Hello, Robert
> Thanks, this looks way better. Some more comments:
>
> - I don't think there's any good reason for this patch to change the
> calling convention for ShmemInitHash(). Maybe that's a good idea and
> maybe it isn't, but it's a separate issue from what this patch is
> doing, and if we're going to do it at all, it should be discussed
> separately. Let's leave it out of this patch.
>
> - I would not provide an option to change the number of freelist
> mutexes. Let's dump DEFAULT_MUTEXES_NUM and MAX_MUTEXES_NUM and have
> FREELIST_MUTEXES_NUM. The value of 32 which you selected is fine with
> me.
>
> - The change to LOG2_NUM_LOCK_PARTITIONS in lwlock.h looks like an
> independent change. Again, leave it out of this patch and submit it
> separately with its own benchmarking data if you want to argue for it.
>
> I think if you make these changes this patch will be quite a bit
> smaller and in pretty good shape.
>
> Thanks for working on this.
>
Here is a corrected patch. I decided to keep a small change in
InitLocks procedure (lock.c) since ShmemInitHash description clearly
states "For a table whose maximum size is certain, [init_size] should
be equal to max_size". Also I added two items to my TODO list to send
more patches as soon (and if) this patch will be applied.
Thanks for your contribution to this patch.
diff --git a/src/backend/storage/lmgr/lock.c b/src/backend/storage/lmgr/lock.c
index e3e9599..5296e77 100644
--- a/src/backend/storage/lmgr/lock.c
+++ b/src/backend/storage/lmgr/lock.c
@@ -374,18 +374,10 @@ void
InitLocks(void)
{
HASHCTL info;
- long init_table_size,
- max_table_size;
+ long max_table_size;
bool found;
/*
- * Compute init/max size to request for lock hashtables. Note these
- * calculations must agree with LockShmemSize!
- */
- max_table_size = NLOCKENTS();
- init_table_size = max_table_size / 2;
-
- /*
* Allocate hash table for LOCK structs. This stores per-locked-object
* information.
*/
@@ -393,16 +385,16 @@ InitLocks(void)
info.keysize = sizeof(LOCKTAG);
info.entrysize = sizeof(LOCK);
info.num_partitions = NUM_LOCK_PARTITIONS;
+ max_table_size = NLOCKENTS();
LockMethodLockHash = ShmemInitHash("LOCK hash",
- init_table_size,
+ max_table_size,
max_table_size,
&info,
HASH_ELEM | HASH_BLOBS | HASH_PARTITION);
/* Assume an average of 2 holders per lock */
max_table_size *= 2;
- init_table_size *= 2;
/*
* Allocate hash table for PROCLOCK structs. This stores
@@ -414,7 +406,7 @@ InitLocks(void)
info.num_partitions = NUM_LOCK_PARTITIONS;
LockMethodProcLockHash = ShmemInitHash("PROCLOCK hash",
- init_table_size,
+ max_table_size,
max_table_size,
&info,
HASH_ELEM | HASH_FUNCTION | HASH_PARTITION);
diff --git a/src/backend/utils/hash/dynahash.c b/src/backend/utils/hash/dynahash.c
index 24a53da..1a3244a 100644
--- a/src/backend/utils/hash/dynahash.c
+++ b/src/backend/utils/hash/dynahash.c
@@ -15,7 +15,7 @@
* 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
@@ -111,6 +111,11 @@
#define DEF_DIRSIZE 256
#define DEF_FFACTOR 1 /* default fill factor */
+/*
+ * Number of mutexes and respectively number of nentries and freeLists
+ * (see below). Should be power of 2.
+ */
+#define MUTEXES_NUM 32
/* A hash bucket is a linked list of HASHELEMENTs */
typedef HASHELEMENT *HASHBUCKET;
@@ -128,12 +133,23 @@ typedef HASHBUCKET *HASHSEGMENT;
*/
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 */
+ /*
+ * There are two fields declared below: nentries and freeList. nentries
+ * stores current number of entries in a hash table. freeList is a linked
+ * list of free elements.
+ *
+ * To keep these fields consistent in a partitioned table we need to
+ * synchronize access to them using a spinlock. But it turned out that a
+ * single spinlock can create a bottleneck. To prevent lock contention an
+ * array of MUTEXES_NUM spinlocks is used. Each spinlock protects one
+ * element of nentries and freeList arrays.
+ *
+ * If hash table is not partitioned only nentries[0] and freeList[0] are
+ * used and spinlocks are not used at all.
+ */
+ slock_t mutex[MUTEXES_NUM]; /* array of spinlocks */
+ long nentries[MUTEXES_NUM]; /* number of entries */
+ HASHELEMENT *freeList[MUTEXES_NUM]; /* lists of free elements */
/* These fields can change, but not in a partitioned table */
/* Also, dsize can't change in a shared table, even if unpartitioned */
@@ -166,6 +182,8 @@ struct HASHHDR
#define IS_PARTITIONED(hctl) ((hctl)->num_partitions != 0)
+#define FREELIST_IDX(hctl, hashcode) (IS_PARTITIONED(hctl) ? hashcode % MUTEXES_NUM : 0)
+
/*
* Top control structure for a hashtable --- in a shared table, each backend
* has its own copy (OK since no fields change at runtime)
@@ -219,10 +237,10 @@ static long hash_accesses,
*/
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);
@@ -282,6 +300,10 @@ hash_create(const char *tabname, long nelem, HASHCTL *info, int flags)
{
HTAB *hashp;
HASHHDR *hctl;
+ int i,
+ partitions_number,
+ nelem_alloc,
+ nelem_alloc_first;
/*
* For shared hash tables, we have a local hash header (HTAB struct) that
@@ -482,10 +504,34 @@ hash_create(const char *tabname, long nelem, HASHCTL *info, int flags)
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")));
+ /*
+ * If hash table is partitioned all freeLists have equal number of
+ * elements. Otherwise only freeList[0] is used.
+ */
+ if (IS_PARTITIONED(hashp->hctl))
+ partitions_number = MUTEXES_NUM;
+ else
+ partitions_number = 1;
+
+ nelem_alloc = nelem / partitions_number;
+ if (nelem_alloc == 0)
+ nelem_alloc = 1;
+
+ if (nelem_alloc * partitions_number < nelem)
+ /* Make sure all memory will be used */
+ nelem_alloc_first = nelem - nelem_alloc * (partitions_number - 1);
+ else
+ nelem_alloc_first = nelem_alloc;
+
+ for (i = 0; i < partitions_number; 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)
@@ -503,9 +549,6 @@ hdefault(HTAB *hashp)
MemSet(hctl, 0, sizeof(HASHHDR));
- hctl->nentries = 0;
- hctl->freeList = NULL;
-
hctl->dsize = DEF_DIRSIZE;
hctl->nsegs = 0;
@@ -572,12 +615,14 @@ init_htab(HTAB *hashp, long nelem)
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 < MUTEXES_NUM; i++)
+ SpinLockInit(&(hctl->mutex[i]));
/*
* Divide number of elements by the fill factor to determine a desired
@@ -648,7 +693,7 @@ init_htab(HTAB *hashp, long nelem)
"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;
}
@@ -769,7 +814,7 @@ hash_stats(const char *where, HTAB *hashp)
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);
@@ -863,6 +908,7 @@ hash_search_with_hash_value(HTAB *hashp,
HASHBUCKET currBucket;
HASHBUCKET *prevBucketPtr;
HashCompareFunc match;
+ int freelist_idx = FREELIST_IDX(hctl, hashvalue);
#if HASH_STATISTICS
hash_accesses++;
@@ -885,7 +931,7 @@ hash_search_with_hash_value(HTAB *hashp,
* 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->nentries[0] / (long) (hctl->max_bucket + 1) >= hctl->ffactor &&
!has_seq_scans(hashp))
(void) expand_table(hashp);
}
@@ -943,20 +989,20 @@ hash_search_with_hash_value(HTAB *hashp,
{
/* if partitioned, must lock to touch nentries and freeList */
if (IS_PARTITIONED(hctl))
- SpinLockAcquire(&hctl->mutex);
+ SpinLockAcquire(&(hctl->mutex[freelist_idx]));
- Assert(hctl->nentries > 0);
- hctl->nentries--;
+ Assert(hctl->nentries[freelist_idx] > 0);
+ hctl->nentries[freelist_idx]--;
/* 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];
+ hctl->freeList[freelist_idx] = currBucket;
if (IS_PARTITIONED(hctl))
- SpinLockRelease(&hctl->mutex);
+ SpinLockRelease(&hctl->mutex[freelist_idx]);
/*
* better hope the caller is synchronizing access to this
@@ -982,7 +1028,7 @@ hash_search_with_hash_value(HTAB *hashp,
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 */
@@ -1175,41 +1221,70 @@ hash_update_hash_key(HTAB *hashp,
* 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->mutex[freelist_idx]);
/* try to get an entry from the freelist */
- newElement = hctl->freeList;
+ newElement = hctl->freeList[freelist_idx];
+
if (newElement != NULL)
- break;
+ {
+ /* remove entry from freelist, bump nentries */
+ hctl->freeList[freelist_idx] = newElement->link;
+ hctl->nentries[freelist_idx]++;
+ if (IS_PARTITIONED(hctl))
+ SpinLockRelease(&hctl->mutex[freelist_idx]);
+
+ return newElement;
+ }
- /* no free elements. allocate another chunk of buckets */
if (IS_PARTITIONED(hctl))
- SpinLockRelease(&hctl->mutex);
+ SpinLockRelease(&hctl->mutex[freelist_idx]);
- 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 */
- /* remove entry from freelist, bump nentries */
- hctl->freeList = newElement->link;
- hctl->nentries++;
+ /* try to borrow element from another partition */
+ borrow_from_idx = freelist_idx;
+ for (;;)
+ {
+ borrow_from_idx = (borrow_from_idx + 1) % MUTEXES_NUM;
+ if (borrow_from_idx == freelist_idx)
+ break;
- if (IS_PARTITIONED(hctl))
- SpinLockRelease(&hctl->mutex);
+ SpinLockAcquire(&(hctl->mutex[borrow_from_idx]));
+ newElement = hctl->freeList[borrow_from_idx];
+
+ if (newElement != NULL)
+ {
+ hctl->freeList[borrow_from_idx] = newElement->link;
+ SpinLockRelease(&(hctl->mutex[borrow_from_idx]));
- return newElement;
+ SpinLockAcquire(&hctl->mutex[freelist_idx]);
+ hctl->nentries[freelist_idx]++;
+ SpinLockRelease(&hctl->mutex[freelist_idx]);
+
+ break;
+ }
+
+ SpinLockRelease(&(hctl->mutex[borrow_from_idx]));
+ }
+
+ return newElement;
+ }
+ }
}
/*
@@ -1218,11 +1293,21 @@ get_hash_entry(HTAB *hashp)
long
hash_get_num_entries(HTAB *hashp)
{
+ int i;
+ long sum = hashp->hctl->nentries[0];
+
/*
* 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 < MUTEXES_NUM; i++)
+ sum += hashp->hctl->nentries[i];
+
+ return sum;
}
/*
@@ -1530,9 +1615,9 @@ seg_alloc(HTAB *hashp)
* allocate some new elements and link them into the 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;
@@ -1563,14 +1648,14 @@ element_alloc(HTAB *hashp, int nelem)
/* if partitioned, must lock to touch freeList */
if (IS_PARTITIONED(hctl))
- SpinLockAcquire(&hctl->mutex);
+ SpinLockAcquire(&hctl->mutex[freelist_idx]);
/* freelist could be nonempty if two backends did this concurrently */
- firstElement->link = hctl->freeList;
- hctl->freeList = prevElement;
+ firstElement->link = hctl->freeList[freelist_idx];
+ hctl->freeList[freelist_idx] = prevElement;
if (IS_PARTITIONED(hctl))
- SpinLockRelease(&hctl->mutex);
+ SpinLockRelease(&hctl->mutex[freelist_idx]);
return true;
}
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