netdev - Re: Network performance with small packets

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Date:	Thu, 3 Feb 2011 07:59:26 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Shirley Ma <mashirle@...ibm.com>
Cc:	Krishna Kumar2 <krkumar2@...ibm.com>,
	David Miller <davem@...emloft.net>, kvm@...r.kernel.org,
	mashirle@...ux.vnet.ibm.com, netdev@...r.kernel.org,
	netdev-owner@...r.kernel.org, Sridhar Samudrala <sri@...ibm.com>,
	Steve Dobbelstein <steved@...ibm.com>
Subject: Re: Network performance with small packets

On Wed, Feb 02, 2011 at 01:41:33PM -0800, Shirley Ma wrote:
> On Wed, 2011-02-02 at 23:20 +0200, Michael S. Tsirkin wrote:
> > > On Wed, 2011-02-02 at 22:17 +0200, Michael S. Tsirkin wrote:
> > > > Well, this is also the only case where the queue is stopped, no?
> > > Yes. I got some debugging data, I saw that sometimes there were so
> > many
> > > packets were waiting for free in guest between vhost_signal & guest
> > xmit
> > > callback.
> > 
> > What does this mean?
> 
> Let's look at the sequence here:
> 
> guest start_xmit()
> 	xmit_skb()
> 	if ring is full,
> 		enable_cb()
> 
> guest skb_xmit_done()
> 	disable_cb,
>         printk free_old_xmit_skbs <-- it was between more than 1/2 to
> full ring size
> 	printk vq->num_free 
> 
> vhost handle_tx()
> 	if (guest interrupt is enabled)
> 		signal guest to free xmit buffers
> 
> So between guest queue full/stopped queue/enable call back to guest
> receives the callback from host to free_old_xmit_skbs, there were about
> 1/2 to full ring size descriptors available. I thought there were only a
> few. (I disabled your vhost patch for this test.)


The expected number is vq->num - max skb frags - 2.

> 
> > > Looks like the time spent too long from vhost_signal to guest
> > > xmit callback?
> > 
> > 
> > 
> > > > > I tried to accumulate multiple guest to host notifications for
> > TX
> > > > xmits,
> > > > > it did help multiple streams TCP_RR results;
> > > > I don't see a point to delay used idx update, do you?
> > > 
> > > It might cause per vhost handle_tx processed more packets.
> > 
> > I don't understand. It's a couple of writes - what is the issue?
> 
> Oh, handle_tx could process more packets per loop for multiple streams
> TCP_RR case. I need to print out the data rate per loop to confirm this.
> 
> Shirley
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