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Add pinning_backends column to the pg_buffercache extension.

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The new column shows how many backends have a buffer pinned. That can
be useful during development or to diagnose production issues
e.g. caused by vacuum waiting for cleanup locks.

To handle upgrades transparently - the extension might be used in
views - deal with callers expecting the old number of columns.

Reviewed by Fujii Masao and Rajeev rastogi.
This commit is contained in:
Andres Freund
2014-08-22 00:28:37 +02:00
parent ce486056ec
commit f57791985a
6 changed files with 58 additions and 6 deletions
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37
contrib/pg_buffercache/pg_buffercache_pages.c
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@ -15,7 +15,8 @@
#include "storage/bufmgr.h"
#define NUM_BUFFERCACHE_PAGES_ELEM 8
#define NUM_BUFFERCACHE_PAGES_MIN_ELEM 8
#define NUM_BUFFERCACHE_PAGES_ELEM 9
PG_MODULE_MAGIC;
@ -33,6 +34,12 @@ typedef struct
bool isvalid;
bool isdirty;
uint16 usagecount;
/*
* 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.
*/
int32 pinning_backends;
} BufferCachePagesRec;
@ -60,6 +67,7 @@ pg_buffercache_pages(PG_FUNCTION_ARGS)
MemoryContext oldcontext;
BufferCachePagesContext *fctx; /* User function context. */
TupleDesc tupledesc;
TupleDesc expected_tupledesc;
HeapTuple tuple;
if (SRF_IS_FIRSTCALL())
@ -75,8 +83,23 @@ pg_buffercache_pages(PG_FUNCTION_ARGS)
/* Create a user function context for cross-call persistence */
fctx = (BufferCachePagesContext *) palloc(sizeof(BufferCachePagesContext));
/*
* To smoothly support upgrades from version 1.0 of this extension
* transparently handle the (non-)existance of the pinning_backends
* column. We unfortunately have to get the result type for that... -
* we can't use the result type determined by the function definition
* without potentially crashing when somebody uses the old (or even
* wrong) function definition though.
*/
if (get_call_result_type(fcinfo, NULL, &expected_tupledesc) != TYPEFUNC_COMPOSITE)
elog(ERROR, "return type must be a row type");
if (expected_tupledesc->natts < NUM_BUFFERCACHE_PAGES_MIN_ELEM ||
expected_tupledesc->natts > NUM_BUFFERCACHE_PAGES_ELEM)
elog(ERROR, "incorrect number of output arguments");
/* Construct a tuple descriptor for the result rows. */
tupledesc = CreateTemplateTupleDesc(NUM_BUFFERCACHE_PAGES_ELEM, false);
tupledesc = CreateTemplateTupleDesc(expected_tupledesc->natts, false);
TupleDescInitEntry(tupledesc, (AttrNumber) 1, "bufferid",
INT4OID, -1, 0);
TupleDescInitEntry(tupledesc, (AttrNumber) 2, "relfilenode",
@ -94,6 +117,10 @@ pg_buffercache_pages(PG_FUNCTION_ARGS)
TupleDescInitEntry(tupledesc, (AttrNumber) 8, "usage_count",
INT2OID, -1, 0);
if (expected_tupledesc->natts == NUM_BUFFERCACHE_PAGES_ELEM)
TupleDescInitEntry(tupledesc, (AttrNumber) 9, "pinning_backends",
INT4OID, -1, 0);
fctx->tupdesc = BlessTupleDesc(tupledesc);
/* Allocate NBuffers worth of BufferCachePagesRec records. */
@ -131,6 +158,7 @@ pg_buffercache_pages(PG_FUNCTION_ARGS)
fctx->record[i].forknum = bufHdr->tag.forkNum;
fctx->record[i].blocknum = bufHdr->tag.blockNum;
fctx->record[i].usagecount = bufHdr->usage_count;
fctx->record[i].pinning_backends = bufHdr->refcount;
if (bufHdr->flags & BM_DIRTY)
fctx->record[i].isdirty = true;
@ -185,6 +213,8 @@ pg_buffercache_pages(PG_FUNCTION_ARGS)
nulls[5] = true;
nulls[6] = true;
nulls[7] = true;
/* unused for v1.0 callers, but the array is always long enough */
nulls[8] = true;
}
else
{
@ -202,6 +232,9 @@ pg_buffercache_pages(PG_FUNCTION_ARGS)
nulls[6] = false;
values[7] = Int16GetDatum(fctx->record[i].usagecount);
nulls[7] = false;
/* unused for v1.0 callers, but the array is always long enough */
values[8] = Int32GetDatum(fctx->record[i].pinning_backends);
nulls[8] = false;
}
/* Build and return the tuple. */