From: Tom Lane <tgl@sss.pgh.pa.us>
To: Robert Haas <robertmhaas@gmail.com>
cc: 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
In-reply-to: 
 <CA+TgmoY9qNYtdx8xh463f=82NosFw4PgthFV4UeP7WhvWu_cbQ@mail.gmail.com>
References: <6eb3b24b-c0eb-b8ca-f847-609704e2a370@pgmasters.net>
 <3b7b7f90-db46-8c37-c4f7-443330c3ae33@pgmasters.net>
 <20170317.142313.232290068.horiguchi.kyotaro@lab.ntt.co.jp>
 <20171031.184622.92520047.horiguchi.kyotaro@lab.ntt.co.jp>
 <CAB7nPqSGdesvS-odcP+RRJaBOssPazwKURf1GAUYZED4dKSmiQ@mail.gmail.com>
 <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>
Comments: In-reply-to Robert Haas <robertmhaas@gmail.com>
	message dated "Thu, 30 Nov 2017 11:11:07 -0500"
MIME-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-ID: <85586.1512101829.1@sss.pgh.pa.us>
Date: Thu, 30 Nov 2017 23:17:09 -0500
Message-ID: <85587.1512101829@sss.pgh.pa.us>
Precedence: bulk

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