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

[Avi Kivity]

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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