From: | Bruce Momjian <bruce(at)momjian(dot)us> |
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To: | Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com> |
Cc: | "Tsunakawa, Takayuki" <tsunakawa(dot)takay(at)jp(dot)fujitsu(dot)com>, "andres(at)anarazel(dot)de" <andres(at)anarazel(dot)de>, Robert Haas <robertmhaas(at)gmail(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Kyotaro HORIGUCHI <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp>, "Ideriha, Takeshi" <ideriha(dot)takeshi(at)jp(dot)fujitsu(dot)com>, "pgsql-hackers(at)lists(dot)postgresql(dot)org" <pgsql-hackers(at)lists(dot)postgresql(dot)org>, "alvherre(at)alvh(dot)no-ip(dot)org" <alvherre(at)alvh(dot)no-ip(dot)org>, "michael(dot)paquier(at)gmail(dot)com" <michael(dot)paquier(at)gmail(dot)com>, "david(at)pgmasters(dot)net" <david(at)pgmasters(dot)net>, "craig(at)2ndquadrant(dot)com" <craig(at)2ndquadrant(dot)com> |
Subject: | Re: Protect syscache from bloating with negative cache entries |
Date: | 2019-02-13 20:24:02 |
Message-ID: | 20190213202402.GA6933@momjian.us |
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Lists: | pgsql-hackers |
On Tue, Feb 12, 2019 at 02:53:40AM +0100, Tomas Vondra wrote:
> Right. But the logic behind time-based approach is that evicting such
> entries should not cause any issues exactly because they are accessed
> infrequently. It might incur some latency when we need them for the
> first time after the eviction, but IMHO that's acceptable (although I
> see Andres did not like that).
>
> FWIW we might even evict entries after some time passes since inserting
> them into the cache - that's what memcached et al do, IIRC. The logic is
> that frequently accessed entries will get immediately loaded back (thus
> keeping cache hit ratio high). But there are reasons why the other dbs
> do that - like not having any cache invalidation (unlike us).
Agreed. If this fixes 90% of the issues people will have, and it
applies to the 99.9% of users who will never tune this, it is a clear
win. If we want to add something that requires tuning later, we can
consider it once the non-tuning solution is done.
> That being said, having a "minimal size" threshold before starting with
> the time-based eviction may be a good idea.
Agreed. I see the minimal size as a way to keep the systems tables in
cache, which we know we will need for the next query.
--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com
+ As you are, so once was I. As I am, so you will be. +
+ Ancient Roman grave inscription +
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