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Don't bother to lock bufmgr partitions in pg_buffercache.

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That makes the view a lot less disruptive to use on a production system.
Without the locks, you don't get a consistent snapshot across all buffers,
but that's OK. It wasn't a very useful guarantee in practice.

Ivan Kartyshov, reviewed by Tomas Vondra and Robert Haas.

Discusssion: <f9d6cab2-73a7-7a84-55a8-07dcb8516ae5@postgrespro.ru>
This commit is contained in:
Heikki Linnakangas 2016-09-29 13:16:30 +03:00
parent 6ad8ac6026
commit 6e654546fb
1 changed files with 4 additions and 19 deletions
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23
contrib/pg_buffercache/pg_buffercache_pages.c
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@ -135,18 +135,13 @@ pg_buffercache_pages(PG_FUNCTION_ARGS)
/* Return to original context when allocating transient memory */
MemoryContextSwitchTo(oldcontext);
/*
* To get a consistent picture of the buffer state, we must lock all
* partitions of the buffer map. Needless to say, this is horrible
* for concurrency. Must grab locks in increasing order to avoid
* possible deadlocks.
*/
for (i = 0; i < NUM_BUFFER_PARTITIONS; i++)
LWLockAcquire(BufMappingPartitionLockByIndex(i), LW_SHARED);
/*
* Scan through all the buffers, saving the relevant fields in the
* fctx->record structure.
*
* We don't hold the partition locks, so we don't get a consistent
* snapshot across all buffers, but we do grab the buffer header
* locks, so the information of each buffer is self-consistent.
*/
for (i = 0; i < NBuffers; i++)
{
@ -179,16 +174,6 @@ pg_buffercache_pages(PG_FUNCTION_ARGS)
UnlockBufHdr(bufHdr, buf_state);
}
/*
* And release locks. We do this in reverse order for two reasons:
* (1) Anyone else who needs more than one of the locks will be trying
* to lock them in increasing order; we don't want to release the
* other process until it can get all the locks it needs. (2) This
* avoids O(N^2) behavior inside LWLockRelease.
*/
for (i = NUM_BUFFER_PARTITIONS; --i >= 0;)
LWLockRelease(BufMappingPartitionLockByIndex(i));
}
funcctx = SRF_PERCALL_SETUP();