Re: About to add WAL write/fsync statistics to pg_stat_wal view

On 2021-03-03 20:27, Masahiro Ikeda wrote:
> On 2021-03-03 16:30, Fujii Masao wrote:
>> On 2021/03/03 14:33, Masahiro Ikeda wrote:
>>> On 2021-02-24 16:14, Fujii Masao wrote:
>>>> On 2021/02/15 11:59, Masahiro Ikeda wrote:
>>>>> On 2021-02-10 00:51, David G. Johnston wrote:
>>>>>> On Thu, Feb 4, 2021 at 4:45 PM Masahiro Ikeda
>>>>>> <ikedamsh(at)oss(dot)nttdata(dot)com> wrote:
>>>>>>
>>>>>>> I pgindented the patches.
>>>>>>
>>>>>> ... <function>XLogWrite</function>, which is invoked during an
>>>>>> <function>XLogFlush</function> request (see ...).  This is also
>>>>>> incremented by the WAL receiver during replication.
>>>>>>
>>>>>> ("which normally called" should be "which is normally called" or
>>>>>> "which normally is called" if you want to keep true to the
>>>>>> original)
>>>>>> You missed the adding the space before an opening parenthesis here
>>>>>> and
>>>>>> elsewhere (probably copy-paste)
>>>>>>
>>>>>> is ether -> is either
>>>>>> "This parameter is off by default as it will repeatedly query the
>>>>>> operating system..."
>>>>>> ", because" -> "as"
>>>>>
>>>>> Thanks, I fixed them.
>>>>>
>>>>>> wal_write_time and the sync items also need the note: "This is
>>>>>> also
>>>>>> incremented by the WAL receiver during replication."
>>>>>
>>>>> I skipped changing it since I separated the stats for the WAL
>>>>> receiver
>>>>> in pg_stat_wal_receiver.
>>>>>
>>>>>> "The number of times it happened..." -> " (the tally of this event
>>>>>> is
>>>>>> reported in wal_buffers_full in....) This is undesirable because
>>>>>> ..."
>>>>>
>>>>> Thanks, I fixed it.
>>>>>
>>>>>> I notice that the patch for WAL receiver doesn't require
>>>>>> explicitly
>>>>>> computing the sync statistics but does require computing the write
>>>>>> statistics.  This is because of the presence of issue_xlog_fsync
>>>>>> but
>>>>>> absence of an equivalent pg_xlog_pwrite.  Additionally, I observe
>>>>>> that
>>>>>> the XLogWrite code path calls pgstat_report_wait_*() while the WAL
>>>>>> receiver path does not.  It seems technically straight-forward to
>>>>>> refactor here to avoid the almost-duplicated logic in the two
>>>>>> places,
>>>>>> though I suspect there may be a trade-off for not adding another
>>>>>> function call to the stack given the importance of WAL processing
>>>>>> (though that seems marginalized compared to the cost of actually
>>>>>> writing the WAL).  Or, as Fujii noted, go the other way and don't
>>>>>> have
>>>>>> any shared code between the two but instead implement the WAL
>>>>>> receiver
>>>>>> one to use pg_stat_wal_receiver instead.  In either case, this
>>>>>> half-and-half implementation seems undesirable.
>>>>>
>>>>> OK, as Fujii-san mentioned, I separated the WAL receiver stats.
>>>>> (v10-0002-Makes-the-wal-receiver-report-WAL-statistics.patch)
>>>>
>>>> Thanks for updating the patches!
>>>>
>>>>
>>>>> I added the infrastructure code to communicate the WAL receiver
>>>>> stats messages between the WAL receiver and the stats collector,
>>>>> and
>>>>> the stats for WAL receiver is counted in pg_stat_wal_receiver.
>>>>> What do you think?
>>>>
>>>> On second thought, this idea seems not good. Because those stats are
>>>> collected between multiple walreceivers, but other values in
>>>> pg_stat_wal_receiver is only related to the walreceiver process
>>>> running
>>>> at that moment. IOW, it seems strange that some values show dynamic
>>>> stats and the others show collected stats, even though they are in
>>>> the same view pg_stat_wal_receiver. Thought?
>>>
>>> OK, I fixed it.
>>> The stats collected in the WAL receiver is exposed in pg_stat_wal
>>> view in v11 patch.
>>
>> Thanks for updating the patches! I'm now reading 001 patch.
>>
>> + /* Check whether the WAL file was synced to disk right now */
>> + if (enableFsync &&
>> + (sync_method == SYNC_METHOD_FSYNC ||
>> + sync_method == SYNC_METHOD_FSYNC_WRITETHROUGH ||
>> + sync_method == SYNC_METHOD_FDATASYNC))
>> + {
>>
>> Isn't it better to make issue_xlog_fsync() return immediately
>> if enableFsync is off, sync_method is open_sync or open_data_sync,
>> to simplify the code more?
>
> Thanks for the comments.
> I added the above code in v12 patch.
>
>>
>> + /*
>> + * Send WAL statistics only if WalWriterDelay has elapsed to
>> minimize
>> + * the overhead in WAL-writing.
>> + */
>> + if (rc & WL_TIMEOUT)
>> + pgstat_send_wal();
>>
>> On second thought, this change means that it always takes
>> wal_writer_delay
>> before walwriter's WAL stats is sent after XLogBackgroundFlush() is
>> called.
>> For example, if wal_writer_delay is set to several seconds, some
>> values in
>> pg_stat_wal would be not up-to-date meaninglessly for those seconds.
>> So I'm thinking to withdraw my previous comment and it's ok to send
>> the stats every after XLogBackgroundFlush() is called. Thought?
>
> Thanks, I didn't notice that.
>
> Although PGSTAT_STAT_INTERVAL is 500msec, wal_writer_delay's
> default value is 200msec and it may be set shorter time.
>
> Why don't to make another way to check the timestamp?
>
> + /*
> + * Don't send a message unless it's been at least
> PGSTAT_STAT_INTERVAL
> + * msec since we last sent one
> + */
> + now = GetCurrentTimestamp();
> + if (TimestampDifferenceExceeds(last_report, now,
> PGSTAT_STAT_INTERVAL))
> + {
> + pgstat_send_wal();
> + last_report = now;
> + }
> +
>
> Although I worried that it's better to add the check code in
> pgstat_send_wal(),
> I didn't do so because to avoid to double check PGSTAT_STAT_INTERVAL.
> pgstat_send_wal() is invoked pg_report_stat() and it already checks the
> PGSTAT_STAT_INTERVAL.

I forgot to remove an unused variable.
The attached v13 patch is fixed.

Regards
--
Masahiro Ikeda
NTT DATA CORPORATION