Re: [Qemu-devel] [RFC] Next gen kvm api

[Anthony Liguori]

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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