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Optimize CPU cache efficiency on dict while it's being rehashed (#5692)

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when find a key  ,if redis is rehashing, currently we should lookup both tables (ht[0] and ht[1]).
if we use the key's index comparing to the rehashidx,if index < rehashidx,then  we can conclude: 
1. it is rehashing(rehashidx is -1 if it is not rehashing) 
2. we can't find key in ht[0] so just continue to find key in ht[1]

The possible performance gain here, is not the looping over the linked list (which is empty),
but rather the lookup in the table (which could be a cache miss).
---------

Co-authored-by: zhangshihua003 <zhangshihua003@ke.com>
Co-authored-by: sundb <sundbcn@gmail.com>
Co-authored-by: Oran Agra <oran@redislabs.com>
Co-authored-by: judeng <abc3844@126.com>
This commit is contained in:
erpeng 2023-10-31 15:57:26 +08:00 committed by GitHub
parent f7fa481156
commit 4bbb2b0152
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1 changed files with 4 additions and 0 deletions
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4
src/dict.c
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@ -563,6 +563,7 @@ static dictEntry *dictGenericDelete(dict *d, const void *key, int nofree) {
for (table = 0; table <= 1; table++) {
idx = h & DICTHT_SIZE_MASK(d->ht_size_exp[table]);
if (table == 0 && (long)idx < d->rehashidx) continue;
he = d->ht_table[table][idx];
prevHe = NULL;
while(he) {
@ -672,6 +673,7 @@ dictEntry *dictFind(dict *d, const void *key)
h = dictHashKey(d, key);
for (table = 0; table <= 1; table++) {
idx = h & DICTHT_SIZE_MASK(d->ht_size_exp[table]);
if (table == 0 && (long)idx < d->rehashidx) continue;
he = d->ht_table[table][idx];
while(he) {
void *he_key = dictGetKey(he);
@ -1450,6 +1452,7 @@ void *dictFindPositionForInsert(dict *d, const void *key, dictEntry **existing)
return NULL;
for (table = 0; table <= 1; table++) {
idx = hash & DICTHT_SIZE_MASK(d->ht_size_exp[table]);
if (table == 0 && (long)idx < d->rehashidx) continue;
/* Search if this slot does not already contain the given key */
he = d->ht_table[table][idx];
while(he) {
@ -1496,6 +1499,7 @@ dictEntry *dictFindEntryByPtrAndHash(dict *d, const void *oldptr, uint64_t hash)
if (dictSize(d) == 0) return NULL; /* dict is empty */
for (table = 0; table <= 1; table++) {
idx = hash & DICTHT_SIZE_MASK(d->ht_size_exp[table]);
if (table == 0 && (long)idx < d->rehashidx) continue;
he = d->ht_table[table][idx];
while(he) {
if (oldptr == dictGetKey(he))