/*
*******************************************************************************
* Implementation of (2^1+,2) cuckoo hashing, where 2^1+ indicates that each
* hash bucket contains 2^n cells, for n >= 1, and 2 indicates that two hash
* functions are employed. The original cuckoo hashing algorithm was described
* in:
*
* Pagh, R., F.F. Rodler (2004) Cuckoo Hashing. Journal of Algorithms
* 51(2):122-144.
*
* Generalization of cuckoo hashing was discussed in:
*
* Erlingsson, U., M. Manasse, F. McSherry (2006) A cool and practical
* alternative to traditional hash tables. In Proceedings of the 7th
* Workshop on Distributed Data and Structures (WDAS'06), Santa Clara, CA,
* January 2006.
*
* This implementation uses precisely two hash functions because that is the
* fewest that can work, and supporting multiple hashes is an implementation
* burden. Here is a reproduction of Figure 1 from Erlingsson et al. (2006)
* that shows approximate expected maximum load factors for various
* configurations:
*
* | #cells/bucket |
* #hashes | 1 | 2 | 4 | 8 |
* --------+-------+-------+-------+-------+
* 1 | 0.006 | 0.006 | 0.03 | 0.12 |
* 2 | 0.49 | 0.86 |>0.93< |>0.96< |
* 3 | 0.91 | 0.97 | 0.98 | 0.999 |
* 4 | 0.97 | 0.99 | 0.999 | |
*
* The number of cells per bucket is chosen such that a bucket fits in one cache
* line. So, on 32- and 64-bit systems, we use (8,2) and (4,2) cuckoo hashing,
* respectively.
*
******************************************************************************/
#define JEMALLOC_CKH_C_
#include "jemalloc/internal/jemalloc_internal.h"
/******************************************************************************/
/* Function prototypes for non-inline static functions. */
static bool ckh_grow(tsd_t *tsd, ckh_t *ckh);
static void ckh_shrink(tsd_t *tsd, ckh_t *ckh);
/******************************************************************************/
/*
* Search bucket for key and return the cell number if found; SIZE_T_MAX
* otherwise.
*/
JEMALLOC_INLINE_C size_t
ckh_bucket_search(ckh_t *ckh, size_t bucket, const void *key)
{
ckhc_t *cell;
unsigned i;
for (i = 0; i < (ZU(1) << LG_CKH_BUCKET_CELLS); i++) {
cell = &ckh->tab[(bucket << LG_CKH_BUCKET_CELLS) + i];
if (cell->key != NULL && ckh->keycomp(key, cell->key))
return ((bucket << LG_CKH_BUCKET_CELLS) + i);
}
return (SIZE_T_MAX);
}
/*
* Search table for key and return cell number if found; SIZE_T_MAX otherwise.
*/
JEMALLOC_INLINE_C size_t
ckh_isearch(ckh_t *ckh, const void *key)
{
size_t hashes[2], bucket, cell;
assert(ckh != NULL);
ckh->hash(key, hashes);
/* Search primary bucket. */
bucket = hashes[0] & ((ZU(1) << ckh->lg_curbuckets) - 1);
cell = ckh_bucket_search(ckh, bucket, key);
if (cell != SIZE_T_MAX)
return (cell);
/* Search secondary bucket. */
bucket = hashes[1] & ((ZU(1) << ckh->lg_curbuckets) - 1);
cell = ckh_bucket_search(ckh, bucket, key);
return (cell);
}
JEMALLOC_INLINE_C bool
ckh_try_bucket_insert(ckh_t *ckh, size_t bucket, const void *key,
const void *data)
{
ckhc_t *cell;
unsigned offset, i;
/*
* Cycle through the cells in the bucket, starting at a random position.
* The randomness avoids worst-case search overhead as buckets fill up.
*/
offset = (unsigned)prng_lg_range_u64(&ckh->prng_state,
LG_CKH_BUCKET_CELLS);
for (i = 0; i < (ZU(1) << LG_CKH_BUCKET_CELLS); i++) {
cell = &ckh->tab[(bucket << LG_CKH_BUCKET_CELLS) +
((i + offset) & ((ZU(1) << LG_CKH_BUCKET_CELLS) - 1))];
if (cell->key == NULL) {
cell->key = key;
cell->data = data;
ckh->count++;
return (false);
}
}
return (true);
}
/*
* No space is available in bucket. Randomly evict an item, then try to find an
* alternate location for that item. Iteratively repeat this
* eviction/relocation procedure until either success or detection of an
* eviction/relocation bucket cycle.
*/
JEMALLOC_INLINE_C bool
ckh_evict_reloc_insert(ckh_t *ckh, size_t argbucket, void const **argkey,
void const **argdata)
{
const void *key, *data, *tkey, *tdata;
ckhc_t *cell;
size_t hashes[2], bucket, tbucket;
unsigned i;
bucket = argbucket;
key = *argkey;
data = *argdata;
while (true) {
/*
* Choose a random item within the bucket to evict. This is
* critical to correct function, because without (eventually)
* evicting all items within a bucket during iteration, it
* would be possible to get stuck in an infinite loop if there
* were an item for which both hashes indicated the same
* bucket.
*/
i = (unsigned)prng_lg_range_u64(&ckh->prng_state,
LG_CKH_BUCKET_CELLS);
cell = &ckh->tab[(bucket << LG_CKH_BUCKET_CELLS) + i];
assert(cell->key != NULL);
/* Swap cell->{key,data} and {key,data} (evict). */
tkey = cell->key; tdata = cell->data;
cell->key = key; cell->data = data;
key = tkey; data = tdata;
#ifdef CKH_COUNT
ckh->nrelocs++;
#endif
/* Find the alternate bucket for the evicted item. */
ckh->hash(key, hashes);
tbucket = hashes[1] & ((ZU(1) << ckh->lg_curbuckets) - 1);
if (tbucket == bucket) {
tbucket = hashes[0] & ((ZU(1) << ckh->lg_curbuckets)
- 1);
/*
* It may be that (tbucket == bucket) still, if the
* item's hashes both indicate this bucket. However,
* we are guaranteed to eventually escape this bucket
* during iteration, assuming pseudo-random item
* selection (true randomness would make infinite
* looping a remote possibility). The reason we can
* never get trapped forever is that there are two
* cases:
*
* 1) This bucket == argbucket, so we will quickly
* detect an eviction cycle and terminate.
* 2) An item was evicted to this bucket from another,
* which means that at least one item in this bucket
* has hashes that indicate distinct buckets.
*/
}
/* Check for a cycle. */
if (tbucket == argbucket) {
*argkey = key;
*argdata = data;
return (true);
}
bucket = tbucket;
if (!ckh_try_bucket_insert(ckh, bucket, key, data))
return (false);
}
}
JEMALLOC_INLINE_C bool
ckh_try_insert(ckh_t *ckh, void const**argkey, void const**argdata)
{
size_t hashes[2], bucket;
const void *key = *argkey;
const void *data = *argdata;
ckh->hash(key, hashes);
/* Try to insert in primary bucket. */
bucket = hashes[0] & ((ZU(1) << ckh->lg_curbuckets) - 1);
if (!ckh_try_bucket_insert(ckh, bucket, key, data))
return (false);
/* Try to insert in secondary bucket. */
bucket = hashes[1] & ((ZU(1) << ckh->lg_curbuckets) - 1);
if (!ckh_try_bucket_insert(ckh, bucket, key, data))
return (false);
/*
* Try to find a place for this item via iterative eviction/relocation.
*/
return (ckh_evict_reloc_insert(ckh, bucket, argkey, argdata));
}
/*
* Try to rebuild the hash table from scratch by inserting all items from the
* old table into the new.
*/
JEMALLOC_INLINE_C bool
ckh_rebuild(ckh_t *ckh, ckhc_t *aTab)
{
size_t count, i, nins;
const void *key, *data;
count = ckh->count;
ckh->count = 0;
for (i = nins = 0; nins < count; i++) {
if (aTab[i].key != NULL) {
key = aTab[i].key;
data = aTab[i].data;
if (ckh_try_insert(ckh, &key, &data)) {
ckh->count = count;
return (true);
}
nins++;
}
}
return (false);
}
static bool
ckh_grow(tsd_t *tsd, ckh_t *ckh)
{
bool ret;
ckhc_t *tab, *ttab;
unsigned lg_prevbuckets, lg_curcells;
#ifdef CKH_COUNT
ckh->ngrows++;
#endif
/*
* It is possible (though unlikely, given well behaved hashes) that the
* table will have to be doubled more than once in order to create a
* usable table.
*/
lg_prevbuckets = ckh->lg_curbuckets;
lg_curcells = ckh->lg_curbuckets + LG_CKH_BUCKET_CELLS;
while (true) {
size_t usize;
lg_curcells++;
usize = sa2u(sizeof(ckhc_t) << lg_curcells, CACHELINE);
if (unlikely(usize == 0 || usize > HUGE_MAXCLASS)) {
ret = true;
goto label_return;
}
tab = (ckhc_t *)ipallocztm(tsd_tsdn(tsd), usize, CACHELINE,
true, NULL, true, arena_ichoose(tsd, NULL));
if (tab == NULL) {
ret = true;
goto label_return;
}
/* Swap in new table. */
ttab = ckh->tab;
ckh->tab = tab;
tab = ttab;
ckh->lg_curbuckets = lg_curcells - LG_CKH_BUCKET_CELLS;
if (!ckh_rebuild(ckh, tab)) {
idalloctm(tsd_tsdn(tsd), tab, NULL, true, true);
break;
}
/* Rebuilding failed, so back out partially rebuilt table. */
idalloctm(tsd_tsdn(tsd), ckh->tab, NULL, true, true);
ckh->tab = tab;
ckh->lg_curbuckets = lg_prevbuckets;
}
ret = false;
label_return:
return (ret);
}
static void
ckh_shrink(tsd_t *tsd, ckh_t *ckh)
{
ckhc_t *tab, *ttab;
size_t usize;
unsigned lg_prevbuckets, lg_curcells;
/*
* It is possible (though unlikely, given well behaved hashes) that the
* table rebuild will fail.
*/
lg_prevbuckets = ckh->lg_curbuckets;
lg_curcells = ckh->lg_curbuckets + LG_CKH_BUCKET_CELLS - 1;
usize = sa2u(sizeof(ckhc_t) << lg_curcells, CACHELINE);
if (unlikely(usize == 0 || usize > HUGE_MAXCLASS))
return;
tab = (ckhc_t *)ipallocztm(tsd_tsdn(tsd), usize, CACHELINE, true, NULL,
true, arena_ichoose(tsd, NULL));
if (tab == NULL) {
/*
* An OOM error isn't worth propagating, since it doesn't
* prevent this or future operations from proceeding.
*/
return;
}
/* Swap in new table. */
ttab = ckh->tab;
ckh->tab = tab;
tab = ttab;
ckh->lg_curbuckets = lg_curcells - LG_CKH_BUCKET_CELLS;
if (!ckh_rebuild(ckh, tab)) {
idalloctm(tsd_tsdn(tsd), tab, NULL, true, true);
#ifdef CKH_COUNT
ckh->nshrinks++;
#endif
return;
}
/* Rebuilding failed, so back out partially rebuilt table. */
idalloctm(tsd_tsdn(tsd), ckh->tab, NULL, true, true);
ckh->tab = tab;
ckh->lg_curbuckets = lg_prevbuckets;
#ifdef CKH_COUNT
ckh->nshrinkfails++;
#endif
}
bool
ckh_new(tsd_t *tsd, ckh_t *ckh, size_t minitems, ckh_hash_t *hash,
ckh_keycomp_t *keycomp)
{
bool ret;
size_t mincells, usize;
unsigned lg_mincells;
assert(minitems > 0);
assert(hash != NULL);
assert(keycomp != NULL);
#ifdef CKH_COUNT
ckh->ngrows = 0;
ckh->nshrinks = 0;
ckh->nshrinkfails = 0;
ckh->ninserts = 0;
ckh->nrelocs = 0;
#endif
ckh->prng_state = 42; /* Value doesn't really matter. */
ckh->count = 0;
/*
* Find the minimum power of 2 that is large enough to fit minitems
* entries. We are using (2+,2) cuckoo hashing, which has an expected
* maximum load factor of at least ~0.86, so 0.75 is a conservative load
* factor that will typically allow mincells items to fit without ever
* growing the table.
*/
assert(LG_CKH_BUCKET_CELLS > 0);
mincells = ((minitems + (3 - (minitems % 3))) / 3) << 2;
for (lg_mincells = LG_CKH_BUCKET_CELLS;
(ZU(1) << lg_mincells) < mincells;
lg_mincells++)
; /* Do nothing. */
ckh->lg_minbuckets = lg_mincells - LG_CKH_BUCKET_CELLS;
ckh->lg_curbuckets = lg_mincells - LG_CKH_BUCKET_CELLS;
ckh->hash = hash;
ckh->keycomp = keycomp;
usize = sa2u(sizeof(ckhc_t) << lg_mincells, CACHELINE);
if (unlikely(usize == 0 || usize > HUGE_MAXCLASS)) {
ret = true;
goto label_return;
}
ckh->tab = (ckhc_t *)ipallocztm(tsd_tsdn(tsd), usize, CACHELINE, true,
NULL, true, arena_ichoose(tsd, NULL));
if (ckh->tab == NULL) {
ret = true;
goto label_return;
}
ret = false;
label_return:
return (ret);
}
void
ckh_delete(tsd_t *tsd, ckh_t *ckh)
{
assert(ckh != NULL);
#ifdef CKH_VERBOSE
malloc_printf(
"%s(%p): ngrows: %"FMTu64", nshrinks: %"FMTu64","
" nshrinkfails: %"FMTu64", ninserts: %"FMTu64","
" nrelocs: %"FMTu64"\n", __func__, ckh,
(unsigned long long)ckh->ngrows,
(unsigned long long)ckh->nshrinks,
(unsigned long long)ckh->nshrinkfails,
(unsigned long long)ckh->ninserts,
(unsigned long long)ckh->nrelocs);
#endif
idalloctm(tsd_tsdn(tsd), ckh->tab, NULL, true, true);
if (config_debug)
memset(ckh, JEMALLOC_FREE_JUNK, sizeof(ckh_t));
}
size_t
ckh_count(ckh_t *ckh)
{
assert(ckh != NULL);
return (ckh->count);
}
bool
ckh_iter(ckh_t *ckh, size_t *tabind, void **key, void **data)
{
size_t i, ncells;
for (i = *tabind, ncells = (ZU(1) << (ckh->lg_curbuckets +
LG_CKH_BUCKET_CELLS)); i < ncells; i++) {
if (ckh->tab[i].key != NULL) {
if (key != NULL)
*key = (void *)ckh->tab[i].key;
if (data != NULL)
*data = (void *)ckh->tab[i].data;
*tabind = i + 1;
return (false);
}
}
return (true);
}
bool
ckh_insert(tsd_t *tsd, ckh_t *ckh, const void *key, const void *data)
{
bool ret;
assert(ckh != NULL);
assert(ckh_search(ckh, key, NULL, NULL));
#ifdef CKH_COUNT
ckh->ninserts++;
#endif
while (ckh_try_insert(ckh, &key, &data)) {
if (ckh_grow(tsd, ckh)) {
ret = true;
goto label_return;
}
}
ret = false;
label_return:
return (ret);
}
bool
ckh_remove(tsd_t *tsd, ckh_t *ckh, const void *searchkey, void **key,
void **data)
{
size_t cell;
assert(ckh != NULL);
cell = ckh_isearch(ckh, searchkey);
if (cell != SIZE_T_MAX) {
if (key != NULL)
*key = (void *)ckh->tab[cell].key;
if (data != NULL)
*data = (void *)ckh->tab[cell].data;
ckh->tab[cell].key = NULL;
ckh->tab[cell].data = NULL; /* Not necessary. */
ckh->count--;
/* Try to halve the table if it is less than 1/4 full. */
if (ckh->count < (ZU(1) << (ckh->lg_curbuckets
+ LG_CKH_BUCKET_CELLS - 2)) && ckh->lg_curbuckets
> ckh->lg_minbuckets) {
/* Ignore error due to OOM. */
ckh_shrink(tsd, ckh);
}
return (false);
}
return (true);
}
bool
ckh_search(ckh_t *ckh, const void *searchkey, void **key, void **data)
{
size_t cell;
assert(ckh != NULL);
cell = ckh_isearch(ckh, searchkey);
if (cell != SIZE_T_MAX) {
if (key != NULL)
*key = (void *)ckh->tab[cell].key;
if (data != NULL)
*data = (void *)ckh->tab[cell].data;
return (false);
}
return (true);
}
void
ckh_string_hash(const void *key, size_t r_hash[2])
{
hash(key, strlen((const char *)key), 0x94122f33U, r_hash);
}
bool
ckh_string_keycomp(const void *k1, const void *k2)
{
assert(k1 != NULL);
assert(k2 != NULL);
return (strcmp((char *)k1, (char *)k2) ? false : true);
}
void
ckh_pointer_hash(const void *key, size_t r_hash[2])
{
union {
const void *v;
size_t i;
} u;
assert(sizeof(u.v) == sizeof(u.i));
u.v = key;
hash(&u.i, sizeof(u.i), 0xd983396eU, r_hash);
}
bool
ckh_pointer_keycomp(const void *k1, const void *k2)
{
return ((k1 == k2) ? true : false);
}