RE: Protect syscache from bloating with negative cache entries

From: "Tsunakawa, Takayuki" <tsunakawa(dot)takay(at)jp(dot)fujitsu(dot)com>
To: "Ideriha, Takeshi" <ideriha(dot)takeshi(at)jp(dot)fujitsu(dot)com>, Kyotaro HORIGUCHI <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp>
Cc: Andres Freund <andres(at)anarazel(dot)de>, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, Bruce Momjian <bruce(at)momjian(dot)us>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, "PostgreSQL Hackers" <pgsql-hackers(at)lists(dot)postgresql(dot)org>, Michael Paquier <michael(dot)paquier(at)gmail(dot)com>, 'Robert Haas' <robertmhaas(at)gmail(dot)com>, "David Steele" <david(at)pgmasters(dot)net>, Craig Ringer <craig(at)2ndquadrant(dot)com>
Subject: RE: Protect syscache from bloating with negative cache entries
Date: 2019-03-01 01:46:39
Message-ID: 0A3221C70F24FB45833433255569204D1FBBC085@G01JPEXMBYT05
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From: Ideriha, Takeshi/出利葉 健
> [Size=800, iter=1,000,000]
> Master |15.763
> Patched|16.262 (+3%)
> [Size=32768, iter=1,000,000]
> Master |61.3076
> Patched|62.9566 (+2%)

What's the unit, second or millisecond?
Why is the number of digits to the right of the decimal point?

Is the measurement correct? I'm wondering because the difference is larger in the latter case. Isn't the accounting processing almost the sane in both cases?
* former: 16.262 - 15.763 = 4.99
* latter: 62.956 - 61.307 = 16.49

> At least compared to previous HashAg version, the overhead is smaller.
> It has some overhead but is increase by 2 or 3% a little bit?

I think the overhead is sufficiently small. It may get even smaller with a trivial tweak.

You added the new member usedspace at the end of MemoryContextData. The original size of MemoryContextData is 72 bytes, and Intel Xeon's cache line is 64 bytes. So, the new member will be on a separate cache line. Try putting usedspace before the name member.

Takayuki Tsunakawa

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