Horton

Re: Performance improvements for src/port/snprintf.c

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Alexander Kuzmenkov <a.kuzmenkov@postgrespro.ru>, pgsql-hackers@lists.postgresql.org
Date: 2018-09-27T01:18:14Z
Lists: pgsql-hackers 
Hi,

On 2018-09-26 17:57:05 -0700, Andres Freund wrote:
> snprintf time = 1324.87 ms total, 0.000264975 ms per iteration
> pg time = 1434.57 ms total, 0.000286915 ms per iteration
> stbsp time = 552.14 ms total, 0.000110428 ms per iteration

Reading around the interwebz lead me to look at ryu

https://dl.acm.org/citation.cfm?id=3192369
https://github.com/ulfjack/ryu/tree/46f4c5572121a6f1428749fe3e24132c3626c946

That's an algorithm that always generates the minimally sized
roundtrip-safe string output for a floating point number. That makes it
insuitable for the innards of printf, but it very well could be
interesting for e.g. float8out, especially when we currently specify a
"too high" precision to guarantee round-trip safeity.

Greetings,

Andres Freund

Commits

  1. Improve snprintf.c's handling of NaN, Infinity, and minus zero.

  2. Rationalize snprintf.c's handling of "ll" formats.

  3. Provide fast path in snprintf.c for conversion specs that are just "%s".

  4. Make assorted performance improvements in snprintf.c.

  5. Set snprintf.c's maximum number of NL arguments to be 31.

  6. Always use our own versions of *printf().