RE: Re: [Proposal] Add accumulated statistics for wait event

Hi,

I am skeptical about accumulated statistics.

pg_stat_activity now gives necessary information about wait events. It can be easily be used with a polling system that sleeps most of the time to limit the overhead. Measuring the duration of individual wait events is not necessary to know the repartition of the charge.

You can aggregate the results of the polling by application, query, wait events or whatever you want.

I wrote a script for that that can be used interactively or in batch mode to produce reports but many solutions exist .

Best regards

Phil

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________________________________
De : MyungKyu LIM <myungkyu(dot)lim(at)samsung(dot)com>
Envoyé : mardi 24 juillet 2018 12:10
À : Alexander Korotkov; pgsql-hackers(at)postgresql(dot)org
Cc : Woosung Sohn; DoHyung HONG
Objet : RE: Re: [Proposal] Add accumulated statistics for wait event

> On Mon, Jul 23, 2018 at 10:53 AM Michael Paquier <michael(at)paquier(dot)xyz> wrote:
>> What's the performance penalty? I am pretty sure that this is
>> measurable as wait events are stored for a backend for each I/O
>> operation as well, and you are calling a C routine within an inlined
>> function which is designed to be light-weight, doing only a four-byte
>> atomic operation.

> Yes, the question is overhead of measuring durations of individual wait events. It has been proposed before, and there been heated debates about that (see threads [1-3]). It doesn't seem
> to be a conclusion about this feature. The thing to be said for sure:
> performance penalty heavily depends on OS/hardware/workload. In some cases overhead is negligible, but in other cases it appears to be huge.

Thanks for good information.
I agree. Performance penalty is exist.
But wait stats are demandable and useful. In some cases, it is worth sacrificing performance and using it.

So, what do you think about developing as extension? I have another concept proposal.
2. This feature can be implemented as extension if some hooks were provided in following functions,
- pgstat_report_wait_start
- pgstat_report_wait_end
This feature can be turned on/off by on-line config when necessary.

Best regards,
MyungKyu, Lim