Re: [PATCH RFC 0/2] kvm: Improving directed yield in PLE handler

[Raghavendra K T]

On 07/12/2012 01:45 PM, Avi Kivity wrote:
> On 07/11/2012 05:01 PM, Raghavendra K T wrote:
> > On 07/11/2012 07:29 PM, Raghavendra K T wrote:
> > > On 07/11/2012 02:30 PM, Avi Kivity wrote:
> > > > On 07/10/2012 12:47 AM, Andrew Theurer wrote:
> > > > > For the cpu threads in the host that are actually active (in this case
> > > > > 1/2 of them), ~50% of their time is in kernel and ~43% in guest. This
> > > > > is for a no-IO workload, so that's just incredible to see so much cpu
> > > > > wasted. I feel that 2 important areas to tackle are a more scalable
> > > > > yield_to() and reducing the number of pause exits itself (hopefully by
> > > > > just tuning ple_window for the latter).
> > > >
> > > > One thing we can do is autotune ple_window. If a ple exit fails to wake
> > > > anybody (because all vcpus are either running, sleeping, or in ple
> > > > exits) then we deduce we are not overcommitted and we can increase the
> > > > ple window. There's the question of how to decrease it again though.
> > >
> > > I see some problem here, If I interpret situation correctly. What
> > > happens if we have two guests with one VM having no over-commit and
> > > other with high over-commit. (except when we have gang scheduling).
> > Sorry, I meant less load and high load inside the guest.
> >
> > > Rather we should have something tied to VM rather than rigid PLE
> > > window.
>
> The problem occurs even with no overcommit at all.  One vcpu is in a
> legitimately long pause loop.  All those exits accomplish nothing, since
> all vcpus are scheduled.  Better to let it spin in guest mode.

I agree. One idea is we can have a scan_window to limit the scan of all
n vcpus each time we enter vcpu_spin, to say 2*log n initially;

then algorithm would be like;

if (yield fails)
   increase ple_window , increase scan_window

if (yield succeeds)
   decrease ple_window , decrease scan_window


and we have to set limit on what is max and min scan window and max and
min ple_window.

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