Re: [Qemu-devel] RFC: handling "backend too fast" in virtio-net

[Stefan Hajnoczi]

On Mon, Feb 18, 2013 at 02:12:13AM -0800, Luigi Rizzo wrote:
> On Mon, Feb 18, 2013 at 1:50 AM, Stefan Hajnoczi <address@hidden> wrote:
> 
> > On Fri, Feb 15, 2013 at 11:24:29AM +0100, Stefan Hajnoczi wrote:
> > > On Thu, Feb 14, 2013 at 07:21:57PM +0100, Luigi Rizzo wrote:
> > >
> > > CCed Michael Tsirkin
> > >
> > > > virtio-style network devices (where the producer and consumer chase
> > > > each other through a shared memory block) can enter into a
> > > > bad operating regime when the consumer is too fast.
> > > >
> > > > I am hitting this case right now when virtio is used on top of the
> > > > netmap/VALE backend that I posted a few weeks ago: what happens is that
> > > > the backend is so fast that the io thread keeps re-enabling
> > notifications
> > > > every few packets.  This results, on my test machine, in a throughput
> > of
> > > > 250-300Kpps (and extremely unstable, oscillating between 200 and
> > 600Kpps).
> > > >
> > > > I'd like to get some feedback on the following trivial patch to have
> > > > the thread keep spinning for a bounded amount of time when the producer
> > > > is slower than the consumer. This gives a relatively stable throughput
> > > > between 700 and 800 Kpps (we have something similar in our
> > paravirtualized
> > > > e1000 driver, which is slightly faster at 900-1100 Kpps).
> > >
> > > Did you experiment with tx timer instead of bh?  It seems that
> > > hw/virtio-net.c has two tx mitigation strategies - the bh approach that
> > > you've tweaked and a true timer.
> > >
> > > It seems you don't really want tx batching but you do want to avoid
> > > guest->host notifications?
> >
> > One more thing I forgot: virtio-net does not use ioeventfd by default.
> > ioeventfd changes the cost of guest->host notifications because the
> > notification becomes an eventfd signal inside the kernel and kvm.ko then
> > re-enters the guest.
> >
> > This means a guest->host notification becomes a light-weight exit and we
> > don't return from ioctl(KVM_RUN).
> >
> > Perhaps -device virtio-blk-pci,ioeventfd=on will give similar results to
> > your patch?
> >
> 
> is the ioeventfd the mechanism used by vhostnet ?
> If so, Giuseppe Lettieri (in Cc) has tried that with
> a modified netmap backend and experienced the same
> problem -- making the io thread (user or kernel)
> spin a bit more has great benefit on the throughput.

Yes, ioeventfd is how the vhost_net kernel module is notified by the
guest.

It's easy to test, just use -device virtio-net-pci,ioeventfd=on,...

Stefan