Re: guest gettimeofday behavior

[Dor Laor <dlaor@redhat.com>]

On 06/25/2009 04:25 PM, Eran Rom wrote:
> Hi All,
> Am a newbie (to kvm, linux kernel, git, etc.) so apologize in advance for
> missing/inaccurate info.
> I am experiencing inconsistent behavior of guest gettimeofday, described below.
> I have seen prior reference to the problem, however, it was not clear whether
> the issue was solved or not and where.
>
> Below is a description of my setup and the behavior I see.
> Another question is whether oprofile with timer mode would suffer from the same
> problem?
>
> Otherwise I saw that kvm-85 has "generic performance counter msr handling", does
> this mean that oprofile can be executed without the timer mode?
>
> Thanks very much,
> Eran
>
> Setup:
> Guest 32 bit ubuntu with 2.6.27 kernel
> Host 64 bit Suse 10.2 with 2.6.27 kernel
> 1 quadcore Intel Xeon CPU
> I am not sure about the kvm userspace code version.
> Used git to create a repository, and then did
> git checkout kvm-updates-2.6.27 (is that fixed in time?)
>
> 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.
Nevertheless, the tsc clock is not reliable, the host can scale it, or 
go into
deep sleep state.

So either use newer kernel with kvmclock (pv) or change the clock source
into rtc/pit


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