Messages in this thread | | | Date | Mon, 25 Mar 2019 15:07:03 +0100 | From | luca abeni <> | Subject | Re: [PATCH] x86/xen: Add "xen_timer_slop" command line option |
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Hi,
On Mon, 25 Mar 2019 09:43:20 -0400 Boris Ostrovsky <boris.ostrovsky@oracle.com> 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
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