Messages in this thread | | | Date | Tue, 07 Feb 2012 18:02:50 +0200 | From | Avi Kivity <> | Subject | Re: [Qemu-devel] [RFC] Next gen kvm api |
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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.
-- I have a truly marvellous patch that fixes the bug which this signature is too narrow to contain.
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