Horton

snprintf.c hammering memset()

Thomas Munro <thomas.munro@enterprisedb.com>

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From: Thomas Munro <thomas.munro@enterprisedb.com>
To: Pg Hackers <pgsql-hackers@postgresql.org>, Mateusz Guzik <mjguzik@gmail.com>
Date: 2018-10-01T23:09:15Z
Lists: pgsql-hackers 
Hello hackers,

Mateusz Guzik was benchmarking PostgreSQL on FreeBSD investigating the
kqueue thread and complained off-list about a lot of calls to memset()
of size 256KB from dopr() in our snprintf.c code.

Yeah, that says:

    PrintfArgType argtypes[NL_ARGMAX + 2];
...
    MemSet(argtypes, 0, sizeof(argtypes));

PrintfArgType is an enum, and we define NL_ARGMAX as 16 if the OS
didn't already define it.  On FreeBSD 11, NL_ARGMAX was defined as 99
in <limits.h>.  On FreeBSD 12, it is defined as 65536... ouch.  On a
Debian box I see it is 4096.

Is there any reason to use the OS definition here?

-- 
Thomas Munro
http://www.enterprisedb.com

Commits

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