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ghash: Fix poor performance with densely populated keyspaces

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Sequential integers would be densely packed in the table, leaving the
high-index buckets unused and causing abnormally long probes for many
operations. This was especially noticeable with failed lookups and
when "aging" the table by repeatedly inserting and removing integers
from a narrow range using g_direct_hash() as the hashing function.

The solution is to multiply the hash by a small prime before applying
the modulo. The compiler optimizes this to a few left shifts and adds, so
the constant overhead is small, and the entries will be spread out,
yielding a lower average probe count.
This commit is contained in:
Hans Petter Jansson 2018-07-10 12:21:59 +02:00
parent 0e5f9aa143
commit 0dee62973c
1 changed files with 12 additions and 2 deletions
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14
glib/ghash.c
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@ -334,6 +334,16 @@ g_hash_table_set_shift_from_size (GHashTable *hash_table, gint size)
g_hash_table_set_shift (hash_table, shift); g_hash_table_set_shift (hash_table, shift);
} }
static inline guint
g_hash_table_hash_to_index (GHashTable *hash_table, guint hash)
{
/* Multiply the hash by a small prime before applying the modulo. This
* prevents the table from becoming densely packed, even with a poor hash
* function. A densely packed table would have poor performance on
* workloads with many failed lookups or a high degree of churn. */
return (hash * 11) % hash_table->mod;
}
/* /*
* g_hash_table_lookup_node: * g_hash_table_lookup_node:
* @hash_table: our #GHashTable * @hash_table: our #GHashTable
@ -382,7 +392,7 @@ g_hash_table_lookup_node (GHashTable *hash_table,
*hash_return = hash_value; *hash_return = hash_value;
node_index = hash_value % hash_table->mod; node_index = g_hash_table_hash_to_index (hash_table, hash_value);
node_hash = hash_table->hashes[node_index]; node_hash = hash_table->hashes[node_index];
while (!HASH_IS_UNUSED (node_hash)) while (!HASH_IS_UNUSED (node_hash))
@ -602,7 +612,7 @@ g_hash_table_resize (GHashTable *hash_table)
if (!HASH_IS_REAL (node_hash)) if (!HASH_IS_REAL (node_hash))
continue; continue;
hash_val = node_hash % hash_table->mod; hash_val = g_hash_table_hash_to_index (hash_table, node_hash);
while (!HASH_IS_UNUSED (new_hashes[hash_val])) while (!HASH_IS_UNUSED (new_hashes[hash_val]))
{ {