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);
}
> > 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. Do you have anything in
mind on how to reduce vhost latency?
Thanks
Shirley
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