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    SubjectRe: [PATCH v4 0/3] KVM: Dynamic Halt-Polling
    On Tue, Sep 1, 2015 at 5:29 PM, Wanpeng Li <wanpeng.li@hotmail.com> wrote:
    > On 9/2/15 7:24 AM, David Matlack wrote:
    >>
    >> On Tue, Sep 1, 2015 at 3:58 PM, Wanpeng Li <wanpeng.li@hotmail.com> 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!

    >
    <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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