Messages in this thread | | | Date | Tue, 07 Feb 2012 10:21:27 -0600 | From | Anthony Liguori <> | Subject | Re: [Qemu-devel] [RFC] Next gen kvm api |
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On 02/07/2012 10:18 AM, Jan Kiszka wrote: > 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?
Depends on whether you're scheduling a kthread or a userspace process, no? If you're eventually going to end up in userspace, you have to do the full heavy weight exit.
If you're scheduling to a kthread, it's better to do the type of trickery that ioeventfd does and just turn it into a function call.
Regards,
Anthony Liguori
> > Jan >
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