Subject: Re: Protect syscache from bloating with negative cache entries
To: "Ideriha, Takeshi" <ideriha.takeshi@jp.fujitsu.com>,
 "'bruce@momjian.us'" <bruce@momjian.us>,
 "Tsunakawa, Takayuki" <tsunakawa.takay@jp.fujitsu.com>
Cc: 'Kyotaro HORIGUCHI' <horiguchi.kyotaro@lab.ntt.co.jp>,
 "GavinFlower@archidevsys.co.nz" <GavinFlower@archidevsys.co.nz>,
 "robertmhaas@gmail.com" <robertmhaas@gmail.com>,
 "tgl@sss.pgh.pa.us" <tgl@sss.pgh.pa.us>,
 "pgsql-hackers@lists.postgresql.org" <pgsql-hackers@lists.postgresql.org>,
 "alvherre@alvh.no-ip.org" <alvherre@alvh.no-ip.org>,
 "andres@anarazel.de" <andres@anarazel.de>,
 "michael.paquier@gmail.com" <michael.paquier@gmail.com>,
 "david@pgmasters.net" <david@pgmasters.net>,
 "Jim.Nasby@bluetreble.com" <Jim.Nasby@bluetreble.com>,
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From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Message-ID: <ec91c81f-c1a5-e4fe-e344-cef963aecfd7@2ndquadrant.com>
Date: Tue, 5 Feb 2019 18:27:52 +0100
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Hi,

I find it a bit surprising there are almost no results demonstrating the
impact of the proposed changes on some typical workloads. It touches
code (syscache, ...) that is quite sensitive performance-wise, and
adding even just a little bit of overhead may hurt significantly. Even
on systems that don't have issues with cache bloat, etc.

I think this is something we need - benchmarks measuring the overhead on
a bunch of workloads (both typical and corner cases). Especially when
there was a limit on cache size in the past, and it was removed because
it was too expensive / hurting in some cases. I can't imagine committing
any such changes without this information.

This is particularly important as the patch was about one particular
issue (bloat due to negative entries) initially, but then the scope grew
quite a it. AFAICS the thread now talks about these workloads:

* negative entries (due to search_path lookups etc.)
* many tables accessed randomly
* many tables with only a small subset accessed frequently
* many tables with subsets accessed in subsets (due to pooling)
* ...

Unfortunately, some of those cases seems somewhat contradictory (i.e.
what works for one hurts the other), so I doubt it's possible to improve
all of them at once. But that makes the bencharking even more important.


regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services