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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andres Freund <andres@anarazel.de>
Cc: pgsql-hackers@lists.postgresql.org, Thomas Munro <thomas.munro@enterprisedb.com>
Cc: Andrew Gierth <andrew@tao11.riddles.org.uk>
Cc: Alexander Kuzmenkov <a.kuzmenkov@postgrespro.ru>
Subject: Re: Performance improvements for src/port/snprintf.c
Date: Wed, 03 Oct 2018 12:22:13 -0400
Message-ID: <27041.1538583733@sss.pgh.pa.us> (raw)
In-Reply-To: <20181003161454.ozeioucsimjg5g6m@alap3.anarazel.de>
References: <20180927004022.lfhffbj3roxzf5ca@alap3.anarazel.de>
	<20180927005705.shbl3n5hq3t65n2j@alap3.anarazel.de>
	<87r2hffwda.fsf@news-spur.riddles.org.uk>
	<CAEepm=1-3AG2O25VNnLXsjCM3EHBLW=jyJtpp_AMGG6a-GRJZA@mail.gmail.com>
	<6FEC34CD-B8C0-44E2-8D1F-2320568BDC78@anarazel.de>
	<30551.1538517271@sss.pgh.pa.us>
	<20181003060936.zul7mfkcu5w7rotg@alap3.anarazel.de>
	<25583.1538569214@sss.pgh.pa.us>
	<20181003155207.b3lqmovuv2c5c4id@alap3.anarazel.de>
	<26510.1538582852@sss.pgh.pa.us>
	<20181003161454.ozeioucsimjg5g6m@alap3.anarazel.de>

Andres Freund <andres@anarazel.de> writes:
> On 2018-10-03 12:07:32 -0400, Tom Lane wrote:
>> [ scratches head ... ]  How would that work?  Seems like it necessarily
>> adds a strlen() call to whatever we'd be doing otherwise.  palloc isn't
>> going to be any faster just from asking it for slightly fewer bytes.
>> I think there might be something wrong with your test scenario ...
>> or there's more noise in the numbers than you thought.

> I guess the difference is that we're more likely to find reusable chunks
> in aset.c and/or need fewer OS allocations.  As the memory is going to
> be touched again very shortly afterwards, the cache effects probably are
> neglegible.

> The strlen definitely shows up in profiles, it just seems to save at
> least as much as it costs.

> Doesn't strike me as THAT odd?

What it strikes me as is excessively dependent on one particular test
scenario.  I don't mind optimizations that are tradeoffs between
well-understood costs, but this smells like handwaving that's going to
lose as much or more often than winning, once it hits the real world.

			regards, tom lane




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