Re: Protect syscache from bloating with negative cache entries

From: Kyotaro HORIGUCHI <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp>
To: tgl(at)sss(dot)pgh(dot)pa(dot)us
Cc: robertmhaas(at)gmail(dot)com, andres(at)anarazel(dot)de, michael(dot)paquier(at)gmail(dot)com, david(at)pgmasters(dot)net, Jim(dot)Nasby(at)bluetreble(dot)com, craig(at)2ndquadrant(dot)com, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Protect syscache from bloating with negative cache entries
Date: 2017-12-26 09:19:16
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At Fri, 22 Dec 2017 13:47:16 +0900 (Tokyo Standard Time), Kyotaro HORIGUCHI <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp> wrote in <20171222(dot)134716(dot)88479707(dot)horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp>
> Anyway, I think we are reached to a consensus that the
> time-tick-based expiration is promising. So I'll work on the way
> as the first step.

So this is the patch. It gets simpler.

# I became to think that the second step is not needed.

I'm not sure that no syscache aceess happens outside a statement
but the operations that lead to the bloat seem to be performed
while processing of a statement. So statement timestamp seems
sufficient as the aging clock.

At first I tried the simple strategy that removes entries that
have been left alone for 30 minutes or more but I still want to
alleviate the quick bloat (by non-reused entries) so I introduced
together a clock-sweep like aging mechanism. An entry is created
with naccessed = 0, then incremented up to 2 each time it is
accessed. Removal side decrements naccessed of entriies older
than 600 seconds then actually removes if it becomes 0. Entries
that just created and not used will go off in 600 seconds and
entries that have been accessed several times have 1800 seconds'
grace after the last acccess.

We could shrink bucket array together but I didn't since it is
not so large and is prone to grow up to the same size shortly


Kyotaro Horiguchi
NTT Open Source Software Center

Attachment Content-Type Size
Remove_old_syscache_entries_patch_v1.patch text/x-patch 5.7 KB

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