Re: Erratic gettimeofday() behavior with KVM and newer Linux kernels?

[Anthony Liguori <anthony-rdkfGonbjUSkNkDKm+mE6A@public.gmane.org>]

Avi Kivity wrote:
> Anthony Liguori wrote:
>> Avi Kivity wrote:
>>> Anthony Liguori wrote:
>>>> Avi Kivity wrote:
>>>>> Anthony Liguori wrote:
>>>>>> It's pretty erratic.  If I run if a few times at any rate, 
>>>>>> sometimes I get 0.0 and sometimes I get a more sane result.
>>>>>
>>>>> I expect the newer kernel is sometimes disqualifying the timestamp 
>>>>> counter from being used as a clocksource.   Running pinned to a 
>>>>> cpu may improve things.
>>>>
>>>> Things get better if I disable the PM timer and if I pin to a 
>>>> single CPU.
>>>>
>>>
>>> Can you post the magic command line for future reference?
>>
>> I disabled CONFIG_X86_PM_TIMER in the kernel config and then used:
>>
>> sudo taskset 1 qemu  -hda /dev/volumes/kvm-dev -m 2000 -vnc :3 
>> -kernel /build/kvm/arch/i386/boot/bzImage -append "root=0301 ro"
>>
>
> Ah, there's probably a kernel command line to select the clock without 
> resorting to config options.  Somebody probably wrote a book about it.
>
>> However, this doesn't appear to be a perfect solution.  If I launch 
>> the VM redirecting everything to the serial port, I again start 
>> having problems with virtbench.  I guess the shear rate of interrupts 
>> using the serial port throws off the accuracy of the clock?
>
> I'd have guessed that there wouldn't be many writes to the serial port 
> when testing is in progress.

It's less stable then I thought.  You were right btw, the guest is 
deciding the TSC is too unreliable.  It happens every once and while 
(although the taskset/no pm timer combination helps a whole lot).

Regards,

Anthony Liguori

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