[ILUG] Timekeeping in virtual machines

[Niall O Broin]

Timekeeping in virtual machines has definite issues, whether the host  
software be Xen or VMware (or presumably some of the others, though  
my experience is mostly with Xen and VMware) and my specific question  
now concerns VMware workstation, where I have a VM running SLES9. The  
matter is discussed at http://www.vmware.com/pdf/vmware_timekeeping.pdf
and what I currently have is

tools.syncTime = TRUE

in the vmx file, and in the kernel options line

clock=pmtr

I've also tried

clock=pit nosmp noapic nolapic

in the kernel options line but it was no different from clock=pmtr.

NTP client software is NOT running in the VM and now time tracks the  
host reasonably, but jerkily, like this:

while true; do here=`date +%s`; there=`ssh host "date +%s"`; echo -n  
" $[$there - $here]";sleep 1; done

4 5 4 5 5 6 5 5 6 5 6 6 7 6 7 7 7 7 8 8 8 9 8 8 1 0 1 1 1 1 2 2 2 2 3  
2 2 3 4 3 3 4 3 4 4 4 4 5 5 5 5 5 6 5 6 6 7 6 7 7 7 7 7 8 7 8

For my purposes now it's OK, but I can certainly see situations where  
it would not be. For instance:

while true; do then=`date +%s`; sleep 5;now=`date +%s`; echo -n " $ 
[$now - $then]"; done

5 5 13 5 5 5 5 5 5 5 5 5 5 5 13 5 5 6 5 5 5 5 5 5 5 13 5 5 5 5 6 5 5  
5 5 5 5 13 5 5 5 5 5 5 5 5 5 5 5 15 5 5 5 5 5 5 5 5 5 5 5 13 5 5 5 5  
5 5 5 5 5 5 5 14 5 5 5 5 5 5 5 5 5 5 5 13 5 5 5 5 5 5 5 5 5 5 5 13 5

Sometimes after sleeping for 5 seconds, you see a wallclock advance  
of much more. I can definitely imagine scenarios where this would be  
bad.

I have also tried the suggestion in "Guest Clock Synchronization With  
Non-VMware Software" in the referenced document, but after 20 minutes  
up, the guest was two
minutes behind the host with ntpq -p showing

      remote           refid      st t when poll reach   delay    
offset  jitter
======================================================================== 
======
*tardis.local    84.16.251.78     3 u   13   64  377    0.061   
122898. 584.872



To be honest, I expect that "time tracks the host reasonably, but  
jerkily" is as good as I'm going to get but if anybody has any other  
suggestions, I'm all ears - or eyes.


Niall