gettimeofday order of magnitude slower with pmtimer, which is default

[bert hubert <bert.hubert@netherlabs.nl>]

Hi everybody,

For my open source nameserver (http://www.powerdns.com) I need to do quite a
number of gettimeofday calls. I've pared it down to almost the bare minimum
of 1 gettimeofday per packet sent and received. With these calls, I can make
stats like http://ds9a.nl/tmp/rrd/ which my users need so they can verify
the proper performance of the nameserver.

Yesterday, together with Zwane, I discovered each gettimeofday call costs me
4 usec on some boxes and almost nothing on others. We did a fruitless chase
for vsyscall/sysenter happening but the problem turned out to be
CONFIG_X86_PM_TIMER.

This problem has been discussed before
http://www.ussg.iu.edu/hypermail/linux/kernel/0411.1/2135.html

Not only is the pm timer slow by design, it also needs to be read multiple
times to work around a bug in certain hardware.

What is new is that this option is now dependent on CONFIG_EMBEDDED. Unless
you select this option, the PM Timer will always be used.

Would a patch removing the link to EMBEDDED and adding a warning that while
this timer is of high quality, it is slow, be welcome?

Thanks.

-- 
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http://netherlabs.nl              Open and Closed source services