Date: Tue, 5 Feb 2019 19:05:26 -0300
From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Cc: Bruce Momjian <bruce@momjian.us>,
	"andres@anarazel.de" <andres@anarazel.de>,
	Robert Haas <robertmhaas@gmail.com>, Tom Lane <tgl@sss.pgh.pa.us>,
	Kyotaro HORIGUCHI <horiguchi.kyotaro@lab.ntt.co.jp>,
	"Ideriha, Takeshi" <ideriha.takeshi@jp.fujitsu.com>,
	"pgsql-hackers@lists.postgresql.org" <pgsql-hackers@lists.postgresql.org>,
	"Tsunakawa, Takayuki" <tsunakawa.takay@jp.fujitsu.com>,
	"michael.paquier@gmail.com" <michael.paquier@gmail.com>,
	"david@pgmasters.net" <david@pgmasters.net>,
	"craig@2ndquadrant.com" <craig@2ndquadrant.com>
Subject: Re: Protect syscache from bloating with negative cache entries
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On 2019-Feb-05, Tomas Vondra wrote:

> I don't think we need to remove the expired entries right away, if there
> are only very few of them. The cleanup requires walking the hash table,
> which means significant fixed cost. So if there are only few expired
> entries (say, less than 25% of the cache), we can just leave them around
> and clean them if we happen to stumble on them (although that may not be
> possible with dynahash, which has no concept of expiration) of before
> enlarging the hash table.

I think seqscanning the hash table is going to be too slow; Ideriha-san
idea of having a dlist with the entries in LRU order (where each entry
is moved to head of list when it is touched) seemed good: it allows you
to evict older ones when the time comes, without having to scan the rest
of the entries.  Having a dlist means two more pointers on each cache
entry AFAIR, so it's not a huge amount of memory.

> So if we want to address this case too (and we probably want), we may
> need to discard the old cache memory context someho (e.g. rebuild the
> cache in a new one, and copy the non-expired entries). Which is a nice
> opportunity to do the "full" cleanup, of course.

Yeah, we probably don't want to do this super frequently though.

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services