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Re-add GUC track_wal_io_timing

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This commit is a rework of 2421e9a51d20, about which Andres Freund has
raised some concerns as it is valuable to have both track_io_timing and
track_wal_io_timing in some cases, as the WAL write and fsync paths can
be a major bottleneck for some workloads.  Hence, it can be relevant to
not calculate the WAL timings in environments where pg_test_timing
performs poorly while capturing some IO data under track_io_timing for
the non-WAL IO paths.  The opposite can be also true: it should be
possible to disable the non-WAL timings and enable the WAL timings (the
previous GUC setups allowed this possibility).

track_wal_io_timing is added back in this commit, controlling if WAL
timings should be calculated in pg_stat_io for the read, fsync and write
paths, as done previously with pg_stat_wal.  pg_stat_wal previously
tracked only the sync and write parts (now removed), read stats is new
data tracked in pg_stat_io, all three are aggregated if
track_wal_io_timing is enabled.  The read part matters during recovery
or if a XLogReader is used.

Extra note: more control over if the types of timings calculated in
pg_stat_io could be done with a GUC that lists pairs of (IOObject,IOOp).

Reported-by: Andres Freund <andres@anarazel.de>
Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Co-authored-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/3opf2wh2oljco6ldyqf7ukabw3jijnnhno6fjb4mlu6civ5h24@fcwmhsgmlmzu
This commit is contained in:
Michael Paquier 2025-02-26 09:49:59 +09:00
parent a5cbdeb98a
commit 6c349d83b6
15 changed files with 96 additions and 32 deletions
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32
doc/src/sgml/config.sgml
View File

@ -8378,9 +8378,11 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
displayed in <link linkend="monitoring-pg-stat-database-view"> displayed in <link linkend="monitoring-pg-stat-database-view">
<structname>pg_stat_database</structname></link>, <structname>pg_stat_database</structname></link>,
<link linkend="monitoring-pg-stat-io-view"> <link linkend="monitoring-pg-stat-io-view">
<structname>pg_stat_io</structname></link>, in the output of the <structname>pg_stat_io</structname></link> (if <varname>object</varname>
is not <literal>wal</literal>), in the output of the
<link linkend="pg-stat-get-backend-io"> <link linkend="pg-stat-get-backend-io">
<function>pg_stat_get_backend_io()</function></link> function, in the <function>pg_stat_get_backend_io()</function></link> function (if
<varname>object</varname> is not <literal>wal</literal>), in the
output of <xref linkend="sql-explain"/> when the <literal>BUFFERS</literal> output of <xref linkend="sql-explain"/> when the <literal>BUFFERS</literal>
option is used, in the output of <xref linkend="sql-vacuum"/> when option is used, in the output of <xref linkend="sql-vacuum"/> when
the <literal>VERBOSE</literal> option is used, by autovacuum the <literal>VERBOSE</literal> option is used, by autovacuum
@ -8393,6 +8395,32 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
</listitem> </listitem>
</varlistentry> </varlistentry>
<varlistentry id="guc-track-wal-io-timing" xreflabel="track_wal_io_timing">
<term><varname>track_wal_io_timing</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>track_wal_io_timing</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Enables timing of WAL I/O calls. This parameter is off by default,
as it will repeatedly query the operating system for the current time,
which may cause significant overhead on some platforms.
You can use the <application>pg_test_timing</application> tool to
measure the overhead of timing on your system.
I/O timing information is displayed in
<link linkend="monitoring-pg-stat-io-view">
<structname>pg_stat_io</structname></link> for the
<varname>object</varname> <literal>wal</literal> and in the output of
the <link linkend="pg-stat-get-backend-io">
<function>pg_stat_get_backend_io()</function></link> function for the
<varname>object</varname> <literal>wal</literal>.
Only superusers and users with the appropriate <literal>SET</literal>
privilege can change this setting.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-track-functions" xreflabel="track_functions"> <varlistentry id="guc-track-functions" xreflabel="track_functions">
<term><varname>track_functions</varname> (<type>enum</type>) <term><varname>track_functions</varname> (<type>enum</type>)
<indexterm> <indexterm>

31
doc/src/sgml/monitoring.sgml
View File

@ -183,6 +183,11 @@ postgres 27093 0.0 0.0 30096 2752 ? Ss 11:34 0:00 postgres: ser
of block read, write, extend, and fsync times. of block read, write, extend, and fsync times.
</para> </para>
<para>
The parameter <xref linkend="guc-track-wal-io-timing"/> enables monitoring
of WAL read, write and fsync times.
</para>
<para> <para>
Normally these parameters are set in <filename>postgresql.conf</filename> so Normally these parameters are set in <filename>postgresql.conf</filename> so
that they apply to all server processes, but it is possible to turn that they apply to all server processes, but it is possible to turn
@ -2723,7 +2728,11 @@ description | Waiting for a newly initialized WAL file to reach durable storage
</para> </para>
<para> <para>
Time spent in read operations in milliseconds (if Time spent in read operations in milliseconds (if
<xref linkend="guc-track-io-timing"/> is enabled, otherwise zero) <xref linkend="guc-track-io-timing"/> is enabled and
<varname>object</varname> is not <literal>wal</literal>,
or if <xref linkend="guc-track-wal-io-timing"/> is enabled
and <varname>object</varname> is <literal>wal</literal>,
otherwise zero)
</para> </para>
</entry> </entry>
</row> </row>
@ -2757,7 +2766,11 @@ description | Waiting for a newly initialized WAL file to reach durable storage
</para> </para>
<para> <para>
Time spent in write operations in milliseconds (if Time spent in write operations in milliseconds (if
<xref linkend="guc-track-io-timing"/> is enabled, otherwise zero) <xref linkend="guc-track-io-timing"/> is enabled and
<varname>object</varname> is not <literal>wal</literal>,
or if <xref linkend="guc-track-wal-io-timing"/> is enabled
and <varname>object</varname> is <literal>wal</literal>,
otherwise zero)
</para> </para>
</entry> </entry>
</row> </row>
@ -2816,8 +2829,12 @@ description | Waiting for a newly initialized WAL file to reach durable storage
<structfield>extend_time</structfield> <type>double precision</type> <structfield>extend_time</structfield> <type>double precision</type>
</para> </para>
<para> <para>
Time spent in extend operations in milliseconds (if Time spent in extend operations in milliseconds. (if
<xref linkend="guc-track-io-timing"/> is enabled, otherwise zero) <xref linkend="guc-track-io-timing"/> is enabled and
<varname>object</varname> is not <literal>wal</literal>,
or if <xref linkend="guc-track-wal-io-timing"/> is enabled
and <varname>object</varname> is <literal>wal</literal>,
otherwise zero)
</para> </para>
</entry> </entry>
</row> </row>
@ -2887,7 +2904,11 @@ description | Waiting for a newly initialized WAL file to reach durable storage
</para> </para>
<para> <para>
Time spent in fsync operations in milliseconds (if Time spent in fsync operations in milliseconds (if
<xref linkend="guc-track-io-timing"/> is enabled, otherwise zero) <xref linkend="guc-track-io-timing"/> is enabled and
<varname>object</varname> is not <literal>wal</literal>,
or if <xref linkend="guc-track-wal-io-timing"/> is enabled
and <varname>object</varname> is <literal>wal</literal>,
otherwise zero)
</para> </para>
</entry> </entry>
</row> </row>

4
doc/src/sgml/wal.sgml
View File

@ -808,7 +808,7 @@
<para> <para>
There are two internal functions to write WAL data to disk: There are two internal functions to write WAL data to disk:
<function>XLogWrite</function> and <function>issue_xlog_fsync</function>. <function>XLogWrite</function> and <function>issue_xlog_fsync</function>.
When <xref linkend="guc-track-io-timing"/> is enabled, the total When <xref linkend="guc-track-wal-io-timing"/> is enabled, the total
amounts of time <function>XLogWrite</function> writes and amounts of time <function>XLogWrite</function> writes and
<function>issue_xlog_fsync</function> syncs WAL data to disk are counted as <function>issue_xlog_fsync</function> syncs WAL data to disk are counted as
<varname>write_time</varname> and <varname>fsync_time</varname> in <varname>write_time</varname> and <varname>fsync_time</varname> in
@ -828,7 +828,7 @@
<literal>fsync</literal>, or <literal>fsync_writethrough</literal>, <literal>fsync</literal>, or <literal>fsync_writethrough</literal>,
the write operation moves WAL buffers to kernel cache and the write operation moves WAL buffers to kernel cache and
<function>issue_xlog_fsync</function> syncs them to disk. Regardless <function>issue_xlog_fsync</function> syncs them to disk. Regardless
of the setting of <varname>track_io_timing</varname>, the number of the setting of <varname>track_wal_io_timing</varname>, the number
of times <function>XLogWrite</function> writes and of times <function>XLogWrite</function> writes and
<function>issue_xlog_fsync</function> syncs WAL data to disk are also <function>issue_xlog_fsync</function> syncs WAL data to disk are also
counted as <varname>writes</varname> and <varname>fsyncs</varname> counted as <varname>writes</varname> and <varname>fsyncs</varname>

9
src/backend/access/transam/xlog.c
View File

@ -134,6 +134,7 @@ int CommitSiblings = 5; /* # concurrent xacts needed to sleep */
int wal_retrieve_retry_interval = 5000; int wal_retrieve_retry_interval = 5000;
int max_slot_wal_keep_size_mb = -1; int max_slot_wal_keep_size_mb = -1;
int wal_decode_buffer_size = 512 * 1024; int wal_decode_buffer_size = 512 * 1024;
bool track_wal_io_timing = false;
#ifdef WAL_DEBUG #ifdef WAL_DEBUG
bool XLOG_DEBUG = false; bool XLOG_DEBUG = false;
@ -2437,7 +2438,7 @@ XLogWrite(XLogwrtRqst WriteRqst, TimeLineID tli, bool flexible)
/* /*
* Measure I/O timing to write WAL data, for pg_stat_io. * Measure I/O timing to write WAL data, for pg_stat_io.
*/ */
start = pgstat_prepare_io_time(); start = pgstat_prepare_io_time(track_wal_io_timing);
pgstat_report_wait_start(WAIT_EVENT_WAL_WRITE); pgstat_report_wait_start(WAIT_EVENT_WAL_WRITE);
written = pg_pwrite(openLogFile, from, nleft, startoffset); written = pg_pwrite(openLogFile, from, nleft, startoffset);
@ -3248,7 +3249,7 @@ XLogFileInitInternal(XLogSegNo logsegno, TimeLineID logtli,
errmsg("could not create file \"%s\": %m", tmppath))); errmsg("could not create file \"%s\": %m", tmppath)));
/* Measure I/O timing when initializing segment */ /* Measure I/O timing when initializing segment */
io_start = pgstat_prepare_io_time(); io_start = pgstat_prepare_io_time(track_wal_io_timing);
pgstat_report_wait_start(WAIT_EVENT_WAL_INIT_WRITE); pgstat_report_wait_start(WAIT_EVENT_WAL_INIT_WRITE);
save_errno = 0; save_errno = 0;
@ -3310,7 +3311,7 @@ XLogFileInitInternal(XLogSegNo logsegno, TimeLineID logtli,
} }
/* Measure I/O timing when flushing segment */ /* Measure I/O timing when flushing segment */
io_start = pgstat_prepare_io_time(); io_start = pgstat_prepare_io_time(track_wal_io_timing);
pgstat_report_wait_start(WAIT_EVENT_WAL_INIT_SYNC); pgstat_report_wait_start(WAIT_EVENT_WAL_INIT_SYNC);
if (pg_fsync(fd) != 0) if (pg_fsync(fd) != 0)
@ -8744,7 +8745,7 @@ issue_xlog_fsync(int fd, XLogSegNo segno, TimeLineID tli)
/* /*
* Measure I/O timing to sync the WAL file for pg_stat_io. * Measure I/O timing to sync the WAL file for pg_stat_io.
*/ */
start = pgstat_prepare_io_time(); start = pgstat_prepare_io_time(track_wal_io_timing);
pgstat_report_wait_start(WAIT_EVENT_WAL_SYNC); pgstat_report_wait_start(WAIT_EVENT_WAL_SYNC);
switch (wal_sync_method) switch (wal_sync_method)

2
src/backend/access/transam/xlogreader.c
View File

@ -1557,7 +1557,7 @@ WALRead(XLogReaderState *state,
#ifndef FRONTEND #ifndef FRONTEND
/* Measure I/O timing when reading segment */ /* Measure I/O timing when reading segment */
io_start = pgstat_prepare_io_time(); io_start = pgstat_prepare_io_time(track_wal_io_timing);
pgstat_report_wait_start(WAIT_EVENT_WAL_READ); pgstat_report_wait_start(WAIT_EVENT_WAL_READ);
#endif #endif

2
src/backend/access/transam/xlogrecovery.c
View File

@ -3401,7 +3401,7 @@ retry:
readOff = targetPageOff; readOff = targetPageOff;
/* Measure I/O timing when reading segment */ /* Measure I/O timing when reading segment */
io_start = pgstat_prepare_io_time(); io_start = pgstat_prepare_io_time(track_wal_io_timing);
pgstat_report_wait_start(WAIT_EVENT_WAL_READ); pgstat_report_wait_start(WAIT_EVENT_WAL_READ);
r = pg_pread(readFile, readBuf, XLOG_BLCKSZ, (off_t) readOff); r = pg_pread(readFile, readBuf, XLOG_BLCKSZ, (off_t) readOff);

10
src/backend/storage/buffer/bufmgr.c
View File

@ -1509,7 +1509,7 @@ WaitReadBuffers(ReadBuffersOperation *operation)
io_pages[io_buffers_len++] = BufferGetBlock(buffers[i]); io_pages[io_buffers_len++] = BufferGetBlock(buffers[i]);
} }
io_start = pgstat_prepare_io_time(); io_start = pgstat_prepare_io_time(track_io_timing);
smgrreadv(operation->smgr, forknum, io_first_block, io_pages, io_buffers_len); smgrreadv(operation->smgr, forknum, io_first_block, io_pages, io_buffers_len);
pgstat_count_io_op_time(io_object, io_context, IOOP_READ, io_start, pgstat_count_io_op_time(io_object, io_context, IOOP_READ, io_start,
1, io_buffers_len * BLCKSZ); 1, io_buffers_len * BLCKSZ);
@ -2402,7 +2402,7 @@ ExtendBufferedRelShared(BufferManagerRelation bmr,
} }
} }
io_start = pgstat_prepare_io_time(); io_start = pgstat_prepare_io_time(track_io_timing);
/* /*
* Note: if smgrzeroextend fails, we will end up with buffers that are * Note: if smgrzeroextend fails, we will end up with buffers that are
@ -3857,7 +3857,7 @@ FlushBuffer(BufferDesc *buf, SMgrRelation reln, IOObject io_object,
*/ */
bufToWrite = PageSetChecksumCopy((Page) bufBlock, buf->tag.blockNum); bufToWrite = PageSetChecksumCopy((Page) bufBlock, buf->tag.blockNum);
io_start = pgstat_prepare_io_time(); io_start = pgstat_prepare_io_time(track_io_timing);
/* /*
* bufToWrite is either the shared buffer or a copy, as appropriate. * bufToWrite is either the shared buffer or a copy, as appropriate.
@ -4459,7 +4459,7 @@ FlushRelationBuffers(Relation rel)
PageSetChecksumInplace(localpage, bufHdr->tag.blockNum); PageSetChecksumInplace(localpage, bufHdr->tag.blockNum);
io_start = pgstat_prepare_io_time(); io_start = pgstat_prepare_io_time(track_io_timing);
smgrwrite(srel, smgrwrite(srel,
BufTagGetForkNum(&bufHdr->tag), BufTagGetForkNum(&bufHdr->tag),
@ -5905,7 +5905,7 @@ IssuePendingWritebacks(WritebackContext *wb_context, IOContext io_context)
sort_pending_writebacks(wb_context->pending_writebacks, sort_pending_writebacks(wb_context->pending_writebacks,
wb_context->nr_pending); wb_context->nr_pending);
io_start = pgstat_prepare_io_time(); io_start = pgstat_prepare_io_time(track_io_timing);
/* /*
* Coalesce neighbouring writes, but nothing else. For that we iterate * Coalesce neighbouring writes, but nothing else. For that we iterate

4
src/backend/storage/buffer/localbuf.c
View File

@ -244,7 +244,7 @@ GetLocalVictimBuffer(void)
PageSetChecksumInplace(localpage, bufHdr->tag.blockNum); PageSetChecksumInplace(localpage, bufHdr->tag.blockNum);
io_start = pgstat_prepare_io_time(); io_start = pgstat_prepare_io_time(track_io_timing);
/* And write... */ /* And write... */
smgrwrite(oreln, smgrwrite(oreln,
@ -414,7 +414,7 @@ ExtendBufferedRelLocal(BufferManagerRelation bmr,
} }
} }
io_start = pgstat_prepare_io_time(); io_start = pgstat_prepare_io_time(track_io_timing);
/* actually extend relation */ /* actually extend relation */
smgrzeroextend(bmr.smgr, fork, first_block, extend_by, false); smgrzeroextend(bmr.smgr, fork, first_block, extend_by, false);

4
src/backend/storage/smgr/md.c
View File

@ -1391,7 +1391,7 @@ register_dirty_segment(SMgrRelation reln, ForkNumber forknum, MdfdVec *seg)
ereport(DEBUG1, ereport(DEBUG1,
(errmsg_internal("could not forward fsync request because request queue is full"))); (errmsg_internal("could not forward fsync request because request queue is full")));
io_start = pgstat_prepare_io_time(); io_start = pgstat_prepare_io_time(track_io_timing);
if (FileSync(seg->mdfd_vfd, WAIT_EVENT_DATA_FILE_SYNC) < 0) if (FileSync(seg->mdfd_vfd, WAIT_EVENT_DATA_FILE_SYNC) < 0)
ereport(data_sync_elevel(ERROR), ereport(data_sync_elevel(ERROR),
@ -1790,7 +1790,7 @@ mdsyncfiletag(const FileTag *ftag, char *path)
need_to_close = true; need_to_close = true;
} }
io_start = pgstat_prepare_io_time(); io_start = pgstat_prepare_io_time(track_io_timing);
/* Sync the file. */ /* Sync the file. */
result = FileSync(file, WAIT_EVENT_DATA_FILE_SYNC); result = FileSync(file, WAIT_EVENT_DATA_FILE_SYNC);

3
src/backend/utils/activity/pgstat_backend.c
View File

@ -24,6 +24,7 @@
#include "postgres.h" #include "postgres.h"
#include "access/xlog.h"
#include "storage/bufmgr.h" #include "storage/bufmgr.h"
#include "utils/memutils.h" #include "utils/memutils.h"
#include "utils/pgstat_internal.h" #include "utils/pgstat_internal.h"
@ -43,7 +44,7 @@ void
pgstat_count_backend_io_op_time(IOObject io_object, IOContext io_context, pgstat_count_backend_io_op_time(IOObject io_object, IOContext io_context,
IOOp io_op, instr_time io_time) IOOp io_op, instr_time io_time)
{ {
Assert(track_io_timing); Assert(track_io_timing || track_wal_io_timing);
if (!pgstat_tracks_backend_bktype(MyBackendType)) if (!pgstat_tracks_backend_bktype(MyBackendType))
return; return;

14
src/backend/utils/activity/pgstat_io.c
View File

@ -83,20 +83,22 @@ pgstat_count_io_op(IOObject io_object, IOContext io_context, IOOp io_op,
} }
/* /*
* Initialize the internal timing for an IO operation. * Initialize the internal timing for an IO operation, depending on an
* IO timing GUC.
*/ */
instr_time instr_time
pgstat_prepare_io_time(void) pgstat_prepare_io_time(bool track_io_guc)
{ {
instr_time io_start; instr_time io_start;
if (track_io_timing) if (track_io_guc)
INSTR_TIME_SET_CURRENT(io_start); INSTR_TIME_SET_CURRENT(io_start);
else else
{ {
/* /*
* There is no need to set io_start when an IO timing GUC is disabled, * There is no need to set io_start when an IO timing GUC is disabled.
* still initialize it to zero to avoid compiler warnings. * Initialize it to zero to avoid compiler warnings and to let
* pgstat_count_io_op_time() know that timings should be ignored.
*/ */
INSTR_TIME_SET_ZERO(io_start); INSTR_TIME_SET_ZERO(io_start);
} }
@ -119,7 +121,7 @@ void
pgstat_count_io_op_time(IOObject io_object, IOContext io_context, IOOp io_op, pgstat_count_io_op_time(IOObject io_object, IOContext io_context, IOOp io_op,
instr_time start_time, uint32 cnt, uint64 bytes) instr_time start_time, uint32 cnt, uint64 bytes)
{ {
if (track_io_timing) if (!INSTR_TIME_IS_ZERO(start_time))
{ {
instr_time io_time; instr_time io_time;

9
src/backend/utils/misc/guc_tables.c
View File

@ -1501,6 +1501,15 @@ struct config_bool ConfigureNamesBool[] =
false, false,
NULL, NULL, NULL NULL, NULL, NULL
}, },
{
{"track_wal_io_timing", PGC_SUSET, STATS_CUMULATIVE,
gettext_noop("Collects timing statistics for WAL I/O activity."),
NULL
},
&track_wal_io_timing,
false,
NULL, NULL, NULL
},
{ {
{"update_process_title", PGC_SUSET, PROCESS_TITLE, {"update_process_title", PGC_SUSET, PROCESS_TITLE,

1
src/backend/utils/misc/postgresql.conf.sample
View File

@ -641,6 +641,7 @@
#track_counts = on #track_counts = on
#track_cost_delay_timing = off #track_cost_delay_timing = off
#track_io_timing = off #track_io_timing = off
#track_wal_io_timing = off
#track_functions = none # none, pl, all #track_functions = none # none, pl, all
#stats_fetch_consistency = cache # cache, none, snapshot #stats_fetch_consistency = cache # cache, none, snapshot

1
src/include/access/xlog.h
View File

@ -54,6 +54,7 @@ extern PGDLLIMPORT char *wal_consistency_checking_string;
extern PGDLLIMPORT bool log_checkpoints; extern PGDLLIMPORT bool log_checkpoints;
extern PGDLLIMPORT int CommitDelay; extern PGDLLIMPORT int CommitDelay;
extern PGDLLIMPORT int CommitSiblings; extern PGDLLIMPORT int CommitSiblings;
extern PGDLLIMPORT bool track_wal_io_timing;
extern PGDLLIMPORT int wal_decode_buffer_size; extern PGDLLIMPORT int wal_decode_buffer_size;
extern PGDLLIMPORT int CheckPointSegments; extern PGDLLIMPORT int CheckPointSegments;

2
src/include/pgstat.h
View File

@ -564,7 +564,7 @@ extern bool pgstat_bktype_io_stats_valid(PgStat_BktypeIO *backend_io,
BackendType bktype); BackendType bktype);
extern void pgstat_count_io_op(IOObject io_object, IOContext io_context, extern void pgstat_count_io_op(IOObject io_object, IOContext io_context,
IOOp io_op, uint32 cnt, uint64 bytes); IOOp io_op, uint32 cnt, uint64 bytes);
extern instr_time pgstat_prepare_io_time(void); extern instr_time pgstat_prepare_io_time(bool track_io_guc);
extern void pgstat_count_io_op_time(IOObject io_object, IOContext io_context, extern void pgstat_count_io_op_time(IOObject io_object, IOContext io_context,
IOOp io_op, instr_time start_time, IOOp io_op, instr_time start_time,
uint32 cnt, uint64 bytes); uint32 cnt, uint64 bytes);