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set.c: tweak LRU first-time insertion policy

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Pushing new elements to the front tends to assign extra weight to that
elements, at the expense of other elements that are already in the cache.
The idea is then to try first-time insertion somewhere in the middle.
Further attempts suggest that the "pivot" should be closer to the end.

Cache performance for "apt-shell <<<unmet", previous commit:
hit=62375 miss=17252

Cache performance for "apt-shell <<<unmet", this commit:
hit=65085 miss=14542
This commit is contained in:
Alexey Tourbin 2011-01-07 06:06:00 +03:00
parent cc5cafb9d7
commit fad9df878b
1 changed files with 17 additions and 4 deletions
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21
lib/set.c
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@ -751,6 +751,8 @@ int decode_set(const char *str, int Mshift, unsigned *v)
static
int cache_decode_set(const char *str, int Mshift, unsigned *v)
{
const int cache_size = 192;
const int pivot_size = 172;
unsigned *v_start = v, *v_end;
struct cache_ent {
struct cache_ent *next;
@ -764,6 +766,7 @@ int cache_decode_set(const char *str, int Mshift, unsigned *v)
struct cache_ent *cache;
// lookup in the cache
struct cache_ent *cur = cache, *prev = NULL;
struct cache_ent *pivot_cur = NULL, *pivot_prev = NULL;
unsigned hash = str[0] | (str[2] << 8) | (str[3] << 16);
int count = 0;
while (cur) {
@ -793,6 +796,10 @@ int cache_decode_set(const char *str, int Mshift, unsigned *v)
break;
prev = cur;
cur = cur->next;
if (count == pivot_size) {
pivot_cur = cur;
pivot_prev = prev;
}
}
// miss, decode
int c = decode_base62_golomb(str + 2, Mshift, v);
@ -800,7 +807,6 @@ int cache_decode_set(const char *str, int Mshift, unsigned *v)
return c;
v_end = v_start + c;
// truncate
int cache_size = 192;
if (count >= cache_size) {
free(cur);
prev->next = NULL;
@ -840,9 +846,16 @@ int cache_decode_set(const char *str, int Mshift, unsigned *v)
}
strcpy(cur->str, str);
cur->hash = hash;
// push to front
cur->next = cache;
cache = cur;
// pivotal insertion!
if (count >= cache_size) {
cur->next = pivot_cur;
pivot_prev->next = cur;
}
// early bird, push to front
else {
cur->next = cache;
cache = cur;
}
return c;
}