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Fix prefetchNextBucketEntries so it fetches the correct next 2 buckets. #2394

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Merged
merged 1 commit into from
Aug 5, 2025
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Fix prefetchNextBucketEntries so it fetches the correct next 2 buckets. #2394

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16 changes: 9 additions & 7 deletions src/hashtable.c
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Original file line number Diff line number Diff line change
Expand Up @@ -992,16 +992,16 @@ static void prefetchBucketEntries(bucket *b) {
}
}

/* Returns the child bucket if chained, otherwise the next bucket in the table. returns NULL if neither exists. */
static bucket *getNextBucket(bucket *current_bucket, size_t bucket_index, hashtable *ht, int table_index) {
/* Returns the child bucket if 'current_bucket' is chained. Otherwise, returns the bucket
* at 'next_top_level_index' in the table. Returns NULL if neither exists. */
static bucket *getNextBucket(bucket *current_bucket, size_t next_top_level_index, hashtable *ht, int table_index) {
bucket *next_bucket = NULL;
if (current_bucket->chained) {
next_bucket = getChildBucket(current_bucket);
} else {
size_t table_size = numBuckets(ht->bucket_exp[table_index]);
size_t next_index = bucket_index + 1;
if (next_index < table_size) {
next_bucket = &ht->tables[table_index][next_index];
if (next_top_level_index < table_size) {
next_bucket = &ht->tables[table_index][next_top_level_index];
}
}
return next_bucket;
Expand All @@ -1012,7 +1012,7 @@ static bucket *getNextBucket(bucket *current_bucket, size_t bucket_index, hashta
* - The next of the next bucket
* It attempts to bring this data closer to the L1 cache to reduce future memory access latency.
*
* Cache state before this function is called(due to last call for this function):
* Cache state before this function is called (due to last call for this function):
* 1. The current bucket and its entries are likely already in cache.
* 2. The next bucket is in cache.
*/
Expand All @@ -1021,7 +1021,9 @@ static void prefetchNextBucketEntries(iter *iter, bucket *current_bucket) {
bucket *next_bucket = getNextBucket(current_bucket, next_index, iter->hashtable, iter->table);
if (next_bucket) {
prefetchBucketEntries(next_bucket);
bucket *next_next_bucket = getNextBucket(next_bucket, next_index + 1, iter->hashtable, iter->table);
/* Calculate the target top-level index for the next-next bucket. */
if (!current_bucket->chained) next_index++;
bucket *next_next_bucket = getNextBucket(next_bucket, next_index, iter->hashtable, iter->table);
if (next_next_bucket) {
valkey_prefetch(next_next_bucket);
}
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