Re: Protect syscache from bloating with negative cache entries

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Andres Freund <andres(at)anarazel(dot)de>
Cc: Kyotaro HORIGUCHI <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Michael Paquier <michael(dot)paquier(at)gmail(dot)com>, David Steele <david(at)pgmasters(dot)net>, Jim Nasby <Jim(dot)Nasby(at)bluetreble(dot)com>, Craig Ringer <craig(at)2ndquadrant(dot)com>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Protect syscache from bloating with negative cache entries
Date: 2018-03-01 19:24:56
Message-ID: CA+TgmoYBBjna12H8iJtuhrQGhbpuLeiHoE5dCbJBugn=MiPfog@mail.gmail.com
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On Thu, Mar 1, 2018 at 1:54 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> So this is hardcoded, without any sort of cache pressure logic? Doesn't
> that mean we'll often *severely* degrade performance if a backend is
> idle for a while?

Well, it is true that if we flush cache entries that haven't been used
in a long time, a backend that is idle for a long time might be a bit
slow when (and if) it eventually becomes non-idle, because it may have
to reload some of those flushed entries. On the other hand, a backend
that holds onto a large number of cache entries that it's not using
for tens of minutes at a time degrades the performance of the whole
system unless, of course, you're running on a machine that is under no
memory pressure at all. I don't understand why people keep acting as
if holding onto cache entries regardless of how infrequently they're
being used is an unalloyed good.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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