[CentOS-virt] Status of kernel divider option in 2.6.9-78.0.1

Wed Sep 17 18:35:30 UTC 2008
Akemi Yagi <amyagi at gmail.com>

On Wed, Sep 17, 2008 at 11:23 AM, Jeff <jlar310 at gmail.com> wrote:
> Under 4.6, we recompiled the kernel with HZ=100 for improved
> time-keeping in VMware guests. I've read about the backporting of the
> divider patch into RHEL/CentOS 4.7, but it sounds like it also comes
> with some bugs. I have been unable to determine the current status of
> the divider option in the latest 4.7 kernel update. I have
> experimented with "divider=10" and it works with no major problems,
> except that the guest clock drifts ahead very slightly. So far, it
> looks like several seconds of drift per day. Not a show-stopper, but
> still a concern. With HZ=100, our time sync was spot on. In both cases
> we are using "clock=pit nosmp noapic" and tools.syncTime="TRUE". If it
> matters, our host machines are CentOS 4.6 x86_64.
> Can anyone comment on the current status of bugs with the divider
> option? Is anyone getting accurate time sync with divider=10 with
> 2.6.9-78.0.1? Is there anything else to watch out for? Is there any
> fix for the slight time drift with divider=10?
> I am primarily concerned with i686.
> Currently, the SRPM for -78.0.1 is missing from the mirrors, so if I
> want to update, I have to rely on the divider option for my VMware
> guests. It also appears that Tru has not yet added any -78 RPMs to his
> kernel-vm project.

As far as I can tell from my limited experience, the clock issue
occurs regardless of the method taken, namely, kernel-vm (100Hz
kernel) or divider=10. (Of course, this is with older kernels, not
-78)  Both options work fine when it comes to idle %cpu.  One
improvement of the 4.7 kernel is that it does not crash even when
divider= is used together with clock=pit like it does with CentOS-5

Yes, the srpm file for -78.0.1 is missing (see
http://bugs.centos.org/view.php?id=3132 ).  But we can try -78 for
performing a test.