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

[Michael S. Tsirkin]

On Thu, Nov 01, 2012 at 02:49:18PM +0530, Amit Shah wrote:
> 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

It will also complicate switching to/from vhost-net.

If this patch helps serial but degrades speed for -net I'm inclined
to simply make serial and net user different codepaths.

-- 
MST