NTP problem for virtual RHEL 4 server on VmWare

[Eric Sisler]

Kenneth Holter <kenneho.ndu at gmail.com> wrote:

> What puzzles me is why this VM drifts so much, while most of our other VMs
> doens't (at least not more than NTPd can handle).

The Linux 2.6.x kernel is largely the culprit.  Are your other VMs 2.4
kernels?  The kernel frequency HZ changed between the 2.4.x & 2.6.x
kernels.  A snippet of a *much* earlier posting.  Unfortunately I
don't have the name to give credit where it's due:

"The 2.4 kernels have HZ set to 100, while 2.6 kernels have HZ set to
1000.  This value affects the frequency of clock interrupts that are
generated, and those are per-CPU (that's why the problem gets worse
when you go SMP).  This change in frequency is the main reason for the
clock problems for Linux guests.  That is why your RH 7.3 works OK,
while RHEL 4 doesn't.  Another drawback of this change in value of HZ
is that 2.6 kernels will waste more CPU cycles than 2.4 kernels when
idling.  Much more.  When you have several SMP Linux 2.6 guests, just
to keep the kernels running will waste a lot of the available CPU time.

One recommendation from VmWare is to recompile the 2.6 kernel with HZ
reduced back to 100 (as it was in 2.4 kernels).  I haven't yet
attempted this, but I heard reports that it works nicely.

Apperently the behaviour of Windows is much closer to 2.4 Linux
kernels.  Hence you don't see the problem that much there."

As I mentioned earlier, adding a small patch to set the kernel clock
HZ back to 100 & building custom RPMs does work, but can be time
consuming if you have many servers to maintain. I *think* I have notes
about the process somewhere if anyone's interested.

-Eric