[ntp:questions] Timekeeping best practices for virtualized systems

Ryan Malayter malayter at gmail.com
Wed Apr 21 13:12:57 UTC 2010

On Wed, Apr 21, 2010 at 2:17 AM, Rob <nomail at example.com> wrote:

> I am running different Linux kernels (older and newer) under VMware ESX
> version 4 but the timekeeping is still worse compared to a physical machine.
> You should expect the offset and jitter to be in the several-milliseconds
> range on a local network.
> It seems that each method of providing timing to the virtual machine has
> disadvantages.

I think non-paravirtualized VMs will always be at a serious
disadvantage for timekeeping because of the way interrupts are
coalesced and the VMs get de-sceduled for variable periods of time.
The only alternative I can think of would be to patch the time-related
functions in kernels of all guest OS with some sort of pass-through to
a host-controlled system timer infrastrcuture, and do no
synchronization whatsoever inside of guest OS.

I am going to run some tests to see if running Windows Time Service or
NTP on Windows guests instead of VMware Tools time synchronization is
actually better. The article in question only covers Linux.

VMware's previous advice was to use VMware tools time synchronization
on both Linux and Windows, so I think perhaps they just haven't gotten
around to re-testing NTP-based solutions on Windows.

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