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Add pg_buffercache_usage_counts() to contrib/pg_buffercache. 

It was pointed out that pg_buffercache_summary()'s report of
the overall average usage count isn't that useful, and what
would be more helpful in many cases is to report totals for
each possible usage count.  Add a new function to do it like
that.  Since pg_buffercache 1.4 is already new for v16,
we don't need to create a new extension version; we'll just
define this as part of 1.4.

Nathan Bossart

Discussion: https://postgr.es/m/20230130233040.GA2800702@nathanxps13
This commit is contained in:
Tom Lane 2023-04-07 14:25:45 -04:00
parent e056c557ae
commit f3fa31327e
5 changed files with 173 additions and 4 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

14
contrib/pg_buffercache/expected/pg_buffercache.out
View File

@ -17,6 +17,12 @@ from pg_buffercache_summary();
t | t | t
(1 row)
SELECT count(*) > 0 FROM pg_buffercache_usage_counts() WHERE buffers >= 0;
?column?
----------
t
(1 row)
-- Check that the functions / views can't be accessed by default. To avoid
-- having to create a dedicated user, use the pg_database_owner pseudo-role.
SET ROLE pg_database_owner;
@ -26,6 +32,8 @@ SELECT * FROM pg_buffercache_pages() AS p (wrong int);
ERROR: permission denied for function pg_buffercache_pages
SELECT * FROM pg_buffercache_summary();
ERROR: permission denied for function pg_buffercache_summary
SELECT * FROM pg_buffercache_usage_counts();
ERROR: permission denied for function pg_buffercache_usage_counts
RESET role;
-- Check that pg_monitor is allowed to query view / function
SET ROLE pg_monitor;
@ -41,3 +49,9 @@ SELECT buffers_used + buffers_unused > 0 FROM pg_buffercache_summary();
t
(1 row)
SELECT count(*) > 0 FROM pg_buffercache_usage_counts();
?column?
----------
t
(1 row)

11
contrib/pg_buffercache/pg_buffercache--1.3--1.4.sql
View File

@ -12,6 +12,17 @@ CREATE FUNCTION pg_buffercache_summary(
AS 'MODULE_PATHNAME', 'pg_buffercache_summary'
LANGUAGE C PARALLEL SAFE;
CREATE FUNCTION pg_buffercache_usage_counts(
OUT usage_count int4,
OUT buffers int4,
OUT dirty int4,
OUT pinned int4)
RETURNS SETOF record
AS 'MODULE_PATHNAME', 'pg_buffercache_usage_counts'
LANGUAGE C PARALLEL SAFE;
-- Don't want these to be available to public.
REVOKE ALL ON FUNCTION pg_buffercache_summary() FROM PUBLIC;
GRANT EXECUTE ON FUNCTION pg_buffercache_summary() TO pg_monitor;
REVOKE ALL ON FUNCTION pg_buffercache_usage_counts() FROM PUBLIC;
GRANT EXECUTE ON FUNCTION pg_buffercache_usage_counts() TO pg_monitor;

43
contrib/pg_buffercache/pg_buffercache_pages.c
View File

@ -18,6 +18,7 @@
#define NUM_BUFFERCACHE_PAGES_MIN_ELEM 8
#define NUM_BUFFERCACHE_PAGES_ELEM 9
#define NUM_BUFFERCACHE_SUMMARY_ELEM 5
#define NUM_BUFFERCACHE_USAGE_COUNTS_ELEM 4
PG_MODULE_MAGIC;
@ -61,6 +62,7 @@ typedef struct
*/
PG_FUNCTION_INFO_V1(pg_buffercache_pages);
PG_FUNCTION_INFO_V1(pg_buffercache_summary);
PG_FUNCTION_INFO_V1(pg_buffercache_usage_counts);
Datum
pg_buffercache_pages(PG_FUNCTION_ARGS)
@ -304,3 +306,44 @@ pg_buffercache_summary(PG_FUNCTION_ARGS)
PG_RETURN_DATUM(result);
}
Datum
pg_buffercache_usage_counts(PG_FUNCTION_ARGS)
{
ReturnSetInfo *rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;
int usage_counts[BM_MAX_USAGE_COUNT + 1] = {0};
int dirty[BM_MAX_USAGE_COUNT + 1] = {0};
int pinned[BM_MAX_USAGE_COUNT + 1] = {0};
Datum values[NUM_BUFFERCACHE_USAGE_COUNTS_ELEM];
bool nulls[NUM_BUFFERCACHE_USAGE_COUNTS_ELEM] = {0};
InitMaterializedSRF(fcinfo, 0);
for (int i = 0; i < NBuffers; i++)
{
BufferDesc *bufHdr = GetBufferDescriptor(i);
uint32 buf_state = pg_atomic_read_u32(&bufHdr->state);
int usage_count;
usage_count = BUF_STATE_GET_USAGECOUNT(buf_state);
usage_counts[usage_count]++;
if (buf_state & BM_DIRTY)
dirty[usage_count]++;
if (BUF_STATE_GET_REFCOUNT(buf_state) > 0)
pinned[usage_count]++;
}
for (int i = 0; i < BM_MAX_USAGE_COUNT + 1; i++)
{
values[0] = Int32GetDatum(i);
values[1] = Int32GetDatum(usage_counts[i]);
values[2] = Int32GetDatum(dirty[i]);
values[3] = Int32GetDatum(pinned[i]);
tuplestore_putvalues(rsinfo->setResult, rsinfo->setDesc, values, nulls);
}
return (Datum) 0;
}

4
contrib/pg_buffercache/sql/pg_buffercache.sql
View File

@ -10,15 +10,19 @@ select buffers_used + buffers_unused > 0,
buffers_pinned <= buffers_used
from pg_buffercache_summary();
SELECT count(*) > 0 FROM pg_buffercache_usage_counts() WHERE buffers >= 0;
-- Check that the functions / views can't be accessed by default. To avoid
-- having to create a dedicated user, use the pg_database_owner pseudo-role.
SET ROLE pg_database_owner;
SELECT * FROM pg_buffercache;
SELECT * FROM pg_buffercache_pages() AS p (wrong int);
SELECT * FROM pg_buffercache_summary();
SELECT * FROM pg_buffercache_usage_counts();
RESET role;
-- Check that pg_monitor is allowed to query view / function
SET ROLE pg_monitor;
SELECT count(*) > 0 FROM pg_buffercache;
SELECT buffers_used + buffers_unused > 0 FROM pg_buffercache_summary();
SELECT count(*) > 0 FROM pg_buffercache_usage_counts();

105
doc/src/sgml/pgbuffercache.sgml
View File

@ -22,9 +22,10 @@
</indexterm>
<para>
The module provides the <function>pg_buffercache_pages()</function>
function, wrapped in the <structname>pg_buffercache</structname> view, and
the <function>pg_buffercache_summary()</function> function.
This module provides the <function>pg_buffercache_pages()</function>
function (wrapped in the <structname>pg_buffercache</structname> view),
the <function>pg_buffercache_summary()</function> function, and the
<function>pg_buffercache_usage_counts()</function> function.
</para>
<para>
@ -39,6 +40,12 @@
row summarizing the state of the shared buffer cache.
</para>
<para>
The <function>pg_buffercache_usage_counts()</function> function returns a set
of records, each row describing the number of buffers with a given usage
count.
</para>
<para>
By default, use is restricted to superusers and roles with privileges of the
<literal>pg_monitor</literal> role. Access may be granted to others
@ -243,7 +250,7 @@
<structfield>usagecount_avg</structfield> <type>float8</type>
</para>
<para>
Average usagecount of used shared buffers
Average usage count of used shared buffers
</para></entry>
</row>
</tbody>
@ -266,6 +273,84 @@
</para>
</sect2>
<sect2>
<title>The <function>pg_buffercache_usage_counts()</function> Function</title>
<para>
The definitions of the columns exposed by the function are shown in
<xref linkend="pgbuffercache_usage_counts-columns"/>.
</para>
<table id="pgbuffercache_usage_counts-columns">
<title><function>pg_buffercache_usage_counts()</function> Output Columns</title>
<tgroup cols="1">
<thead>
<row>
<entry role="catalog_table_entry"><para role="column_definition">
Column Type
</para>
<para>
Description
</para></entry>
</row>
</thead>
<tbody>
<row>
<entry role="catalog_table_entry"><para role="column_definition">
<structfield>usage_count</structfield> <type>int4</type>
</para>
<para>
A possible buffer usage count
</para></entry>
</row>
<row>
<entry role="catalog_table_entry"><para role="column_definition">
<structfield>buffers</structfield> <type>int4</type>
</para>
<para>
Number of buffers with the usage count
</para></entry>
</row>
<row>
<entry role="catalog_table_entry"><para role="column_definition">
<structfield>dirty</structfield> <type>int4</type>
</para>
<para>
Number of dirty buffers with the usage count
</para></entry>
</row>
<row>
<entry role="catalog_table_entry"><para role="column_definition">
<structfield>pinned</structfield> <type>int4</type>
</para>
<para>
Number of pinned buffers with the usage count
</para></entry>
</row>
</tbody>
</tgroup>
</table>
<para>
The <function>pg_buffercache_usage_counts()</function> function returns a
set of rows summarizing the states of all shared buffers, aggregated over
the possible usage count values. Similar and more detailed information is
provided by the <structname>pg_buffercache</structname> view, but
<function>pg_buffercache_usage_counts()</function> is significantly cheaper.
</para>
<para>
Like the <structname>pg_buffercache</structname> view,
<function>pg_buffercache_usage_counts()</function> does not acquire buffer
manager locks. Therefore concurrent activity can lead to minor inaccuracies
in the result.
</para>
</sect2>
<sect2 id="pgbuffercache-sample-output">
<title>Sample Output</title>
@ -300,6 +385,18 @@ regression=# SELECT * FROM pg_buffercache_summary();
--------------+----------------+---------------+----------------+----------------
248 | 2096904 | 39 | 0 | 3.141129
(1 row)
regression=# SELECT * FROM pg_buffercache_usage_counts();
usage_count | buffers | dirty | pinned
-------------+---------+-------+--------
0 | 14650 | 0 | 0
1 | 1436 | 671 | 0
2 | 102 | 88 | 0
3 | 23 | 21 | 0
4 | 9 | 7 | 0
5 | 164 | 106 | 0
(6 rows)
</screen>
</sect2>