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

This PR fixes a bug in prefetchNextBucketEntries which is used in the
hashtable iterator. The current version of prefetchNextBucketEntries
does not correctly prefetch the next two buckets that will be iterated
over. This is due next_index being incremented twice (once in
prefetchNextBucketEntries and again in getNextBucket).

Fix:

1. Add check in prefetchNextBucketEntries to ensure we pass the correct
   'next_index' to getNextBucket.
2. The extra increment was removed from getNextBucket.

Signed-off-by: Nicky Khorasani <[email protected]>

Author: Nicky-2000

src/hashtable.c

@@ -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;
@@ -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.
  */
@@ -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);
         }