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

[Anthony Liguori]

Paul Brook wrote:
> > You see time drift with 100hz timers in the guest??  That makes very
> > little sense as even without hr timers, the host should have no problem
> > delivering a 10ms timer.
> >     
>
> If your host has HZ=100 and no HR timers than a 10ms interval is fairly 
> borderline, and it's not that hard to end up missing an interrupt, especially 
> under heavy load.
>   

If the host is otherwise idle, then the guest should have ample time to 
run.  With /dev/rtc, you should be able to get a reliably 1ms timer.

Gleb: are you perhaps using a qcow2 file in conjunction with -snapshot?  
If that's the case, and you're doing an IO intensive workload, then you 
would be expanding the qcow2 file and updating the metadata is 
synchronous.  This could cause lost time for the guest.

If you are using qcow2, can you still reproduce with a raw file and not 
using -snapshot?

> If part of qemu gets swapped out then all bets are off, and you can easily 
> stall for significant fractions of a second. No amount of host high 
> resolution time support will help you there.
>   

Running a steady workload, you aren't going to be partially swapped.

Regards,

Anthony Liguori

> Paul
>