On Wed, Feb 02, 2011 at 09:10:35AM -0800, Shirley Ma wrote:
> On Wed, 2011-02-02 at 17:47 +0200, Michael S. Tsirkin wrote:
> > On Wed, Feb 02, 2011 at 07:39:45AM -0800, Shirley Ma wrote:
> > > On Wed, 2011-02-02 at 12:48 +0200, Michael S. Tsirkin wrote:
> > > > Yes, I think doing this in the host is much simpler,
> > > > just send an interrupt after there's a decent amount
> > > > of space in the queue.
> > > >
> > > > Having said that the simple heuristic that I coded
> > > > might be a bit too simple.
> > >
> > > >From the debugging out what I have seen so far (a single small
> > message
> > > TCP_STEAM test), I think the right approach is to patch both guest
> > and
> > > vhost.
> >
> > One problem is slowing down the guest helps here.
> > So there's a chance that just by adding complexity
> > in guest driver we get a small improvement :(
> >
> > We can't rely on a patched guest anyway, so
> > I think it is best to test guest and host changes separately.
> >
> > And I do agree something needs to be done in guest too,
> > for example when vqs share an interrupt, we
> > might invoke a callback when we see vq is not empty
> > even though it's not requested. Probably should
> > check interrupts enabled here?
>
> Yes, I modified xmit callback something like below:
>
> static void skb_xmit_done(struct virtqueue *svq)
> {
> struct virtnet_info *vi = svq->vdev->priv;
>
> /* Suppress further interrupts. */
> virtqueue_disable_cb(svq);
>
> /* We were probably waiting for more output buffers. */
> if (netif_queue_stopped(vi->dev)) {
> free_old_xmit_skbs(vi);
> if (virtqueue_free_size(svq) <= svq->vring.num / 2) {
> virtqueue_enable_cb(svq);
> return;
> }
> }
> netif_wake_queue(vi->dev);
> }
OK, but this should have no effect with a vhost patch
which should ensure that we don't get an interrupt
until the queue is at least half empty.
Right?
> > > The problem I have found is a regression for single small
> > > message TCP_STEAM test. Old kernel works well for TCP_STREAM, only
> > new
> > > kernel has problem.
> >
> > Likely new kernel is faster :)
>
> > > For Steven's problem, it's multiple stream TCP_RR issues, the old
> > guest
> > > doesn't perform well, so does new guest kernel. We tested reducing
> > vhost
> > > signaling patch before, it didn't help the performance at all.
> > >
> > > Thanks
> > > Shirley
> >
> > Yes, it seems unrelated to tx interrupts.
>
> The issue is more likely related to latency.
Could be. Why do you think so?
> Do you have anything in
> mind on how to reduce vhost latency?
>
> Thanks
> Shirley
Hmm, bypassing the bridge might help a bit.
Are you using tap+bridge or macvtap?
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