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

[Stefan Hajnoczi]

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 :).

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.

Stefan