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.janes@gmail.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
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company