* to hash_create. This prevents any attempt to split buckets on-the-fly.
* Therefore, each hash bucket chain operates independently, and no fields
* of the hash header change after init except nentries and freeList.
- * A partitioned table uses a spinlock to guard changes of those two fields.
+ * A partitioned table uses spinlocks to guard changes of those fields.
* This lets any subset of the hash buckets be treated as a separately
* lockable partition. We expect callers to use the low-order bits of a
* lookup key's hash value as a partition number --- this will work because
#define DEF_DIRSIZE 256
#define DEF_FFACTOR 1 /* default fill factor */
+/* Number of freelists to be used for a partitioned hash table. */
+#define NUM_FREELISTS 32
/* A hash bucket is a linked list of HASHELEMENTs */
typedef HASHELEMENT *HASHBUCKET;
/* A hash segment is an array of bucket headers */
typedef HASHBUCKET *HASHSEGMENT;
+/*
+ * Using array of FreeListData instead of separate arrays of mutexes, nentries
+ * and freeLists prevents, at least partially, sharing one cache line between
+ * different mutexes (see below).
+ */
+typedef struct
+{
+ slock_t mutex; /* spinlock */
+ long nentries; /* number of entries */
+ HASHELEMENT *freeList; /* list of free elements */
+} FreeListData;
+
/*
* Header structure for a hash table --- contains all changeable info
*
*/
struct HASHHDR
{
- /* In a partitioned table, take this lock to touch nentries or freeList */
- slock_t mutex; /* unused if not partitioned table */
-
- /* These fields change during entry addition/deletion */
- long nentries; /* number of entries in hash table */
- HASHELEMENT *freeList; /* linked list of free elements */
+ /*
+ * The freelist can become a point of contention on high-concurrency hash
+ * tables, so we use an array of freelist, each with its own mutex and
+ * nentries count, instead of just a single one.
+ *
+ * If hash table is not partitioned only freeList[0] is used and spinlocks
+ * are not used at all.
+ */
+ FreeListData freeList[NUM_FREELISTS];
/* These fields can change, but not in a partitioned table */
/* Also, dsize can't change in a shared table, even if unpartitioned */
#define IS_PARTITIONED(hctl) ((hctl)->num_partitions != 0)
+#define FREELIST_IDX(hctl, hashcode) \
+ (IS_PARTITIONED(hctl) ? hashcode % NUM_FREELISTS : 0)
+
/*
* Top control structure for a hashtable --- in a shared table, each backend
* has its own copy (OK since no fields change at runtime)
*/
static void *DynaHashAlloc(Size size);
static HASHSEGMENT seg_alloc(HTAB *hashp);
-static bool element_alloc(HTAB *hashp, int nelem);
+static bool element_alloc(HTAB *hashp, int nelem, int freelist_idx);
static bool dir_realloc(HTAB *hashp);
static bool expand_table(HTAB *hashp);
-static HASHBUCKET get_hash_entry(HTAB *hashp);
+static HASHBUCKET get_hash_entry(HTAB *hashp, int freelist_idx);
static void hdefault(HTAB *hashp);
static int choose_nelem_alloc(Size entrysize);
static bool init_htab(HTAB *hashp, long nelem);
if ((flags & HASH_SHARED_MEM) ||
nelem < hctl->nelem_alloc)
{
- if (!element_alloc(hashp, (int) nelem))
- ereport(ERROR,
- (errcode(ERRCODE_OUT_OF_MEMORY),
- errmsg("out of memory")));
+ int i,
+ freelist_partitions,
+ nelem_alloc,
+ nelem_alloc_first;
+
+ /*
+ * If hash table is partitioned all freeLists have equal number of
+ * elements. Otherwise only freeList[0] is used.
+ */
+ if (IS_PARTITIONED(hashp->hctl))
+ freelist_partitions = NUM_FREELISTS;
+ else
+ freelist_partitions = 1;
+
+ nelem_alloc = nelem / freelist_partitions;
+ if (nelem_alloc == 0)
+ nelem_alloc = 1;
+
+ /* Make sure all memory will be used */
+ if (nelem_alloc * freelist_partitions < nelem)
+ nelem_alloc_first =
+ nelem - nelem_alloc * (freelist_partitions - 1);
+ else
+ nelem_alloc_first = nelem_alloc;
+
+ for (i = 0; i < freelist_partitions; i++)
+ {
+ int temp = (i == 0) ? nelem_alloc_first : nelem_alloc;
+
+ if (!element_alloc(hashp, temp, i))
+ ereport(ERROR,
+ (errcode(ERRCODE_OUT_OF_MEMORY),
+ errmsg("out of memory")));
+ }
}
if (flags & HASH_FIXED_SIZE)
MemSet(hctl, 0, sizeof(HASHHDR));
- hctl->nentries = 0;
- hctl->freeList = NULL;
-
hctl->dsize = DEF_DIRSIZE;
hctl->nsegs = 0;
HASHSEGMENT *segp;
int nbuckets;
int nsegs;
+ int i;
/*
* initialize mutex if it's a partitioned table
*/
if (IS_PARTITIONED(hctl))
- SpinLockInit(&hctl->mutex);
+ for (i = 0; i < NUM_FREELISTS; i++)
+ SpinLockInit(&(hctl->freeList[i].mutex));
/*
* Divide number of elements by the fill factor to determine a desired
"HIGH MASK ", hctl->high_mask,
"LOW MASK ", hctl->low_mask,
"NSEGS ", hctl->nsegs,
- "NENTRIES ", hctl->nentries);
+ "NENTRIES ", hash_get_num_entries(hctl));
#endif
return true;
}
where, hashp->hctl->accesses, hashp->hctl->collisions);
fprintf(stderr, "hash_stats: entries %ld keysize %ld maxp %u segmentcount %ld\n",
- hashp->hctl->nentries, (long) hashp->hctl->keysize,
+ hash_get_num_entries(hashp), (long) hashp->hctl->keysize,
hashp->hctl->max_bucket, hashp->hctl->nsegs);
fprintf(stderr, "%s: total accesses %ld total collisions %ld\n",
where, hash_accesses, hash_collisions);
HASHBUCKET currBucket;
HASHBUCKET *prevBucketPtr;
HashCompareFunc match;
+ int freelist_idx = FREELIST_IDX(hctl, hashvalue);
#if HASH_STATISTICS
hash_accesses++;
* order of these tests is to try to check cheaper conditions first.
*/
if (!IS_PARTITIONED(hctl) && !hashp->frozen &&
- hctl->nentries / (long) (hctl->max_bucket + 1) >= hctl->ffactor &&
+ hctl->freeList[0].nentries / (long) (hctl->max_bucket + 1) >= hctl->ffactor &&
!has_seq_scans(hashp))
(void) expand_table(hashp);
}
{
/* if partitioned, must lock to touch nentries and freeList */
if (IS_PARTITIONED(hctl))
- SpinLockAcquire(&hctl->mutex);
+ SpinLockAcquire(&(hctl->freeList[freelist_idx].mutex));
- Assert(hctl->nentries > 0);
- hctl->nentries--;
+ Assert(hctl->freeList[freelist_idx].nentries > 0);
+ hctl->freeList[freelist_idx].nentries--;
/* remove record from hash bucket's chain. */
*prevBucketPtr = currBucket->link;
/* add the record to the freelist for this table. */
- currBucket->link = hctl->freeList;
- hctl->freeList = currBucket;
+ currBucket->link = hctl->freeList[freelist_idx].freeList;
+ hctl->freeList[freelist_idx].freeList = currBucket;
if (IS_PARTITIONED(hctl))
- SpinLockRelease(&hctl->mutex);
+ SpinLockRelease(&hctl->freeList[freelist_idx].mutex);
/*
* better hope the caller is synchronizing access to this
elog(ERROR, "cannot insert into frozen hashtable \"%s\"",
hashp->tabname);
- currBucket = get_hash_entry(hashp);
+ currBucket = get_hash_entry(hashp, freelist_idx);
if (currBucket == NULL)
{
/* out of memory */
* create a new entry if possible
*/
static HASHBUCKET
-get_hash_entry(HTAB *hashp)
+get_hash_entry(HTAB *hashp, int freelist_idx)
{
- HASHHDR *hctl = hashp->hctl;
+ HASHHDR *hctl = hashp->hctl;
HASHBUCKET newElement;
+ int borrow_from_idx;
for (;;)
{
/* if partitioned, must lock to touch nentries and freeList */
if (IS_PARTITIONED(hctl))
- SpinLockAcquire(&hctl->mutex);
+ SpinLockAcquire(&hctl->freeList[freelist_idx].mutex);
/* try to get an entry from the freelist */
- newElement = hctl->freeList;
+ newElement = hctl->freeList[freelist_idx].freeList;
+
if (newElement != NULL)
break;
- /* no free elements. allocate another chunk of buckets */
if (IS_PARTITIONED(hctl))
- SpinLockRelease(&hctl->mutex);
+ SpinLockRelease(&hctl->freeList[freelist_idx].mutex);
- if (!element_alloc(hashp, hctl->nelem_alloc))
+ /* no free elements. allocate another chunk of buckets */
+ if (!element_alloc(hashp, hctl->nelem_alloc, freelist_idx))
{
- /* out of memory */
- return NULL;
+ if (!IS_PARTITIONED(hctl))
+ return NULL; /* out of memory */
+
+ /* try to borrow element from another partition */
+ borrow_from_idx = freelist_idx;
+ for (;;)
+ {
+ borrow_from_idx = (borrow_from_idx + 1) % NUM_FREELISTS;
+ if (borrow_from_idx == freelist_idx)
+ break;
+
+ SpinLockAcquire(&(hctl->freeList[borrow_from_idx].mutex));
+ newElement = hctl->freeList[borrow_from_idx].freeList;
+
+ if (newElement != NULL)
+ {
+ hctl->freeList[borrow_from_idx].freeList = newElement->link;
+ SpinLockRelease(&(hctl->freeList[borrow_from_idx].mutex));
+
+ SpinLockAcquire(&hctl->freeList[freelist_idx].mutex);
+ hctl->freeList[freelist_idx].nentries++;
+ SpinLockRelease(&hctl->freeList[freelist_idx].mutex);
+
+ break;
+ }
+
+ SpinLockRelease(&(hctl->freeList[borrow_from_idx].mutex));
+ }
+
+ return newElement;
}
}
/* remove entry from freelist, bump nentries */
- hctl->freeList = newElement->link;
- hctl->nentries++;
+ hctl->freeList[freelist_idx].freeList = newElement->link;
+ hctl->freeList[freelist_idx].nentries++;
if (IS_PARTITIONED(hctl))
- SpinLockRelease(&hctl->mutex);
+ SpinLockRelease(&hctl->freeList[freelist_idx].mutex);
return newElement;
}
long
hash_get_num_entries(HTAB *hashp)
{
+ int i;
+ long sum = hashp->hctl->freeList[0].nentries;
+
/*
* We currently don't bother with the mutex; it's only sensible to call
* this function if you've got lock on all partitions of the table.
*/
- return hashp->hctl->nentries;
+
+ if (!IS_PARTITIONED(hashp->hctl))
+ return sum;
+
+ for (i = 1; i < NUM_FREELISTS; i++)
+ sum += hashp->hctl->freeList[i].nentries;
+
+ return sum;
}
/*
}
/*
- * allocate some new elements and link them into the free list
+ * allocate some new elements and link them into the indicated free list
*/
static bool
-element_alloc(HTAB *hashp, int nelem)
+element_alloc(HTAB *hashp, int nelem, int freelist_idx)
{
- HASHHDR *hctl = hashp->hctl;
+ HASHHDR *hctl = hashp->hctl;
Size elementSize;
HASHELEMENT *firstElement;
HASHELEMENT *tmpElement;
/* if partitioned, must lock to touch freeList */
if (IS_PARTITIONED(hctl))
- SpinLockAcquire(&hctl->mutex);
+ SpinLockAcquire(&hctl->freeList[freelist_idx].mutex);
/* freelist could be nonempty if two backends did this concurrently */
- firstElement->link = hctl->freeList;
- hctl->freeList = prevElement;
+ firstElement->link = hctl->freeList[freelist_idx].freeList;
+ hctl->freeList[freelist_idx].freeList = prevElement;
if (IS_PARTITIONED(hctl))
- SpinLockRelease(&hctl->mutex);
+ SpinLockRelease(&hctl->freeList[freelist_idx].mutex);
return true;
}
projects / postgresql / commitdiff