From: Aleksander Alekseev <a.alekseev@postgrespro.ru>
To: Robert Haas <robertmhaas@gmail.com>
Cc: pgsql-hackers <pgsql-hackers@postgresql.org>
Cc: Anastasia Lubennikova <a.lubennikova@postgrespro.ru>
Subject: Re: Patch: fix lock contention for HASHHDR.mutex
Date: Thu, 11 Feb 2016 17:11:01 +0300
Message-ID: <20160211171101.33f34160@fujitsu> (raw)
In-Reply-To: <CA+TgmoZm=Uowt8a_XaSfooGwufeeLJ861NTADiCEOpyFehV8Wg@mail.gmail.com>
References: <CA+TgmoZKfuJog2MoZrmPsrVh4b8owruPU3+Nm_7bMgVcc8U3WQ@mail.gmail.com>
<20160208192857.GA316589@alvherre.pgsql>
<20160210112400.22c0b677@fujitsu>
<CA+TgmoZm=Uowt8a_XaSfooGwufeeLJ861NTADiCEOpyFehV8Wg@mail.gmail.com>
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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.
--
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http://www.postgresql.org/mailpref/pgsql-hackersAttachments:dynahash-optimization-v14.patch (text/x-patch, 13.1K) downloaddiff --git a/src/backend/storage/lmgr/lock.c b/src/backend/storage/lmgr/lock.cindex 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.cindex 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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To: pgsql-hackers@postgresql.org
Cc: a.alekseev@postgrespro.ru, robertmhaas@gmail.com, a.lubennikova@postgrespro.ru
Subject: Re: Patch: fix lock contention for HASHHDR.mutex
In-Reply-To: <20160211171101.33f34160@fujitsu>
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