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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Bruce Momjian <bruce@momjian.us>
Cc: Kyotaro HORIGUCHI <horiguchi.kyotaro@lab.ntt.co.jp>
Cc: Ideriha, Takeshi <ideriha.takeshi@jp.fujitsu.com>
Cc: pgsql-hackers@lists.postgresql.org <pgsql-hackers@lists.postgresql.org>
Cc: Tsunakawa, Takayuki <tsunakawa.takay@jp.fujitsu.com>
Cc: alvherre@alvh.no-ip.org <alvherre@alvh.no-ip.org>
Cc: andres@anarazel.de <andres@anarazel.de>
Cc: michael.paquier@gmail.com <michael.paquier@gmail.com>
Cc: david@pgmasters.net <david@pgmasters.net>
Cc: craig@2ndquadrant.com <craig@2ndquadrant.com>
Subject: Re: Protect syscache from bloating with negative cache entries
Date: Fri, 18 Jan 2019 15:57:17 -0500
Message-ID: <1391.1547845037@sss.pgh.pa.us> (raw)
In-Reply-To: <CA+TgmoaQVtw=D8sDe78NwrOAPmJFjsR6XWtQ29C=fquoBvhCVw@mail.gmail.com>
References: <4E72940DA2BF16479384A86D54D0988A6F1BCB6F@G01JPEXMBKW04>
	<20181019.173457.68080786.horiguchi.kyotaro@lab.ntt.co.jp>
	<4E72940DA2BF16479384A86D54D0988A6F1F4165@G01JPEXMBKW04>
	<20181127.193622.252197705.horiguchi.kyotaro@lab.ntt.co.jp>
	<4E72940DA2BF16479384A86D54D0988A6F3D09C1@G01JPEXMBKW04>
	<4E72940DA2BF16479384A86D54D0988A6F3D0D5F@G01JPEXMBKW04>
	<7925.1547397565@sss.pgh.pa.us>
	<CA+TgmoYPWOHB0uPwSza350mH6n58KWeJKAyfuFbR11HXgGMvTw@mail.gmail.com>
	<20190117194851.GG10895@momjian.us>
	<CA+TgmoaQVtw=D8sDe78NwrOAPmJFjsR6XWtQ29C=fquoBvhCVw@mail.gmail.com>

Robert Haas <robertmhaas@gmail.com> writes:
> On Thu, Jan 17, 2019 at 2:48 PM Bruce Momjian <bruce@momjian.us> wrote:
>> Unfortunately, because we have not found something we are happy with, we
>> have done nothing.  I agree LRU can be expensive.  What if we do some
>> kind of clock sweep and expiration like we do for shared buffers?  I
>> think the trick is figuring how frequently to do the sweep.  What if we
>> mark entries as unused every 10 queries, mark them as used on first use,
>> and delete cache entries that have not be used in the past 10 queries.

> I still think wall-clock time is a perfectly reasonable heuristic.

The easy implementations of that involve putting gettimeofday() calls
into hot code paths, which would be a Bad Thing.  But maybe we could
do this only at transaction or statement start, and piggyback on the
gettimeofday() calls that already happen at those times.

			regards, tom lane




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  Cc: tgl@sss.pgh.pa.us, robertmhaas@gmail.com, bruce@momjian.us, horiguchi.kyotaro@lab.ntt.co.jp, ideriha.takeshi@jp.fujitsu.com, pgsql-hackers@lists.postgresql.org, tsunakawa.takay@jp.fujitsu.com, alvherre@alvh.no-ip.org, andres@anarazel.de, michael.paquier@gmail.com, david@pgmasters.net, craig@2ndquadrant.com
  Subject: Re: Protect syscache from bloating with negative cache entries
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