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Fix pg_buffercache to release buffer partition locks in reverse order,

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and add a note about why.  This is not tremendously important right now,
probably, but it will get more urgent if NUM_BUFFER_PARTITIONS is increased
as much as proposed.
This commit is contained in:
Tom Lane 2007-07-16 21:20:36 +00:00
parent 82b3684672
commit 2a275e6d3c
1 changed files with 11 additions and 4 deletions
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15
contrib/pg_buffercache/pg_buffercache_pages.c
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@ -3,7 +3,7 @@
* pg_buffercache_pages.c * pg_buffercache_pages.c
* display some contents of the buffer cache * display some contents of the buffer cache
* *
* $PostgreSQL: pgsql/contrib/pg_buffercache/pg_buffercache_pages.c,v 1.12 2007/04/07 16:09:14 momjian Exp $ * $PostgreSQL: pgsql/contrib/pg_buffercache/pg_buffercache_pages.c,v 1.13 2007/07/16 21:20:36 tgl Exp $
*------------------------------------------------------------------------- *-------------------------------------------------------------------------
*/ */
#include "postgres.h" #include "postgres.h"
@ -110,7 +110,8 @@ pg_buffercache_pages(PG_FUNCTION_ARGS)
/* /*
* To get a consistent picture of the buffer state, we must lock all * To get a consistent picture of the buffer state, we must lock all
* partitions of the buffer map. Needless to say, this is horrible * partitions of the buffer map. Needless to say, this is horrible
* for concurrency... * for concurrency. Must grab locks in increasing order to avoid
* possible deadlocks.
*/ */
for (i = 0; i < NUM_BUFFER_PARTITIONS; i++) for (i = 0; i < NUM_BUFFER_PARTITIONS; i++)
LWLockAcquire(FirstBufMappingLock + i, LW_SHARED); LWLockAcquire(FirstBufMappingLock + i, LW_SHARED);
@ -145,8 +146,14 @@ pg_buffercache_pages(PG_FUNCTION_ARGS)
UnlockBufHdr(bufHdr); UnlockBufHdr(bufHdr);
} }
/* Release Buffer map. */ /*
for (i = 0; i < NUM_BUFFER_PARTITIONS; i++) * 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(FirstBufMappingLock + i); LWLockRelease(FirstBufMappingLock + i);
} }