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    SubjectRe: kvm & dyntick
    Ingo Molnar wrote:
    > * Ingo Molnar <mingo@elte.hu> wrote:
    >
    >
    >>> dyntick-enabled guest:
    >>> - reduce the load on the host when the guest is idling
    >>> (currently an idle guest consumes a few percent cpu)
    >>>
    >> yeah. KVM under -rt already works with dynticks enabled on both the
    >> host and the guest. (but it's more optimal to use a dedicated
    >> hypercall to set the next guest-interrupt)
    >>
    >
    > using the dynticks code from the -rt kernel makes the overhead of an
    > idle guest go down by a factor of 10-15:
    >
    > PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
    > 2556 mingo 15 0 598m 159m 157m R 1.5 8.0 0:26.20 qemu
    >
    >

    As usual, great news.

    > ( for this to work on my system i have added a 'hyper' clocksource
    > hypercall API for KVM guests to use - this is needed instead of the
    > running-to-slowly TSC. )
    >

    What's the problem with the TSC? The only issue I'm aware of is that
    the tsc might go backwards if the vcpu is migrated to another host cpu
    (easily fixed).

    A pv clocksource makes sense in any case.

    --
    Do not meddle in the internals of kernels, for they are subtle and quick to panic.

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