From: Rusty Russell <rusty@rustcorp•com.au>
To: Mark McLoughlin <markmc@redhat•com>
Cc: netdev@vger•kernel.org, virtualization@lists•linux-foundation.org
Subject: Re: [PATCH 3/4] virtio_net: don't free buffers in xmit ring
Date: Tue, 2 Jun 2009 23:13:52 +0930 [thread overview]
Message-ID: <200906022313.52808.rusty@rustcorp.com.au> (raw)
In-Reply-To: <1243930422.9146.60.camel@blaa>
On Tue, 2 Jun 2009 05:43:42 pm Mark McLoughlin wrote:
> On Fri, 2009-05-29 at 23:46 +0930, Rusty Russell wrote:
> > The virtio_net driver is complicated by the two methods of freeing old
> > xmit buffers (in addition to freeing old ones at the start of the xmit
> > path).
> >
> > The original code used a 1/10 second timer attached to xmit_free(),
> > reset on every xmit. Before we orphaned skbs on xmit, the
> > transmitting userspace could block with a full socket until the timer
> > fired, the skb destructor was called, and they were re-woken.
>
> The timer was actually added to solve a hang when trying to unload
> nf_conntrack AFAIR - nf_conntrack was blocking on the skb being freed
> and we never actually freed it.
>
> I think skb_orphan() is enough to prevent this, is it?
Yep.
> > Unfortunately, VIRTIO_F_NOTIFY_ON_EMPTY makes quite a lot of work: a
> > host which is faster than the guest will fire the interrupt every xmit
> > packet (slowing the guest down further).
>
> Ouch. So, does simply disabling host support for
> VIRTIO_F_NOTIFY_ON_EMPTY speed up current guests?
That was my original change. It cuts the interrupts, but there's enough else
going on that there's not a reliably-measurable speedup (this is with lguest,
haven't tested with kvm).
Cheers,
Rusty.
prev parent reply other threads:[~2009-06-02 13:43 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2009-05-29 14:16 [PATCH 3/4] virtio_net: don't free buffers in xmit ring Rusty Russell
2009-06-02 8:13 ` Mark McLoughlin
2009-06-02 11:41 ` Mark McLoughlin
2009-06-02 14:07 ` Rusty Russell
2009-06-02 13:43 ` Rusty Russell [this message]
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