Re: virtio performance analysis

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, May 02, 2013 at 09:13:29AM +0530, nitesh narayan lal wrote:
> Hi,
> I am currently working on the virtio performance analysis in PowerPC .

That's nice.

> I was looking at the virtio front-end code and had done tracing by
> adding WARN_ON condition in skb_recv_done(),xmit_skb_done(
> ) and virtqueue_kick().

That's a bit heavy-handed. Why not just use ftrace?

> What I had seen is virtqueue_kick() internally it calls iowrite16()
> which will cause an exit to QEMU,

You probably should be looking at vhost-net and not
userspace virtio-net if you are interested in performance.

> now either I send a packets from
> Guest or receive a packets to guest sart_xmit() will be called and
> inside start_xmit there is a call to virtqueue_kick() causing Guest
> exit. Also for every packet or ack sent from Guest there is an
> exception received while sending or receiving the next packet/ack.

Not exactly, we can buffer many events in
case guest and host are running in parallel.
See vring_need_event and its uses.

> Due to all of the above factors mentioned their will be an increase in
> signal, EXTINT and guest exits

Increase as compared to what?

>  and hence it will effect the CPU
> performance.
> This is what my analysis is so far, it would be great if I could get
> some help on this such that whether it seems appropriate or not ?
> Regards
> Nitesh Narayan Lal
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