Re: Perf Benchmarking and regression.

On Fri, Jun 3, 2016 at 1:43 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
>> I really don't get it. There's nothing in any set of guidelines for
>> setting shared_buffers that I've ever seen which would cause people to
>> avoid this scenario.
>
> The "roughly 1/4" of memory guideline already mostly avoids it? It's
> hard to constantly re-dirty a written-back page within 30s, before the
> 10% (background)/20% (foreground) limits apply; if your shared buffers
> are larger than the 10%/20% limits (which only apply to *available* not
> total memory btw).

I've always heard that guideline as "roughly 1/4, but not more than
about 8GB" - and the number of people with more than 32GB of RAM is
going to just keep going up.

>> You're the first person I've ever heard describe this as a
>> misconfiguration.
>
> Huh? People tried addressing this problem for *years* with bigger /
> smaller shared buffers, but couldn't easily.

I'm saying that setting 8GB of shared_buffers on a system with
lotsamem is not widely regarded as misconfiguration.

> I'm inclined to give up and disable backend_flush_after (not the rest),
> because it's new and by far the "riskiest". But I do think it's a
> disservice for the majority of our users.

I think that's the right course of action. I wasn't arguing for
disabling either of the other two.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
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