On 2021/05/12 19:19, Fujii Masao wrote:
>
>
> On 2021/05/11 18:46, Masahiro Ikeda wrote:
>>
>>
>> On 2021/05/11 16:44, Fujii Masao wrote:
>>>
>>>
>>> On 2021/04/28 9:10, Masahiro Ikeda wrote:
>>>>
>>>>
>>>> On 2021/04/27 21:56, Fujii Masao wrote:
>>>>>
>>>>>
>>>>> On 2021/04/26 10:11, Masahiro Ikeda wrote:
>>>>>>
>>>>>> First patch has only the changes for pg_stat_wal view.
>>>>>> ("v6-0001-performance-improvements-of-reporting-wal-stats-without-introducing-a-new-variable.patch")
>>>>>>
>>>>>>
>>>>>>
>>>>>
>>>>> + pgWalUsage.wal_records == prevWalUsage.wal_records &&
>>>>> + walStats.wal_write == 0 && walStats.wal_sync == 0 &&
>>>>>> WalStats.m_wal_write should be checked here instead of
>>>>>> walStats.wal_write?
>>>>
>>>> Thanks! Yes, I'll fix it.
>>>
>>> Thanks!
>>
>> Thanks for your comments!
>> I fixed them.
>
> Thanks for updating the patch!
>
> if ((pgStatTabList == NULL || pgStatTabList->tsa_used == 0) &&
> pgStatXactCommit == 0 && pgStatXactRollback == 0 &&
> + pgWalUsage.wal_records == prevWalUsage.wal_records &&
> + WalStats.m_wal_write == 0 && WalStats.m_wal_sync == 0 &&
>
> I'm just wondering if the above WAL activity counters need to be checked.
> Maybe it's not necessary because "pgStatXactCommit == 0 && pgStatXactRollback
> == 0"
> is checked? IOW, is there really the case where WAL activity counters are
> updated
> even when both pgStatXactCommit and pgStatXactRollback are zero?
Thanks for checking.
I haven't found the case yet. But, I added the condition because there is a
discussion that it's safer.
(It seems the mail thread chain is broken, Sorry...)
I copy the discussion at the time below.
https://www.postgresql.org/message-id/20210330.172843.267174731834281075.horikyota.ntt%40gmail.com
>>>> 3) Doing if (memcmp(&WalStats, &all_zeroes, sizeof(PgStat_MsgWal)) == 0)
>>>> just to figure out if there's been any changes isn't all that
>>>> cheap. This is regularly exercised in read-only workloads too. Seems
>>>> adding a boolean WalStats.have_pending = true or such would be
>>>> better.
>>>> 4) For plain backends pgstat_report_wal() is called by
>>>> pgstat_report_stat() - but it is not checked as part of the "Don't
>>>> expend a clock check if nothing to do" check at the top. It's
>>>> probably rare to have pending wal stats without also passing one of
>>>> the other conditions, but ...
>>>
>>> I added the logic to check if the stats counters are updated or not in
>>> pgstat_report_stat() using not only generated wal record but also write/sync
>>> counters, and it can skip to call reporting function.
>>
>> I removed the checking code whether the wal stats counters were updated or
>> not
>> in pgstat_report_stat() since I couldn't understand the valid case yet.
>> pgstat_report_stat() is called by backends when the transaction is finished.
>> This means that the condition seems always pass.
>
> Doesn't the same holds for all other counters? If you are saying that
> "wal counters should be zero if all other stats counters are zero", we
> need an assertion to check that and a comment to explain that.
>
> I personally find it safer to add the WAL-stats condition to the
> fast-return check, rather than addin such assertion.
> + if (pgWalUsage.wal_records != prevWalUsage.wal_records)
> + {
> + WalUsage walusage;
> +
> + /*
> + * Calculate how much WAL usage counters were increased by
> substracting
> + * the previous counters from the current ones. Fill the results in
> + * WAL stats message.
> + */
> + MemSet(&walusage, 0, sizeof(WalUsage));
> + WalUsageAccumDiff(&walusage, &pgWalUsage, &prevWalUsage);
>
> Isn't it better to move the code "prevWalUsage = pgWalUsage" into here?
> Because it's necessary only when pgWalUsage.wal_records !=
> prevWalUsage.wal_records.
Yes, I fixed it.
Regards,
--
Masahiro Ikeda
NTT DATA CORPORATION
diff --git a/src/backend/executor/instrument.c b/src/backend/executor/instrument.c
index 2b106d8473..355778028e 100644
--- a/src/backend/executor/instrument.c
+++ b/src/backend/executor/instrument.c
@@ -17,6 +17,12 @@
#include "executor/instrument.h"
+/*
+ * Buffer and generated WAL usage counters.
+ *
+ * The counters are accumulated values. There are infrastructures
+ * to add the values and calculate the difference within a specific period.
+ */
BufferUsage pgBufferUsage;
static BufferUsage save_pgBufferUsage;
WalUsage pgWalUsage;
diff --git a/src/backend/postmaster/checkpointer.c b/src/backend/postmaster/checkpointer.c
index e7e6a2a459..1761694a5b 100644
--- a/src/backend/postmaster/checkpointer.c
+++ b/src/backend/postmaster/checkpointer.c
@@ -505,7 +505,7 @@ CheckpointerMain(void)
pgstat_send_bgwriter();
/* Send WAL statistics to the stats collector. */
- pgstat_report_wal();
+ pgstat_send_wal(true);
/*
* If any checkpoint flags have been set, redo the loop to handle the
@@ -583,7 +583,7 @@ HandleCheckpointerInterrupts(void)
BgWriterStats.m_requested_checkpoints++;
ShutdownXLOG(0, 0);
pgstat_send_bgwriter();
- pgstat_report_wal();
+ pgstat_send_wal(true);
/* Normal exit from the checkpointer is here */
proc_exit(0); /* done */
diff --git a/src/backend/postmaster/pgstat.c b/src/backend/postmaster/pgstat.c
index e94f5f55c7..b65bcd5c9b 100644
--- a/src/backend/postmaster/pgstat.c
+++ b/src/backend/postmaster/pgstat.c
@@ -133,8 +133,8 @@ PgStat_MsgWal WalStats;
/*
* 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 substracting
+ * pgstat_send_wal(). This is used to calculate how much WAL usage
+ * happens between pgstat_send_wal() calls, by substracting
* the previous counters from the current ones.
*/
static WalUsage prevWalUsage;
@@ -852,9 +852,19 @@ pgstat_report_stat(bool disconnect)
TabStatusArray *tsa;
int i;
- /* Don't expend a clock check if nothing to do */
+ /*
+ * Don't expend a clock check if nothing to do.
+ *
+ * Note: regarding to WAL statistics counters, it's not enough to check
+ * only whether the wal record is generated or not. If a read transaction
+ * is executed, wal records aren't nomally generated (although HOT makes
+ * wal records). But, just writes and syncs the wal data may happen when
+ * to flush buffers.
+ */
if ((pgStatTabList == NULL || pgStatTabList->tsa_used == 0) &&
pgStatXactCommit == 0 && pgStatXactRollback == 0 &&
+ pgWalUsage.wal_records == prevWalUsage.wal_records &&
+ WalStats.m_wal_write == 0 && WalStats.m_wal_sync == 0 &&
!have_function_stats && !disconnect)
return;
@@ -948,7 +958,7 @@ pgstat_report_stat(bool disconnect)
pgstat_send_funcstats();
/* Send WAL statistics */
- pgstat_report_wal();
+ pgstat_send_wal(true);
/* Finally send SLRU statistics */
pgstat_send_slru();
@@ -2918,7 +2928,7 @@ void
pgstat_initialize(void)
{
/*
- * Initialize prevWalUsage with pgWalUsage so that pgstat_report_wal() can
+ * Initialize prevWalUsage with pgWalUsage so that pgstat_send_wal() can
* calculate how much pgWalUsage counters are increased by substracting
* prevWalUsage from pgWalUsage.
*/
@@ -3030,44 +3040,6 @@ pgstat_send_bgwriter(void)
MemSet(&BgWriterStats, 0, sizeof(BgWriterStats));
}
-/* ----------
- * pgstat_report_wal() -
- *
- * Calculate how much WAL usage counters are increased and send
- * WAL statistics to the collector.
- *
- * Must be called by processes that generate WAL.
- * ----------
- */
-void
-pgstat_report_wal(void)
-{
- WalUsage walusage;
-
- /*
- * Calculate how much WAL usage counters are increased by substracting the
- * previous counters from the current ones. Fill the results in WAL stats
- * message.
- */
- MemSet(&walusage, 0, sizeof(WalUsage));
- WalUsageAccumDiff(&walusage, &pgWalUsage, &prevWalUsage);
-
- WalStats.m_wal_records = walusage.wal_records;
- WalStats.m_wal_fpi = walusage.wal_fpi;
- WalStats.m_wal_bytes = walusage.wal_bytes;
-
- /*
- * Send WAL stats message to the collector.
- */
- if (!pgstat_send_wal(true))
- return;
-
- /*
- * Save the current counters for the subsequent calculation of WAL usage.
- */
- prevWalUsage = pgWalUsage;
-}
-
/* ----------
* pgstat_send_wal() -
*
@@ -3075,24 +3047,38 @@ pgstat_report_wal(void)
*
* If 'force' is not set, WAL stats message is only sent if enough time has
* passed since last one was sent to reach PGSTAT_STAT_INTERVAL.
- *
- * Return true if the message is sent, and false otherwise.
* ----------
*/
-bool
+void
pgstat_send_wal(bool force)
{
- /* We assume this initializes to zeroes */
- static const PgStat_MsgWal all_zeroes;
static TimestampTz sendTime = 0;
/*
- * This function can be called even if nothing at all has happened. In
- * this case, avoid sending a completely empty message to the stats
- * collector.
+ * First, to check the WAL stats counters were updated.
+ *
+ * Even if the 'force' is true, we don't need to send the stats if the
+ * counters were not updated.
+ *
+ * We can know whether the counters were updated or not to check only
+ * three counters. So, for performance, we don't allocate another memory
+ * spaces and check the all stats like pgstat_send_slru().
+ *
+ * The current 'wal_records' is the same as the previous one means that
+ * wal records weren't generated. So, the counters of 'wal_fpi',
+ * 'wal_bytes', 'm_wal_buffers_full' are not updated neither.
+ *
+ * It's not enough to check the number of generated wal records, for
+ * example the walwriter may write/sync the WAL although it doesn't
+ * generate wal records. 'm_wal_write' and 'm_wal_sync' are zero means the
+ * counters of time spent are zero too.
*/
- if (memcmp(&WalStats, &all_zeroes, sizeof(PgStat_MsgWal)) == 0)
- return false;
+ if (pgWalUsage.wal_records == prevWalUsage.wal_records &&
+ WalStats.m_wal_write == 0 && WalStats.m_wal_sync == 0)
+ {
+ Assert(WalStats.m_wal_buffers_full == 0);
+ return;
+ }
if (!force)
{
@@ -3100,13 +3086,40 @@ pgstat_send_wal(bool force)
/*
* Don't send a message unless it's been at least PGSTAT_STAT_INTERVAL
- * msec since we last sent one.
+ * msec since we last sent one to avoid overloading the stats
+ * collector.
*/
if (!TimestampDifferenceExceeds(sendTime, now, PGSTAT_STAT_INTERVAL))
- return false;
+ return;
sendTime = now;
}
+ /*
+ * Set the counters related to generated WAL data if the counters were
+ * updated.
+ */
+ if (pgWalUsage.wal_records != prevWalUsage.wal_records)
+ {
+ WalUsage walusage;
+
+ /*
+ * Calculate how much WAL usage counters were increased by substracting
+ * the previous counters from the current ones. Fill the results in
+ * WAL stats message.
+ */
+ MemSet(&walusage, 0, sizeof(WalUsage));
+ WalUsageAccumDiff(&walusage, &pgWalUsage, &prevWalUsage);
+
+ WalStats.m_wal_records = walusage.wal_records;
+ WalStats.m_wal_fpi = walusage.wal_fpi;
+ WalStats.m_wal_bytes = walusage.wal_bytes;
+
+ /*
+ * Save the current counters for the subsequent calculation of WAL usage.
+ */
+ prevWalUsage = pgWalUsage;
+ }
+
/*
* Prepare and send the message
*/
@@ -3117,8 +3130,6 @@ pgstat_send_wal(bool force)
* Clear out the statistics buffer, so it can be re-used.
*/
MemSet(&WalStats, 0, sizeof(WalStats));
-
- return true;
}
/* ----------
diff --git a/src/include/executor/instrument.h b/src/include/executor/instrument.h
index fc87eed4fb..31dbfc832a 100644
--- a/src/include/executor/instrument.h
+++ b/src/include/executor/instrument.h
@@ -16,6 +16,12 @@
#include "portability/instr_time.h"
+/*
+ * The accumulated counters for buffer usage.
+ *
+ * The reason the counters are accumulated values is to avoid unexpected
+ * reset because many callers may use them.
+ */
typedef struct BufferUsage
{
int64 shared_blks_hit; /* # of shared buffer hits */
@@ -32,6 +38,15 @@ typedef struct BufferUsage
instr_time blk_write_time; /* time spent writing */
} BufferUsage;
+/*
+ * The accumulated counters for generated WAL usage.
+ *
+ * The reason the counters are accumulated values is the same as BufferUsage's one.
+ * And the reason to store only generated WAL usage and doesn't store WAL I/O
+ * activity, is that this is assumed for knowing the WAL usage in per query or
+ * transaction. So, common resources for the cluster like WAL I/O activity is
+ * not stored.
+ */
typedef struct WalUsage
{
int64 wal_records; /* # of WAL records produced */
diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index 72ff4a06d6..7727e83455 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -1091,8 +1091,7 @@ extern void pgstat_twophase_postabort(TransactionId xid, uint16 info,
extern void pgstat_send_archiver(const char *xlog, bool failed);
extern void pgstat_send_bgwriter(void);
-extern void pgstat_report_wal(void);
-extern bool pgstat_send_wal(bool force);
+extern void pgstat_send_wal(bool force);
/* ----------
* Support functions for the SQL-callable functions to
