Re: Wait events monitoring future development

On Mon, Aug 8, 2016 at 11:47:11PM +0200, Ilya Kosmodemiansky wrote:
> On Mon, Aug 8, 2016 at 7:03 PM, Bruce Momjian <bruce(at)momjian(dot)us> wrote:
> > It seems asking users to run pg_test_timing before deploying to check
> > the overhead would be sufficient.
>
> I'am not sure. Time measurement for waits is slightly more complicated
> than a time measurement for explain analyze: a good workload plus
> using gettimeofday in a straightforward manner can cause huge
> overhead. Thats why a proper testing is important - if we can see a
> significant performance drop if we have for example large
> shared_buffers with the same concurrency, that shows gettimeofday is
> too expensive to use. Am I correct, that we do not have such accurate
> tests now?

Well, if we find that pg_test_timing is insufficient, we can perhaps add
a parallel test option to that utility.

> My another concern is, that it is a bad idea to release a feature,
> which allegedly has huge performance impact even if it is not turned
> on by default. I often meet people who do not use exceptions in
> plpgsql because a tip "A block containing an EXCEPTION clause is
> significantly more expensive to enter ..." in PostgreSQL documentation

Well, if we document that is can be slow, it is up to the user to decide
if they want to use it.

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

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