netdev - Re: [PATCH 2/2] virtio_net: remove send completion interrupts and avoid TX queue overrun through packet drop

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Message-ID: <87tyetbcem.fsf@rustcorp.com.au>
Date:	Thu, 24 Mar 2011 10:46:49 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	"Michael S. Tsirkin" <mst@...hat.com>,
	Shirley Ma <mashirle@...ibm.com>
Cc:	Herbert Xu <herbert@...dor.hengli.com.au>, davem@...emloft.net,
	kvm@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH 2/2] virtio_net: remove send completion interrupts and avoid TX queue overrun through packet drop

On Tue, 22 Mar 2011 13:36:50 +0200, "Michael S. Tsirkin" <mst@...hat.com> wrote:
> On Mon, Mar 21, 2011 at 11:03:07AM -0700, Shirley Ma wrote:
> > On Fri, 2011-03-18 at 18:41 -0700, Shirley Ma wrote:
> > > > > +       /* Drop packet instead of stop queue for better
> > > performance
> > > > */
> > > > 
> > > > I would like to see some justification as to why this is the right
> > > > way to go and not just papering over the real problem. 
> > > 
> > > Fair. KVM guest virtio_net TX queue stop/restart is pretty expensive,
> > > which involves:
> > > 
> > > 1. Guest enable callback: one memory barrier, interrupt flag set
> > 
> > Missed this cost: for history reason, it also involves a guest exit from
> > I/O write (PCI_QUEUE_NOTIFY).
> 
> OK, after some research, it looks like the reason was the tx timer that
> qemu used to use. So the hack of avoiding the add_buf call will
> avoid this kick and so break these hosts.
> I guess we can add a feature bit to detect a new host
> and so avoid the kick. We are running low on feature bits
> unfortunately, but just fo testing, could you quantify the difference
> that this makes using the following patch:

Performance would suffer for those ancient qemus if we didn't do this,
but it wouldn't be fatal to them.

I think we should just remove it; the standard certainly doesn't mention
it.

Cheers,
Rusty.
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