Re: [PATCH v2] KVM: halt-polling: poll if emulated lapic timer will fire soon

[Yang Zhang]

On 2016/5/24 2:04, David Matlack wrote:
> On Sun, May 22, 2016 at 6:26 PM, Yang Zhang <yang.zhang.wz@xxxxxxxxx> wrote:
> > On 2016/5/21 2:37, David Matlack wrote:
> > > It's not obvious to me why polling for a timer interrupt would improve
> > > context switch latency. Can you explain a bit more?
> >
> >
> > We have a workload which using high resolution timer(less than 1ms) inside
> > guest. It rely on the timer to wakeup itself. Sometimes the timer is
> > expected to fired just after the VCPU is blocked due to execute halt
> > instruction. But the thread who is running in the CPU will turn off the
> > hardware interrupt for long time due to disk access. This will cause the
> > timer interrupt been blocked until the interrupt is re-open.
>
> Does this happen on the idle thread (swapper)? If not, halt-polling
> may not help; it only polls if there are no other runnable threads.

Yes, there is no runnable task inside guest.

> > For optimization, we let VCPU to poll for a while if the next timer will
> > arrive soon before schedule out. And the result shows good when running
> > several workloads inside guest.
>
> Thanks for the explanation, I appreciate it.
>
> > --
> > best regards
> > yang


--
best regards
yang