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

[Wanpeng Li]

On 9/2/15 9:49 AM, David Matlack wrote:
> On Tue, Sep 1, 2015 at 5:29 PM, Wanpeng Li <wanpeng.li@xxxxxxxxxxx> wrote:
> > On 9/2/15 7:24 AM, David Matlack wrote:
> > > On Tue, Sep 1, 2015 at 3:58 PM, Wanpeng Li <wanpeng.li@xxxxxxxxxxx> wrote:
> <snip>
> > > > Why this can happen?
> > > Ah, probably because I'm missing 9c8fd1ba220 (KVM: x86: optimize delivery
> > > of TSC deadline timer interrupt). I don't think the edge case exists in
> > > the latest kernel.
> >
> > Yeah, hope we both(include Peter Kieser) can test against latest kvm tree to
> > avoid confusing. The reason to introduce the adaptive halt-polling toggle is
> > to handle the "edge case" as you mentioned above. So I think we can make
> > more efforts improve v4 instead. I will improve v4 to handle short halt
> > today. ;-)
> That's fine. It's just easier to convey my ideas with a patch. FYI the
> other reason for the toggle patch was to add the timer for kvm_vcpu_block,
> which I think is the only way to get dynamic halt-polling right. Feel free
> to work on top of v4!

I introduce your idea to shrink/grow poll time in v5 by detecting 
long/short halt and the performance looks good. Many thanks your help, 
David! ;-)

Regards,
Wanpeng Li

> <snip>
> > > > Did you test your patch against a windows guest?
> > > I have not. I tested against a 250HZ linux guest to check how it performs
> > > against a ticking guest. Presumably, windows should be the same, but at a
> > > higher tick rate. Do you have a test for Windows?
> >
> > I just test the idle vCPUs usage.
> >
> >
> > V4 for windows 10:
> >
> > +-----------------+----------------+-----------------------+
> > |                                 | |
> > |
> > |  w/o halt-poll           |  w/ halt-poll          | dynamic(v4) halt-poll
> > |
> > +-----------------+----------------+-----------------------+
> > |                                 | |
> > |
> > |    ~2.1%                    |    ~3.0%                  | ~2.4%
> > |
> > +-----------------+----------------+-----------------------+
> I'm not seeing the same results with v4. With a 250HZ ticking guest
> I see 15% c0 with halt_poll_ns=2000000 and 1.27% with halt_poll_ns=0.
> Are you running one vcpu per pcpu?
>
> (The reason for the overhead: the new tracepoint shows each vcpu is
> alternating between 0 and 500 us.)
>
> > V4  for linux guest:
> >
> > +-----------------+----------------+-------------------+
> > |                 |                |                   |
> > |  w/o halt-poll  |  w/ halt-poll  | dynamic halt-poll |
> > +-----------------+----------------+-------------------+
> > |                 |                |                   |
> > |    ~0.9%        |    ~1.8%       |     ~1.2%         |
> > +-----------------+----------------+-------------------+
> >
> >
> > Regards,
> > Wanpeng Li

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