT-PIC pcnet_cs
8: 2 XT-PIC rtc
11: 8 XT-PIC i82365
12: 182918 XT-PIC PS/2 Mouse
13: 1 XT-PIC fpu
14: 1232265 XT-PIC ide0
15: 7 XT-PIC ide1
NMI: 0
In 2.4.* a couple of lines where added to this file LOC & ERR (this time is the
output of a SMP machine):
> cat /proc/interrupts
CPU0 CPU1
0: 1243498 1214548 IO-APIC-edge timer
1: 8949 8958 IO-APIC-edge keyboard
2: 0 0 XT-PIC cascade
5: 11286 10161 IO-APIC-edge soundblaster
8: 1 0 IO-APIC-edge rtc
9: 27422 27407 IO-APIC-edge 3c503
12: 113645 113873 IO-APIC-edge PS/2 Mouse
13: 0 0 XT-PIC fpu
14: 22491 24012 IO-APIC-edge ide0
15: 2183 2415 IO-APIC-edge ide1
17: 30564 30414 IO-APIC-level eth0
18: 177 164 IO-APIC-level bttv
NMI: 2457961 2457959
LOC: 2457882 2457881
ERR: 2155
NMI is incremented in this case because every timer interrupt generates a NMI
(Non Maskable Interrupt) which is used by the NMI Watchdog to detect lockups.
LOC is the local interrupt counter of the internal APIC of every CPU.
ERR is incremented in the case of errors in the IO-APIC bus (the bus that
connects the CPUs in a SMP system. This means that an error has been detected,
the IO-APIC automatically retry the transmission, so it should not be a big
problem, but you should read the SMP-FAQ.
In 2.6.2* /proc/interrupts was expanded again. This time the goal was for
/proc/interrupts to display every IRQ vector in use by the system, not
just those considered 'most important'. The new vectors are:
THR -- interrupt raised when a machine check threshold counter
(typically counting ECC corrected errors of memory or cache) exceeds
a configurable threshold. Only available on some systems.
TRM -- a thermal event interrupt occurs when a temperature threshold
has been exceeded for the CPU. This interrupt may also be generated
when the temperature drops back to normal.
SPU -- a spurious interrupt is some interrupt that was raised then lowered
by some IO device before it could be fully processed by the APIC. Hence
the APIC sees the interrupt but does not know what device it came from.
For this case the APIC will generate the interrupt with a IRQ vector
of 0xff. This might also be generated by chipset bugs.
RES, CAL, TLB -- rescheduling, call and TLB flush interrupts are
sent from one CPU to another per the needs of the OS. Typically,
their statistics are used by kernel developers and interested users to
determine the occurrence of interrupts of the given type.
The above IRQ vectors are displayed only when relevant. For example,
the threshold vector does not exist on x86_64 platforms. Others are
suppressed when the system is a uniprocessor. As of this writing, only
i386 and x86_64 platforms support the new IRQ vector displays.
Of some interest is the introduction of the /proc/irq directory to 2.4.
It could be used to set IRQ to CPU affinity, this means that you can "hook" an
IRQ to only one CPU, or to exclude a CPU of handling IRQs. The contents of the
irq subdir is one subdir for each IRQ, and two files; default_smp_affinity and
prof_cpu_mask.
For example
> ls /proc/irq/
0 10 12 14 16 18 2 4 6 8 prof_cpu_mask
1 11 13 15 17 19 3 5 7 9 default_smp_affinity
> ls /proc/irq/0/
smp_affinity
smp_affinity is a bitmask, in which you can specify which CPUs can handle the
IRQ, you can set it by doing:
> echo 1 > /proc/irq/10/smp_affinity
This means that only the first CPU will handle the IRQ, but you can also echo
5 which means that only the first and third CPU can handle the IRQ.
The contents of each smp_affinity file is the same by default:
> cat /proc/irq/0/smp_affinity
ffffffff
There is an alternate interface, smp_affinity_list which allows specifying
a cpu range instead of a bitmask:
> cat /proc/irq/0/smp_affinity_list
1024-1031
The default_smp_affinity mask applies to all non-active IRQs, which are the
IRQs which have not yet been allocated/activated, and hence which lack a
/proc/irq/[0-9]* directory.
The node file on an SMP system shows the node to which the device using the IRQ
reports itself as being attached. This hardware locality information does not
include information about any possible driver locality preference.
prof_cpu_mask specifies which CPUs are to be profiled by the system wide
profiler. Default value is ffffffff (all cpus if there are only 32 of them).
The way IRQs are routed is handled by the IO-APIC, and it's Round Robin
between all the CPUs which are allowed to handle it. As usual the kernel has
more info than you and does a better job than you, so the defaults are the
best choice for almost everyone. [Note this applies only to those IO-APIC's
that support "Round Robin" interrupt distribution.]
There are three more important subdirectories in /proc: net, scsi, and sys.
The general rule is that the contents, or even the existence of these
directories, depend on your kernel configuration. If SCSI is not enabled, the
directory scsi may not exist. The same is true with the net, which is there
only when networking support is present in the running kernel.
The slabinfo file gives information about memory usage at the slab level.
Linux uses slab pools for memory management above page level in version 2.2.
Commonly used objects have their own slab pool (such as network buffers,
directory cache, and so on).
..............................................................................
> cat /proc/buddyinfo
Node 0, zone DMA 0 4 5 4 4 3 ...
Node 0, zone Normal 1 0 0 1 101 8 ...
Node 0, zone HighMem 2 0 0 1 1 0 ...
External fragmentation is a problem under some workloads, and buddyinfo is a
useful tool for helping diagnose these problems. Buddyinfo will give you a
clue as to how big an area you can safely allocate, or why a previous
allocation failed.
Each column represents the number of pages of a certain order which are
available. In this case, there are 0 chunks of 2^0*PAGE_SIZE available in
ZONE_DMA, 4 chunks of 2^1*PAGE_SIZE in ZONE_DMA, 101 chunks of 2^4*PAGE_SIZE
available in ZONE_NORMAL, etc...
More information relevant to external fragmentation can be found in
pagetypeinfo.
> cat /proc/pagetypeinfo
Page block order: 9
Pages per block: 512
Free pages count per migrate type at order 0 1 2 3 4 5 6 7 8 9 10
Node 0, zone DMA, type Unmovable 0 0 0 1 1 1 1 1 1 1 0
Node 0, zone DMA, type Reclaimable 0 0 0 0 0 0 0 0 0 0 0
Node 0, zone DMA, type Movable 1 1 2 1 2 1 1 0 1 0 2
Node 0, zone DMA, type Reserve 0 0 0 0 0 0 0 0 0 1 0
Node 0, zone DMA, type Isolate 0 0 0 0 0 0 0 0 0 0 0
Node 0, zone DMA32, type Unmovable 103 54 77 1 1 1 11 8 7 1 9
Node 0, zone DMA32, type Reclaimable 0 0 2 1 0 0 0 0 1 0 0
Node 0, zone DMA32, type Movable 169 152 113 91 77 54 39 13 6 1 452
Node 0, zone DMA32, type Reserve 1 2 2 2 2 0 1 1 1 1 0
Node 0, zone DMA32, type Isolate 0 0 0 0 0 0 0 0 0 0 0
Number of blocks type Unmovable Reclaimable Movable Reserve Isolate
Node 0, zone DMA 2 0 5 1 0
Node 0, zone DMA32 41 6 967 2 0
Fragmentation avoidance in the kernel works by grouping pages of different
migrate types into the same contiguous regions of memory called page blocks.
A page block is typically the size of the default hugepage size e.g. 2MB on
X86-64. By keeping pages grouped based on their ability to move, the kernel
can reclaim pages within a page block to satisfy a high-order allocation.
The pagetypinfo begins with information on the size of a page block. It
then gives the same type of information as buddyinfo except broken down
by migrate-type and finishes with details on how many page blocks of each
type exist.
If min_free_kbytes has been tuned correctly (recommendations made by hugeadm
from libhugetlbfs https://github.com/libhugetlbfs/libhugetlbfs/), one can
make an estimate of the likely number of huge pages that can be allocated
at a given point in time. All the "Movable" blocks should be allocatable
unless memory has been mlock()'d. Some of the Reclaimable blocks should
also be allocatable although a lot of filesystem metadata may have to be
reclaimed to achieve this.
..............................................................................
meminfo:
Provides information about distribution and utilization of memory. This
varies by architecture and compile options. The following is from a
16GB PIII, which has highmem enabled. You may not have all of these fields.
> cat /proc/meminfo
MemTotal: 16344972 kB
MemFree: 13634064 kB
MemAvailable: 14836172 kB
Buffers: 3656 kB
Cached: 1195708 kB
SwapCached: 0 kB
Active: 891636 kB
Inactive: 1077224 kB
HighTotal: 15597528 kB
HighFree: 13629632 kB
LowTotal: 747444 kB
LowFree: 4432 kB
SwapTotal: 0 kB
SwapFree: 0 kB
Dirty: 968 kB
Writeback: 0 kB
AnonPages: 861800 kB
Mapped: 280372 kB
Shmem: 644 kB
Slab: 284364 kB
SReclaimable: 159856 kB
SUnreclaim: 124508 kB
PageTables: 24448 kB
NFS_Unstable: 0 kB
Bounce: 0 kB
WritebackTmp: 0 kB
CommitLimit: 7669796 kB
Committed_AS: 100056 kB
VmallocTotal: 112216 kB
VmallocUsed: 428 kB
VmallocChunk: 111088 kB
AnonHugePages: 49152 kB
ShmemHugePages: 0 kB
ShmemPmdMapped: 0 kB
MemTotal: Total usable ram (i.e. physical ram minus a few reserved
bits and the kernel binary code)
MemFree: The sum of LowFree+HighFree
MemAvailable: An estimate of how much memory is available for starting new
applications, without swapping. Calculated from MemFree,
SReclaimable, the size of the file LRU lists, and the low
watermarks in each zone.
The estimate takes into account that the system needs some
page cache to function well, and that not all reclaimable
slab will be reclaimable, due to items being in use. The
impact of those factors will vary from system to system.
Buffers: Relatively temporary storage for raw disk blocks
shouldn't get tremendously large (20MB or so)
Cached: in-memory cache for files read from the disk (the
pagecache). Doesn't include SwapCached
SwapCached: Memory that once was swapped out, is swapped back in but
still also is in the swapfile (if memory is needed it
doesn't need to be swapped out AGAIN because it is already
in the swapfile. This saves I/O)
Active: Memory that has been used more recently and usually not
reclaimed unless absolutely necessary.
Inactive: Memory which has been less recently used. It is more
eligible to be reclaimed for other purposes
HighTotal:
HighFree: Highmem is all memory above ~860MB of physical memory
Highmem areas are for use by userspace programs, or
for the pagecache. The kernel must use tricks to access
this memory, making it slower to access than lowmem.
LowTotal:
LowFree: Lowmem is memory which can be used for everything that
highmem can be used for, but it is also available for the
kernel's use for its own data structures. Among many
other things, it is where everything from the Slab is
allocated. Bad things happen when you're out of lowmem.
SwapTotal: total amount of swap space available
SwapFree: Memory which has been evicted from RAM, and is temporarily
on the disk
Dirty: Memory which is waiting to get written back to the disk
Writeback: Memory which is actively being written back to the disk
AnonPages: Non-file backed pages mapped into userspace page tables
AnonHugePages: Non-file backed huge pages mapped into userspace page tables
Mapped: files which have been mmaped, such as libraries
Shmem: Total memory used by shared memory (shmem) and tmpfs
ShmemHugePages: Memory used by shared memory (shmem) and tmpfs allocated
with huge pages
ShmemPmdMapped: Shared memory mapped into userspace with huge pages
Slab: in-kernel data structures cache
SReclaimable: Part of Slab, that might be reclaimed, such as caches
SUnreclaim: Part of Slab, that cannot be reclaimed on memory pressure
PageTables: amount of memory dedicated to the lowest level of page
tables.
NFS_Unstable: NFS pages sent to the server, but not yet committed to stable
storage
Bounce: Memory used for block device "bounce buffers"
WritebackTmp: Memory used by FUSE for temporary writeback buffers
CommitLimit: Based on the overcommit ratio ('vm.overcommit_ratio'),
this is the total amount of memory currently available to
be allocated on the system. This limit is only adhered to
if strict overcommit accounting is enabled (mode 2 in
'vm.overcommit_memory').
The CommitLimit is calculated with the following formula:
CommitLimit = ([total RAM pages] - [total huge TLB pages]) *
overcommit_ratio / 100 + [total swap pages]
For example, on a system with 1G of physical RAM and 7G
of swap with a `vm.overcommit_ratio` of 30 it would
yield a CommitLimit of 7.3G.
For more details, see the memory overcommit documentation
in vm/overcommit-accounting.
Committed_AS: The amount of memory presently allocated on the system.
The committed memory is a sum of all of the memory which
has been allocated by processes, even if it has not been
"used" by them as of yet. A process which malloc()'s 1G
of memory, but only touches 300M of it will show up as
using 1G. This 1G is memory which has been "committed" to
by the VM and can be used at any time by the allocating
application. With strict overcommit enabled on the system
(mode 2 in 'vm.overcommit_memory'),allocations which would
exceed the CommitLimit (detailed above) will not be permitted.
This is useful if one needs to guarantee that processes will
not fail due to lack of memory once that memory has been
successfully allocated.
VmallocTotal: total size of vmalloc memory area
VmallocUsed: amount of vmalloc area which is used
VmallocChunk: largest contiguous block of vmalloc area which is free
..............................................................................
vmallocinfo:
Provides information about vmalloced/vmaped areas. One line per area,
containing the virtual address range of the area, size in bytes,
caller information of the creator, and optional information depending
on the kind of area :
pages=nr number of pages
phys=addr if a physical address was specified
ioremap I/O mapping (ioremap() and friends)
vmalloc vmalloc() area
vmap vmap()ed pages
user VM_USERMAP area
vpages buffer for pages pointers was vmalloced (huge area)
N<node>=nr (Only on NUMA kernels)
Number of pages allocated on memory node <node>
> cat /proc/vmallocinfo
0xffffc20000000000-0xffffc20000201000 2101248 alloc_large_system_hash+0x204 ...
/0x2c0 pages=512 vmalloc N0=128 N1=128 N2=128 N3=128
0xffffc20000201000-0xffffc20000302000 1052672 alloc_large_system_hash+0x204 ...
/0x2c0 pages=256 vmalloc N0=64 N1=64 N2=64 N3=64
0xffffc20000302000-0xffffc20000304000 8192 acpi_tb_verify_table+0x21/0x4f...
phys=7fee8000 ioremap
0xffffc20000304000-0xffffc20000307000 12288 acpi_tb_verify_table+0x21/0x4f...
phys=7fee7000 ioremap
0xffffc2000031d000-0xffffc2000031f000 8192 init_vdso_vars+0x112/0x210
0xffffc2000031f000-0xffffc2000032b000 49152 cramfs_uncompress_init+0x2e ...
/0x80 pages=11 vmalloc N0=3 N1=3 N2=2 N3=3
0xffffc2000033a000-0xffffc2000033d000 12288 sys_swapon+0x640/0xac0 ...
pages=2 vmalloc N1=2
0xffffc20000347000-0xffffc2000034c000 20480 xt_alloc_table_info+0xfe ...
/0x130 [x_tables] pages=4 vmalloc N0=4
0xffffffffa0000000-0xffffffffa000f000 61440 sys_init_module+0xc27/0x1d00 ...
pages=14 vmalloc N2=14
0xffffffffa000f000-0xffffffffa0014000 20480 sys_init_module+0xc27/0x1d00 ...
pages=4 vmalloc N1=4
0xffffffffa0014000-0xffffffffa0017000 12288 sys_init_module+0xc27/0x1d00 ...
pages=2 vmalloc N1=2
0xffffffffa0017000-0xffffffffa0022000 45056 sys_init_module+0xc27/0x1d00 ...
pages=10 vmalloc N0=10
..............................................................................
softirqs:
Provides counts of softirq handlers serviced since boot time, for each cpu.
> cat /proc/softirqs
CPU0 CPU1 CPU2 CPU3
HI: 0 0 0 0
TIMER: 27166 27120 27097 27034
NET_TX: 0 0 0 17
NET_RX: 42 0 0 39
BLOCK: 0 0 107 1121
TASKLET: 0 0 0 290
SCHED: 27035 26983 26971 26746
HRTIMER: 0 0 0 0
RCU: 1678 1769 2178 2250
1.3 IDE devices in /proc/ide
----------------------------
The subdirectory /proc/ide contains information about all IDE devices of which
the kernel is aware. There is one subdirectory for each IDE controller, the
file drivers and a link for each IDE device, pointing to the device directory
in the controller specific subtree.
The file drivers contains general information about the drivers used for the
IDE devices:
> cat /proc/ide/drivers
ide-cdrom version 4.53
ide-disk version 1.08
More detailed information can be found in the controller specific
subdirectories. These are named ide0, ide1 and so on. Each of these
directories contains the files shown in table 1-6.
Table 1-6: IDE controller info in /proc/ide/ide?
..............................................................................
File Content
channel IDE channel (0 or 1)
config Configuration (only for PCI/IDE bridge)
mate Mate name
model Type/Chipset of IDE controller
..............................................................................
Each device connected to a controller has a separate subdirectory in the
controllers directory. The files listed in table 1-7 are contained in these
directories.
Table 1-7: IDE device information
..............................................................................
File Content
cache The cache
capacity Capacity of the medium (in 512Byte blocks)
driver driver and version
geometry physical and logical geometry
identify device identify block
media media type
model device identifier
settings device setup
smart_thresholds IDE disk management thresholds
smart_values IDE disk management values
..............................................................................
The most interesting file is settings. This file contains a nice overview of
the drive parameters:
# cat /proc/ide/ide0/hda/settings
name value min max mode
---- ----- --- --- ----
bios_cyl 526 0 65535 rw
bios_head 255 0 255 rw
bios_sect 63 0 63 rw
breada_readahead 4 0 127 rw
bswap 0 0 1 r
file_readahead 72 0 2097151 rw
io_32bit 0 0 3 rw
keepsettings 0 0 1 rw
max_kb_per_request 122 1 127 rw
multcount 0 0 8 rw
nice1 1 0 1 rw
nowerr 0 0 1 rw
pio_mode write-only 0 255 w
slow 0 0 1 rw
unmaskirq 0 0 1 rw
using_dma 0 0 1 rw
1.4 Networking info in /proc/net
--------------------------------
The subdirectory /proc/net follows the usual pattern. Table 1-8 shows the
additional values you get for IP version 6 if you configure the kernel to
support this. Table 1-9 lists the files and their meaning.
Table 1-8: IPv6 info in /proc/net
..............................................................................
File Content
udp6 UDP sockets (IPv6)
tcp6 TCP sockets (IPv6)
raw6 Raw device statistics (IPv6)
igmp6 IP multicast addresses, which this host joined (IPv6)
if_inet6 List of IPv6 interface addresses
ipv6_route Kernel routing table for IPv6
rt6_stats Global IPv6 routing tables statistics
sockstat6 Socket statistics (IPv6)
snmp6 Snmp data (IPv6)
..............................................................................
Table 1-9: Network info in /proc/net
..............................................................................
File Content
arp Kernel ARP table
dev network devices with statistics
dev_mcast the Layer2 multicast groups a device is listening too
(interface index, label, number of references, number of bound
addresses).
dev_stat network device status
ip_fwchains Firewall chain linkage
ip_fwnames Firewall chain names
ip_masq Directory containing the masquerading tables
ip_masquerade Major masquerading table
netstat Network statistics
raw raw device statistics
route Kernel routing table
rpc Directory containing rpc info
rt_cache Routing cache
snmp SNMP data
sockstat Socket statistics
tcp TCP sockets
udp UDP sockets
unix UNIX domain sockets
wireless Wireless interface data (Wavelan etc)
igmp IP multicast addresses, which this host joined
psched Global packet scheduler parameters.
netlink List of PF_NETLINK sockets
ip_mr_vifs List of multicast virtual interfaces
ip_mr_cache List of multicast routing cache
..............................................................................
You can use this information to see which network devices are available in
your system and how much traffic was routed over those devices:
> cat /proc/net/dev
Inter-|Receive |[...
face |bytes packets errs drop fifo frame compressed multicast|[...
lo: 908188 5596 0 0 0 0 0 0 [...
ppp0:15475140 20721 410 0 0 410 0 0 [...
eth0: 614530 7085 0 0 0 0 0 1 [...
...] Transmit
...] bytes packets errs drop fifo colls carrier compressed
...] 908188 5596 0 0 0 0 0 0
...] 1375103 17405 0 0 0 0 0 0
...] 1703981 5535 0 0 0 3 0 0
In addition, each Channel Bond interface has its own directory. For
example, the bond0 device will have a directory called /proc/net/bond0/.
It will contain information that is specific to that bond, such as the
current slaves of the bond, the link status of the slaves, and how
many times the slaves link has failed.
1.5 SCSI info
-------------
If you have a SCSI host adapter in your system, you'll find a subdirectory
named after the driver for this adapter in /proc/scsi. You'll also see a list
of all recognized SCSI devices in /proc/scsi:
>cat /proc/scsi/scsi
Attached devices:
Host: scsi0 Channel: 00 Id: 00 Lun: 00
Vendor: IBM Model: DGHS09U Rev: 03E0
Type: Direct-Access ANSI SCSI revision: 03
Host: scsi0 Channel: 00 Id: 06 Lun: 00
Vendor: PIONEER Model: CD-ROM DR-U06S Rev: 1.04
Type: CD-ROM ANSI SCSI revision: 02
The directory named after the driver has one file for each adapter found in
the system. These files contain information about the controller, including
the used IRQ and the IO address range. The amount of information shown is
dependent on the adapter you use. The example shows the output for an Adaptec
AHA-2940 SCSI adapter:
> cat /proc/scsi/aic7xxx/0
Adaptec AIC7xxx driver version: 5.1.19/3.2.4
Compile Options:
TCQ Enabled By Default : Disabled
AIC7XXX_PROC_STATS : Disabled
AIC7XXX_RESET_DELAY : 5
Adapter Configuration:
SCSI Adapter: Adaptec AHA-294X Ultra SCSI host adapter
Ultra Wide Controller
PCI MMAPed I/O Base: 0xeb001000
Adapter SEEPROM Config: SEEPROM found and used.
Adaptec SCSI BIOS: Enabled
IRQ: 10
SCBs: Active 0, Max Active 2,
Allocated 15, HW 16, Page 255
Interrupts: 160328
BIOS Control Word: 0x18b6
Adapter Control Word: 0x005b
Extended Translation: Enabled
Disconnect Enable Flags: 0xffff
Ultra Enable Flags: 0x0001
Tag Queue Enable Flags: 0x0000
Ordered Queue Tag Flags: 0x0000
Default Tag Queue Depth: 8
Tagged Queue By Device array for aic7xxx host instance 0:
{255,255,255,255,255,255,255,255,255,255,255,255,255,255,255,255}
Actual queue depth per device for aic7xxx host instance 0:
{1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1}
Statistics:
(scsi0:0:0:0)
Device using Wide/Sync transfers at 40.0 MByte/sec, offset 8
Transinfo settings: current(12/8/1/0), goal(12/8/1/0), user(12/15/1/0)
Total transfers 160151 (74577 reads and 85574 writes)
(scsi0:0:6:0)
Device using Narrow/Sync transfers at 5.0 MByte/sec, offset 15
Transinfo settings: current(50/15/0/0), goal(50/15/0/0), user(50/15/0/0)
Total transfers 0 (0 reads and 0 writes)
1.6 Parallel port info in /proc/parport
---------------------------------------
The directory /proc/parport contains information about the parallel ports of
your system. It has one subdirectory for each port, named after the port
number (0,1,2,...).
These directories contain the four files shown in Table 1-10.
Table 1-10: Files in /proc/parport
..............................................................................
File Content
autoprobe Any IEEE-1284 device ID information that has been acquired.
devices list of the device drivers using that port. A + will appear by the
name of the device currently using the port (it might not appear
against any).
hardware Parallel port's base address, IRQ line and DMA channel.
irq IRQ that parport is using for that port. This is in a separate
file to allow you to alter it by writing a new value in (IRQ
number or none).
..............................................................................
1.7 TTY info in /proc/tty
-------------------------
Information about the available and actually used tty's can be found in the
directory /proc/tty.You'll find entries for drivers and line disciplines in
this directory, as shown in Table 1-11.
Table 1-11: Files in /proc/tty
..............................................................................
File Content
drivers list of drivers and their usage
ldiscs registered line disciplines
driver/serial usage statistic and status of single tty lines
..............................................................................
To see which tty's are currently in use, you can simply look into the file
/proc/tty/drivers:
> cat /proc/tty/drivers
pty_slave /dev/pts 136 0-255 pty:slave
pty_master /dev/ptm 128 0-255 pty:master
pty_slave /dev/ttyp 3 0-255 pty:slave
pty_master /dev/pty 2 0-255 pty:master
serial /dev/cua 5 64-67 serial:callout
serial /dev/ttyS 4 64-67 serial
/dev/tty0 /dev/tty0 4 0 system:vtmaster
/dev/ptmx /dev/ptmx 5 2 system
/dev/console /dev/console 5 1 system:console
/dev/tty /dev/tty 5 0 system:/dev/tty
unknown /dev/tty 4 1-63 console
1.8 Miscellaneous kernel statistics in /proc/stat
-------------------------------------------------
Various pieces of information about kernel activity are available in the
/proc/stat file. All of the numbers reported in this file are aggregates
since the system first booted. For a quick look, simply cat the file:
> cat /proc/stat
cpu 2255 34 2290 22625563 6290 127 456 0 0 0
cpu0 1132 34 1441 11311718 3675 127 438 0 0 0
cpu1 1123 0 849 11313845 2614 0 18 0 0 0
intr 114930548 113199788 3 0 5 263 0 4 [... lots more numbers ...]
ctxt 1990473
btime 1062191376
processes 2915
procs_running 1
procs_blocked 0
softirq 183433 0 21755 12 39 1137 231 21459 2263
The very first "cpu" line aggregates the numbers in all of the other "cpuN"
lines. These numbers identify the amount of time the CPU has spent performing
different kinds of work. Time units are in USER_HZ (typically hundredths of a
second). The meanings of the columns are as follows, from left to right:
- user: normal processes executing in user mode
- nice: niced processes executing in user mode
- system: processes executing in kernel mode
- idle: twiddling thumbs
- iowait: In a word, iowait stands for waiting for I/O to complete. But there
are several problems:
1. Cpu will not wait for I/O to complete, iowait is the time that a task is
waiting for I/O to complete. When cpu goes into idle state for
outstanding task io, another task will be scheduled on this CPU.
2. In a multi-core CPU, the task waiting for I/O to complete is not running
on any CPU, so the iowait of each CPU is difficult to calculate.
3. The value of iowait field in /proc/stat will decrease in certain
conditions.
So, the iowait is not reliable by reading from /proc/stat.
- irq: servicing interrupts
- softirq: servicing softirqs
- steal: involuntary wait
- guest: running a normal guest
- guest_nice: running a niced guest
The "intr" line gives counts of interrupts serviced since boot time, for each
of the possible system interrupts. The first column is the total of all
interrupts serviced including unnumbered architecture specific interrupts;
each subsequent column is the total for that particular numbered interrupt.
Unnumbered interrupts are not shown, only summed into the total.
The "ctxt" line gives the total number of context switches across all CPUs.
The "btime" line gives the time at which the system booted, in seconds since
the Unix epoch.
The "processes" line gives the number of processes and threads created, which
includes (but is not limited to) those created by calls to the fork() and
clone() system calls.
The "procs_running" line gives the total number of threads that are
running or ready to run (i.e., the total number of runnable threads).
The "procs_blocked" line gives the number of processes currently blocked,
waiting for I/O to complete.
The "softirq" line gives counts of softirqs serviced since boot time, for each
of the possible system softirqs. The first column is the total of all
softirqs serviced; each subsequent column is the total for that particular
softirq.
1.9 Ext4 file system parameters
-------------------------------
Information about mounted ext4 file systems can be found in
/proc/fs/ext4. Each mounted filesystem will have a directory in
/proc/fs/ext4 based on its device name (i.e., /proc/fs/ext4/hdc or
/proc/fs/ext4/dm-0). The files in each per-device directory are shown
in Table 1-12, below.
Table 1-12: Files in /proc/fs/ext4/<devname>
..............................................................................
File Content
mb_groups details of multiblock allocator buddy cache of free blocks
..............................................................................
2.0 /proc/consoles
------------------
Shows registered system console lines.
To see which character device lines are currently used for the system console
/dev/console, you may simply look into the file /proc/consoles:
> cat /proc/consoles
tty0 -WU (ECp) 4:7
ttyS0 -W- (Ep) 4:64
The columns are:
device name of the device
operations R = can do read operations
W = can do write operations
U = can do unblank
flags E = it is enabled
C = it is preferred console
B = it is primary boot console
p = it is used for printk buffer
b = it is not a TTY but a Braille device
a = it is safe to use when cpu is offline
major:minor major and minor number of the device separated by a colon
------------------------------------------------------------------------------
Summary
------------------------------------------------------------------------------
The /proc file system serves information about the running system. It not only
allows access to process data but also allows you to request the kernel status
by reading files in the hierarchy.
The directory structure of /proc reflects the types of information and makes
it easy, if not obvious, where to look for specific data.
------------------------------------------------------------------------------
------------------------------------------------------------------------------
CHAPTER 2: MODIFYING SYSTEM PARAMETERS
------------------------------------------------------------------------------
------------------------------------------------------------------------------
In This Chapter
------------------------------------------------------------------------------
* Modifying kernel parameters by writing into files found in /proc/sys
* Exploring the files which modify certain parameters
* Review of the /proc/sys file tree
------------------------------------------------------------------------------
A very interesting part of /proc is the directory /proc/sys. This is not only
a source of information, it also allows you to change parameters within the
kernel. Be very careful when attempting this. You can optimize your system,
but you can also cause it to crash. Never alter kernel parameters on a
production system. Set up a development machine and test to make sure that
everything works the way you want it to. You may have no alternative but to
reboot the machine once an error has been made.
To change a value, simply echo the new value into the file. An example is
given below in the section on the file system data. You need to be root to do
this. You can create your own boot script to perform this every time your
system boots.
The files in /proc/sys can be used to fine tune and monitor miscellaneous and
general things in the operation of the Linux kernel. Since some of the files
can inadvertently disrupt your system, it is advisable to read both
documentation and source before actually making adjustments. In any case, be
very careful when writing to any of these files. The entries in /proc may
change slightly between the 2.1.* and the 2.2 kernel, so if there is any doubt
review the kernel documentation in the directory /usr/src/linux/Documentation.
This chapter is heavily based on the documentation included in the pre 2.2
kernels, and became part of it in version 2.2.1 of the Linux kernel.
Please see: Documentation/sysctl/ directory for descriptions of these
entries.
------------------------------------------------------------------------------
Summary
------------------------------------------------------------------------------
Certain aspects of kernel behavior can be modified at runtime, without the
need to recompile the kernel, or even to reboot the system. The files in the
/proc/sys tree can not only be read, but also modified. You can use the echo
command to write value into these files, thereby changing the default settings
of the kernel.
------------------------------------------------------------------------------
------------------------------------------------------------------------------
CHAPTER 3: PER-PROCESS PARAMETERS
------------------------------------------------------------------------------
3.1 /proc/<pid>/oom_adj & /proc/<pid>/oom_score_adj- Adjust the oom-killer score
--------------------------------------------------------------------------------
These file can be used to adjust the badness heuristic used to select which
process gets killed in out of memory conditions.
The badness heuristic assigns a value to each candidate task ranging from 0
(never kill) to 1000 (always kill) to determine which process is targeted. The
units are roughly a proportion along that range of allowed memory the process
may allocate from based on an estimation of its current memory and swap use.
For example, if a task is using all allowed memory, its badness score will be
1000. If it is using half of its allowed memory, its score will be 500.
There is an additional factor included in the badness score: the current memory
and swap usage is discounted by 3% for root processes.
The amount of "allowed" memory depends on the context in which the oom killer
was called. If it is due to the memory assigned to the allocating task's cpuset
being exhausted, the allowed memory represents the set of mems assigned to that
cpuset. If it is due to a mempolicy's node(s) being exhausted, the allowed
memory represents the set of mempolicy nodes. If it is due to a memory
limit (or swap limit) being reached, the allowed memory is that configured
limit. Finally, if it is due to the entire system being out of memory, the
allowed memory represents all allocatable resources.
The value of /proc/<pid>/oom_score_adj is added to the badness score before it
is used to determine which task to kill. Acceptable values range from -1000
(OOM_SCORE_ADJ_MIN) to +1000 (OOM_SCORE_ADJ_MAX). This allows userspace to
polarize the preference for oom killing either by always preferring a certain
task or completely disabling it. The lowest possible value, -1000, is
equivalent to disabling oom killing entirely for that task since it will always
report a badness score of 0.
Consequently, it is very simple for userspace to define the amount of memory to
consider for each task. Setting a /proc/<pid>/oom_score_adj value of +500, for
example, is roughly equivalent to allowing the remainder of tasks sharing the
same system, cpuset, mempolicy, or memory controller resources to use at least
50% more memory. A value of -500, on the other hand, would be roughly
equivalent to discounting 50% of the task's allowed memory from being considered
as scoring against the task.
For backwards compatibility with previous kernels, /proc/<pid>/oom_adj may also
be used to tune the badness score. Its acceptable values range from -16
(OOM_ADJUST_MIN) to +15 (OOM_ADJUST_MAX) and a special value of -17
(OOM_DISABLE) to disable oom killing entirely for that task. Its value is
scaled linearly with /proc/<pid>/oom_score_adj.
The value of /proc/<pid>/oom_score_adj may be reduced no lower than the last
value set by a CAP_SYS_RESOURCE process. To reduce the value any lower
requires CAP_SYS_RESOURCE.
Caveat: when a parent task is selected, the oom killer will sacrifice any first
generation children with separate address spaces instead, if possible. This
avoids servers and important system daemons from being killed and loses the
minimal amount of work.
3.2 /proc/<pid>/oom_score - Display current oom-killer score
-------------------------------------------------------------
This file can be used to check the current score used by the oom-killer is for
any given <pid>. Use it together with /proc/<pid>/oom_score_adj to tune which
process should be killed in an out-of-memory situation.
3.3 /proc/<pid>/io - Display the IO accounting fields
-------------------------------------------------------
This file contains IO statistics for each running process
Example
-------
test:/tmp # dd if=/dev/zero of=/tmp/test.dat &
[1] 3828
test:/tmp # cat /proc/3828/io
rchar: 323934931
wchar: 323929600
syscr: 632687
syscw: 632675
read_bytes: 0
write_bytes: 323932160
cancelled_write_bytes: 0
Description
-----------
rchar
-----
I/O counter: chars read
The number of bytes which this task has caused to be read from storage. This
is simply the sum of bytes which this process passed to read() and pread().
It includes things like tty IO and it is unaffected by whether or not actual
physical disk IO was required (the read might have been satisfied from
pagecache)
wchar
-----
I/O counter: chars written
The number of bytes which this task has caused, or shall cause to be written
to disk. Similar caveats apply here as with rchar.
syscr
-----
I/O counter: read syscalls
Attempt to count the number of read I/O operations, i.e. syscalls like read()
and pread().
syscw
-----
I/O counter: write syscalls
Attempt to count the number of write I/O operations, i.e. syscalls like
write() and pwrite().
read_bytes
----------
I/O counter: bytes read
Attempt to count the number of bytes which this process really did cause to
be fetched from the storage layer. Done at the submit_bio() level, so it is
accurate for block-backed filesystems. <please add status regarding NFS and
CIFS at a later time>
write_bytes
-----------
I/O counter: bytes written
Attempt to count the number of bytes which this process caused to be sent to
the storage layer. This is done at page-dirtying time.
cancelled_write_bytes
---------------------
The big inaccuracy here is truncate. If a process writes 1MB to a file and
then deletes the file, it will in fact perform no writeout. But it will have
been accounted as having caused 1MB of write.
In other words: The number of bytes which this process caused to not happen,
by truncating pagecache. A task can cause "negative" IO too. If this task
truncates some dirty pagecache, some IO which another task has been accounted
for (in its write_bytes) will not be happening. We _could_ just subtract that
from the truncating task's write_bytes, but there is information loss in doing
that.
Note
----
At its current implementation state, this is a bit racy on 32-bit machines: if
process A reads process B's /proc/pid/io while process B is updating one of
those 64-bit counters, process A could see an intermediate result.
More information about this can be found within the taskstats documentation in
Documentation/accounting.
3.4 /proc/<pid>/coredump_filter - Core dump filtering settings
---------------------------------------------------------------
When a process is dumped, all anonymous memory is written to a core file as
long as the size of the core file isn't limited. But sometimes we don't want
to dump some memory segments, for example, huge shared memory or DAX.
Conversely, sometimes we want to save file-backed memory segments into a core
file, not only the individual files.
/proc/<pid>/coredump_filter allows you to customize which memory segments
will be dumped when the <pid> process is dumped. coredump_filter is a bitmask
of memory types. If a bit of the bitmask is set, memory segments of the
corresponding memory type are dumped, otherwise they are not dumped.
The following 9 memory types are supported:
- (bit 0) anonymous private memory
- (bit 1) anonymous shared memory
- (bit 2) file-backed private memory
- (bit 3) file-backed shared memory
- (bit 4) ELF header pages in file-backed private memory areas (it is
effective only if the bit 2 is cleared)
- (bit 5) hugetlb private memory
- (bit 6) hugetlb shared memory
- (bit 7) DAX private memory
- (bit 8) DAX shared memory
Note that MMIO pages such as frame buffer are never dumped and vDSO pages
are always dumped regardless of the bitmask status.
Note that bits 0-4 don't affect hugetlb or DAX memory. hugetlb memory is
only affected by bit 5-6, and DAX is only affected by bits 7-8.
The default value of coredump_filter is 0x33; this means all anonymous memory
segments, ELF header pages and hugetlb private memory are dumped.
If you don't want to dump all shared memory segments attached to pid 1234,
write 0x31 to the process's proc file.
$ echo 0x31 > /proc/1234/coredump_filter
When a new process is created, the process inherits the bitmask status from its
parent. It is useful to set up coredump_filter before the program runs.
For example:
$ echo 0x7 > /proc/self/coredump_filter
$ ./some_program
3.5 /proc/<pid>/mountinfo - Information about mounts
--------------------------------------------------------
This file contains lines of the form:
36 35 98:0 /mnt1 /mnt2 rw,noatime master:1 - ext3 /dev/root rw,errors=continue
(1)(2)(3) (4) (5) (6) (7) (8) (9) (10) (11)
(1) mount ID: unique identifier of the mount (may be reused after umount)
(2) parent ID: ID of parent (or of self for the top of the mount tree)
(3) major:minor: value of st_dev for files on filesystem
(4) root: root of the mount within the filesystem
(5) mount point: mount point relative to the process's root
(6) mount options: per mount options
(7) optional fields: zero or more fields of the form "tag[:value]"
(8) separator: marks the end of the optional fields
(9) filesystem type: name of filesystem of the form "type[.subtype]"
(10) mount source: filesystem specific information or "none"
(11) super options: per super block options
Parsers should ignore all unrecognised optional fields. Currently the
possible optional fields are:
shared:X mount is shared in peer group X
master:X mount is slave to peer group X
propagate_from:X mount is slave and receives propagation from peer group X (*)
unbindable mount is unbindable
(*) X is the closest dominant peer group under the process's root. If
X is the immediate master of the mount, or if there's no dominant peer
group under the same root, then only the "master:X" field is present
and not the "propagate_from:X" field.
For more information on mount propagation see:
Documentation/filesystems/sharedsubtree.txt
3.6 /proc/<pid>/comm & /proc/<pid>/task/<tid>/comm
--------------------------------------------------------
These files provide a method to access a tasks comm value. It also allows for
a task to set its own or one of its thread siblings comm value. The comm value
is limited in size compared to the cmdline value, so writing anything longer
then the kernel's TASK_COMM_LEN (currently 16 chars) will result in a truncated
comm value.
3.7 /proc/<pid>/task/<tid>/children - Information about task children
-------------------------------------------------------------------------
This file provides a fast way to retrieve first level children pids
of a task pointed by <pid>/<tid> pair. The format is a space separated
stream of pids.
Note the "first level" here -- if a child has own children they will
not be listed here, one needs to read /proc/<children-pid>/task/<tid>/children
to obtain the descendants.
Since this interface is intended to be fast and cheap it doesn't
guarantee to provide precise results and some children might be
skipped, especially if they've exited right after we printed their
pids, so one need to either stop or freeze processes being inspected
if precise results are needed.
3.8 /proc/<pid>/fdinfo/<fd> - Information about opened file
---------------------------------------------------------------
This file provides information associated with an opened file. The regular
files have at least three fields -- 'pos', 'flags' and mnt_id. The 'pos'
represents the current offset of the opened file in decimal form [see lseek(2)
for details], 'flags' denotes the octal O_xxx mask the file has been
created with [see open(2) for details] and 'mnt_id' represents mount ID of
the file system containing the opened file [see 3.5 /proc/<pid>/mountinfo
for details].
A typical output is
pos: 0
flags: 0100002
mnt_id: 19
All locks associated with a file descriptor are shown in its fdinfo too.
lock: 1: FLOCK ADVISORY WRITE 359 00:13:11691 0 EOF
The files such as eventfd, fsnotify, signalfd, epoll among the regular pos/flags
pair provide additional information particular to the objects they represent.
Eventfd files
~~~~~~~~~~~~~
pos: 0
flags: 04002
mnt_id: 9
eventfd-count: 5a
where 'eventfd-count' is hex value of a counter.
Signalfd files
~~~~~~~~~~~~~~
pos: 0
flags: 04002
mnt_id: 9
sigmask: 0000000000000200
where 'sigmask' is hex value of the signal mask associated
with a file.
Epoll files
~~~~~~~~~~~
pos: 0
flags: 02
mnt_id: 9
tfd: 5 events: 1d data: ffffffffffffffff
where 'tfd' is a target file descriptor number in decimal form,
'events' is events mask being watched and the 'data' is data
associated with a target [see epoll(7) for more details].
Fsnotify files
~~~~~~~~~~~~~~
For inotify files the format is the following
pos: 0
flags: 02000000
inotify wd:3 ino:9e7e sdev:800013 mask:800afce ignored_mask:0 fhandle-bytes:8 fhandle-type:1 f_handle:7e9e0000640d1b6d
where 'wd' is a watch descriptor in decimal form, ie a target file
descriptor number, 'ino' and 'sdev' are inode and device where the
target file resides and the 'mask' is the mask of events, all in hex
form [see inotify(7) for more details].
If the kernel was built with exportfs support, the path to the target
file is encoded as a file handle. The file handle is provided by three
fields 'fhandle-bytes', 'fhandle-type' and 'f_handle', all in hex
format.
If the kernel is built without exportfs support the file handle won't be
printed out.
If there is no inotify mark attached yet the 'inotify' line will be omitted.
For fanotify files the format is
pos: 0
flags: 02
mnt_id: 9
fanotify flags:10 event-flags:0
fanotify mnt_id:12 mflags:40 mask:38 ignored_mask:40000003
fanotify ino:4f969 sdev:800013 mflags:0 mask:3b ignored_mask:40000000 fhandle-bytes:8 fhandle-type:1 f_handle:69f90400c275b5b4
where fanotify 'flags' and 'event-flags' are values used in fanotify_init
call, 'mnt_id' is the mount point identifier, 'mflags' is the value of
flags associated with mark which are tracked separately from events
mask. 'ino', 'sdev' are target inode and device, 'mask' is the events
mask and 'ignored_mask' is the mask of events which are to be ignored.
All in hex format. Incorporation of 'mflags', 'mask' and 'ignored_mask'
does provide information about flags and mask used in fanotify_mark
call [see fsnotify manpage for details].
While the first three lines are mandatory and always printed, the rest is
optional and may be omitted if no marks created yet.
Timerfd files
~~~~~~~~~~~~~
pos: 0
flags: 02
mnt_id: 9
clockid: 0
ticks: 0
settime flags: 01
it_value: (0, 49406829)
it_interval: (1, 0)
where 'clockid' is the clock type and 'ticks' is the number of the timer expirations
that have occurred [see timerfd_create(2) for details]. 'settime flags' are
flags in octal form been used to setup the timer [see timerfd_settime(2) for
details]. 'it_value' is remaining time until the timer exiration.
'it_interval' is the interval for the timer. Note the timer might be set up
with TIMER_ABSTIME option which will be shown in 'settime flags', but 'it_value'
still exhibits timer's remaining time.
3.9 /proc/<pid>/map_files - Information about memory mapped files
---------------------------------------------------------------------
This directory contains symbolic links which represent memory mapped files
the process is maintaining. Example output:
| lr-------- 1 root root 64 Jan 27 11:24 333c600000-333c620000 -> /usr/lib64/ld-2.18.so
| lr-------- 1 root root 64 Jan 27 11:24 333c81f000-333c820000 -> /usr/lib64/ld-2.18.so
| lr-------- 1 root root 64 Jan 27 11:24 333c820000-333c821000 -> /usr/lib64/ld-2.18.so
| ...
| lr-------- 1 root root 64 Jan 27 11:24 35d0421000-35d0422000 -> /usr/lib64/libselinux.so.1
| lr-------- 1 root root 64 Jan 27 11:24 400000-41a000 -> /usr/bin/ls
The name of a link represents the virtual memory bounds of a mapping, i.e.
vm_area_struct::vm_start-vm_area_struct::vm_end.
The main purpose of the map_files is to retrieve a set of memory mapped
files in a fast way instead of parsing /proc/<pid>/maps or
/proc/<pid>/smaps, both of which contain many more records. At the same
time one can open(2) mappings from the listings of two processes and
comparing their inode numbers to figure out which anonymous memory areas
are actually shared.
3.10 /proc/<pid>/timerslack_ns - Task timerslack value
---------------------------------------------------------
This file provides the value of the task's timerslack value in nanoseconds.
This value specifies a amount of time that normal timers may be deferred
in order to coalesce timers and avoid unnecessary wakeups.
This allows a task's interactivity vs power consumption trade off to be
adjusted.
Writing 0 to the file will set the tasks timerslack to the default value.
Valid values are from 0 - ULLONG_MAX
An application setting the value must have PTRACE_MODE_ATTACH_FSCREDS level
permissions on the task specified to change its timerslack_ns value.
3.11 /proc/<pid>/patch_state - Livepatch patch operation state
-----------------------------------------------------------------
When CONFIG_LIVEPATCH is enabled, this file displays the value of the
patch state for the task.
A value of '-1' indicates that no patch is in transition.
A value of '0' indicates that a patch is in transition and the task is
unpatched. If the patch is being enabled, then the task hasn't been
patched yet. If the patch is being disabled, then the task has already
been unpatched.
A value of '1' indicates that a patch is in transition and the task is
patched. If the patch is being enabled, then the task has already been
patched. If the patch is being disabled, then the task hasn't been
unpatched yet.
------------------------------------------------------------------------------
Configuring procfs
------------------------------------------------------------------------------
4.1 Mount options
---------------------
The following mount options are supported:
hidepid= Set /proc/<pid>/ access mode.
gid= Set the group authorized to learn processes information.
hidepid=0 means classic mode - everybody may access all /proc/<pid>/ directories
(default).
hidepid=1 means users may not access any /proc/<pid>/ directories but their
own. Sensitive files like cmdline, sched*, status are now protected against
other users. This makes it impossible to learn whether any user runs
specific program (given the program doesn't reveal itself by its behaviour).
As an additional bonus, as /proc/<pid>/cmdline is unaccessible for other users,
poorly written programs passing sensitive information via program arguments are
now protected against local eavesdroppers.
hidepid=2 means hidepid=1 plus all /proc/<pid>/ will be fully invisible to other
users. It doesn't mean that it hides a fact whether a process with a specific
pid value exists (it can be learned by other means, e.g. by "kill -0 $PID"),
but it hides process' uid and gid, which may be learned by stat()'ing
/proc/<pid>/ otherwise. It greatly complicates an intruder's task of gathering
information about running processes, whether some daemon runs with elevated
privileges, whether other user runs some sensitive program, whether other users
run any program at all, etc.
gid= defines a group authorized to learn processes information otherwise
prohibited by hidepid=. If you use some daemon like identd which needs to learn
information about processes information, just add identd to this group.
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