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Commit13453ee

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Add pg_buffercache_evict() function for testing.
When testing buffer pool logic, it is useful to be able to evictarbitrary blocks. This function can be used in SQL queries over thepg_buffercache view to set up a wide range of buffer pool states. Ofcourse, buffer mappings might change concurrently so you might evict ablock other than the one you had in mind, and another session mightbring it back in at any time. That's OK for the intended purpose ofsetting up developer testing scenarios, and more complicated interlockingschemes to give stronger guararantees about that would likely be lessflexible for actual testing work anyway. Superuser-only.Author: Palak Chaturvedi <chaturvedipalak1911@gmail.com>Author: Thomas Munro <thomas.munro@gmail.com> (docs, small tweaks)Reviewed-by: Nitin Jadhav <nitinjadhavpostgres@gmail.com>Reviewed-by: Andres Freund <andres@anarazel.de>Reviewed-by: Cary Huang <cary.huang@highgo.ca>Reviewed-by: Cédric Villemain <cedric.villemain+pgsql@abcsql.com>Reviewed-by: Jim Nasby <jim.nasby@gmail.com>Reviewed-by: Maxim Orlov <orlovmg@gmail.com>Reviewed-by: Thomas Munro <thomas.munro@gmail.com>Reviewed-by: Melanie Plageman <melanieplageman@gmail.com>Discussion:https://postgr.es/m/CALfch19pW48ZwWzUoRSpsaV9hqt0UPyaBPC4bOZ4W+c7FF566A@mail.gmail.com
1 parent0ea51ba commit13453ee

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8 files changed

+127
-8
lines changed

8 files changed

+127
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lines changed

‎contrib/pg_buffercache/Makefile

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -8,7 +8,7 @@ OBJS = \
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EXTENSION = pg_buffercache
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DATA = pg_buffercache--1.2.sql pg_buffercache--1.2--1.3.sql\
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pg_buffercache--1.1--1.2.sql pg_buffercache--1.0--1.1.sql\
11-
pg_buffercache--1.3--1.4.sql
11+
pg_buffercache--1.3--1.4.sql pg_buffercache--1.4--1.5.sql
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PGFILEDESC = "pg_buffercache - monitoring of shared buffer cache in real-time"
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REGRESS = pg_buffercache

‎contrib/pg_buffercache/meson.build

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Original file line numberDiff line numberDiff line change
@@ -22,6 +22,7 @@ install_data(
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'pg_buffercache--1.2--1.3.sql',
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'pg_buffercache--1.2.sql',
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'pg_buffercache--1.3--1.4.sql',
25+
'pg_buffercache--1.4--1.5.sql',
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'pg_buffercache.control',
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kwargs: contrib_data_args,
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)
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Original file line numberDiff line numberDiff line change
@@ -0,0 +1,6 @@
1+
\echo Use"ALTER EXTENSION pg_buffercache UPDATE TO '1.5'" to load this file. \quit
2+
3+
CREATEFUNCTIONpg_buffercache_evict(INint)
4+
RETURNS bool
5+
AS'MODULE_PATHNAME','pg_buffercache_evict'
6+
LANGUAGE C PARALLEL SAFE VOLATILE STRICT;
Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -1,5 +1,5 @@
11
# pg_buffercache extension
22
comment = 'examine the shared buffer cache'
3-
default_version = '1.4'
3+
default_version = '1.5'
44
module_pathname = '$libdir/pg_buffercache'
55
relocatable = true

‎contrib/pg_buffercache/pg_buffercache_pages.c

Lines changed: 20 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -63,6 +63,7 @@ typedef struct
6363
PG_FUNCTION_INFO_V1(pg_buffercache_pages);
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PG_FUNCTION_INFO_V1(pg_buffercache_summary);
6565
PG_FUNCTION_INFO_V1(pg_buffercache_usage_counts);
66+
PG_FUNCTION_INFO_V1(pg_buffercache_evict);
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Datum
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pg_buffercache_pages(PG_FUNCTION_ARGS)
@@ -347,3 +348,22 @@ pg_buffercache_usage_counts(PG_FUNCTION_ARGS)
347348

348349
return (Datum)0;
349350
}
351+
352+
/*
353+
* Try to evict a shared buffer.
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*/
355+
Datum
356+
pg_buffercache_evict(PG_FUNCTION_ARGS)
357+
{
358+
Bufferbuf=PG_GETARG_INT32(0);
359+
360+
if (!superuser())
361+
ereport(ERROR,
362+
(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
363+
errmsg("must be superuser to use pg_buffercache_evict function")));
364+
365+
if (buf<1||buf>NBuffers)
366+
elog(ERROR,"bad buffer ID: %d",buf);
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368+
PG_RETURN_BOOL(EvictUnpinnedBuffer(buf));
369+
}

‎doc/src/sgml/pgbuffercache.sgml

Lines changed: 33 additions & 6 deletions
Original file line numberDiff line numberDiff line change
@@ -11,6 +11,8 @@
1111
<para>
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The <filename>pg_buffercache</filename> module provides a means for
1313
examining what's happening in the shared buffer cache in real time.
14+
It also offers a low-level way to evict data from it, for testing
15+
purposes.
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</para>
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<indexterm>
@@ -21,11 +23,16 @@
2123
<primary>pg_buffercache_summary</primary>
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</indexterm>
2325

26+
<indexterm>
27+
<primary>pg_buffercache_evict</primary>
28+
</indexterm>
29+
2430
<para>
2531
This module provides the <function>pg_buffercache_pages()</function>
2632
function (wrapped in the <structname>pg_buffercache</structname> view),
27-
the <function>pg_buffercache_summary()</function> function, and the
28-
<function>pg_buffercache_usage_counts()</function> function.
33+
the <function>pg_buffercache_summary()</function> function, the
34+
<function>pg_buffercache_usage_counts()</function> function and
35+
the <function>pg_buffercache_evict()</function> function.
2936
</para>
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3138
<para>
@@ -47,9 +54,15 @@
4754
</para>
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<para>
50-
By default, use is restricted to superusers and roles with privileges of the
51-
<literal>pg_monitor</literal> role. Access may be granted to others
52-
using <command>GRANT</command>.
57+
By default, use of the above functions is restricted to superusers and roles
58+
with privileges of the <literal>pg_monitor</literal> role. Access may be
59+
granted to others using <command>GRANT</command>.
60+
</para>
61+
62+
<para>
63+
The <function>pg_buffercache_evict()</function> function allows a block to
64+
be evicted from the buffer pool given a buffer identifier. Use of this
65+
function is restricted to superusers only.
5366
</para>
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5568
<sect2 id="pgbuffercache-pg-buffercache">
@@ -351,7 +364,21 @@
351364
</para>
352365
</sect2>
353366

354-
<sect2 id="pgbuffercache-sample-output">
367+
<sect2 id="pgbuffercache-pg-buffercache-evict">
368+
<title>The <structname>pg_buffercache_evict</structname> Function</title>
369+
<para>
370+
The <function>pg_buffercache_evict()</function> function takes a buffer
371+
identifier, as shown in the <structfield>bufferid</structfield> column of
372+
the <structname>pg_buffercache</structname> view. It returns true on success,
373+
and false if the buffer wasn't valid, if it couldn't be evicted because it
374+
was pinned, or if it became dirty again after an attempt to write it out.
375+
The result is immediately out of date upon return, as the buffer might
376+
become valid again at any time due to concurrent activity. The function is
377+
intended for developer testing only.
378+
</para>
379+
</sect2>
380+
381+
<sect2 id="pgbuffercache-sample-output">
355382
<title>Sample Output</title>
356383

357384
<screen>

‎src/backend/storage/buffer/bufmgr.c

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Original file line numberDiff line numberDiff line change
@@ -6003,3 +6003,66 @@ ResOwnerPrintBufferPin(Datum res)
60036003
{
60046004
returnDebugPrintBufferRefcount(DatumGetInt32(res));
60056005
}
6006+
6007+
/*
6008+
* Try to evict the current block in a shared buffer.
6009+
*
6010+
* This function is intended for testing/development use only!
6011+
*
6012+
* To succeed, the buffer must not be pinned on entry, so if the caller had a
6013+
* particular block in mind, it might already have been replaced by some other
6014+
* block by the time this function runs. It's also unpinned on return, so the
6015+
* buffer might be occupied again by the time control is returned, potentially
6016+
* even by the same block. This inherent raciness without other interlocking
6017+
* makes the function unsuitable for non-testing usage.
6018+
*
6019+
* Returns true if the buffer was valid and it has now been made invalid.
6020+
* Returns false if it wasn't valid, if it couldn't be evicted due to a pin,
6021+
* or if the buffer becomes dirty again while we're trying to write it out.
6022+
*/
6023+
bool
6024+
EvictUnpinnedBuffer(Bufferbuf)
6025+
{
6026+
BufferDesc*desc;
6027+
uint32buf_state;
6028+
boolresult;
6029+
6030+
/* Make sure we can pin the buffer. */
6031+
ResourceOwnerEnlarge(CurrentResourceOwner);
6032+
ReservePrivateRefCountEntry();
6033+
6034+
Assert(!BufferIsLocal(buf));
6035+
desc=GetBufferDescriptor(buf-1);
6036+
6037+
/* Lock the header and check if it's valid. */
6038+
buf_state=LockBufHdr(desc);
6039+
if ((buf_state&BM_VALID)==0)
6040+
{
6041+
UnlockBufHdr(desc,buf_state);
6042+
return false;
6043+
}
6044+
6045+
/* Check that it's not pinned already. */
6046+
if (BUF_STATE_GET_REFCOUNT(buf_state)>0)
6047+
{
6048+
UnlockBufHdr(desc,buf_state);
6049+
return false;
6050+
}
6051+
6052+
PinBuffer_Locked(desc);/* releases spinlock */
6053+
6054+
/* If it was dirty, try to clean it once. */
6055+
if (buf_state&BM_DIRTY)
6056+
{
6057+
LWLockAcquire(BufferDescriptorGetContentLock(desc),LW_SHARED);
6058+
FlushBuffer(desc,NULL,IOOBJECT_RELATION,IOCONTEXT_NORMAL);
6059+
LWLockRelease(BufferDescriptorGetContentLock(desc));
6060+
}
6061+
6062+
/* This will return false if it becomes dirty or someone else pins it. */
6063+
result=InvalidateVictimBuffer(desc);
6064+
6065+
UnpinBuffer(desc);
6066+
6067+
returnresult;
6068+
}

‎src/include/storage/bufmgr.h

Lines changed: 2 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -305,6 +305,8 @@ extern bool BgBufferSync(struct WritebackContext *wb_context);
305305
externvoidLimitAdditionalPins(uint32*additional_pins);
306306
externvoidLimitAdditionalLocalPins(uint32*additional_pins);
307307

308+
externboolEvictUnpinnedBuffer(Bufferbuf);
309+
308310
/* in buf_init.c */
309311
externvoidInitBufferPool(void);
310312
externSizeBufferShmemSize(void);

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