Re: [Qemu-devel] [Bug 1066055] Re: Network performance regression with vde_switch

[Amit Shah <amit.shah@redhat.com>]

On (Tue) 23 Oct 2012 [14:55:03], Stefan Hajnoczi wrote:
> On Mon, Oct 22, 2012 at 06:50:00AM -0700, Edivaldo de Araujo Pereira wrote:
> > I didn't take enough time to uderstand the code, so unfortunately I fear there is not much I could do to solve the problem, apart from trying your suggestions. But I'll try to spend a little more time on it, until we find a solution.
> 
> I've thought a little about how to approach this.  Amit, here's a brain
> dump:
> 
> The simplest solution is to make virtqueue_avail_bytes() use the old
> behavior of stopping early.
> 
> However, I wonder if we can actually *improve* performance of existing
> code by changing virtio-net.c:virtio_net_receive().  The intuition is
> that calling virtio_net_has_buffers() (internally calls
> virtqueue_avail_bytes()) followed by virtqueue_pop() is suboptimal
> because we're repeatedly traversing the descriptor chain.
> 
> We can get rid of this repetition.  A side-effect of this is that we no
> longer need to call virtqueue_avail_bytes() from virtio-net.c.  Here's
> how:
> 
> The common case in virtio_net_receive() is that we have buffers and they
> are large enough for the received packet.  So to optimize for this case:
> 
> 1. Take the VirtQueueElement off the vring but don't increment
>    last_avail_idx yet.  (This is essentially a "peek" operation.)
> 
> 2. If there is an error or we drop the packet because the
>    VirtQueueElement is too small, just bail out and we'll grab the same
>    VirtQueueElement again next time.
> 
> 3. When we've committed filling in this VirtQueueElement, increment
>    last_avail_idx.  This is the point of no return.
> 
> Essentially we're splitting pop() into peek() and consume().  Peek()
> grabs the VirtQueueElement but does not increment last_avail_idx.
> Consume() simply increments last_avail_idx and maybe the EVENT_IDX
> optimization stuff.
> 
> Whether this will improve performance, I'm not sure.  Perhaps
> virtio_net_has_buffers() pulls most descriptors into the CPU's cache and
> following up with virtqueue_pop() is very cheap already.  But the idea
> here is to avoid the virtio_net_has_buffers() because we'll find out
> soon enough when we try to pop :).

This sounds doable -- adding mst for comments.

> Another approach would be to drop virtio_net_has_buffers() but continue
> to use virtqueue_pop().  We'd keep the same VirtQueueElem stashed in
> VirtIONet across virtio_net_receive() calls in the case where we drop
> the packet.  I don't like this approach very much though because it gets
> tricky when the guest modifies the vring memory, resets the virtio
> device, etc across calls.

Right.

Also, save/load will become slightly complicated in both these
cases, but it might be worth it.

Michael, can you comment pls?


		Amit