Re: wal stats questions - Mailing list pgsql-hackers

At Tue, 30 Mar 2021 09:41:24 +0900, Masahiro Ikeda <ikedamsh@oss.nttdata.com> wrote in 
> I update the patch since there were my misunderstanding points.
> 
> On 2021/03/26 16:20, Masahiro Ikeda wrote:
> > Thanks for many your suggestions!
> > I made the patch to handle the issues.
> > 
> >> 1) What is the motivation to have both prevWalUsage and pgWalUsage,
> >>    instead of just accumulating the stats since the last submission?
> >>    There doesn't seem to be any comment explaining it? Computing
> >>    differences to previous values, and copying to prev*, isn't free. I
> >>    assume this is out of a fear that the stats could get reset before
> >>    they're used for instrument.c purposes - but who knows?
> > 
> > I removed the unnecessary code copying pgWalUsage and just reset the
> > pgWalUsage after reporting the stats in pgstat_report_wal().
> 
> I didn't change this.
> 
> >> 2) Why is there both pgstat_send_wal() and pgstat_report_wal()? With the
> >>    former being used by wal writer, the latter by most other processes?
> >>    There again don't seem to be comments explaining this.
> > 
> > I added the comments why two functions are separated.
> > (But is it better to merge them?)
> 
> I updated the comments for following reasons.
> 
> 
> >> 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.

pgstat_send_wal() is called mainly from pgstat_report_wal() which
consumes pgWalStats counters and WalWriterMain() which
doesn't. Checking on pgWalStats counters isn't so complex that we need
to avoid that in wal writer, thus *I* think pgstat_send_wal() and
pgstat_report_wal() can be consolidated.  Even if you have a strong
opinion that wal writer should call a separate function, the name
should be something other than pgstat_send_wal() since it ignores
pgWalUsage counters, which are supposed to be included in "WAL stats".


> I thought to implement if the WAL stats counters were not updated, skip to
> send all statistics including the counters for databases and so on. But I
> think it's not good because it may take more time to be reflected the
> generated stats by read-only transaction.

Ur, yep.

> > Although I added the condition which the write/sync counters are updated or
> > not, I haven't understood the following case yet...Sorry. I checked related
> > code and tested to insert large object, but I couldn't. I'll investigate more
> > deeply, but if you already know the function which leads the following case,
> > please let me know.
> 
> I understood the above case (Fujii-san, thanks for your advice in person).
> When to flush buffers, for example, to select buffer replacement victim,
> it requires a WAL flush if the buffer is dirty. So, to check the WAL stats
> counters are updated or not, I check the number of generated wal record and
> the counter of syncing in pgstat_report_wal().
> 
> The reason why not to check the counter of writing is that if to write is
> happened, to sync is happened too in the above case. I added the comments in
> the patch.

Mmm..  Although I couldn't read you clearly, The fact that a flush may
happen without a write means the reverse at the same time, a write may
happen without a flush.  For asynchronous commits, WAL-write happens
alone unaccompanied by a flush.  And the corresponding flush would
happen later without writes.

regards.

-- 
Kyotaro Horiguchi
NTT Open Source Software Center