Feedback-ID: id4a34324:Fastmail
Date: Sat, 11 Feb 2023 14:04:13 -0800
From: Andres Freund <andres@anarazel.de>
To: Heikki Linnakangas <hlinnaka@iki.fi>
Cc: vignesh C <vignesh21@gmail.com>, pgsql-hackers@postgresql.org,
	Thomas Munro <thomas.munro@gmail.com>,
	Melanie Plageman <melanieplageman@gmail.com>,
	Yura Sokolov <y.sokolov@postgrespro.ru>,
	Robert Haas <robertmhaas@gmail.com>,
	David Rowley <dgrowleyml@gmail.com>
Subject: Re: refactoring relation extension and BufferAlloc(), faster COPY
Message-ID: <20230211220413.mrbdmlmgmaqzrcff@awork3.anarazel.de>
References: <20221029025420.eplyow6k7tgu6he3@awork3.anarazel.de>
 <CALDaNm3prG-F9DS2G8od9QyrE5HefMvmk5oqvEs64n+tPbkd8g@mail.gmail.com>
 <20230110020749.uvi7dfrdvdqlhgal@awork3.anarazel.de>
 <CAApHDvrxsvdodymBXoazvT19KgV9aHd5ccFntRMC2OsAd=LAkw@mail.gmail.com>
 <419312fd-9255-078c-c3e3-f0525f911d7f@iki.fi>
 <f5164e7a-eef6-8972-75a3-8ac622ed0c6e@iki.fi>
 <20230211213651.pu4ns2764fxpyit5@awork3.anarazel.de>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20230211213651.pu4ns2764fxpyit5@awork3.anarazel.de>
Archived-At: <https://www.postgresql.org/message-id/20230211220413.mrbdmlmgmaqzrcff%40awork3.anarazel.de>
Precedence: bulk

On 2023-02-11 13:36:51 -0800, Andres Freund wrote:
> Even though it's not a correctness issue, it seems to me that
> DropRelationsAllBuffers() etc ought to check if the buffer is BM_TAG_VALID,
> before doing anything further.  Particularly in DropRelationsAllBuffers(), the
> check we do for each buffer isn't cheap. Doing it for buffers that don't even
> have a tag seems .. not smart.

There's a small regression for a single relation, but after that it's a clear
benefit.

32GB shared buffers, empty. The test creates N new relations and then rolls
back.

		tps		tps
num relations	HEAD		precheck
1		46.11		45.22
2		43.24		44.87
4		35.14		44.20
8		28.72		42.79

I don't understand the regression at 1, TBH.  I think it must be a random code
layout issue, because the same pre-check in DropRelationBuffers() (exercised
via TRUNCATE of a newly created relation), shows a tiny speedup.