Re: track_io_timing default setting

On 12/10/21 17:22, Laurenz Albe wrote:
> On Fri, 2021-12-10 at 10:20 -0500, Tom Lane wrote:
>> Jeff Janes <jeff(dot)janes(at)gmail(dot)com> writes:
>>> Can we change the default setting of track_io_timing to on?
>>
>> That adds a very significant amount of overhead on some platforms
>> (gettimeofday is not cheap if it requires a kernel call).  And I
>> doubt the claim that the average Postgres user needs this, and
>> doubt even more that they need it on all the time.
>> So I'm -1 on the idea.
>
> I set "track_io_timing" to "on" all the time, same as "log_lock_waits",
> so I'd want them both on by default.
>

IMHO those options have very different overhead - log_lock_waits logs
only stuff that exceeds deadlock_timeout (1s by default), so the amount
of gettimeofday() calls is miniscule compared to calling it for every
I/O request.

I wonder if we could simply do the thing we usually do when measuring
expensive stuff - measure just a small sample. That is, we wouldn't
measure timing for every I/O request, but just a small fraction. For
cases with a lot of I/O requests that should give pretty good image.

That's not a simple "change GUC default" patch, but it's not a very
complicated patch either.

regards

--
Tomas Vondra
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