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LRU: cache SDS strings in the eviction pool.

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To destroy and recreate the pool[].key element is slow, so we allocate
in pool[].cached SDS strings that can account up to 255 chars keys and
try to reuse them. This provides a solid 20% performance improvement
in real world workload alike benchmarks.
This commit is contained in:
antirez 2016-07-12 12:31:37 +02:00
parent 965905c9f2
commit e64bf05f43
1 changed files with 29 additions and 13 deletions
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42
src/evict.c
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@ -44,10 +44,12 @@
* greater idle times to the right (ascending order).
*
* Empty entries have the key pointer set to NULL. */
#define MAXMEMORY_EVICTION_POOL_SIZE 16
#define EVPOOL_SIZE 16
#define EVPOOL_CACHED_SDS_SIZE 255
struct evictionPoolEntry {
unsigned long long idle; /* Object idle time. */
sds key; /* Key name. */
sds cached; /* Cached SDS object for key name. */
};
/* ----------------------------------------------------------------------------
@ -96,7 +98,7 @@ unsigned long long estimateObjectIdleTime(robj *o) {
* Redis uses an approximation of the LRU algorithm that runs in constant
* memory. Every time there is a key to expire, we sample N keys (with
* N very small, usually in around 5) to populate a pool of best keys to
* evict of M keys (the pool size is defined by MAXMEMORY_EVICTION_POOL_SIZE).
* evict of M keys (the pool size is defined by EVPOOL_SIZE).
*
* The N keys sampled are added in the pool of good keys to expire (the one
* with an old access time) if they are better than one of the current keys
@ -116,10 +118,11 @@ struct evictionPoolEntry *evictionPoolAlloc(void) {
struct evictionPoolEntry *ep;
int j;
ep = zmalloc(sizeof(*ep)*MAXMEMORY_EVICTION_POOL_SIZE);
for (j = 0; j < MAXMEMORY_EVICTION_POOL_SIZE; j++) {
ep = zmalloc(sizeof(*ep)*EVPOOL_SIZE);
for (j = 0; j < EVPOOL_SIZE; j++) {
ep[j].idle = 0;
ep[j].key = NULL;
ep[j].cached = sdsnewlen(NULL,EVPOOL_CACHED_SDS_SIZE);
}
return ep;
}
@ -158,33 +161,45 @@ void evictionPoolPopulate(dict *sampledict, dict *keydict, struct evictionPoolEn
* First, find the first empty bucket or the first populated
* bucket that has an idle time smaller than our idle time. */
k = 0;
while (k < MAXMEMORY_EVICTION_POOL_SIZE &&
while (k < EVPOOL_SIZE &&
pool[k].key &&
pool[k].idle < idle) k++;
if (k == 0 && pool[MAXMEMORY_EVICTION_POOL_SIZE-1].key != NULL) {
if (k == 0 && pool[EVPOOL_SIZE-1].key != NULL) {
/* Can't insert if the element is < the worst element we have
* and there are no empty buckets. */
continue;
} else if (k < MAXMEMORY_EVICTION_POOL_SIZE && pool[k].key == NULL) {
} else if (k < EVPOOL_SIZE && pool[k].key == NULL) {
/* Inserting into empty position. No setup needed before insert. */
} else {
/* Inserting in the middle. Now k points to the first element
* greater than the element to insert. */
if (pool[MAXMEMORY_EVICTION_POOL_SIZE-1].key == NULL) {
if (pool[EVPOOL_SIZE-1].key == NULL) {
/* Free space on the right? Insert at k shifting
* all the elements from k to end to the right. */
memmove(pool+k+1,pool+k,
sizeof(pool[0])*(MAXMEMORY_EVICTION_POOL_SIZE-k-1));
sizeof(pool[0])*(EVPOOL_SIZE-k-1));
} else {
/* No free space on right? Insert at k-1 */
k--;
/* Shift all elements on the left of k (included) to the
* left, so we discard the element with smaller idle time. */
sdsfree(pool[0].key);
if (pool[0].key != pool[0].cached) sdsfree(pool[0].key);
memmove(pool,pool+1,sizeof(pool[0])*k);
}
}
pool[k].key = sdsdup(key);
/* Try to reuse the cached SDS string allocated in the pool entry,
* because allocating and deallocating this object is costly
* (according to the profiler, not my fantasy. Remember:
* premature optimizbla bla bla bla. */
int klen = sdslen(key);
if (klen > EVPOOL_CACHED_SDS_SIZE) {
pool[k].key = sdsdup(key);
} else {
memcpy(pool[k].cached,key,klen+1);
sdssetlen(pool[k].cached,klen);
pool[k].key = pool[k].cached;
}
pool[k].idle = idle;
}
}
@ -269,12 +284,13 @@ int freeMemoryIfNeeded(void) {
while(bestkey == NULL) {
evictionPoolPopulate(dict, db->dict, db->eviction_pool);
/* Go backward from best to worst element to evict. */
for (k = MAXMEMORY_EVICTION_POOL_SIZE-1; k >= 0; k--) {
for (k = EVPOOL_SIZE-1; k >= 0; k--) {
if (pool[k].key == NULL) continue;
de = dictFind(dict,pool[k].key);
/* Remove the entry from the pool. */
sdsfree(pool[k].key);
if (pool[k].key != pool[k].cached)
sdsfree(pool[k].key);
pool[k].key = NULL;
pool[k].idle = 0;