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    SubjectRe: [Qemu-devel] [RFC] Next gen kvm api
    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.


    I have a truly marvellous patch that fixes the bug which this
    signature is too narrow to contain.

     \ /
      Last update: 2012-02-07 17:05    [W:0.027 / U:8.192 seconds]
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