Re: [PERFORM] syslog performance when logging big statements

Jeff <threshar@threshar.is-a-geek.com> writes:
> On Jul 8, 2008, at 8:24 AM, Achilleas Mantzios wrote:
>> File sizes of about 3M result in actual logging output of ~ 10Mb.
>> In this case, the INSERT *needs* 20 minutes to return. This is
>> because the logging through syslog seems to severely slow the system.
>> If instead, i use stderr, even with logging_collector=on, the same
>> statement needs 15 seconds to return.

> In syslog.conf is the destination for PG marked with a "-"? (ie -/var/
> log/pg.log) which tells syslog to not sync after each line logged.
> That could explain a large chunk of the difference in time.

I experimented with this a bit here. There definitely is an O(N^2)
penalty from the repeated strchr() calls, but it doesn't really start
to hurt till 1MB or so statement length. Even with that patched,
syslog logging pretty much sucks performance-wise. Here are the numbers
I got on a Fedora 8 workstation, testing the time to log a statement of
the form SELECT length('123456...lots of data, no newlines...7890');

statement length 1MB 10MB

CVS HEAD 2523ms 215588ms
+ patch to fix repeated strchr 529ms 36734ms
after turning off syslogd's fsync 569ms 5881ms
PG_SYSLOG_LIMIT 1024, fsync on 216ms 2532ms
PG_SYSLOG_LIMIT 1024, no fsync 242ms 2692ms
For comparison purposes:
logging statements to stderr 155ms 2042ms
execute statement without logging 42ms 520ms

This machine is running a cheap IDE drive that caches writes, so
the lack of difference between fsync off and fsync on is not too
surprising --- on a machine with server-grade drives there'd be
a lot more difference. (The fact that there is a difference in
the 10MB case probably reflects filling the drive's write cache.)

On my old HPUX machine, where fsync really works (and the syslogd
doesn't seem to allow turning it off), the 1MB case takes
195957ms with the strchr patch, 22922ms at PG_SYSLOG_LIMIT=1024.

So there's a fairly clear case to be made for fixing the repeated
strchr, but I also think that there's a case for jacking up
PG_SYSLOG_LIMIT. As far as I can tell the current value of 128
was chosen *very* conservatively without thought for performance:
http://archives.postgresql.org/pgsql-hackers/2000-05/msg01242.php

At the time we were looking at evidence that the then-current
Linux syslogd got tummyache with messages over about 1KB:
http://archives.postgresql.org/pgsql-hackers/2000-05/msg00880.php

Some experimentation with the machines I have handy now says that

Fedora 8: truncates messages at 2KB (including syslog's header)
HPUX 10.20 (ancient): ditto
Mac OS X 10.5.3: drops messages if longer than about 1900 bytes

So it appears to me that setting PG_SYSLOG_LIMIT = 1024 would be
perfectly safe on current systems (and probably old ones too),
and would give at least a factor of two speedup for logging long
strings --- more like a factor of 8 if syslogd is fsync'ing.

Comments? Anyone know of systems where this is too high?
Perhaps we should make that change only in HEAD, not in the
back branches, or crank it up only to 512 in the back branches?

regards, tom lane

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