From: Tom Lane <tgl@sss.pgh.pa.us>
To: Thomas Munro <thomas.munro@enterprisedb.com>
Cc: Alvaro Herrera <alvherre@2ndquadrant.com>
Cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: Allowing printf("%m") only where it actually works
Date: Sun, 12 Aug 2018 15:35:09 -0400
Message-ID: <3067.1534102509@sss.pgh.pa.us> (raw)
In-Reply-To: <1437.1534100930@sss.pgh.pa.us>
References: <20180528150125.j2x3mp62ssbeg3h5@alvherre.pgsql>
<2685.1527520461@sss.pgh.pa.us>
<8770.1533931511@sss.pgh.pa.us>
<16896.1533935042@sss.pgh.pa.us>
<14925.1534040609@sss.pgh.pa.us>
<1437.1534100930@sss.pgh.pa.us>
I wrote:
> ... So we would be taking a hit on most platforms, but I've not really
> seen sprintf as a major component of very many profiles. Moreover,
> at least for the elog/ereport use-case, we'd be buying back some
> nontrivial part of that hit by getting rid of expand_fmt_string().
> Also worth noting is that we've never made any effort at all to
> micro-optimize snprintf.c, so maybe there's some gold to be mined
> there to reduce the penalty.
Oh, the plot thickens: apparently, a very large chunk of the time
in that test scenario went into the %g format item, which I think
we can all agree isn't performance-critical for Postgres. Changing
the test case to test %lld in place of %g, I get (on the same
five platforms as before)
RHEL6:
snprintf time = 357.634 ms total, 0.000357634 ms per iteration
pg_snprintf time = 281.708 ms total, 0.000281708 ms per iteration
ratio = 0.788
macOS:
snprintf time = 155.86 ms total, 0.00015586 ms per iteration
pg_snprintf time = 104.074 ms total, 0.000104074 ms per iteration
ratio = 0.668
FreeBSD:
snprintf time = 268.883 ms total, 0.000268883 ms per iteration
pg_snprintf time = 185.294 ms total, 0.000185294 ms per iteration
ratio = 0.689
OpenBSD:
snprintf time = 276.418 ms total, 0.000276418 ms per iteration
pg_snprintf time = 153.334 ms total, 0.000153334 ms per iteration
ratio = 0.555
NetBSD:
snprintf time = 1174.13 ms total, 0.00117413 ms per iteration
pg_snprintf time = 1508.82 ms total, 0.00150882 ms per iteration
ratio = 1.285
So there's actually a plausible argument to be made that we'd
get a net speed win on most platforms and test cases.
regards, tom lane
Attachments:timeprintf-2.c (text/x-c, 1.3K) download
#include "postgres_fe.h"
#include "portability/instr_time.h"
#include "snprintf.c"
int
main(int argc, char **argv)
{
int count = 0;
char buffer[1000];
instr_time start;
instr_time stop;
double elapsed;
double elapsed2;
int i;
if (argc > 1)
count = atoi(argv[1]);
if (count <= 0)
count = 1000000;
INSTR_TIME_SET_CURRENT(start);
for (i = 0; i < count; i++)
{
snprintf(buffer, sizeof(buffer),
"%d %lld %s",
42, (long long) 42,
"01234567890012345678900123456789001234567890012345678900123456789001234567890012345678900123456789001234567890");
}
INSTR_TIME_SET_CURRENT(stop);
INSTR_TIME_SUBTRACT(stop, start);
elapsed = INSTR_TIME_GET_MILLISEC(stop);
printf("snprintf time = %g ms total, %g ms per iteration\n",
elapsed, elapsed / count);
INSTR_TIME_SET_CURRENT(start);
for (i = 0; i < count; i++)
{
pg_snprintf(buffer, sizeof(buffer),
"%d %lld %s",
42, (long long) 42,
"01234567890012345678900123456789001234567890012345678900123456789001234567890012345678900123456789001234567890");
}
INSTR_TIME_SET_CURRENT(stop);
INSTR_TIME_SUBTRACT(stop, start);
elapsed2 = INSTR_TIME_GET_MILLISEC(stop);
printf("pg_snprintf time = %g ms total, %g ms per iteration\n",
elapsed2, elapsed2 / count);
printf("ratio = %.3f\n", elapsed2 / elapsed);
return 0;
}
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To: pgsql-hackers@postgresql.org
Cc: tgl@sss.pgh.pa.us, thomas.munro@enterprisedb.com, alvherre@2ndquadrant.com, pgsql-hackers@lists.postgresql.org
Subject: Re: Allowing printf("%m") only where it actually works
In-Reply-To: <3067.1534102509@sss.pgh.pa.us>
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