Re: guest gettimeofday behavior

[Eran Rom <eranr@il.ibm.com>]

Dor Laor <dlaor <at> redhat.com> writes:

> 
> On 06/25/2009 04:25 PM, Eran Rom wrote:
> > Behavior:
> > Running a code doing:
> > t1 = gettimeofday
> > t2 = gettimeofday
> > while t2-t1<  5 minutes {
> >      sleep(1)
> >      t2 = gettimeofday
> > }
> >
> > Ran it 10 times, each time in a 'newly launched' VM, 
> > halting it after the test.
> > 8 out of 10 times the wall clock showed 5 minutes
> > 1 time 4 minutes and 40 seconds
> > 1 time 0 seconds
> >    
> 
> I'm not sure what's your 'wall clock' value, 
> there is not printf in your 
> script.
the 'wall clock' is host time.
Here is what I am doing form the host:
#! /bin/bash
date
ssh $GuestIP run_gettimeofday_binary
date

> Nevertheless, the tsc clock is not reliable, 
> the host can scale it, or 
> go into
> deep sleep state.
indeed my current clock source is tsc

> 
> So either use newer kernel with kvmclock (pv) 
> or change the clock source
> into rtc/pit
>
No rtc/pit in my available clock sources, however, 
I assume its a question of kernel compilation parameters, 
is that right?
What is the advantage of rtc/pit/kvmclock (pv)over tsc?
Also, is kvmclock (pv) better than the other two options. 
the thing is that I am 'confined' to use 2.6.27, 
and if I decide on kvmclock, i will need to add it
'manually'
 
Thanks,
Eran
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