Hi,
On Wed, Mar 05, 2025 at 05:45:57PM +0800, Xuneng Zhou wrote:
> Subject: Clarification Needed on WAL Pending Checks in Patchset
>
> Hi,
>
> Thank you for the patchset. I’ve spent some time learning and reviewing it
> and have 2 comments.
Thanks for looking at it!
> I noticed that in patches prior to patch 6, the function
> pgstat_backend_wal_have_pending() was implemented as follows:
>
> /*
> * To determine whether any WAL activity has occurred since last time, not
> * only the number of generated WAL records but also the numbers of WAL
> * writes and syncs need to be checked. Because even transactions that
> * generate no WAL records can write or sync WAL data when flushing the
> * data pages.
> */
> static bool
> pgstat_backend_wal_have_pending(void)
> {
> PgStat_PendingWalStats pending_wal;
>
> pending_wal = PendingBackendStats.pending_wal;
>
> return pgWalUsage.wal_records != prevBackendWalUsage.wal_records ||
> pending_wal.wal_write != 0 || pending_wal.wal_sync != 0;
> }
>
> Starting with patch 7, it seems the implementation was simplified to:
>
> /*
> * To determine whether WAL usage happened.
> */
> static bool
> pgstat_backend_wal_have_pending(void)
> {
> return pgWalUsage.wal_records != prevBackendWalUsage.wal_records;
> }
That's right. This is due to 2421e9a51d2 that removed PgStat_PendingWalStats.
> Meanwhile, the cluster-level check in the function
> pgstat_wal_have_pending_cb() still performs the additional checks:
>
> bool
> pgstat_wal_have_pending_cb(void)
> {
> return pgWalUsage.wal_records != prevWalUsage.wal_records ||
> PendingWalStats.wal_write != 0 ||
> PendingWalStats.wal_sync != 0;
> }
Not since 2421e9a51d2. It looks like that you are looking at code prior to
2421e9a51d2.
> Another one is on:
> Bertrand Drouvot <[email protected]> 于2025年3月3日周一 18:47写道：
>
> > Hi,
> >
> > On Mon, Mar 03, 2025 at 09:17:30AM +0000, Bertrand Drouvot wrote:
> > > hmm, that would work as long as PGSTAT_BACKEND_FLUSH_ALL represents
> > things
> > > that need to be called from pgstat_report_wal(). But I think that's open
> > > door for issue should be add a new PGSTAT_BACKEND_FLUSH_XXX where XXX is
> > not
> > > related to pgstat_report_wal() at all. So, I'm tempted to keep it as it
> > is.
> >
> > I just realized that pgstat_flush_backend() is not correct in 0001. Indeed
> > we check:
> >
> > "
> > if (pg_memory_is_all_zeros(&PendingBackendStats,
> > sizeof(struct PgStat_BackendPending)))
> > return false;
> > "
> >
> > but the WAL stats are not part of PgStat_BackendPending... So we only check
> > for IO pending stats here. I'm not sure WAL stats could be non empty if IO
> > stats are but the attached now also takes care of pending WAL stats here.
>
> I've noticed that here's a function for checking if there are any backend
> stats waiting for flush:
> /*
> * Check if there are any backend stats waiting for flush.
> */
> bool
> pgstat_backend_have_pending_cb(void)
> {
> return (!pg_memory_is_all_zeros(&PendingBackendStats,
> sizeof(struct PgStat_BackendPending)));
> }
That's right.
The reason I did not add the extra check there is because I have in mind to
replace the pg_memory_is_all_zeros() checks by a new global variable and also
replace
the checks in pgstat_flush_backend() by a call to
pgstat_backend_have_pending_cb()
(see 0002 in [1]). It means that all of that would be perfectly clean if
0002 in [1] goes in.
But yeah, if 0002 in [1] does not go in, then your concern is valid, so adding
the extra check in the attached.
Thanks for the review!
[1]:
https://www.postgresql.org/message-id/Z8WYf1jyy4MwOveQ%40ip-10-97-1-34.eu-west-3.compute.internal
Regards,
--
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com
>From c9500ba1ed93f97aee2f9f4a97bcbfaaff998cad Mon Sep 17 00:00:00 2001
From: Bertrand Drouvot <[email protected]>
Date: Mon, 6 Jan 2025 10:00:00 +0000
Subject: [PATCH v14] per backend WAL statistics
Now that commit 9aea73fc61 added backend-level statistics to pgstats (and
per backend IO statistics) we can more easily add per backend statistics.
This commit adds per backend WAL statistics using the same layer as pg_stat_wal,
except that it is now possible to know how much WAL activity is happening in each
backend rather than an overall aggregate of all the activity. A function called
pg_stat_get_backend_wal() is added to access this data depending on the
PID of a backend.
The same limitation as in 9aea73fc61 persists, meaning that Auxiliary processes
are not included in this set of statistics.
XXX: bump catalog version
XXX: bump of stats file format not required, as backend stats do not
persist on disk.
---
doc/src/sgml/monitoring.sgml | 19 ++++++
src/backend/utils/activity/pgstat_backend.c | 70 ++++++++++++++++++++-
src/backend/utils/activity/pgstat_wal.c | 1 +
src/backend/utils/adt/pgstatfuncs.c | 26 +++++++-
src/include/catalog/pg_proc.dat | 7 +++
src/include/pgstat.h | 37 +++++------
src/include/utils/pgstat_internal.h | 3 +-
src/test/regress/expected/stats.out | 14 +++++
src/test/regress/sql/stats.sql | 6 ++
9 files changed, 160 insertions(+), 23 deletions(-)
15.1% doc/src/sgml/
42.6% src/backend/utils/activity/
14.7% src/backend/utils/adt/
8.3% src/include/catalog/
4.2% src/include/utils/
8.0% src/test/regress/expected/
6.1% src/test/regress/sql/
diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml
index 16646f560e8..b1710680705 100644
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -4866,6 +4866,25 @@ description | Waiting for a newly initialized WAL file to reach durable storage
</para></entry>
</row>
+ <row>
+ <entry id="pg-stat-get-backend-wal" role="func_table_entry"><para role="func_signature">
+ <indexterm>
+ <primary>pg_stat_get_backend_wal</primary>
+ </indexterm>
+ <function>pg_stat_get_backend_wal</function> ( <type>integer</type> )
+ <returnvalue>record</returnvalue>
+ </para>
+ <para>
+ Returns WAL statistics about the backend with the specified
+ process ID. The output fields are exactly the same as the ones in the
+ <structname>pg_stat_wal</structname> view.
+ </para>
+ <para>
+ The function does not return WAL statistics for the checkpointer,
+ the background writer, the startup process and the autovacuum launcher.
+ </para></entry>
+ </row>
+
<row>
<entry role="func_table_entry"><para role="func_signature">
<indexterm>
diff --git a/src/backend/utils/activity/pgstat_backend.c b/src/backend/utils/activity/pgstat_backend.c
index a9343b7b59e..01e072a9bfd 100644
--- a/src/backend/utils/activity/pgstat_backend.c
+++ b/src/backend/utils/activity/pgstat_backend.c
@@ -38,6 +38,14 @@
*/
static PgStat_BackendPending PendingBackendStats;
+/*
+ * WAL usage counters saved from pgWalUsage at the previous call to
+ * pgstat_report_wal(). This is used to calculate how much WAL usage
+ * happens between pgstat_report_wal() calls, by subtracting
+ * the previous counters from the current ones.
+ */
+static WalUsage prevBackendWalUsage;
+
/*
* Utility routines to report I/O stats for backends, kept here to avoid
* exposing PendingBackendStats to the outside world.
@@ -184,6 +192,57 @@ pgstat_flush_backend_entry_io(PgStat_EntryRef *entry_ref)
MemSet(&PendingBackendStats.pending_io, 0, sizeof(PgStat_PendingIO));
}
+/*
+ * To determine whether WAL usage happened.
+ */
+static bool
+pgstat_backend_wal_have_pending(void)
+{
+ return pgWalUsage.wal_records != prevBackendWalUsage.wal_records;
+}
+
+/*
+ * Flush out locally pending backend WAL statistics. Locking is managed
+ * by the caller.
+ */
+static void
+pgstat_flush_backend_entry_wal(PgStat_EntryRef *entry_ref)
+{
+ PgStatShared_Backend *shbackendent;
+ PgStat_WalCounters *bktype_shstats;
+ WalUsage wal_usage_diff = {0};
+
+ /*
+ * This function can be called even if nothing at all has happened. Avoid
+ * taking lock for nothing in that case.
+ */
+ if (!pgstat_backend_wal_have_pending())
+ return;
+
+ shbackendent = (PgStatShared_Backend *) entry_ref->shared_stats;
+ bktype_shstats = &shbackendent->stats.wal_counters;
+
+ /*
+ * We don't update the WAL usage portion of the local WalStats elsewhere.
+ * Calculate how much WAL usage counters were increased by subtracting the
+ * previous counters from the current ones.
+ */
+ WalUsageAccumDiff(&wal_usage_diff, &pgWalUsage, &prevBackendWalUsage);
+
+#define WALSTAT_ACC(fld, var_to_add) \
+ (bktype_shstats->fld += var_to_add.fld)
+ WALSTAT_ACC(wal_buffers_full, wal_usage_diff);
+ WALSTAT_ACC(wal_records, wal_usage_diff);
+ WALSTAT_ACC(wal_fpi, wal_usage_diff);
+ WALSTAT_ACC(wal_bytes, wal_usage_diff);
+#undef WALSTAT_ACC
+
+ /*
+ * Save the current counters for the subsequent calculation of WAL usage.
+ */
+ prevBackendWalUsage = pgWalUsage;
+}
+
/*
* Flush out locally pending backend statistics
*
@@ -199,7 +258,8 @@ pgstat_flush_backend(bool nowait, bits32 flags)
return false;
if (pg_memory_is_all_zeros(&PendingBackendStats,
- sizeof(struct PgStat_BackendPending)))
+ sizeof(struct PgStat_BackendPending))
+ && !pgstat_backend_wal_have_pending())
return false;
entry_ref = pgstat_get_entry_ref_locked(PGSTAT_KIND_BACKEND, InvalidOid,
@@ -211,6 +271,9 @@ pgstat_flush_backend(bool nowait, bits32 flags)
if (flags & PGSTAT_BACKEND_FLUSH_IO)
pgstat_flush_backend_entry_io(entry_ref);
+ if (flags & PGSTAT_BACKEND_FLUSH_WAL)
+ pgstat_flush_backend_entry_wal(entry_ref);
+
pgstat_unlock_entry(entry_ref);
return false;
@@ -223,7 +286,8 @@ bool
pgstat_backend_have_pending_cb(void)
{
return (!pg_memory_is_all_zeros(&PendingBackendStats,
- sizeof(struct PgStat_BackendPending)));
+ sizeof(struct PgStat_BackendPending))
+ || pgstat_backend_wal_have_pending());
}
/*
@@ -258,6 +322,8 @@ pgstat_create_backend(ProcNumber procnum)
pgstat_unlock_entry(entry_ref);
MemSet(&PendingBackendStats, 0, sizeof(PgStat_BackendPending));
+
+ prevBackendWalUsage = pgWalUsage;
}
/*
diff --git a/src/backend/utils/activity/pgstat_wal.c b/src/backend/utils/activity/pgstat_wal.c
index 5d3da4b674e..16a1ecb4d90 100644
--- a/src/backend/utils/activity/pgstat_wal.c
+++ b/src/backend/utils/activity/pgstat_wal.c
@@ -52,6 +52,7 @@ pgstat_report_wal(bool force)
/* flush wal stats */
(void) pgstat_wal_flush_cb(nowait);
+ pgstat_flush_backend(nowait, PGSTAT_BACKEND_FLUSH_WAL);
/* flush IO stats */
pgstat_flush_io(nowait);
diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 9172e1cb9d2..662ce46cbc2 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -1609,8 +1609,8 @@ pg_stat_get_backend_io(PG_FUNCTION_ARGS)
/*
* pg_stat_wal_build_tuple
*
- * Helper routine for pg_stat_get_wal() returning one tuple based on the
- * contents of wal_counters.
+ * Helper routine for pg_stat_get_wal() and pg_stat_get_backend_wal()
+ * returning one tuple based on the contents of wal_counters.
*/
static Datum
pg_stat_wal_build_tuple(PgStat_WalCounters wal_counters,
@@ -1659,6 +1659,28 @@ pg_stat_wal_build_tuple(PgStat_WalCounters wal_counters,
PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc, values, nulls)));
}
+/*
+ * Returns WAL statistics for a backend with given PID.
+ */
+Datum
+pg_stat_get_backend_wal(PG_FUNCTION_ARGS)
+{
+ int pid;
+ PgStat_Backend *backend_stats;
+ PgStat_WalCounters bktype_stats;
+
+ pid = PG_GETARG_INT32(0);
+ backend_stats = pgstat_fetch_stat_backend_by_pid(pid, NULL);
+
+ if (!backend_stats)
+ PG_RETURN_NULL();
+
+ bktype_stats = backend_stats->wal_counters;
+
+ /* save tuples with data from this PgStat_WalCounters */
+ return (pg_stat_wal_build_tuple(bktype_stats, backend_stats->stat_reset_timestamp));
+}
+
/*
* Returns statistics of WAL activity
*/
diff --git a/src/include/catalog/pg_proc.dat b/src/include/catalog/pg_proc.dat
index cd9422d0bac..3e35f8b8e99 100644
--- a/src/include/catalog/pg_proc.dat
+++ b/src/include/catalog/pg_proc.dat
@@ -5954,6 +5954,13 @@
proargmodes => '{o,o,o,o,o}',
proargnames => '{wal_records,wal_fpi,wal_bytes,wal_buffers_full,stats_reset}',
prosrc => 'pg_stat_get_wal' },
+{ oid => '8037', descr => 'statistics: backend WAL activity',
+ proname => 'pg_stat_get_backend_wal', provolatile => 'v',
+ proparallel => 'r', prorettype => 'record', proargtypes => 'int4',
+ proallargtypes => '{int4,int8,int8,numeric,int8,timestamptz}',
+ proargmodes => '{i,o,o,o,o,o}',
+ proargnames => '{backend_pid,wal_records,wal_fpi,wal_bytes,wal_buffers_full,stats_reset}',
+ prosrc => 'pg_stat_get_backend_wal' },
{ oid => '6248', descr => 'statistics: information about WAL prefetching',
proname => 'pg_stat_get_recovery_prefetch', prorows => '1', proretset => 't',
provolatile => 'v', prorettype => 'record', proargtypes => '',
diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index 4aad10b0b6d..06359b9157d 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -340,24 +340,6 @@ typedef struct PgStat_IO
PgStat_BktypeIO stats[BACKEND_NUM_TYPES];
} PgStat_IO;
-typedef struct PgStat_Backend
-{
- TimestampTz stat_reset_timestamp;
- PgStat_BktypeIO io_stats;
-} PgStat_Backend;
-
-/* ---------
- * PgStat_BackendPending Non-flushed backend stats.
- * ---------
- */
-typedef struct PgStat_BackendPending
-{
- /*
- * Backend statistics store the same amount of IO data as PGSTAT_KIND_IO.
- */
- PgStat_PendingIO pending_io;
-} PgStat_BackendPending;
-
typedef struct PgStat_StatDBEntry
{
PgStat_Counter xact_commit;
@@ -500,6 +482,25 @@ typedef struct PgStat_WalStats
TimestampTz stat_reset_timestamp;
} PgStat_WalStats;
+typedef struct PgStat_Backend
+{
+ TimestampTz stat_reset_timestamp;
+ PgStat_BktypeIO io_stats;
+ PgStat_WalCounters wal_counters;
+} PgStat_Backend;
+
+/* ---------
+ * PgStat_BackendPending Non-flushed backend stats.
+ * ---------
+ */
+typedef struct PgStat_BackendPending
+{
+ /*
+ * Backend statistics store the same amount of IO data as PGSTAT_KIND_IO.
+ */
+ PgStat_PendingIO pending_io;
+} PgStat_BackendPending;
+
/*
* Functions in pgstat.c
*/
diff --git a/src/include/utils/pgstat_internal.h b/src/include/utils/pgstat_internal.h
index 36d228e3558..d5557e6e998 100644
--- a/src/include/utils/pgstat_internal.h
+++ b/src/include/utils/pgstat_internal.h
@@ -622,7 +622,8 @@ extern void pgstat_archiver_snapshot_cb(void);
/* flags for pgstat_flush_backend() */
#define PGSTAT_BACKEND_FLUSH_IO (1 << 0) /* Flush I/O statistics */
-#define PGSTAT_BACKEND_FLUSH_ALL (PGSTAT_BACKEND_FLUSH_IO)
+#define PGSTAT_BACKEND_FLUSH_WAL (1 << 1) /* Flush WAL statistics */
+#define PGSTAT_BACKEND_FLUSH_ALL (PGSTAT_BACKEND_FLUSH_IO | PGSTAT_BACKEND_FLUSH_WAL)
extern bool pgstat_flush_backend(bool nowait, bits32 flags);
extern bool pgstat_backend_flush_cb(bool nowait);
diff --git a/src/test/regress/expected/stats.out b/src/test/regress/expected/stats.out
index 093e6368dbb..b3c303c98cb 100644
--- a/src/test/regress/expected/stats.out
+++ b/src/test/regress/expected/stats.out
@@ -832,6 +832,8 @@ SELECT sessions > :db_stat_sessions FROM pg_stat_database WHERE datname = (SELEC
SELECT num_requested AS rqst_ckpts_before FROM pg_stat_checkpointer \gset
-- Test pg_stat_wal (and make a temp table so our temp schema exists)
SELECT wal_bytes AS wal_bytes_before FROM pg_stat_wal \gset
+-- Test pg_stat_get_backend_wal (and make a temp table so our temp schema exists)
+SELECT wal_bytes AS backend_wal_bytes_before from pg_stat_get_backend_wal(pg_backend_pid()) \gset
CREATE TEMP TABLE test_stats_temp AS SELECT 17;
DROP TABLE test_stats_temp;
-- Checkpoint twice: The checkpointer reports stats after reporting completion
@@ -851,6 +853,18 @@ SELECT wal_bytes > :wal_bytes_before FROM pg_stat_wal;
t
(1 row)
+SELECT pg_stat_force_next_flush();
+ pg_stat_force_next_flush
+--------------------------
+
+(1 row)
+
+SELECT wal_bytes > :backend_wal_bytes_before FROM pg_stat_get_backend_wal(pg_backend_pid());
+ ?column?
+----------
+ t
+(1 row)
+
-- Test pg_stat_get_backend_idset() and some allied functions.
-- In particular, verify that their notion of backend ID matches
-- our temp schema index.
diff --git a/src/test/regress/sql/stats.sql b/src/test/regress/sql/stats.sql
index 0a44e14d9f4..ad3f7b7e66a 100644
--- a/src/test/regress/sql/stats.sql
+++ b/src/test/regress/sql/stats.sql
@@ -423,6 +423,9 @@ SELECT num_requested AS rqst_ckpts_before FROM pg_stat_checkpointer \gset
-- Test pg_stat_wal (and make a temp table so our temp schema exists)
SELECT wal_bytes AS wal_bytes_before FROM pg_stat_wal \gset
+-- Test pg_stat_get_backend_wal (and make a temp table so our temp schema exists)
+SELECT wal_bytes AS backend_wal_bytes_before from pg_stat_get_backend_wal(pg_backend_pid()) \gset
+
CREATE TEMP TABLE test_stats_temp AS SELECT 17;
DROP TABLE test_stats_temp;
@@ -435,6 +438,9 @@ CHECKPOINT;
SELECT num_requested > :rqst_ckpts_before FROM pg_stat_checkpointer;
SELECT wal_bytes > :wal_bytes_before FROM pg_stat_wal;
+SELECT pg_stat_force_next_flush();
+SELECT wal_bytes > :backend_wal_bytes_before FROM pg_stat_get_backend_wal(pg_backend_pid());
+
-- Test pg_stat_get_backend_idset() and some allied functions.
-- In particular, verify that their notion of backend ID matches
-- our temp schema index.
--
2.34.1
