[CentOS] Good news for virtual EL-4.x boxes

Mon Apr 21 15:42:59 UTC 2008
Akemi Yagi <amyagi at gmail.com>

On Mon, Apr 21, 2008 at 8:27 AM, Stephen John Smoogen <smooge at 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:


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.