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Commit4b4b680

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Make backend local tracking of buffer pins memory efficient.
Since the dawn of time (aka Postgres95) multiple pins of the samebuffer by one backend have been optimized not to modify the sharedrefcount more than once. This optimization has always used a NBuffersized array in each backend keeping track of a backend's pins.That array (PrivateRefCount) was one of the biggest per-backend memoryallocations, depending on the shared_buffers setting. Besides thewaste of memory it also has proven to be a performance bottleneck whenassertions are enabled as we make sure that there's no remaining pinsleft at the end of transactions. Also, on servers with lots of memoryand a correspondingly high shared_buffers setting the amount of randommemory accesses can also lead to poor cpu cache efficiency.Because of these reasons a backend's buffers pins are now kept trackof in a small statically sized array that overflows into a hash tablewhen necessary. Benchmarks have shown neutral to positive performanceresults with considerably lower memory usage.Patch by me, review by Robert Haas.Discussion: 20140321182231.GA17111@alap3.anarazel.de
1 parentc6eaa88 commit4b4b680

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4 files changed

+391
-87
lines changed

4 files changed

+391
-87
lines changed

‎contrib/pg_buffercache/pg_buffercache_pages.c

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -37,7 +37,7 @@ typedef struct
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/*
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* An int32 is sufficiently large, as MAX_BACKENDS prevents a buffer from
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* being pinned by too many backends and each backend will only pin once
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* because of bufmgr.c's PrivateRefCountarray.
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* because of bufmgr.c's PrivateRefCountinfrastructure.
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*/
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int32pinning_backends;
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}BufferCachePagesRec;

‎src/backend/storage/buffer/buf_init.c

Lines changed: 3 additions & 36 deletions
Original file line numberDiff line numberDiff line change
@@ -20,7 +20,6 @@
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BufferDesc*BufferDescriptors;
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char*BufferBlocks;
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int32*PrivateRefCount;
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/*
@@ -50,16 +49,9 @@ int32 *PrivateRefCount;
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*
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* refcount --Counts the number of processes holding pins on a buffer.
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*A buffer is pinned during IO and immediately after a BufferAlloc().
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*Pins must be released before end of transaction.
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*
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* PrivateRefCount -- Each buffer also has a private refcount that keeps
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*track of the number of times the buffer is pinned in the current
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*process. This is used for two purposes: first, if we pin a
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*a buffer more than once, we only need to change the shared refcount
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*once, thus only lock the shared state once; second, when a transaction
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*aborts, it should only unpin the buffers exactly the number of times it
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*has pinned them, so that it will not blow away buffers of another
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*backend.
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*Pins must be released before end of transaction. For efficiency the
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*shared refcount isn't increased if a individual backend pins a buffer
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*multiple times. Check the PrivateRefCount infrastructure in bufmgr.c.
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*/
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@@ -129,31 +121,6 @@ InitBufferPool(void)
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StrategyInitialize(!foundDescs);
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}
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/*
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* Initialize access to shared buffer pool
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*
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* This is called during backend startup (whether standalone or under the
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* postmaster). It sets up for this backend's access to the already-existing
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* buffer pool.
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*
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* NB: this is called before InitProcess(), so we do not have a PGPROC and
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* cannot do LWLockAcquire; hence we can't actually access stuff in
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* shared memory yet. We are only initializing local data here.
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* (See also InitBufferPoolBackend, over in bufmgr.c.)
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*/
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void
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InitBufferPoolAccess(void)
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{
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/*
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* Allocate and zero local arrays of per-buffer info.
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*/
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PrivateRefCount= (int32*)calloc(NBuffers,sizeof(int32));
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if (!PrivateRefCount)
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ereport(FATAL,
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(errcode(ERRCODE_OUT_OF_MEMORY),
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errmsg("out of memory")));
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}
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/*
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* BufferShmemSize
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*

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