Subject: Re: [PATCHv2 10/14] virtio_net: limit xmit polling

From: Rusty Russell <rusty-8n+1lVoiYb80n/[email-masked]>
Date: Mon, 23 May 2011 11:37:15 +0930

On Sun, 22 May 2011 15:10:08 +0300, "Michael S. Tsirkin" <[email-masked]> wrote:
> On Sat, May 21, 2011 at 11:49:59AM +0930, Rusty Russell wrote:
> > On Fri, 20 May 2011 02:11:56 +0300, "Michael S. Tsirkin" <[email-masked]> wrote:
> > > Current code might introduce a lot of latency variation
> > > if there are many pending bufs at the time we
> > > attempt to transmit a new one. This is bad for
> > > real-time applications and can't be good for TCP either.
> > 
> > Do we have more than speculation to back that up, BTW?
> 
> Need to dig this up: I thought we saw some reports of this on the list?

I think so too, but a reference needs to be here too.

It helps to have exact benchmarks on what's being tested, otherwise we
risk unexpected interaction with the other optimization patches.

> > >  	struct sk_buff *skb;
> > >  	unsigned int len;
> > > -
> > > -	while ((skb = virtqueue_get_buf(vi->svq, &len)) != NULL) {
> > > +	bool c;
> > > +	int n;
> > > +
> > > +	/* We try to free up at least 2 skbs per one sent, so that we'll get
> > > +	 * all of the memory back if they are used fast enough. */
> > > +	for (n = 0;
> > > +	     ((c = virtqueue_get_capacity(vi->svq) < capacity) || n < 2) &&
> > > +	     ((skb = virtqueue_get_buf(vi->svq, &len)));
> > > +	     ++n) {
> > >  		pr_debug("Sent skb %p\n", skb);
> > >  		vi->dev->stats.tx_bytes += skb->len;
> > >  		vi->dev->stats.tx_packets++;
> > >  		dev_kfree_skb_any(skb);
> > >  	}
> > > +	return !c;
> > 
> > This is for() abuse :)
> > 
> > Why is the capacity check in there at all?  Surely it's simpler to try
> > to free 2 skbs each time around?
> 
> This is in case we can't use indirect: we want to free up
> enough buffers for the following add_buf to succeed.

Sure, or we could just count the frags of the skb we're taking out,
which would be accurate for both cases and far more intuitive.

ie. always try to free up twice as much as we're about to put in.

Can we hit problems with OOM?  Sure, but no worse than now...

The problem is that this "virtqueue_get_capacity()" returns the worst
case, not the normal case.  So using it is deceptive.

> I just wanted to localize the 2+MAX_SKB_FRAGS logic that tries to make
> sure we have enough space in the buffer. Another way to do
> that is with a define :).

To do this properly, we should really be using the actual number of sg
elements needed, but we'd have to do most of xmit_skb beforehand so we
know how many.

Cheers,
Rusty.