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    SubjectRe: [Qemu-devel] [RFC] Next gen kvm api
    On 2012-02-07 17:02, Avi Kivity wrote:
    > On 02/07/2012 05:17 PM, Anthony Liguori wrote:
    >> On 02/07/2012 06:03 AM, Avi Kivity wrote:
    >>> On 02/06/2012 09:11 PM, Anthony Liguori wrote:
    >>>>
    >>>> I'm not so sure. ioeventfds and a future mmio-over-socketpair have
    >>>> to put the
    >>>> kthread to sleep while it waits for the other end to process it.
    >>>> This is
    >>>> effectively equivalent to a heavy weight exit. The difference in
    >>>> cost is
    >>>> dropping to userspace which is really neglible these days (< 100
    >>>> cycles).
    >>>
    >>> On what machine did you measure these wonderful numbers?
    >>
    >> A syscall is what I mean by "dropping to userspace", not the cost of a
    >> heavy weight exit.
    >
    > Ah. But then ioeventfd has that as well, unless the other end is in the
    > kernel too.
    >
    >> I think a heavy weight exit is still around a few thousand cycles.
    >>
    >> Any nehalem class or better processor should have a syscall cost of
    >> around that unless I'm wildly mistaken.
    >>
    >
    > That's what I remember too.
    >
    >>>
    >>> But I agree a heavyweight exit is probably faster than a double
    >>> context switch
    >>> on a remote core.
    >>
    >> I meant, if you already need to take a heavyweight exit (and you do to
    >> schedule something else on the core), than the only additional cost is
    >> taking a syscall return to userspace *first* before scheduling another
    >> process. That overhead is pretty low.
    >
    > Yeah.
    >

    Isn't there another level in between just scheduling and full syscall
    return if the user return notifier has some real work to do?

    Jan

    --
    Siemens AG, Corporate Technology, CT T DE IT 1
    Corporate Competence Center Embedded Linux


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