DHCP/UDP performance tuning..

Scott S. Bertilson scott at nts.umn.edu
Tue Mar 27 23:10:05 UTC 2007

> On 27 Mar 2007, at 21:52, Cory Meyer wrote:
> > Next steps would be to try and
> > reduce the dependency on fsync().
> 	Look to your syslog.conf, and consider which subsystems you need
> 	immediate logging for.  Search the output from `man syslog.conf'
> 	for `omit sync'.  A few servers ago, this bought us a big improvement
> 	on a saturated box which was overdue for replacement.
> 	Best regards,
> 	Niall O'Reilly
> 	University College Dublin IT Services

  Yes YES YESSS!!!!   This is almost assuredly the
problem.  Check to see which files are handling your
DHCP activity logging and insert a "-" in front of
the filename in the "syslog.conf" file.  This
restores your syslogd to the behavior that used
to be the default before the default fsync after
every message madness set in with the Linux version
of syslogd.
  The reason it drags your DHCP server so badly
is that it is sending log messages via a Unix
domain socket which allows syslogd to block it
while it is fsync-ing after every message sent to
the log file.
  I might be wrong about this, but this really
killed us when we started moving services from
Solaris to Linux.
			Scott Bertilson
			University of Minnesota / OIT / NTS

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