On 2012-02-07 17:21, Anthony Liguori wrote: > 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 Kthreads can't return, of course. User space threads /may/ do so. And then there needs to be a differences between host and guest in the tracked MSRs. I think to recall it's a question of another few hundred cycles. Jan > 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 >> > -- 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
