Re: New statistics for tuning WAL buffer size

[Fujii Masao]

On 2020/09/09 13:57, Masahiro Ikeda wrote:

On 2020-09-07 16:19, Fujii Masao wrote:

On 2020/09/07 9:58, Masahiro Ikeda wrote:

Thanks for the review and advice!

On 2020-09-03 16:05, Fujii Masao wrote:

On 2020/09/02 18:56, Masahiro Ikeda wrote:

+/* ----------
+ * Backend types
+ * ----------

You seem to forget to add "*/" into the above comment.
This issue could cause the following compiler warning.
../../src/include/pgstat.h:761:1: warning: '/*' within block comment [-Wcomment]

Thanks for the comment. I fixed.

Thanks for the fix! But why are those comments necessary?

Sorry about that. This comment is not necessary.
I removed it.

The pg_stat_walwriter is not security restricted now, so ordinary users can 
access it.
It has the same security level as pg_stat_archiver. If you have any comments, 
please let me know.

+       <structfield>dirty_writes</structfield> <type>bigint</type>

I guess that the column name "dirty_writes" derived from
the DTrace probe name. Isn't this name confusing? We should
rename it to "wal_buffers_full" or something?

I agree and rename it to "wal_buffers_full".

+/* ----------
+ * PgStat_MsgWalWriter            Sent by the walwriter to update statistics.

This comment seems not accurate because backends also send it.

+/*
+ * WAL writes statistics counter is updated in XLogWrite function
+ */
+extern PgStat_MsgWalWriter WalWriterStats;

This comment seems not right because the counter is not updated in XLogWrite().

Right. I fixed it to "Sent by each backend and background workers to update WAL 
statistics."
In the future, other statistics will be included so I remove the function's 
name.

+-- There will surely and maximum one record
+select count(*) = 1 as ok from pg_stat_walwriter;

What about changing this comment to "There must be only one record"?

Thanks, I fixed.

+                    WalWriterStats.m_xlog_dirty_writes++;
                     LWLockRelease(WALWriteLock);

Since WalWriterStats.m_xlog_dirty_writes doesn't need to be protected
with WALWriteLock, isn't it better to increment that after releasing the lock?

Thanks, I fixed.

+CREATE VIEW pg_stat_walwriter AS
+    SELECT
+        pg_stat_get_xlog_dirty_writes() AS dirty_writes,
+        pg_stat_get_walwriter_stat_reset_time() AS stats_reset;
+
 CREATE VIEW pg_stat_progress_vacuum AS

In system_views.sql, the definition of pg_stat_walwriter should be
placed just after that of pg_stat_bgwriter not pg_stat_progress_analyze.

OK, I fixed it.

     }
-
     /*
      * We found an existing collector stats file. Read it and put all the

You seem to accidentally have removed the empty line here.

Sorry about that. I fixed it.

-                 errhint("Target must be \"archiver\" or \"bgwriter\".")));
+                 errhint("Target must be \"archiver\" or \"bgwriter\" or
\"walwriter\".")));

There are two "or" in the message, but the former should be replaced with ","?

Thanks, I fixed.


On 2020-09-05 18:40, Magnus Hagander wrote:

On Fri, Sep 4, 2020 at 5:42 AM Fujii Masao
<masao.fu...@oss.nttdata.com> wrote:

On 2020/09/04 11:50, tsunakawa.ta...@fujitsu.com wrote:

From: Fujii Masao <masao.fu...@oss.nttdata.com>

I changed the view name from pg_stat_walwrites to

pg_stat_walwriter.

I think it is better to match naming scheme with other views

like

pg_stat_bgwriter,

which is for bgwriter statistics but it has the statistics

related to backend.

I prefer the view name pg_stat_walwriter for the consistency with
other view names. But we also have pg_stat_wal_receiver. Which
makes me think that maybe pg_stat_wal_writer is better for
the consistency. Thought? IMO either of them works for me.
I'd like to hear more opinons about this.

I think pg_stat_bgwriter is now a misnomer, because it contains

the backends' activity.  Likewise, pg_stat_walwriter leads to
misunderstanding because its information is not limited to WAL
writer.

How about simply pg_stat_wal?  In the future, we may want to

include WAL reads in this view, e.g. reading undo logs in zheap.

Sounds reasonable.

+1.

pg_stat_bgwriter has had the "wrong name" for quite some time now --
it became even more apparent when the checkpointer was split out to
it's own process, and that's not exactly a recent change. And it had
allocs in it from day one...

I think naming it for what the data in it is ("wal") rather than which
process deals with it ("walwriter") is correct, unless the statistics
can be known to only *ever* affect one type of process. (And then
different processes can affect different columns in the view). As a
general rule -- and that's from what I can tell exactly what's being
proposed.

Thanks for your comments. I agree with your opinions.
I changed the view name to "pg_stat_wal".


I fixed the code to send the WAL statistics from not only backend and walwriter
but also checkpointer, walsender and autovacuum worker.

Good point! Thanks for updating the patch!


@@ -604,6 +604,7 @@ heap_vacuum_rel(Relation onerel, VacuumParams *params,
                          onerel->rd_rel->relisshared,
                          Max(new_live_tuples, 0),
                          vacrelstats->new_dead_tuples);
+    pgstat_send_wal();

I guess that you changed heap_vacuum_rel() as above so that autovacuum
workers can send WAL stats. But heap_vacuum_rel() can be called by
the processes (e.g., backends) other than autovacuum workers? Also
what happens if autovacuum workers just do ANALYZE only? In that case,
heap_vacuum_rel() may not be called.

Currently autovacuum worker reports the stats at the exit via
pgstat_beshutdown_hook(). Unlike other processes, autovacuum worker
is not the process that basically keeps running during the service. It exits
after it does vacuum or analyze. So ISTM that it's not bad to report the stats
only at the exit, in autovacuum worker case. There is no need to add extra
code for WAL stats report by autovacuum worker. Thought?

Thanks, I understood. I removed this code.

@@ -1430,6 +1430,9 @@ WalSndWaitForWal(XLogRecPtr loc)
         else
             RecentFlushPtr = GetXLogReplayRecPtr(NULL);
 +        /* Send wal statistics */
+        pgstat_send_wal();

AFAIR logical walsender uses three loops in WalSndLoop(), WalSndWriteData()
and WalSndWaitForWal(). But could you tell me why added pgstat_send_wal()
into WalSndWaitForWal()? I'd like to know why WalSndWaitForWal() is the best
for that purpose.

I checked what function calls XLogBackgroundFlush() which calls
AdvanceXLInsertBuffer() to increment m_wal_buffers_full.

I found that WalSndWaitForWal() calls it, so I added it.

Ok. But XLogBackgroundFlush() calls AdvanceXLInsertBuffer() wit the second 
argument opportunistic=true, so in this case WAL write by wal_buffers full 
seems to never happen. Right? If this understanding is right, 
WalSndWaitForWal() doesn't need to call pgstat_send_wal(). Probably also 
walwriter doesn't need to do that.

The logical rep walsender can generate WAL and call AdvanceXLInsertBuffer() when 
it executes the replication commands like CREATE_REPLICATION_SLOT. But this case 
is already covered by pgstat_report_activity()->pgstat_send_wal() called in 
PostgresMain(), with your patch. So no more calls to pgstat_send_wal() seems 
necessary for logical rep walsender.

Regards,

--
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION