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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Michael Paquier <michael.paquier@gmail.com>
Cc: Kyotaro HORIGUCHI <horiguchi.kyotaro@lab.ntt.co.jp>
Cc: David Steele <david@pgmasters.net>
Cc: Jim Nasby <Jim.Nasby@bluetreble.com>
Cc: Craig Ringer <craig@2ndquadrant.com>
Cc: PostgreSQL mailing lists <pgsql-hackers@postgresql.org>
Subject: Re: Protect syscache from bloating with negative cache entries
Date: Thu, 30 Nov 2017 23:17:09 -0500
Message-ID: <85587.1512101829@sss.pgh.pa.us> (raw)
In-Reply-To: <CA+TgmoY9qNYtdx8xh463f=82NosFw4PgthFV4UeP7WhvWu_cbQ@mail.gmail.com>
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	<20170317.142313.232290068.horiguchi.kyotaro@lab.ntt.co.jp>
	<20171031.184622.92520047.horiguchi.kyotaro@lab.ntt.co.jp>
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	<CA+TgmobPHMmQEEUFDU6XCSj5ArEfyagS1+wisJ8QAY+EsUNbxg@mail.gmail.com>
	<CAB7nPqQvbMc5Exx=Qfx=42Osntqrm-thLLt3wCiLvLaokcsg4Q@mail.gmail.com>
	<84820.1512015443@sss.pgh.pa.us>
	<CA+TgmoY9qNYtdx8xh463f=82NosFw4PgthFV4UeP7WhvWu_cbQ@mail.gmail.com>

Robert Haas <robertmhaas@gmail.com> writes:
> On Wed, Nov 29, 2017 at 11:17 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> The thing that makes me uncomfortable about this is that we used to have a
>> catcache size limitation mechanism, and ripped it out because it had too
>> much overhead (see commit 8b9bc234a).  I'm not sure how we can avoid that
>> problem within a fresh implementation.

> At the risk of beating a dead horse, I still think that the amount of
> wall clock time that has elapsed since an entry was last accessed is
> very relevant.

While I don't object to that statement, I'm not sure how it helps us
here.  If we couldn't afford DLMoveToFront(), doing a gettimeofday()
during each syscache access is surely right out.

			regards, tom lane




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