Re: [Qemu-devel] [QEMU PATCH] kvmclock: advance clock by time window between vm_stop and pre_save

[Marcelo Tosatti]

On Fri, Nov 04, 2016 at 06:39:18PM +0100, Radim Krčmář wrote:
> 2016-11-04 15:07-0200, Marcelo Tosatti:
> > On Fri, Nov 04, 2016 at 04:25:23PM +0100, Radim Krčmář wrote:
> >> > +        /*
> >> > +         * Transition from VM-running to VM-stopped via migration?
> >> > +         * Record when the VM was stopped.
> >> > +         */
> >> > +
> >> > +        if (state == RUN_STATE_FINISH_MIGRATE &&
> >> > +            !migration_in_postcopy(migrate_get_current())) {
> >> > +            clock_gettime(CLOCK_MONOTONIC, &s->t_aftervmstop);
> >> 
> >> How big (more like small) was the clock delta between here and
> >> kvmclock_pre_save with postcopy?
> >> 
> >> Thanks.
> > 
> > qemu-system-x86_64: postcopy_ram_supported_by_host: userfaultfd not
> > available: Function not implemented
> > 
> > But should be about the same as precopy+this patch (guess as i don't 
> > know the postcopy path).
> 
> I was wondering about the improvement we could achieve by not excluding
> postcopy from the time fixup.  (i.e. how much time elapses between
> pausing and migrating the vm in postcopy)
> 
> I would also guess that it is not significant.

Ideally the total should be zero because for certain workloads
any change is problematic...