struct bstring
{
+ /* Hash chain. */
struct bstring *next;
- size_t length;
+ /* Assume the data length is no more than 64k. */
+ unsigned short length;
+ /* The half hash hack. This contains the upper 16 bits of the hash
+ value and is used as a pre-check when comparing two strings and
+ avoids the need to do length or memcmp calls. It proves to be
+ roughly 100% effective. */
+ unsigned short half_hash;
union
{
expand_hash_count. */
unsigned long expand_count;
unsigned long expand_hash_count;
+ /* 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 half_hash_miss_count;
};
/* The old hash function was stolen from SDBM. This is what DB 3.0 uses now,
void *
bcache (const void *addr, int length, struct bcache *bcache)
{
+ unsigned long full_hash;
+ unsigned short half_hash;
int hash_index;
struct bstring *s;
bcache->total_count++;
bcache->total_size += length;
- hash_index = hash (addr, length) % bcache->num_buckets;
+ full_hash = hash (addr, length);
+ half_hash = (full_hash >> 16);
+ hash_index = full_hash % bcache->num_buckets;
- /* Search the hash bucket for a string identical to the caller's. */
+ /* Search the hash bucket for a string identical to the caller's.
+ As a short-circuit first compare the upper part of each hash
+ values. */
for (s = bcache->bucket[hash_index]; s; s = s->next)
- if (s->length == length
- && ! memcmp (&s->d.data, addr, length))
- return &s->d.data;
+ {
+ if (s->half_hash == half_hash)
+ {
+ if (s->length == length
+ && ! memcmp (&s->d.data, addr, length))
+ return &s->d.data;
+ else
+ bcache->half_hash_miss_count++;
+ }
+ }
/* The user's string isn't in the list. Insert it after *ps. */
{
memcpy (&new->d.data, addr, length);
new->length = length;
new->next = bcache->bucket[hash_index];
+ new->half_hash = half_hash;
bcache->bucket[hash_index] = new;
bcache->unique_count++;
c->expand_count);
printf_filtered (" Hash table hashes: %lu\n",
c->total_count + c->expand_hash_count);
+ printf_filtered (" Half hash misses: %lu\n",
+ c->half_hash_miss_count);
printf_filtered (" Hash table population: ");
print_percentage (occupied_buckets, c->num_buckets);
printf_filtered (" Median hash chain length: %3d\n",
Written by Fred Fish <fnf@cygnus.com>
Rewritten by Jim Blandy <jimb@cygnus.com>
- Copyright 1999, 2000, 2002 Free Software Foundation, Inc.
+ Copyright 1999, 2000, 2002, 2003 Free Software Foundation, Inc.
This file is part of GDB.
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.
+ 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. */
+ 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 :-(
+
+*/
struct bcache;
projects / binutils-gdb.git / commitdiff