230 lines
8.8 KiB
C++
230 lines
8.8 KiB
C++
/* Include file cached obstack implementation.
|
|
Written by Fred Fish <fnf@cygnus.com>
|
|
Rewritten by Jim Blandy <jimb@cygnus.com>
|
|
|
|
Copyright (C) 1999-2020 Free Software Foundation, Inc.
|
|
|
|
This file is part of GDB.
|
|
|
|
This program is free software; you can redistribute it and/or modify
|
|
it under the terms of the GNU General Public License as published by
|
|
the Free Software Foundation; either version 3 of the License, or
|
|
(at your option) any later version.
|
|
|
|
This program is distributed in the hope that it will be useful,
|
|
but WITHOUT ANY WARRANTY; without even the implied warranty of
|
|
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
|
|
GNU General Public License for more details.
|
|
|
|
You should have received a copy of the GNU General Public License
|
|
along with this program. If not, see <http://www.gnu.org/licenses/>. */
|
|
|
|
#ifndef BCACHE_H
|
|
#define BCACHE_H 1
|
|
|
|
/* A bcache is a data structure for factoring out duplication in
|
|
read-only structures. You give the bcache some string of bytes S.
|
|
If the bcache already contains a copy of S, it hands you back a
|
|
pointer to its copy. Otherwise, it makes a fresh copy of S, and
|
|
hands you back a pointer to that. In either case, you can throw
|
|
away your copy of S, and use the bcache's.
|
|
|
|
The "strings" in question are arbitrary strings of bytes --- they
|
|
can contain zero bytes. You pass in the length explicitly when you
|
|
call the bcache function.
|
|
|
|
This means that you can put ordinary C objects in a bcache.
|
|
However, if you do this, remember that structs can contain `holes'
|
|
between members, added for alignment. These bytes usually contain
|
|
garbage. If you try to bcache two objects which are identical from
|
|
your code's point of view, but have different garbage values in the
|
|
structure's holes, then the bcache will treat them as separate
|
|
strings, and you won't get the nice elimination of duplicates you
|
|
were hoping for. So, remember to memset your structures full of
|
|
zeros before bcaching them!
|
|
|
|
You shouldn't modify the strings you get from a bcache, because:
|
|
|
|
- You don't necessarily know who you're sharing space with. If I
|
|
stick eight bytes of text in a bcache, and then stick an eight-byte
|
|
structure in the same bcache, there's no guarantee those two
|
|
objects don't actually comprise the same sequence of bytes. If
|
|
they happen to, the bcache will use a single byte string for both
|
|
of them. Then, modifying the structure will change the string. In
|
|
bizarre ways.
|
|
|
|
- Even if you know for some other reason that all that's okay,
|
|
there's another problem. A bcache stores all its strings in a hash
|
|
table. If you modify a string's contents, you will probably change
|
|
its hash value. This means that the modified string is now in the
|
|
wrong place in the hash table, and future bcache probes will never
|
|
find it. So by mutating a string, you give up any chance of
|
|
sharing its space with future duplicates.
|
|
|
|
|
|
Size of bcache VS hashtab:
|
|
|
|
For bcache, the most critical cost is size (or more exactly the
|
|
overhead added by the bcache). It turns out that the bcache is
|
|
remarkably efficient.
|
|
|
|
Assuming a 32-bit system (the hash table slots are 4 bytes),
|
|
ignoring alignment, and limit strings to 255 bytes (1 byte length)
|
|
we get ...
|
|
|
|
bcache: This uses a separate linked list to track the hash chain.
|
|
The numbers show roughly 100% occupancy of the hash table and an
|
|
average chain length of 4. Spreading the slot cost over the 4
|
|
chain elements:
|
|
|
|
4 (slot) / 4 (chain length) + 1 (length) + 4 (chain) = 6 bytes
|
|
|
|
hashtab: This uses a more traditional re-hash algorithm where the
|
|
chain is maintained within the hash table. The table occupancy is
|
|
kept below 75% but we'll assume its perfect:
|
|
|
|
4 (slot) x 4/3 (occupancy) + 1 (length) = 6 1/3 bytes
|
|
|
|
So a perfect hashtab has just slightly larger than an average
|
|
bcache.
|
|
|
|
It turns out that an average hashtab is far worse. Two things
|
|
hurt:
|
|
|
|
- Hashtab's occupancy is more like 50% (it ranges between 38% and
|
|
75%) giving a per slot cost of 4x2 vs 4x4/3.
|
|
|
|
- the string structure needs to be aligned to 8 bytes which for
|
|
hashtab wastes 7 bytes, while for bcache wastes only 3.
|
|
|
|
This gives:
|
|
|
|
hashtab: 4 x 2 + 1 + 7 = 16 bytes
|
|
|
|
bcache 4 / 4 + 1 + 4 + 3 = 9 bytes
|
|
|
|
The numbers of GDB debugging GDB support this. ~40% vs ~70% overhead.
|
|
|
|
|
|
Speed of bcache VS hashtab (the half hash hack):
|
|
|
|
While hashtab has a typical chain length of 1, bcache has a chain
|
|
length of round 4. This means that the bcache will require
|
|
something like double the number of compares after that initial
|
|
hash. In both cases the comparison takes the form:
|
|
|
|
a.length == b.length && memcmp (a.data, b.data, a.length) == 0
|
|
|
|
That is lengths are checked before doing the memcmp.
|
|
|
|
For GDB debugging GDB, it turned out that all lengths were 24 bytes
|
|
(no C++ so only psymbols were cached) and hence, all compares
|
|
required a call to memcmp. As a hack, two bytes of padding
|
|
(mentioned above) are used to store the upper 16 bits of the
|
|
string's hash value and then that is used in the comparison vis:
|
|
|
|
a.half_hash == b.half_hash && a.length == b.length && memcmp
|
|
(a.data, b.data, a.length)
|
|
|
|
The numbers from GDB debugging GDB show this to be a remarkable
|
|
100% effective (only necessary length and memcmp tests being
|
|
performed).
|
|
|
|
Mind you, looking at the wall clock, the same GDB debugging GDB
|
|
showed only marginal speed up (0.780 vs 0.773s). Seems GDB is too
|
|
busy doing something else :-(
|
|
|
|
*/
|
|
|
|
namespace gdb {
|
|
|
|
struct bstring;
|
|
|
|
struct bcache
|
|
{
|
|
/* Allocate a bcache. HASH_FN and COMPARE_FN can be used to pass in
|
|
custom hash, and compare functions to be used by this bcache. If
|
|
HASH_FUNCTION is NULL fast_hash() is used and if COMPARE_FUNCTION is
|
|
NULL memcmp() is used. */
|
|
|
|
explicit bcache (unsigned long (*hash_fn)(const void *,
|
|
int length) = nullptr,
|
|
int (*compare_fn)(const void *, const void *,
|
|
int length) = nullptr)
|
|
: m_hash_function (hash_fn == nullptr ? default_hash : hash_fn),
|
|
m_compare_function (compare_fn == nullptr ? compare : compare_fn)
|
|
{
|
|
}
|
|
|
|
~bcache ();
|
|
|
|
/* Find a copy of the LENGTH bytes at ADDR in BCACHE. If BCACHE has
|
|
never seen those bytes before, add a copy of them to BCACHE. In
|
|
either case, return a pointer to BCACHE's copy of that string.
|
|
Since the cached value is ment to be read-only, return a const
|
|
buffer. If ADDED is not NULL, set *ADDED to true if the bytes
|
|
were newly added to the cache, or to false if the bytes were
|
|
found in the cache. */
|
|
|
|
const void *insert (const void *addr, int length, int *added = nullptr);
|
|
|
|
/* Print statistics on this bcache's memory usage and efficacity at
|
|
eliminating duplication. TYPE should be a string describing the
|
|
kind of data this bcache holds. Statistics are printed using
|
|
`printf_filtered' and its ilk. */
|
|
void print_statistics (const char *type);
|
|
int memory_used ();
|
|
|
|
private:
|
|
|
|
/* All the bstrings are allocated here. */
|
|
struct obstack m_cache {};
|
|
|
|
/* How many hash buckets we're using. */
|
|
unsigned int m_num_buckets = 0;
|
|
|
|
/* Hash buckets. This table is allocated using malloc, so when we
|
|
grow the table we can return the old table to the system. */
|
|
struct bstring **m_bucket = nullptr;
|
|
|
|
/* Statistics. */
|
|
unsigned long m_unique_count = 0; /* number of unique strings */
|
|
long m_total_count = 0; /* total number of strings cached, including dups */
|
|
long m_unique_size = 0; /* size of unique strings, in bytes */
|
|
long m_total_size = 0; /* total number of bytes cached, including dups */
|
|
long m_structure_size = 0; /* total size of bcache, including infrastructure */
|
|
/* Number of times that the hash table is expanded and hence
|
|
re-built, and the corresponding number of times that a string is
|
|
[re]hashed as part of entering it into the expanded table. The
|
|
total number of hashes can be computed by adding TOTAL_COUNT to
|
|
expand_hash_count. */
|
|
unsigned long m_expand_count = 0;
|
|
unsigned long m_expand_hash_count = 0;
|
|
/* Number of times that the half-hash compare hit (compare the upper
|
|
16 bits of hash values) hit, but the corresponding combined
|
|
length/data compare missed. */
|
|
unsigned long m_half_hash_miss_count = 0;
|
|
|
|
/* Hash function to be used for this bcache object. */
|
|
unsigned long (*m_hash_function)(const void *addr, int length);
|
|
|
|
/* Compare function to be used for this bcache object. */
|
|
int (*m_compare_function)(const void *, const void *, int length);
|
|
|
|
/* Default compare function. */
|
|
static int compare (const void *addr1, const void *addr2, int length);
|
|
|
|
/* Default hash function. */
|
|
static unsigned long default_hash (const void *ptr, int length)
|
|
{
|
|
return fast_hash (ptr, length, 0);
|
|
}
|
|
|
|
/* Expand the hash table. */
|
|
void expand_hash_table ();
|
|
};
|
|
|
|
} /* namespace gdb */
|
|
|
|
#endif /* BCACHE_H */
|