From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andres Freund <andres@anarazel.de>
cc: Robert Haas <robertmhaas@gmail.com>,
        Michael Paquier <michael.paquier@gmail.com>,
        Kyotaro HORIGUCHI <horiguchi.kyotaro@lab.ntt.co.jp>,
        David Steele <david@pgmasters.net>,
        Jim Nasby <Jim.Nasby@bluetreble.com>,
        Craig Ringer <craig@2ndquadrant.com>,
        PostgreSQL mailing lists <pgsql-hackers@postgresql.org>
Subject: Re: Protect syscache from bloating with negative cache entries
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Comments: In-reply-to Andres Freund <andres@anarazel.de>
	message dated "Fri, 01 Dec 2017 13:45:34 -0800"
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Date: Fri, 01 Dec 2017 17:03:28 -0500
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Andres Freund <andres@anarazel.de> writes:
> On 2017-12-01 16:40:23 -0500, Tom Lane wrote:
>> I have no faith in either of these proposals, because they both assume
>> that the problem only arises over the course of many minutes.  In the
>> recent complaint about pg_dump causing relcache bloat, it probably does
>> not take nearly that long for the bloat to occur.

> To me that's a bit of a different problem than what I was discussing
> here.  It also actually doesn't seem that hard - if your caches are
> growing fast, you'll continually get hash-resizing of the
> various. Adding cache-pruning to the resizing code doesn't seem hard,
> and wouldn't add meaningful overhead.

That's an interesting way to think about it, as well, though I'm not
sure it's quite that simple.  If you tie this to cache resizing then
the cache will have to grow up to the newly increased size before
you'll prune it again.  That doesn't sound like it will lead to nice
steady-state behavior.

			regards, tom lane