From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andres Freund <andres@anarazel.de>
Cc: Alexander Kuzmenkov <a.kuzmenkov@postgrespro.ru>
Cc: pgsql-hackers@lists.postgresql.org
Subject: Re: Performance improvements for src/port/snprintf.c
Date: Wed, 26 Sep 2018 19:45:07 -0400
Message-ID: <14302.1538005507@sss.pgh.pa.us> (raw)
In-Reply-To: <20180926221814.el5nwmkez6zfmzbz@alap3.anarazel.de>
References: <11787.1534530779@sss.pgh.pa.us>
<2238.1534716465@sss.pgh.pa.us>
<f5dd41da-911c-fd97-1b78-e0f8c0141897@postgrespro.ru>
<15785.1536776055@sss.pgh.pa.us>
<20180926220419.lorxpb564gakdjkz@alap3.anarazel.de>
<20180926221814.el5nwmkez6zfmzbz@alap3.anarazel.de>
Andres Freund <andres@anarazel.de> writes:
> On 2018-09-26 15:04:20 -0700, Andres Freund wrote:
>> I assume this partially is just the additional layers of function calls
>> (psprintf, pvsnprintf, pg_vsnprintf, dopr) that are now done, in
>> addition to pretty much the same work as before (i.e. sprintf("%.*f")).
No, there are no additional layers that weren't there before ---
snprintf.c's snprintf() slots in directly where the platform's did before.
Well, ok, dopr() wasn't there before, but I trust you're not claiming
that glibc's implementation of snprintf() is totally flat either.
I think it's just that snprintf.c is a bit slower in this case. If you
look at glibc's implementation, they've expended a heck of a lot of code
and sweat on it. The only reason we could hope to beat it is that we're
prepared to throw out some functionality, like LC_NUMERIC handling.
> I'm *NOT* proposing that as the actual solution, but as a datapoint, it
> might be interesting that hardcoding the precision and thus allowing use
> ofusing strfromd() instead of sprintf yields a *better* runtime than
> master.
Interesting. strfromd() is a glibc-ism, and a fairly recent one at
that (my RHEL6 box doesn't seem to have it). But we could use it where
available. And it doesn't seem unreasonable to have a fast path for
the specific precision value(s) that float4/8out will actually use.
regards, tom lane
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To: pgsql-hackers@postgresql.org
Cc: tgl@sss.pgh.pa.us, andres@anarazel.de, a.kuzmenkov@postgrespro.ru, pgsql-hackers@lists.postgresql.org
Subject: Re: Performance improvements for src/port/snprintf.c
In-Reply-To: <14302.1538005507@sss.pgh.pa.us>
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