[Mesa-dev] [PATCH 3/4] util: Change util/set to use quadratic probing
Thomas Helland
thomashelland90 at gmail.com
Mon Mar 16 07:33:31 PDT 2015
2015-03-16 6:49 GMT+01:00 Connor Abbott <cwabbott0 at gmail.com>:
> Ok, so I think I managed to find the source of the bug. When inserting
> elements into the set/hash table, we computed the initial probe
> address *before* doing the rehash. In the case where inserting an
> element led to a rehash, this meant that we'd use the wrong starting
> address when actually inserting it, and then afterwards when we'd go
> back and search for it we'd start at a different address => boom.
Thanks for tracking this down!
That does indeed look like it's it.
I'll send an updated version once I get the time.
> Fixing that makes the pubilc shader-db run without any errors, at
> least for me. In addition, while examining the algorithm I found one
> other thing that I've mentioned inline. Obviously, all these comments
> apply equally to the hash table code as well, although I'm not sure
> why the bug didn't affect it as well, or for that matter why we didn't
> notice the bug with the old code as it also seems to have it. As Jason
> said, hash table bugs are really terrible and tricky.
>
> 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 },
>> + { 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);
>
> Here I think we could do (quad_hash + quad_hash * quad_hash) / 2
> instead, and then we can start with quad_hash = 1 since we'll
> correctly calculate start_hash_address + 1 and we'll actually get the
> full range of addresses. Obviously this change needs to be applied to
> the insert path too as well as both paths in the hash table code.
>
>> + 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;
>
> Here's the bug; we need to initialize these two variables after the
> calls to set_rehash() below.
>
>> + 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);
>> }
>>
(.....)
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