netdev - Re: Network performance with small packets

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Message-ID: <1296713354.25430.143.camel@localhost.localdomain>
Date:	Wed, 02 Feb 2011 22:09:14 -0800
From:	Shirley Ma <mashirle@...ibm.com>
To:	"Michael S. Tsirkin" <mst@...hat.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 Thu, 2011-02-03 at 07:59 +0200, Michael S. Tsirkin wrote:
> > 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. 

It was various (up to the ring size 256). This is using indirection
buffers, it returned how many freed descriptors, not number of buffers.

Why do you think it is vq->num - max skb frags - 2 here?

Shirley

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