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Commit4835458

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Allow Pin/UnpinBuffer to operate in a lockfree manner.
Pinning/Unpinning a buffer is a very frequent operation; especially inread-mostly cache resident workloads. Benchmarking shows that in variousscenarios the spinlock protecting a buffer header's state becomes asignificant bottleneck. The problem can be reproduced with pgbench -S onlarger machines, but can be considerably worse for queries which touchthe same buffers over and over at a high frequency (e.g. nested loopsover a small inner table).To allow atomic operations to be used, cram BufferDesc's flags,usage_count, buf_hdr_lock, refcount into a single 32bit atomic variable;that allows to manipulate them together using 32bit compare-and-swapoperations. This requires reducing MAX_BACKENDS to 2^18-1 (which couldbe lifted by using a 64bit field, but it's not a realistic configurationatm).As not all operations can easily implemented in a lockfree manner,implement the previous buf_hdr_lock via a flag bit in the atomicvariable. That way we can continue to lock the header in places whereit's needed, but can get away without acquiring it in the more frequenthot-paths. There's some additional operations which can be done withoutthe lock, but aren't in this patch; but the most important places arecovered.As bufmgr.c now essentially re-implements spinlocks, abstract the delaylogic from s_lock.c into something more generic. It now has already twousers, and more are coming up; there's a follupw patch for lwlock.c atleast.This patch is based on a proof-of-concept written by me, which AlexanderKorotkov made into a fully working patch; the committed version is againrevised by me. Benchmarking and testing has, amongst others, beenprovided by Dilip Kumar, Alexander Korotkov, Robert Haas.On a large x86 system improvements for readonly pgbench, with a highclient count, of a factor of 8 have been observed.Author: Alexander Korotkov and Andres FreundDiscussion: 2400449.GjM57CE0Yg@dinodell
1 parentcf223c3 commit4835458

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10 files changed

+622
-357
lines changed

10 files changed

+622
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lines changed

‎contrib/pg_buffercache/pg_buffercache_pages.c‎

Lines changed: 8 additions & 7 deletions
Original file line numberDiff line numberDiff line change
@@ -148,33 +148,34 @@ pg_buffercache_pages(PG_FUNCTION_ARGS)
148148
*/
149149
for (i=0;i<NBuffers;i++)
150150
{
151-
volatileBufferDesc*bufHdr;
151+
BufferDesc*bufHdr;
152+
uint32buf_state;
152153

153154
bufHdr=GetBufferDescriptor(i);
154155
/* Lock each buffer header before inspecting. */
155-
LockBufHdr(bufHdr);
156+
buf_state=LockBufHdr(bufHdr);
156157

157158
fctx->record[i].bufferid=BufferDescriptorGetBuffer(bufHdr);
158159
fctx->record[i].relfilenode=bufHdr->tag.rnode.relNode;
159160
fctx->record[i].reltablespace=bufHdr->tag.rnode.spcNode;
160161
fctx->record[i].reldatabase=bufHdr->tag.rnode.dbNode;
161162
fctx->record[i].forknum=bufHdr->tag.forkNum;
162163
fctx->record[i].blocknum=bufHdr->tag.blockNum;
163-
fctx->record[i].usagecount=bufHdr->usage_count;
164-
fctx->record[i].pinning_backends=bufHdr->refcount;
164+
fctx->record[i].usagecount=BUF_STATE_GET_USAGECOUNT(buf_state);
165+
fctx->record[i].pinning_backends=BUF_STATE_GET_REFCOUNT(buf_state);
165166

166-
if (bufHdr->flags&BM_DIRTY)
167+
if (buf_state&BM_DIRTY)
167168
fctx->record[i].isdirty= true;
168169
else
169170
fctx->record[i].isdirty= false;
170171

171172
/* Note if the buffer is valid, and has storage created */
172-
if ((bufHdr->flags&BM_VALID)&& (bufHdr->flags&BM_TAG_VALID))
173+
if ((buf_state&BM_VALID)&& (buf_state&BM_TAG_VALID))
173174
fctx->record[i].isvalid= true;
174175
else
175176
fctx->record[i].isvalid= false;
176177

177-
UnlockBufHdr(bufHdr);
178+
UnlockBufHdr(bufHdr,buf_state);
178179
}
179180

180181
/*

‎src/backend/storage/buffer/buf_init.c‎

Lines changed: 2 additions & 5 deletions
Original file line numberDiff line numberDiff line change
@@ -135,12 +135,9 @@ InitBufferPool(void)
135135
BufferDesc*buf=GetBufferDescriptor(i);
136136

137137
CLEAR_BUFFERTAG(buf->tag);
138-
buf->flags=0;
139-
buf->usage_count=0;
140-
buf->refcount=0;
141-
buf->wait_backend_pid=0;
142138

143-
SpinLockInit(&buf->buf_hdr_lock);
139+
pg_atomic_init_u32(&buf->state,0);
140+
buf->wait_backend_pid=0;
144141

145142
buf->buf_id=i;
146143

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