From: Rusty Russell <rusty@rustcorp•com.au>
To: "Michael S. Tsirkin" <mst@redhat•com>, Shirley Ma <mashirle@us•ibm.com>
Cc: Herbert Xu <herbert@gondor•hengli.com.au>,
davem@davemloft•net, kvm@vger•kernel.org, netdev@vger•kernel.org
Subject: Re: [PATCH 2/2] virtio_net: remove send completion interrupts and avoid TX queue overrun through packet drop
Date: Thu, 24 Mar 2011 10:46:49 +1030 [thread overview]
Message-ID: <87tyetbcem.fsf@rustcorp.com.au> (raw)
In-Reply-To: <20110322113649.GA17071@redhat.com>
On Tue, 22 Mar 2011 13:36:50 +0200, "Michael S. Tsirkin" <mst@redhat•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.
next prev parent reply other threads:[~2011-03-24 0:16 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-03-17 0:12 [PATCH 2/2] virtio_net: remove send completion interrupts and avoid TX queue overrun through packet drop Shirley Ma
2011-03-17 5:02 ` Michael S. Tsirkin
2011-03-17 15:18 ` Shirley Ma
2011-03-18 3:28 ` Shirley Ma
2011-03-18 13:15 ` Michael S. Tsirkin
2011-03-18 16:54 ` Shirley Ma
2011-03-17 5:10 ` Rusty Russell
2011-03-17 15:10 ` Shirley Ma
2011-03-18 13:33 ` Herbert Xu
2011-03-19 1:41 ` Shirley Ma
2011-03-21 18:03 ` Shirley Ma
2011-03-22 11:36 ` Michael S. Tsirkin
2011-03-23 2:26 ` Shirley Ma
2011-03-24 0:30 ` Rusty Russell
2011-03-24 4:14 ` Shirley Ma
2011-03-24 14:28 ` Michael S. Tsirkin
2011-03-24 17:46 ` Shirley Ma
2011-03-24 18:10 ` Michael S. Tsirkin
2011-03-25 4:51 ` Rusty Russell
2011-03-25 4:50 ` Rusty Russell
2011-03-27 7:52 ` Michael S. Tsirkin
2011-04-04 6:13 ` Rusty Russell
2011-03-24 0:16 ` Rusty Russell [this message]
2011-03-24 6:39 ` Michael S. Tsirkin
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