Date: Wed, 20 Jun 2001 21:40:48 +0200 From: Poul-Henning Kamp <phk@critter.freebsd.dk> To: Brad Karp <bkarp@icsi.berkeley.edu> Cc: smp@FreeBSD.ORG Subject: Re: clock speedup on SMP boxes Message-ID: <7314.993066048@critter> In-Reply-To: Your message of "Wed, 20 Jun 2001 11:23:23 PDT." <200106201823.f5KINN321282@carp.aciri.org>
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In message <200106201823.f5KINN321282@carp.aciri.org>, Brad Karp writes:
>There were several responses to my initial note that the time of day clock in
>FreeBSD 4.3-RELEASE runs faster than real time (gaining up to an hour every
>day, where the amount of clock speedup is in proportion to the load on the
>CPUs) on my SMP boxes. In these responses, others mentioned they see similar
>behavior on other hardware. I see this behavior on Intel STL2 and Tyan S2567
>motherboards with dual 1 GHz P-IIIs installed.
I'm the author of our timekeeping ("timecounter") code, I heard
about this problem first time today.
Unfortunately part of my lab is already packed down so I can't reproduce
this locally right now.
I don't have a good idea right now what causes this, but I have a number
of ideas I would like people to try out:
0. First off, kill ntpd/ntpdate-from-cron.
1. Try to quanitify the speedup at 100% CPU load with no
interrupts. Basically run one copy of for each CPU
in the system:
main()
{
for(;;);
}
2. Try to quanitify the speedup at 1/NCPU load with no
interrupts. Run just one copy of the above.
3. Increase kernel HZ to 500 and rerun #2.
4. Decrease kernel HZ to 50 and rerun #2.
Please include in your report the contents of /var/run/dmesg.boot
please email your data to timec@phk.freebsd.dk
I'll try to see if I can find out what's going on.
--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
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