Re: [PATCH v4 0/3] KVM: Dynamic Halt-Polling

[Wanpeng Li]

On 9/2/15 6:34 AM, David Matlack wrote:
> On Tue, Sep 1, 2015 at 3:30 PM, Wanpeng Li <wanpeng.li@xxxxxxxxxxx> wrote:
> > On 9/2/15 5:45 AM, David Matlack wrote:
> > > On Thu, Aug 27, 2015 at 2:47 AM, Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
> > > wrote:
> > > > v3 -> v4:
> > > >    * bring back grow vcpu->halt_poll_ns when interrupt arrives and shrinks
> > > >      when idle VCPU is detected
> > > >
> > > > v2 -> v3:
> > > >    * grow/shrink vcpu->halt_poll_ns by *halt_poll_ns_grow or
> > > > /halt_poll_ns_shrink
> > > >    * drop the macros and hard coding the numbers in the param definitions
> > > >    * update the comments "5-7 us"
> > > >    * remove halt_poll_ns_max and use halt_poll_ns as the max halt_poll_ns
> > > > time,
> > > >      vcpu->halt_poll_ns start at zero
> > > >    * drop the wrappers
> > > >    * move the grow/shrink logic before "out:" w/ "if (waited)"
> > > I posted a patchset which adds dynamic poll toggling (on/off switch). I
> > > think
> > > this gives you a good place to build your dynamic growth patch on top. The
> > > toggling patch has close to zero overhead for idle VMs and equivalent
> > > performance VMs doing message passing as always-poll. It's a patch that's
> > > been
> > > in my queue for a few weeks but just haven't had the time to send out. We
> > > can
> > > win even more with your patchset by only polling as much as we need (via
> > > dynamic growth/shrink). It also gives us a better place to stand for
> > > choosing
> > > a default for halt_poll_ns. (We can run experiments and see how high
> > > vcpu->halt_poll_ns tends to grow.)
> > >
> > > The reason I posted a separate patch for toggling is because it adds
> > > timers
> > > to kvm_vcpu_block and deals with a weird edge case (kvm_vcpu_block can get
> > > called multiple times for one halt). To do dynamic poll adjustment

Why this can happen?

> > > correctly,
> > > we have to time the length of each halt. Otherwise we hit some bad edge
> > > cases:
> > >
> > >     v3: v3 had lots of idle overhead. It's because vcpu->halt_poll_ns grew
> > > every
> > >     time we had a long halt. So idle VMs looked like: 0 us -> 500 us -> 1
> > > ms ->
> > >     2 ms -> 4 ms -> 0 us. Ideally vcpu->halt_poll_ns should just stay at 0
> > > when
> > >     the halts are long.
> > >
> > >     v4: v4 fixed the idle overhead problem but broke dynamic growth for
> > > message
> > >     passing VMs. Every time a VM did a short halt, vcpu->halt_poll_ns would
> > > grow.
> > >     That means vcpu->halt_poll_ns will always be maxed out, even when the
> > > halt
> > >     time is much less than the max.
> > >
> > > I think we can fix both edge cases if we make grow/shrink decisions based
> > > on
> > > the length of kvm_vcpu_block rather than the arrival of a guest interrupt
> > > during polling.
> > >
> > > Some thoughts for dynamic growth:
> > >     * Given Windows 10 timer tick (1 ms), let's set the maximum poll time
> > > to
> > >       less than 1ms. 200 us has been a good value for always-poll. We can
> > >       probably go a bit higher once we have your patch. Maybe 500 us?

Did you test your patch against a windows guest?

> > >     * The base case of dynamic growth (the first grow() after being at 0)
> > > should
> > >       be small. 500 us is too big. When I run TCP_RR in my guest I see poll
> > > times
> > >       of < 10 us. TCP_RR is on the lower-end of message passing workload
> > > latency,
> > >       so 10 us would be a good base case.
> >
> > How to get your TCP_RR benchmark?
> >
> > Regards,
> > Wanpeng Li
> Install the netperf package, or build from here:
> http://www.netperf.org/netperf/DownloadNetperf.html
>
> In the vm:
>
> # ./netserver
> # ./netperf -t TCP_RR
>
> Be sure to use an SMP guest (we want TCP_RR to be a cross-core message
> passing workload in order to test halt-polling).

Ah, ok, I use the same benchmark as yours.

Regards,
Wanpeng Li


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