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help / color / mirror / Atom feedFrom: Andres Freund <[email protected]>
To: Melanie Plageman <[email protected]>
Cc: Justin Pryzby <[email protected]>
Cc: Alvaro Herrera <[email protected]>
Cc: Kyotaro Horiguchi <[email protected]>
Cc: Magnus Hagander <[email protected]>
Cc: Pg Hackers <[email protected]>
Cc: Lukas Fittl <[email protected]>
Cc: Thomas Munro <[email protected]>
Subject: Re: pg_stat_bgwriter.buffers_backend is pretty meaningless (and more?)
Date: Tue, 12 Jul 2022 10:18:10 -0700
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Hi,
On 2022-07-11 22:22:28 -0400, Melanie Plageman wrote:
> Yes, per an off list suggestion by you, I have changed the tests to use a
> sum of writes. I've also added a test for IOPATH_LOCAL and fixed some of
> the missing calls to count IO Operations for IOPATH_LOCAL and
> IOPATH_STRATEGY.
>
> I struggled to come up with a way to test writes for a particular
> type of backend are counted correctly since a dirty buffer could be
> written out by another type of backend before the target BackendType has
> a chance to write it out.
I guess temp file writes would be reliably done by one backend... Don't have a
good idea otherwise.
> I also struggled to come up with a way to test IO operations for
> background workers. I'm not sure of a way to deterministically have a
> background worker do a particular kind of IO in a test scenario.
I think it's perfectly fine to not test that - for it to be broken we'd have
to somehow screw up setting the backend type. Everything else is the same as
other types of backends anyway.
If you *do* want to test it, you probably could use
SET parallel_leader_participation = false;
SET force_parallel_mode = 'regress';
SELECT something_triggering_io;
> I'm not sure how to cause a strategy "extend" for testing.
COPY into a table should work. But might be unattractive due to the size of of
the COPY ringbuffer.
> > Would be nice to have something testing that the ringbuffer stats stuff
> > does something sensible - that feels not entirely trivial.
> >
> >
> I've added a test to test that reused strategy buffers are counted as
> allocs. I would like to add a test which checks that if a buffer in the
> ring is pinned and thus not reused, that it is not counted as a strategy
> alloc, but I found it challenging without a way to pause vacuuming, pin
> a buffer, then resume vacuuming.
Yea, that's probably too hard to make reliable to be worth it.
Greetings,
Andres Freund
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Subject: Re: pg_stat_bgwriter.buffers_backend is pretty meaningless (and more?)
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