109 lines
5.1 KiB
Plaintext
109 lines
5.1 KiB
Plaintext
This document explains the thinking about the revamped and streamlined
|
|
nice-levels implementation in the new Linux scheduler.
|
|
|
|
Nice levels were always pretty weak under Linux and people continuously
|
|
pestered us to make nice +19 tasks use up much less CPU time.
|
|
|
|
Unfortunately that was not that easy to implement under the old
|
|
scheduler, (otherwise we'd have done it long ago) because nice level
|
|
support was historically coupled to timeslice length, and timeslice
|
|
units were driven by the HZ tick, so the smallest timeslice was 1/HZ.
|
|
|
|
In the O(1) scheduler (in 2003) we changed negative nice levels to be
|
|
much stronger than they were before in 2.4 (and people were happy about
|
|
that change), and we also intentionally calibrated the linear timeslice
|
|
rule so that nice +19 level would be _exactly_ 1 jiffy. To better
|
|
understand it, the timeslice graph went like this (cheesy ASCII art
|
|
alert!):
|
|
|
|
|
|
A
|
|
\ | [timeslice length]
|
|
\ |
|
|
\ |
|
|
\ |
|
|
\ |
|
|
\|___100msecs
|
|
|^ . _
|
|
| ^ . _
|
|
| ^ . _
|
|
-*----------------------------------*-----> [nice level]
|
|
-20 | +19
|
|
|
|
|
|
|
|
|
|
So that if someone wanted to really renice tasks, +19 would give a much
|
|
bigger hit than the normal linear rule would do. (The solution of
|
|
changing the ABI to extend priorities was discarded early on.)
|
|
|
|
This approach worked to some degree for some time, but later on with
|
|
HZ=1000 it caused 1 jiffy to be 1 msec, which meant 0.1% CPU usage which
|
|
we felt to be a bit excessive. Excessive _not_ because it's too small of
|
|
a CPU utilization, but because it causes too frequent (once per
|
|
millisec) rescheduling. (and would thus trash the cache, etc. Remember,
|
|
this was long ago when hardware was weaker and caches were smaller, and
|
|
people were running number crunching apps at nice +19.)
|
|
|
|
So for HZ=1000 we changed nice +19 to 5msecs, because that felt like the
|
|
right minimal granularity - and this translates to 5% CPU utilization.
|
|
But the fundamental HZ-sensitive property for nice+19 still remained,
|
|
and we never got a single complaint about nice +19 being too _weak_ in
|
|
terms of CPU utilization, we only got complaints about it (still) being
|
|
too _strong_ :-)
|
|
|
|
To sum it up: we always wanted to make nice levels more consistent, but
|
|
within the constraints of HZ and jiffies and their nasty design level
|
|
coupling to timeslices and granularity it was not really viable.
|
|
|
|
The second (less frequent but still periodically occuring) complaint
|
|
about Linux's nice level support was its assymetry around the origo
|
|
(which you can see demonstrated in the picture above), or more
|
|
accurately: the fact that nice level behavior depended on the _absolute_
|
|
nice level as well, while the nice API itself is fundamentally
|
|
"relative":
|
|
|
|
int nice(int inc);
|
|
|
|
asmlinkage long sys_nice(int increment)
|
|
|
|
(the first one is the glibc API, the second one is the syscall API.)
|
|
Note that the 'inc' is relative to the current nice level. Tools like
|
|
bash's "nice" command mirror this relative API.
|
|
|
|
With the old scheduler, if you for example started a niced task with +1
|
|
and another task with +2, the CPU split between the two tasks would
|
|
depend on the nice level of the parent shell - if it was at nice -10 the
|
|
CPU split was different than if it was at +5 or +10.
|
|
|
|
A third complaint against Linux's nice level support was that negative
|
|
nice levels were not 'punchy enough', so lots of people had to resort to
|
|
run audio (and other multimedia) apps under RT priorities such as
|
|
SCHED_FIFO. But this caused other problems: SCHED_FIFO is not starvation
|
|
proof, and a buggy SCHED_FIFO app can also lock up the system for good.
|
|
|
|
The new scheduler in v2.6.23 addresses all three types of complaints:
|
|
|
|
To address the first complaint (of nice levels being not "punchy"
|
|
enough), the scheduler was decoupled from 'time slice' and HZ concepts
|
|
(and granularity was made a separate concept from nice levels) and thus
|
|
it was possible to implement better and more consistent nice +19
|
|
support: with the new scheduler nice +19 tasks get a HZ-independent
|
|
1.5%, instead of the variable 3%-5%-9% range they got in the old
|
|
scheduler.
|
|
|
|
To address the second complaint (of nice levels not being consistent),
|
|
the new scheduler makes nice(1) have the same CPU utilization effect on
|
|
tasks, regardless of their absolute nice levels. So on the new
|
|
scheduler, running a nice +10 and a nice 11 task has the same CPU
|
|
utilization "split" between them as running a nice -5 and a nice -4
|
|
task. (one will get 55% of the CPU, the other 45%.) That is why nice
|
|
levels were changed to be "multiplicative" (or exponential) - that way
|
|
it does not matter which nice level you start out from, the 'relative
|
|
result' will always be the same.
|
|
|
|
The third complaint (of negative nice levels not being "punchy" enough
|
|
and forcing audio apps to run under the more dangerous SCHED_FIFO
|
|
scheduling policy) is addressed by the new scheduler almost
|
|
automatically: stronger negative nice levels are an automatic
|
|
side-effect of the recalibrated dynamic range of nice levels.
|