Re: [PATCH] x86/xen: Add "xen_timer_slop" command line option

[luca abeni]

Hi,

On Mon, 25 Mar 2019 09:43:20 -0400
Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx> wrote:
[...]
> > http://retis.santannapisa.it/luca/XenTimers/
> > (there also is a link to the scripts to be used for reproducing the
> > results). The latencies have been measured by running cyclictest in
> > the guest (see the scripts for details).
> >
> > The picture shows the latencies measured with an unpatched guest
> > kernel and with a guest kernel having TIMER_SLOP set to 1000
> > (arbitrary small value :).
> > All the experiments have been performed booting the hypervisor with
> > a small timer_slop (the hypervisor's one) value. So, they show that
> > decreasing the hypervisor's timer_slop is not enough to measure low
> > latencies with cyclictest.  
> 
> 
> 
> I have a couple of questions:
> * Does it make sense to make this a tunable for other clockevent
> devices as well?
> * This patch adjusts min value. Could max value (ever) need a similar
> adjustment?

Sorry, I do not know much about clockevent devices, so I have no answers
to these questions...

What I can say is that when I repeated the cyclictest experiments on
VMs using a different clockevent device (lapic) I did not measure large
latencies.
So, I guess the "lapic" clockevent device already defaults to a smaller
min value (not sure about other clockevent devices, I do not know how
to test them).



				Luca