[Qemu-devel] Re: [PATCH] virtio: Use ioeventfd for virtqueue notify

[Avi Kivity]

 On 10/05/2010 01:00 PM, rukhsana ansari wrote:
> Hi,
>
> W.r.t:
> >  Note that this is a tradeoff.  If an idle core is available and the
> >  scheduler places the iothread on that core, then the heavyweight exit is
> >  replaced by a lightweight exit + IPI.  If the iothread is co-located with
> >  the vcpu, then we'll take a heavyweight exit in any case.
> >
> Q: Does the kvm kernel code check for such a condition and take a
> heavyweight exit?

No.  The heavyweight exit is caused by a context switch (partial) or 
return to userspace (full).

> >  The first case is very likely if the host cpu is undercommitted and there is
> >  heavy I/O activity.  This is a typical subsystem benchmark scenario (as
> >  opposed to a system benchmark like specvirt).  My feeling is that total
> >  system throughput will be decreased unless the scheduler is clever enough to
> >  place the iothread and vcpu on the same host cpu when the system is
> >  overcommitted.
> >
> >
> Q: Sorry if the answer is obvious here.
> If the heavyweight exit is taken when both threads are assigned to the
> same core, how will the system throughput increase?

Co-locating threads on the same core reduces cross-core traffic.

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