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pg_buffercache: Add pg_buffercache_summary()

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Using pg_buffercache_summary() is significantly cheaper than querying
pg_buffercache and summarizing in SQL.

Author: Melih Mutlu <m.melihmutlu@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Aleksander Alekseev <aleksander@timescale.com>
Reviewed-by: Zhang Mingli <zmlpostgres@gmail.com>
Discussion: https://postgr.es/m/CAGPVpCQAXYo54Q%3D8gqBsS%3Du0uk9qhnnq4%2B710BtUhUisX1XGEg%40mail.gmail.com
This commit is contained in:
Andres Freund 2022-10-13 09:55:46 -07:00
parent 7f8d9cedb3
commit 2589434ae0
8 changed files with 250 additions and 5 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

3
contrib/pg_buffercache/Makefile
View File

@ -7,7 +7,8 @@ OBJS = \
EXTENSION = pg_buffercache
DATA = pg_buffercache--1.2.sql pg_buffercache--1.2--1.3.sql \
pg_buffercache--1.1--1.2.sql pg_buffercache--1.0--1.1.sql
pg_buffercache--1.1--1.2.sql pg_buffercache--1.0--1.1.sql \
pg_buffercache--1.3--1.4.sql
PGFILEDESC = "pg_buffercache - monitoring of shared buffer cache in real-time"
REGRESS = pg_buffercache

33
contrib/pg_buffercache/expected/pg_buffercache.out
View File

@ -8,3 +8,36 @@ from pg_buffercache;
t
(1 row)
select buffers_used + buffers_unused > 0,
buffers_dirty <= buffers_used,
buffers_pinned <= buffers_used
from pg_buffercache_summary();
?column? | ?column? | ?column?
----------+----------+----------
t | t | 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;
SELECT * FROM pg_buffercache;
ERROR: permission denied for view pg_buffercache
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
RESET role;
-- Check that pg_monitor is allowed to query view / function
SET ROLE pg_monitor;
SELECT count(*) > 0 FROM pg_buffercache;
?column?
----------
t
(1 row)
SELECT buffers_used + buffers_unused > 0 FROM pg_buffercache_summary();
?column?
----------
t
(1 row)

1
contrib/pg_buffercache/meson.build
View File

@ -19,6 +19,7 @@ install_data(
'pg_buffercache--1.1--1.2.sql',
'pg_buffercache--1.2--1.3.sql',
'pg_buffercache--1.2.sql',
'pg_buffercache--1.3--1.4.sql',
'pg_buffercache.control',
kwargs: contrib_data_args,
)

17
contrib/pg_buffercache/pg_buffercache--1.3--1.4.sql Normal file
View File

@ -0,0 +1,17 @@
/* contrib/pg_buffercache/pg_buffercache--1.3--1.4.sql */
-- complain if script is sourced in psql, rather than via ALTER EXTENSION
\echo Use "ALTER EXTENSION pg_buffercache UPDATE TO '1.4'" to load this file. \quit
CREATE FUNCTION pg_buffercache_summary(
OUT buffers_used int4,
OUT buffers_unused int4,
OUT buffers_dirty int4,
OUT buffers_pinned int4,
OUT usagecount_avg float8)
AS 'MODULE_PATHNAME', 'pg_buffercache_summary'
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;

2
contrib/pg_buffercache/pg_buffercache.control
View File

@ -1,5 +1,5 @@
# pg_buffercache extension
comment = 'examine the shared buffer cache'
default_version = '1.3'
default_version = '1.4'
module_pathname = '$libdir/pg_buffercache'
relocatable = true

67
contrib/pg_buffercache/pg_buffercache_pages.c
View File

@ -17,6 +17,7 @@
#define NUM_BUFFERCACHE_PAGES_MIN_ELEM 8
#define NUM_BUFFERCACHE_PAGES_ELEM 9
#define NUM_BUFFERCACHE_SUMMARY_ELEM 5
PG_MODULE_MAGIC;
@ -59,6 +60,7 @@ typedef struct
* relation node/tablespace/database/blocknum and dirty indicator.
*/
PG_FUNCTION_INFO_V1(pg_buffercache_pages);
PG_FUNCTION_INFO_V1(pg_buffercache_summary);
Datum
pg_buffercache_pages(PG_FUNCTION_ARGS)
@ -237,3 +239,68 @@ pg_buffercache_pages(PG_FUNCTION_ARGS)
else
SRF_RETURN_DONE(funcctx);
}
Datum
pg_buffercache_summary(PG_FUNCTION_ARGS)
{
Datum result;
TupleDesc tupledesc;
HeapTuple tuple;
Datum values[NUM_BUFFERCACHE_SUMMARY_ELEM];
bool nulls[NUM_BUFFERCACHE_SUMMARY_ELEM];
int32 buffers_used = 0;
int32 buffers_unused = 0;
int32 buffers_dirty = 0;
int32 buffers_pinned = 0;
int64 usagecount_total = 0;
if (get_call_result_type(fcinfo, NULL, &tupledesc) != TYPEFUNC_COMPOSITE)
elog(ERROR, "return type must be a row type");
for (int i = 0; i < NBuffers; i++)
{
BufferDesc *bufHdr;
uint32 buf_state;
/*
* This function summarizes the state of all headers. Locking the
* buffer headers wouldn't provide an improved result as the state of
* the buffer can still change after we release the lock and it'd
* noticeably increase the cost of the function.
*/
bufHdr = GetBufferDescriptor(i);
buf_state = pg_atomic_read_u32(&bufHdr->state);
if (buf_state & BM_VALID)
{
buffers_used++;
usagecount_total += BUF_STATE_GET_USAGECOUNT(buf_state);
if (buf_state & BM_DIRTY)
buffers_dirty++;
}
else
buffers_unused++;
if (BUF_STATE_GET_REFCOUNT(buf_state) > 0)
buffers_pinned++;
}
memset(nulls, 0, sizeof(nulls));
values[0] = Int32GetDatum(buffers_used);
values[1] = Int32GetDatum(buffers_unused);
values[2] = Int32GetDatum(buffers_dirty);
values[3] = Int32GetDatum(buffers_pinned);
if (buffers_used != 0)
values[4] = Float8GetDatum((double) usagecount_total / buffers_used);
else
nulls[4] = true;
/* Build and return the tuple. */
tuple = heap_form_tuple(tupledesc, values, nulls);
result = HeapTupleGetDatum(tuple);
PG_RETURN_DATUM(result);
}

18
contrib/pg_buffercache/sql/pg_buffercache.sql
View File

@ -4,3 +4,21 @@ select count(*) = (select setting::bigint
from pg_settings
where name = 'shared_buffers')
from pg_buffercache;
select buffers_used + buffers_unused > 0,
buffers_dirty <= buffers_used,
buffers_pinned <= buffers_used
from pg_buffercache_summary();
-- 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();
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();

114
doc/src/sgml/pgbuffercache.sgml
View File

@ -16,13 +16,28 @@
<primary>pg_buffercache_pages</primary>
</indexterm>
<indexterm>
<primary>pg_buffercache_summary</primary>
</indexterm>
<para>
The module provides a C function <function>pg_buffercache_pages</function>
that returns a set of records, plus a view
<structname>pg_buffercache</structname> that wraps the function for
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.
</para>
<para>
The <function>pg_buffercache_pages()</function> function returns a set of
records, each row describing the state of one shared buffer entry. The
<structname>pg_buffercache</structname> view wraps the function for
convenient use.
</para>
<para>
The <function>pg_buffercache_summary()</function> function returns a single
row summarizing the state of the shared buffer cache.
</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
@ -164,6 +179,92 @@
</para>
</sect2>
<sect2>
<title>The <function>pg_buffercache_summary()</function> Function</title>
<para>
The definitions of the columns exposed by the function are shown in <xref linkend="pgbuffercache_summary-columns"/>.
</para>
<table id="pgbuffercache_summary-columns">
<title><function>pg_buffercache_summary()</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>buffers_used</structfield> <type>int4</type>
</para>
<para>
Number of unused shared buffers
</para></entry>
</row>
<row>
<entry role="catalog_table_entry"><para role="column_definition">
<structfield>buffers_unused</structfield> <type>int4</type>
</para>
<para>
Number of unused shared buffers
</para></entry>
</row>
<row>
<entry role="catalog_table_entry"><para role="column_definition">
<structfield>buffers_dirty</structfield> <type>int4</type>
</para>
<para>
Number of dirty shared buffers
</para></entry>
</row>
<row>
<entry role="catalog_table_entry"><para role="column_definition">
<structfield>buffers_pinned</structfield> <type>int4</type>
</para>
<para>
Number of pinned shared buffers
</para></entry>
</row>
<row>
<entry role="catalog_table_entry"><para role="column_definition">
<structfield>usagecount_avg</structfield> <type>float8</type>
</para>
<para>
Average usagecount of used shared buffers
</para></entry>
</row>
</tbody>
</tgroup>
</table>
<para>
The <function>pg_buffercache_summary()</function> function returns a
single row summarizing the state of all shared buffers. Similar and more
detailed information is provided by the
<structname>pg_buffercache</structname> view, but
<function>pg_buffercache_summary()</function> is significantly cheaper.
</para>
<para>
Like the <structname>pg_buffercache</structname> view,
<function>pg_buffercache_summary()</function> does not acquire buffer
manager locks. Therefore concurrent activity can lead to minor inaccuracies
in the result.
</para>
</sect2>
<sect2>
<title>Sample Output</title>
@ -191,6 +292,13 @@ regression=# SELECT n.nspname, c.relname, count(*) AS buffers
public | gin_test_tbl | 188
public | spgist_text_tbl | 182
(10 rows)
regression=# SELECT * FROM pg_buffercache_summary();
buffers_used | buffers_unused | buffers_dirty | buffers_pinned | usagecount_avg
--------------+----------------+---------------+----------------+----------------
248 | 2096904 | 39 | 0 | 3.141129
(1 row)
</screen>
</sect2>