Re: [Qemu-devel] [RESEND][PATCH 0/3] Fix guest time drift under heavy load.

[Anthony Liguori <anthony@codemonkey.ws>]

Gleb Natapov wrote:
> On Thu, Nov 06, 2008 at 09:37:56AM -0600, Anthony Liguori wrote:
>   
>
> Yes indeed. With raw image copy benchmark no longer runs enough time to
> produce time drift big enough to be visible. So I ran this disk test
> utility http://69.90.47.6/mybootdisks.com/mybootdisks_com/nu2/bst514.zip
> for ~12 hours and the time drift was 12 secs (if I weren't so lazy and
> wrote bat file to copy c:\windows in a loop I am sure result would be the
> same). This is on completely idle host.
>   

What frequency is the guest running at?  If it's running at 100hz, then 
it missed a tick once every 36 seconds.  This means that the guest 
couldn't run long enough to handle a timer interrupt (which should be a 
relatively small number of cycles) in a 10ms period.

Does this drift go away with the TDF patches?  This almost makes me 
think that we aren't delivering interrupts at the right frequency and 
we're simply accumulating error.  In theory, the TDF patches shouldn't 
help that.

Otherwise, I'm curious if you have any insight into where we're pausing 
for 10ms that's causing the missed interrupt?

We could also be missing ticks somehow.  I think this warrants further 
investigation.

Regards,

Anthony Liguori

>   
>> I think the best ones are going to be intense host workload (and let's  
>> see how much is needed before we start drifting badly) and high guest  
>> frequencies with hosts that lack high resolution timers.  I think with a  
>> high resolution guest and no host overcommit, it should be very  
>> difficult to produce drift regardless of what the guest is doing.
>>
>>     
> Later I'll try to generate load on a host an see how this affects
> guest's time drift.
>
> --
> 			Gleb.
>