Subject: Re: Network performance with small packets

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 1 Feb 2011 07:56:27 +0200

On Mon, Jan 31, 2011 at 05:30:38PM -0800, Sridhar Samudrala wrote:
> On Mon, 2011-01-31 at 18:24 -0600, Steve Dobbelstein wrote:
> > "Michael S. Tsirkin" <[email-masked]> wrote on 01/28/2011 06:16:16 AM:
> > 
> > > OK, so thinking about it more, maybe the issue is this:
> > > tx becomes full. We process one request and interrupt the guest,
> > > then it adds one request and the queue is full again.
> > >
> > > Maybe the following will help it stabilize?
> > > By itself it does nothing, but if you set
> > > all the parameters to a huge value we will
> > > only interrupt when we see an empty ring.
> > > Which might be too much: pls try other values
> > > in the middle: e.g. make bufs half the ring,
> > > or bytes some small value, or packets some
> > > small value etc.
> > >
> > > Warning: completely untested.
> > >
> > > diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> > > index aac05bc..6769cdc 100644
> > > --- a/drivers/vhost/net.c
> > > +++ b/drivers/vhost/net.c
> > > @@ -32,6 +32,13 @@
> > >   * Using this limit prevents one virtqueue from starving others. */
> > >  #define VHOST_NET_WEIGHT 0x80000
> > >
> > > +int tx_bytes_coalesce = 0;
> > > +module_param(tx_bytes_coalesce, int, 0644);
> > > +int tx_bufs_coalesce = 0;
> > > +module_param(tx_bufs_coalesce, int, 0644);
> > > +int tx_packets_coalesce = 0;
> > > +module_param(tx_packets_coalesce, int, 0644);
> > > +
> > >  enum {
> > >     VHOST_NET_VQ_RX = 0,
> > >     VHOST_NET_VQ_TX = 1,
> > > @@ -127,6 +134,9 @@ static void handle_tx(struct vhost_net *net)
> > >     int err, wmem;
> > >     size_t hdr_size;
> > >     struct socket *sock;
> > > +   int bytes_coalesced = 0;
> > > +   int bufs_coalesced = 0;
> > > +   int packets_coalesced = 0;
> > >
> > >     /* TODO: check that we are running from vhost_worker? */
> > >     sock = rcu_dereference_check(vq->private_data, 1);
> > > @@ -196,14 +206,26 @@ static void handle_tx(struct vhost_net *net)
> > >        if (err != len)
> > >           pr_debug("Truncated TX packet: "
> > >               " len %d != %zd\n", err, len);
> > > -      vhost_add_used_and_signal(&net->dev, vq, head, 0);
> > >        total_len += len;
> > > +      packets_coalesced += 1;
> > > +      bytes_coalesced += len;
> > > +      bufs_coalesced += in;
> > 
> > Should this instead be:
> >       bufs_coalesced += out;
> > 
> > Perusing the code I see that earlier there is a check to see if "in" is not
> > zero, and, if so, error out of the loop.  After the check, "in" is not
> > touched until it is added to bufs_coalesced, effectively not changing
> > bufs_coalesced, meaning bufs_coalesced will never trigger the conditions
> > below.
> 
> Yes. It definitely should be 'out'. 'in' should be 0 in the tx path.
> 
> I tried a simpler version of this patch without any tunables by
> delaying the signaling until we come out of the for loop.
> It definitely reduced the number of vmexits significantly for small message
> guest to host stream test and the throughput went up a little.
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 9b3ca10..5f9fae9 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -197,7 +197,7 @@ static void handle_tx(struct vhost_net *net)
>  		if (err != len)
>  			pr_debug("Truncated TX packet: "
>  				 " len %d != %zd\n", err, len);
> -		vhost_add_used_and_signal(&net->dev, vq, head, 0);
> +		vhost_add_used(vq, head, 0);
>  		total_len += len;
>  		if (unlikely(total_len >= VHOST_NET_WEIGHT)) {
>  			vhost_poll_queue(&vq->poll);
> @@ -205,6 +205,8 @@ static void handle_tx(struct vhost_net *net)
>  		}
>  	}
>  
> +	if (total_len > 0)
> +		vhost_signal(&net->dev, vq);
>  	mutex_unlock(&vq->mutex);
>  }
>  
> 
> > 
> > Or am I missing something?
> > 
> > > +      if (unlikely(packets_coalesced > tx_packets_coalesce ||
> > > +              bytes_coalesced > tx_bytes_coalesce ||
> > > +              bufs_coalesced > tx_bufs_coalesce))
> > > +         vhost_add_used_and_signal(&net->dev, vq, head, 0);
> > > +      else
> > > +         vhost_add_used(vq, head, 0);
> > >        if (unlikely(total_len >= VHOST_NET_WEIGHT)) {
> > >           vhost_poll_queue(&vq->poll);
> > >           break;
> > >        }
> > >     }
> > >
> > > +   if (likely(packets_coalesced > tx_packets_coalesce ||
> > > +         bytes_coalesced > tx_bytes_coalesce ||
> > > +         bufs_coalesced > tx_bufs_coalesce))
> > > +      vhost_signal(&net->dev, vq);
> > >     mutex_unlock(&vq->mutex);
> > >  }
> 
> It is possible that we can miss signaling the guest even after
> processing a few pkts, if we don't hit any of these conditions.

Yes. It really should be
   if (likely(packets_coalesced && bytes_coalesced && bufs_coalesced))
      vhost_signal(&net->dev, vq);

> > >
> > 
> > Steve D.
> > 
> > --
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