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 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
