[Mesa-dev] [PATCH 3/4] util: Change util/set to use quadratic probing
Connor Abbott
cwabbott0 at gmail.com
Sat Mar 14 22:28:35 PDT 2015
On Fri, Mar 13, 2015 at 6:37 PM, Thomas Helland
<thomashelland90 at gmail.com> wrote:
> The same rationale applies here as for the hash table.
> Power of two size should give better performance,
> and using the algorithm hash = sh + i/2 + i*i/2
> should result in only distinct hash values when hitting collisions.
> Should give a performance increase as we can do bitmasking instead
> of a modulo operation for fitting the hash in the address space.
> ---
> src/util/set.c | 103 ++++++++++++++++++++++++++++++---------------------------
> src/util/set.h | 1 -
> 2 files changed, 54 insertions(+), 50 deletions(-)
>
> diff --git a/src/util/set.c b/src/util/set.c
> index f01f869..8f0ad0d 100644
> --- a/src/util/set.c
> +++ b/src/util/set.c
> @@ -48,40 +48,46 @@
> uint32_t deleted_key_value;
> const void *deleted_key = &deleted_key_value;
>
> +/**
> + * We chose table sizes that's a power of two.
> + * This is computationally less expensive than primes.
> + * FNV-1a has good avalanche properties, so collision is not an issue.
> + * These tables are sized to have an extra 10% free to avoid
> + * exponential performance degradation as the hash table fills
> + */
> static const struct {
> - uint32_t max_entries, size, rehash;
> + uint32_t max_entries, size;
> } hash_sizes[] = {
> - { 2, 5, 3 },
> - { 4, 7, 5 },
> - { 8, 13, 11 },
> - { 16, 19, 17 },
> - { 32, 43, 41 },
> - { 64, 73, 71 },
> - { 128, 151, 149 },
> - { 256, 283, 281 },
> - { 512, 571, 569 },
> - { 1024, 1153, 1151 },
> - { 2048, 2269, 2267 },
> - { 4096, 4519, 4517 },
> - { 8192, 9013, 9011 },
> - { 16384, 18043, 18041 },
> - { 32768, 36109, 36107 },
> - { 65536, 72091, 72089 },
> - { 131072, 144409, 144407 },
> - { 262144, 288361, 288359 },
> - { 524288, 576883, 576881 },
> - { 1048576, 1153459, 1153457 },
> - { 2097152, 2307163, 2307161 },
> - { 4194304, 4613893, 4613891 },
> - { 8388608, 9227641, 9227639 },
> - { 16777216, 18455029, 18455027 },
> - { 33554432, 36911011, 36911009 },
> - { 67108864, 73819861, 73819859 },
> - { 134217728, 147639589, 147639587 },
> - { 268435456, 295279081, 295279079 },
> - { 536870912, 590559793, 590559791 },
> - { 1073741824, 1181116273, 1181116271 },
> - { 2147483648ul, 2362232233ul, 2362232231ul }
> + { 3, 4 },
Same comment here about increasing the default beginning size and
computing the sizes on the fly.
> + { 7, 8 },
> + { 14, 16 },
> + { 28, 32 },
> + { 57, 64 },
> + { 115, 128 },
> + { 230, 256 },
> + { 460, 512 },
> + { 921, 1024 },
> + { 1843, 2048 },
> + { 3686, 4096 },
> + { 7372, 8192 },
> + { 14745, 16384 },
> + { 29491, 32768 },
> + { 58982, 65536 },
> + { 117964, 131072 },
> + { 235929, 262144 },
> + { 471859, 524288 },
> + { 943718, 1048576 },
> + { 1887436, 2097152 },
> + { 3774873, 4194304 },
> + { 7549747, 8388608 },
> + { 15099494, 16777216 },
> + { 30198988, 33554432 },
> + { 60397977, 67108864 },
> + { 120795955, 134217728 },
> + { 241591910, 268435456 },
> + { 483183820, 536870912 },
> + { 966367641, 1073741824 },
> + { 1932735283ul, 2147483648ul }
> };
>
> static int
> @@ -116,7 +122,6 @@ _mesa_set_create(void *mem_ctx,
>
> ht->size_index = 0;
> ht->size = hash_sizes[ht->size_index].size;
> - ht->rehash = hash_sizes[ht->size_index].rehash;
> ht->max_entries = hash_sizes[ht->size_index].max_entries;
> ht->key_hash_function = key_hash_function;
> ht->key_equals_function = key_equals_function;
> @@ -163,12 +168,12 @@ _mesa_set_destroy(struct set *ht, void (*delete_function)(struct set_entry *entr
> static struct set_entry *
> set_search(const struct set *ht, uint32_t hash, const void *key)
> {
> - uint32_t hash_address;
> + uint32_t start_hash_address = hash & (ht->size - 1);
> + uint32_t hash_address = start_hash_address;
> + // Start at 2, or we will match start_hash_address initially and bail
> + uint32_t quad_hash = 2;
>
> - hash_address = hash % ht->size;
> do {
> - uint32_t double_hash;
> -
> struct set_entry *entry = ht->table + hash_address;
>
> if (entry_is_free(entry)) {
> @@ -179,10 +184,10 @@ set_search(const struct set *ht, uint32_t hash, const void *key)
> }
> }
>
> - double_hash = 1 + hash % ht->rehash;
> -
> - hash_address = (hash_address + double_hash) % ht->size;
> - } while (hash_address != hash % ht->size);
> + hash_address = (start_hash_address + (quad_hash / 2) +
> + ((quad_hash * quad_hash) / 2)) & (ht->size - 1);
> + quad_hash++;
> + } while (hash_address != start_hash_address);
>
> return NULL;
> }
> @@ -225,7 +230,6 @@ set_rehash(struct set *ht, unsigned new_size_index)
> ht->table = table;
> ht->size_index = new_size_index;
> ht->size = hash_sizes[ht->size_index].size;
> - ht->rehash = hash_sizes[ht->size_index].rehash;
> ht->max_entries = hash_sizes[ht->size_index].max_entries;
> ht->entries = 0;
> ht->deleted_entries = 0;
> @@ -250,7 +254,9 @@ set_rehash(struct set *ht, unsigned new_size_index)
> static struct set_entry *
> set_add(struct set *ht, uint32_t hash, const void *key)
> {
> - uint32_t hash_address;
> + uint32_t start_hash_address = hash & (ht->size - 1);
> + uint32_t hash_address = start_hash_address;
> + uint32_t quad_hash = 2;
> struct set_entry *available_entry = NULL;
>
> if (ht->entries >= ht->max_entries) {
> @@ -259,10 +265,8 @@ set_add(struct set *ht, uint32_t hash, const void *key)
> set_rehash(ht, ht->size_index);
> }
>
> - hash_address = hash % ht->size;
> do {
> struct set_entry *entry = ht->table + hash_address;
> - uint32_t double_hash;
>
> if (!entry_is_present(entry)) {
> /* Stash the first available entry we find */
> @@ -288,10 +292,11 @@ set_add(struct set *ht, uint32_t hash, const void *key)
> return entry;
> }
>
> - double_hash = 1 + hash % ht->rehash;
> + hash_address = (start_hash_address + (quad_hash / 2) +
> + ((quad_hash * quad_hash) / 2)) & (ht->size - 1);
> + quad_hash++;
> + } while (hash_address != start_hash_address);
>
> - hash_address = (hash_address + double_hash) % ht->size;
> - } while (hash_address != hash % ht->size);
>
> if (available_entry) {
> if (entry_is_deleted(available_entry))
> @@ -368,7 +373,7 @@ _mesa_set_random_entry(struct set *ht,
> int (*predicate)(struct set_entry *entry))
> {
> struct set_entry *entry;
> - uint32_t i = rand() % ht->size;
> + uint32_t i = rand() & (ht->size - 1);
>
> if (ht->entries == 0)
> return NULL;
> diff --git a/src/util/set.h b/src/util/set.h
> index 9acd2c2..5a1097c 100644
> --- a/src/util/set.h
> +++ b/src/util/set.h
> @@ -46,7 +46,6 @@ struct set {
> uint32_t (*key_hash_function)(const void *key);
> bool (*key_equals_function)(const void *a, const void *b);
> uint32_t size;
> - uint32_t rehash;
> uint32_t max_entries;
> uint32_t size_index;
> uint32_t entries;
> --
> 2.2.1
>
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