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Old 04-21-2008, 03:42 PM
"Akemi Yagi"
 
Default Good news for virtual EL-4.x boxes

On Mon, Apr 21, 2008 at 8:27 AM, Stephen John Smoogen <smooge@gmail.com> wrote:
> Currently, if you are running CentOS-4.x on a Vmware box you end up
> with time skew problems. The main fix has been using a set of
> recompiled kernel that has a HZ that is more fitting with what Vmware
> expects (eg 100 hz). The 4.7 kernels look like they will not need
> this:
>
> http://jons-thoughts.blogspot.com/2008/04/rhel-and-vmware-time-skew-problems.html
>
> Well, I've gotten quite the number of questions over the last few days
> about time skew problems in RHEL4 and VMWare, so figured that I would
> write this.
>
> In RHEL4.7, there will be a kernel feature that allows you to pass
> 'divider=' on the command line in grub. The number must be a divisor
> of 1000, and the most common value is 10 (yielding an effective timer
> interrupt rate of 100Hz). I'm expecting the 4.7 beta to become
> available Real Soon Now(TM), though I have no information as to when
> that will occur. In the meantime, this change is available as a hotfix
> from Red Hat's Global Support Services.
>
> What this does is take the kernel tick rate (which defaults to 1000Hz)
> and divides it by the value of divider, to come up with an effective
> interrupt rate. You can verify that this is working by:
>
> Before making the change, use 'watch --interval=1 cat
> /proc/interrupts' and you'll notice that the timer interrupt is
> incrementing by 1000 every second. After you make the change, you you
> notice with the same command that it is incrementing via 1000/ every
> second.
>
> The vlaue of HZ that is exposed to modules is still 1000, thus not
> breaking module ABI.
>
> This same change is already present in RHEL5.1 today.

Just one word of caution here. The current implementation of the
divider= option in RHEL5.1 has bugs
people might want to be aware of:

https://bugzilla.redhat.com/show_bug.cgi?id=315471

The kernel-vm packages offered by CentOS do not suffer from this
issue. I hope the bugs get fixed soon in 5.1 and would not be present
in 4.7.

Akemi
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