My colleague Jeremy Schneider (CC'd) was recently looking into usage count
distributions for various workloads, and he mentioned that it would be nice
to have an easy way to do $SUBJECT. I've attached a patch that adds a
pg_buffercache_usage_counts() function. This function returns a row per
possible usage count with some basic information about the corresponding
buffers.
postgres=# SELECT * FROM pg_buffercache_usage_counts();
usage_count | buffers | dirty | pinned
-------------+---------+-------+--------
0 | 0 | 0 | 0
1 | 1436 | 671 | 0
2 | 102 | 88 | 0
3 | 23 | 21 | 0
4 | 9 | 7 | 0
5 | 164 | 106 | 0
(6 rows)
This new function provides essentially the same information as
pg_buffercache_summary(), but pg_buffercache_summary() only shows the
average usage count for the buffers in use. If there is interest in this
idea, another approach to consider could be to alter
pg_buffercache_summary() instead.
Thoughts?
--
Nathan Bossart
Amazon Web Services: https://aws.amazon.com
>From b2fd87696185537d2cbb611cf70e743d7e197406 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathandboss...@gmail.com>
Date: Fri, 27 Jan 2023 16:39:43 -0800
Subject: [PATCH v1 1/1] introduce pg_buffercache_usage_counts()
---
.../expected/pg_buffercache.out | 14 +++
.../pg_buffercache--1.3--1.4.sql | 13 +++
contrib/pg_buffercache/pg_buffercache_pages.c | 46 ++++++++
contrib/pg_buffercache/sql/pg_buffercache.sql | 4 +
doc/src/sgml/pgbuffercache.sgml | 101 +++++++++++++++++-
5 files changed, 176 insertions(+), 2 deletions(-)
diff --git a/contrib/pg_buffercache/expected/pg_buffercache.out b/contrib/pg_buffercache/expected/pg_buffercache.out
index 635f01e3b2..b745dc69ea 100644
--- a/contrib/pg_buffercache/expected/pg_buffercache.out
+++ b/contrib/pg_buffercache/expected/pg_buffercache.out
@@ -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)
+
diff --git a/contrib/pg_buffercache/pg_buffercache--1.3--1.4.sql b/contrib/pg_buffercache/pg_buffercache--1.3--1.4.sql
index 8f212dc5e9..f4702e4b4b 100644
--- a/contrib/pg_buffercache/pg_buffercache--1.3--1.4.sql
+++ b/contrib/pg_buffercache/pg_buffercache--1.3--1.4.sql
@@ -15,3 +15,16 @@ 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;
+
+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_usage_counts() FROM PUBLIC;
+GRANT EXECUTE ON FUNCTION pg_buffercache_usage_counts() TO pg_monitor;
diff --git a/contrib/pg_buffercache/pg_buffercache_pages.c b/contrib/pg_buffercache/pg_buffercache_pages.c
index 1c6a2f22ca..f333967c51 100644
--- a/contrib/pg_buffercache/pg_buffercache_pages.c
+++ b/contrib/pg_buffercache/pg_buffercache_pages.c
@@ -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 +306,47 @@ 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;
+
+ if ((buf_state & BM_VALID) == 0)
+ continue;
+
+ 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;
+}
diff --git a/contrib/pg_buffercache/sql/pg_buffercache.sql b/contrib/pg_buffercache/sql/pg_buffercache.sql
index 2e2e0a7451..944fbb1bea 100644
--- a/contrib/pg_buffercache/sql/pg_buffercache.sql
+++ b/contrib/pg_buffercache/sql/pg_buffercache.sql
@@ -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();
diff --git a/doc/src/sgml/pgbuffercache.sgml b/doc/src/sgml/pgbuffercache.sgml
index f5d1901af2..29db3f613b 100644
--- a/doc/src/sgml/pgbuffercache.sgml
+++ b/doc/src/sgml/pgbuffercache.sgml
@@ -22,8 +22,9 @@
<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.
+ 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>
@@ -38,6 +39,12 @@
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
@@ -265,6 +272,84 @@
</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 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 usage counts of all shared buffers. 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>
@@ -299,6 +384,18 @@ 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 | 0 | 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>
--
2.25.1
