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Make backend local tracking of buffer pins memory efficient. 

Since the dawn of time (aka Postgres95) multiple pins of the same
buffer by one backend have been optimized not to modify the shared
refcount more than once. This optimization has always used a NBuffer
sized array in each backend keeping track of a backend's pins.

That array (PrivateRefCount) was one of the biggest per-backend memory
allocations, depending on the shared_buffers setting. Besides the
waste of memory it also has proven to be a performance bottleneck when
assertions are enabled as we make sure that there's no remaining pins
left at the end of transactions. Also, on servers with lots of memory
and a correspondingly high shared_buffers setting the amount of random
memory accesses can also lead to poor cpu cache efficiency.

Because of these reasons a backend's buffers pins are now kept track
of in a small statically sized array that overflows into a hash table
when necessary. Benchmarks have shown neutral to positive performance
results with considerably lower memory usage.

Patch by me, review by Robert Haas.

Discussion: 20140321182231.GA17111@alap3.anarazel.de
This commit is contained in:
Andres Freund 2014-08-30 14:03:21 +02:00
parent c6eaa880ee
commit 4b4b680c3d
4 changed files with 391 additions and 87 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
contrib/pg_buffercache/pg_buffercache_pages.c
View File

@ -37,7 +37,7 @@ typedef struct
/*
* An int32 is sufficiently large, as MAX_BACKENDS prevents a buffer from
* being pinned by too many backends and each backend will only pin once
* because of bufmgr.c's PrivateRefCount array.
* because of bufmgr.c's PrivateRefCount infrastructure.
*/
int32 pinning_backends;
} BufferCachePagesRec;

39
src/backend/storage/buffer/buf_init.c
View File

@ -20,7 +20,6 @@
BufferDesc *BufferDescriptors;
char *BufferBlocks;
int32 *PrivateRefCount;
/*
@ -50,16 +49,9 @@ int32 *PrivateRefCount;
*
* refcount -- Counts the number of processes holding pins on a buffer.
* A buffer is pinned during IO and immediately after a BufferAlloc().
* Pins must be released before end of transaction.
*
* PrivateRefCount -- Each buffer also has a private refcount that keeps
* track of the number of times the buffer is pinned in the current
* process. This is used for two purposes: first, if we pin a
* a buffer more than once, we only need to change the shared refcount
* once, thus only lock the shared state once; second, when a transaction
* aborts, it should only unpin the buffers exactly the number of times it
* has pinned them, so that it will not blow away buffers of another
* backend.
* Pins must be released before end of transaction. For efficiency the
* shared refcount isn't increased if a individual backend pins a buffer
* multiple times. Check the PrivateRefCount infrastructure in bufmgr.c.
*/
@ -129,31 +121,6 @@ InitBufferPool(void)
StrategyInitialize(!foundDescs);
}
/*
* Initialize access to shared buffer pool
*
* This is called during backend startup (whether standalone or under the
* postmaster). It sets up for this backend's access to the already-existing
* buffer pool.
*
* NB: this is called before InitProcess(), so we do not have a PGPROC and
* cannot do LWLockAcquire; hence we can't actually access stuff in
* shared memory yet. We are only initializing local data here.
* (See also InitBufferPoolBackend, over in bufmgr.c.)
*/
void
InitBufferPoolAccess(void)
{
/*
* Allocate and zero local arrays of per-buffer info.
*/
PrivateRefCount = (int32 *) calloc(NBuffers, sizeof(int32));
if (!PrivateRefCount)
ereport(FATAL,
(errcode(ERRCODE_OUT_OF_MEMORY),
errmsg("out of memory")));
}
/*
* BufferShmemSize
*

418
src/backend/storage/buffer/bufmgr.c
View File

@ -65,6 +65,15 @@
#define DROP_RELS_BSEARCH_THRESHOLD 20
typedef struct PrivateRefCountEntry
{
Buffer buffer;
int32 refcount;
} PrivateRefCountEntry;
/* 64 bytes, about the size of a cache line on common systems */
#define REFCOUNT_ARRAY_ENTRIES 8
/* GUC variables */
bool zero_damaged_pages = false;
int bgwriter_lru_maxpages = 100;
@ -85,6 +94,281 @@ static bool IsForInput;
/* local state for LockBufferForCleanup */
static volatile BufferDesc *PinCountWaitBuf = NULL;
/*
* Backend-Private refcount management:
*
* Each buffer also has a private refcount that keeps track of the number of
* times the buffer is pinned in the current process. This is so that the
* shared refcount needs to be modified only once if a buffer is pinned more
* than once by a individual backend. It's also used to check that no buffers
* are still pinned at the end of transactions and when exiting.
*
*
* To avoid - as we used to - requiring an array with NBuffers entries to keep
* track of local buffers we use a small sequentially searched array
* (PrivateRefCountArray) and a overflow hash table (PrivateRefCountHash) to
* keep track of backend local pins.
*
* Until no more than REFCOUNT_ARRAY_ENTRIES buffers are pinned at once, all
* refcounts are kept track of in the array; after that, new array entries
* displace old ones into the hash table. That way a frequently used entry
* can't get "stuck" in the hashtable while infrequent ones clog the array.
*
* Note that in most scenarios the number of pinned buffers will not exceed
* REFCOUNT_ARRAY_ENTRIES.
*/
static struct PrivateRefCountEntry PrivateRefCountArray[REFCOUNT_ARRAY_ENTRIES];
static HTAB *PrivateRefCountHash = NULL;
static int32 PrivateRefCountOverflowed = 0;
static uint32 PrivateRefCountClock = 0;
static PrivateRefCountEntry* GetPrivateRefCountEntry(Buffer buffer, bool create, bool do_move);
static inline int32 GetPrivateRefCount(Buffer buffer);
static void ForgetPrivateRefCountEntry(PrivateRefCountEntry *ref);
/*
* Return the PrivateRefCount entry for the passed buffer.
*
* Returns NULL if create = false is passed and the buffer doesn't have a
* PrivateRefCount entry; allocates a new PrivateRefCountEntry if currently
* none exists and create = true is passed.
*
* If do_move is true - only allowed for create = false - the entry is
* optimized for frequent access.
*
* When a returned refcount entry isn't used anymore it has to be forgotten,
* using ForgetPrivateRefCountEntry().
*
* Only works for shared buffers.
*/
static PrivateRefCountEntry*
GetPrivateRefCountEntry(Buffer buffer, bool create, bool do_move)
{
PrivateRefCountEntry *res;
PrivateRefCountEntry *free = NULL;
bool found = false;
int i;
Assert(!create || do_move);
Assert(BufferIsValid(buffer));
Assert(!BufferIsLocal(buffer));
/*
* First search for references in the array, that'll be sufficient in the
* majority of cases.
*/
for (i = 0; i < REFCOUNT_ARRAY_ENTRIES; i++)
{
res = &PrivateRefCountArray[i];
if (res->buffer == buffer)
return res;
/* Remember where to put a new refcount, should it become necessary. */
if (free == NULL && res->buffer == InvalidBuffer)
free = res;
}
/*
* By here we know that the buffer, if already pinned, isn't residing in
* the array.
*/
res = NULL;
found = false;
/*
* Look up the buffer in the hashtable if we've previously overflowed into
* it.
*/
if (PrivateRefCountOverflowed > 0)
{
res = hash_search(PrivateRefCountHash,
(void *) &buffer,
HASH_FIND,
&found);
}
if (!found)
{
if (!create)
{
/* Neither array nor hash have an entry and no new entry is needed */
return NULL;
}
else if (free != NULL)
{
/* add entry into the free array slot */
free->buffer = buffer;
free->refcount = 0;
return free;
}
else
{
/*
* Move entry from the current clock position in the array into the
* hashtable. Use that slot.
*/
PrivateRefCountEntry *arrayent;
PrivateRefCountEntry *hashent;
/* select victim slot */
arrayent = &PrivateRefCountArray[
PrivateRefCountClock++ % REFCOUNT_ARRAY_ENTRIES];
Assert(arrayent->buffer != InvalidBuffer);
/* enter victim array entry into hashtable */
hashent = hash_search(PrivateRefCountHash,
(void *) &arrayent->buffer,
HASH_ENTER,
&found);
Assert(!found);
hashent->refcount = arrayent->refcount;
/* fill the now free array slot */
arrayent->buffer = buffer;
arrayent->refcount = 0;
PrivateRefCountOverflowed++;
return arrayent;
}
}
else
{
if (!do_move)
{
return res;
}
else if (found && free != NULL)
{
/* move buffer from hashtable into the free array slot */
/* fill array slot */
free->buffer = buffer;
free->refcount = res->refcount;
/* delete from hashtable */
hash_search(PrivateRefCountHash,
(void *) &buffer,
HASH_REMOVE,
&found);
Assert(found);
Assert(PrivateRefCountOverflowed > 0);
PrivateRefCountOverflowed--;
return free;
}
else
{
/*
* Swap the entry in the hash table with the one in the array at the
* current clock position.
*/
PrivateRefCountEntry *arrayent;
PrivateRefCountEntry *hashent;
/* select victim slot */
arrayent = &PrivateRefCountArray[
PrivateRefCountClock++ % REFCOUNT_ARRAY_ENTRIES];
Assert(arrayent->buffer != InvalidBuffer);
/* enter victim entry into the hashtable */
hashent = hash_search(PrivateRefCountHash,
(void *) &arrayent->buffer,
HASH_ENTER,
&found);
Assert(!found);
hashent->refcount = arrayent->refcount;
/* fill now free array entry with previously searched entry */
arrayent->buffer = res->buffer;
arrayent->refcount = res->refcount;
/* and remove the old entry */
hash_search(PrivateRefCountHash,
(void *) &arrayent->buffer,
HASH_REMOVE,
&found);
Assert(found);
/* PrivateRefCountOverflowed stays the same -1 + +1 = 0*/
return arrayent;
}
}
Assert(false); /* unreachable */
return NULL;
}
/*
* Returns how many times the passed buffer is pinned by this backend.
*
* Only works for shared memory buffers!
*/
static inline int32
GetPrivateRefCount(Buffer buffer)
{
PrivateRefCountEntry *ref;
Assert(BufferIsValid(buffer));
Assert(!BufferIsLocal(buffer));
ref = GetPrivateRefCountEntry(buffer, false, false);
if (ref == NULL)
return 0;
return ref->refcount;
}
/*
* Release resources used to track the reference count of a buffer which we no
* longer have pinned and don't want to pin again immediately.
*/
static void
ForgetPrivateRefCountEntry(PrivateRefCountEntry *ref)
{
Assert(ref->refcount == 0);
if (ref >= &PrivateRefCountArray[0] &&
ref < &PrivateRefCountArray[REFCOUNT_ARRAY_ENTRIES])
{
ref->buffer = InvalidBuffer;
}
else
{
bool found;
Buffer buffer = ref->buffer;
hash_search(PrivateRefCountHash,
(void *) &buffer,
HASH_REMOVE,
&found);
Assert(found);
Assert(PrivateRefCountOverflowed > 0);
PrivateRefCountOverflowed--;
}
}
/*
* BufferIsPinned
* True iff the buffer is pinned (also checks for valid buffer number).
*
* NOTE: what we check here is that *this* backend holds a pin on
* the buffer. We do not care whether some other backend does.
*/
#define BufferIsPinned(bufnum) \
( \
!BufferIsValid(bufnum) ? \
false \
: \
BufferIsLocal(bufnum) ? \
(LocalRefCount[-(bufnum) - 1] > 0) \
: \
(GetPrivateRefCount(bufnum) > 0) \
)
static Buffer ReadBuffer_common(SMgrRelation reln, char relpersistence,
ForkNumber forkNum, BlockNumber blockNum,
@ -940,7 +1224,7 @@ retry:
UnlockBufHdr(buf);
LWLockRelease(oldPartitionLock);
/* safety check: should definitely not be our *own* pin */
if (PrivateRefCount[buf->buf_id] != 0)
if (GetPrivateRefCount(buf->buf_id) > 0)
elog(ERROR, "buffer is pinned in InvalidateBuffer");
WaitIO(buf);
goto retry;
@ -999,7 +1283,7 @@ MarkBufferDirty(Buffer buffer)
bufHdr = &BufferDescriptors[buffer - 1];
Assert(PrivateRefCount[buffer - 1] > 0);
Assert(BufferIsPinned(buffer));
/* unfortunately we can't check if the lock is held exclusively */
Assert(LWLockHeldByMe(bufHdr->content_lock));
@ -1046,9 +1330,9 @@ ReleaseAndReadBuffer(Buffer buffer,
if (BufferIsValid(buffer))
{
Assert(BufferIsPinned(buffer));
if (BufferIsLocal(buffer))
{
Assert(LocalRefCount[-buffer - 1] > 0);
bufHdr = &LocalBufferDescriptors[-buffer - 1];
if (bufHdr->tag.blockNum == blockNum &&
RelFileNodeEquals(bufHdr->tag.rnode, relation->rd_node) &&
@ -1059,7 +1343,6 @@ ReleaseAndReadBuffer(Buffer buffer,
}
else
{
Assert(PrivateRefCount[buffer - 1] > 0);
bufHdr = &BufferDescriptors[buffer - 1];
/* we have pin, so it's ok to examine tag without spinlock */
if (bufHdr->tag.blockNum == blockNum &&
@ -1096,8 +1379,11 @@ PinBuffer(volatile BufferDesc *buf, BufferAccessStrategy strategy)
{
int b = buf->buf_id;
bool result;
PrivateRefCountEntry *ref;
if (PrivateRefCount[b] == 0)
ref = GetPrivateRefCountEntry(b + 1, true, true);
if (ref->refcount == 0)
{
LockBufHdr(buf);
buf->refcount++;
@ -1119,8 +1405,9 @@ PinBuffer(volatile BufferDesc *buf, BufferAccessStrategy strategy)
/* If we previously pinned the buffer, it must surely be valid */
result = true;
}
PrivateRefCount[b]++;
Assert(PrivateRefCount[b] > 0);
ref->refcount++;
Assert(ref->refcount > 0);
ResourceOwnerRememberBuffer(CurrentResourceOwner,
BufferDescriptorGetBuffer(buf));
return result;
@ -1143,12 +1430,15 @@ static void
PinBuffer_Locked(volatile BufferDesc *buf)
{
int b = buf->buf_id;
PrivateRefCountEntry *ref;
if (PrivateRefCount[b] == 0)
ref = GetPrivateRefCountEntry(b + 1, true, true);
if (ref->refcount == 0)
buf->refcount++;
UnlockBufHdr(buf);
PrivateRefCount[b]++;
Assert(PrivateRefCount[b] > 0);
ref->refcount++;
Assert(ref->refcount > 0);
ResourceOwnerRememberBuffer(CurrentResourceOwner,
BufferDescriptorGetBuffer(buf));
}
@ -1164,15 +1454,19 @@ PinBuffer_Locked(volatile BufferDesc *buf)
static void
UnpinBuffer(volatile BufferDesc *buf, bool fixOwner)
{
PrivateRefCountEntry *ref;
int b = buf->buf_id;
ref = GetPrivateRefCountEntry(b + 1, false, false);
Assert(ref != NULL);
if (fixOwner)
ResourceOwnerForgetBuffer(CurrentResourceOwner,
BufferDescriptorGetBuffer(buf));
Assert(PrivateRefCount[b] > 0);
PrivateRefCount[b]--;
if (PrivateRefCount[b] == 0)
Assert(ref->refcount > 0);
ref->refcount--;
if (ref->refcount == 0)
{
/* I'd better not still hold any locks on the buffer */
Assert(!LWLockHeldByMe(buf->content_lock));
@ -1197,6 +1491,8 @@ UnpinBuffer(volatile BufferDesc *buf, bool fixOwner)
}
else
UnlockBufHdr(buf);
ForgetPrivateRefCountEntry(ref);
}
}
@ -1702,6 +1998,10 @@ SyncOneBuffer(int buf_id, bool skip_recently_used)
/*
* AtEOXact_Buffers - clean up at end of transaction.
*
* As of PostgreSQL 8.0, buffer pins should get released by the
* ResourceOwner mechanism. This routine is just a debugging
* cross-check that no pins remain.
*/
void
AtEOXact_Buffers(bool isCommit)
@ -1709,6 +2009,36 @@ AtEOXact_Buffers(bool isCommit)
CheckForBufferLeaks();
AtEOXact_LocalBuffers(isCommit);
Assert(PrivateRefCountOverflowed == 0);
}
/*
* Initialize access to shared buffer pool
*
* This is called during backend startup (whether standalone or under the
* postmaster). It sets up for this backend's access to the already-existing
* buffer pool.
*
* NB: this is called before InitProcess(), so we do not have a PGPROC and
* cannot do LWLockAcquire; hence we can't actually access stuff in
* shared memory yet. We are only initializing local data here.
* (See also InitBufferPoolBackend)
*/
void
InitBufferPoolAccess(void)
{
HASHCTL hash_ctl;
memset(&PrivateRefCountArray, 0, sizeof(PrivateRefCountArray));
MemSet(&hash_ctl, 0, sizeof(hash_ctl));
hash_ctl.keysize = sizeof(int32);
hash_ctl.entrysize = sizeof(PrivateRefCountArray);
hash_ctl.hash = oid_hash; /* a bit more efficient than tag_hash */
PrivateRefCountHash = hash_create("PrivateRefCount", 100, &hash_ctl,
HASH_ELEM | HASH_FUNCTION);
}
/*
@ -1754,16 +2084,34 @@ CheckForBufferLeaks(void)
{
#ifdef USE_ASSERT_CHECKING
int RefCountErrors = 0;
Buffer b;
PrivateRefCountEntry *res;
int i;
for (b = 1; b <= NBuffers; b++)
/* check the array */
for (i = 0; i < REFCOUNT_ARRAY_ENTRIES; i++)
{
if (PrivateRefCount[b - 1] != 0)
res = &PrivateRefCountArray[i];
if (res->buffer != InvalidBuffer)
{
PrintBufferLeakWarning(b);
PrintBufferLeakWarning(res->buffer);
RefCountErrors++;
}
}
/* if neccessary search the hash */
if (PrivateRefCountOverflowed)
{
HASH_SEQ_STATUS hstat;
hash_seq_init(&hstat, PrivateRefCountHash);
while ((res = (PrivateRefCountEntry *) hash_seq_search(&hstat)) != NULL)
{
PrintBufferLeakWarning(res->buffer);
RefCountErrors++;
}
}
Assert(RefCountErrors == 0);
#endif
}
@ -1789,7 +2137,7 @@ PrintBufferLeakWarning(Buffer buffer)
else
{
buf = &BufferDescriptors[buffer - 1];
loccount = PrivateRefCount[buffer - 1];
loccount = GetPrivateRefCount(buffer);
backend = InvalidBackendId;
}
@ -2329,7 +2677,7 @@ PrintBufferDescs(void)
i, buf->freeNext,
relpathbackend(buf->tag.rnode, InvalidBackendId, buf->tag.forkNum),
buf->tag.blockNum, buf->flags,
buf->refcount, PrivateRefCount[i]);
buf->refcount, GetPrivateRefCount(i));
}
}
#endif
@ -2343,7 +2691,7 @@ PrintPinnedBufs(void)
for (i = 0; i < NBuffers; ++i, ++buf)
{
if (PrivateRefCount[i] > 0)
if (GetPrivateRefCount(i + 1) > 0)
{
/* theoretically we should lock the bufhdr here */
elog(LOG,
@ -2352,7 +2700,7 @@ PrintPinnedBufs(void)
i, buf->freeNext,
relpath(buf->tag.rnode, buf->tag.forkNum),
buf->tag.blockNum, buf->flags,
buf->refcount, PrivateRefCount[i]);
buf->refcount, GetPrivateRefCount(i + 1));
}
}
}
@ -2509,6 +2857,7 @@ void
ReleaseBuffer(Buffer buffer)
{
volatile BufferDesc *bufHdr;
PrivateRefCountEntry *ref;
if (!BufferIsValid(buffer))
elog(ERROR, "bad buffer ID: %d", buffer);
@ -2524,10 +2873,12 @@ ReleaseBuffer(Buffer buffer)
bufHdr = &BufferDescriptors[buffer - 1];
Assert(PrivateRefCount[buffer - 1] > 0);
ref = GetPrivateRefCountEntry(buffer, false, false);
Assert(ref != NULL);
Assert(ref->refcount > 0);
if (PrivateRefCount[buffer - 1] > 1)
PrivateRefCount[buffer - 1]--;
if (ref->refcount > 1)
ref->refcount--;
else
UnpinBuffer(bufHdr, false);
}
@ -2561,7 +2912,12 @@ IncrBufferRefCount(Buffer buffer)
if (BufferIsLocal(buffer))
LocalRefCount[-buffer - 1]++;
else
PrivateRefCount[buffer - 1]++;
{
PrivateRefCountEntry *ref;
ref = GetPrivateRefCountEntry(buffer, false, true);
Assert(ref != NULL);
ref->refcount++;
}
}
/*
@ -2595,7 +2951,7 @@ MarkBufferDirtyHint(Buffer buffer, bool buffer_std)
bufHdr = &BufferDescriptors[buffer - 1];
Assert(PrivateRefCount[buffer - 1] > 0);
Assert(GetPrivateRefCount(buffer) > 0);
/* here, either share or exclusive lock is OK */
Assert(LWLockHeldByMe(bufHdr->content_lock));
@ -2813,9 +3169,9 @@ LockBufferForCleanup(Buffer buffer)
}
/* There should be exactly one local pin */
if (PrivateRefCount[buffer - 1] != 1)
if (GetPrivateRefCount(buffer) != 1)
elog(ERROR, "incorrect local pin count: %d",
PrivateRefCount[buffer - 1]);
GetPrivateRefCount(buffer));
bufHdr = &BufferDescriptors[buffer - 1];
@ -2880,7 +3236,7 @@ HoldingBufferPinThatDelaysRecovery(void)
if (bufid < 0)
return false;
if (PrivateRefCount[bufid] > 0)
if (GetPrivateRefCount(bufid + 1) > 0)
return true;
return false;
@ -2910,8 +3266,8 @@ ConditionalLockBufferForCleanup(Buffer buffer)
}
/* There should be exactly one local pin */
Assert(PrivateRefCount[buffer - 1] > 0);
if (PrivateRefCount[buffer - 1] != 1)
Assert(GetPrivateRefCount(buffer) > 0);
if (GetPrivateRefCount(buffer) != 1)
return false;
/* Try to acquire lock */

19
src/include/storage/bufmgr.h
View File

@ -55,7 +55,6 @@ extern int target_prefetch_pages;
/* in buf_init.c */
extern PGDLLIMPORT char *BufferBlocks;
extern PGDLLIMPORT int32 *PrivateRefCount;
/* in localbuf.c */
extern PGDLLIMPORT int NLocBuffer;
@ -101,24 +100,6 @@ extern PGDLLIMPORT int32 *LocalRefCount;
(bufnum) != InvalidBuffer \
)
/*
* BufferIsPinned
* True iff the buffer is pinned (also checks for valid buffer number).
*
* NOTE: what we check here is that *this* backend holds a pin on
* the buffer. We do not care whether some other backend does.
*/
#define BufferIsPinned(bufnum) \
( \
!BufferIsValid(bufnum) ? \
false \
: \
BufferIsLocal(bufnum) ? \
(LocalRefCount[-(bufnum) - 1] > 0) \
: \
(PrivateRefCount[(bufnum) - 1] > 0) \
)
/*
* BufferGetBlock
* Returns a reference to a disk page image associated with a buffer.