netdev - Re: [PATCH net-next] virtio_net: replace netdev_alloc_skb_ip_align() with napi_alloc

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Date:	Fri, 18 Mar 2016 09:21:41 +0100
From:	Paolo Abeni <pabeni@...hat.com>
To:	Venkatesh Srinivas <venkateshs@...gle.com>
Cc:	netdev@...r.kernel.org, virtualization@...ts.linux-foundation.org,
	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	"Michael S. Tsirkin" <mst@...hat.com>
Subject: Re: [PATCH net-next] virtio_net: replace
 netdev_alloc_skb_ip_align() with napi_alloc_skb()

On Thu, 2016-03-17 at 18:01 -0700, Venkatesh Srinivas wrote:
> On Thu, Mar 17, 2016 at 7:44 AM, Paolo Abeni <pabeni@...hat.com> wrote:
> >
> > This gives small but noticeable rx performance improvement (2-3%)
> > and will allow exploiting future napi improvement.
> >
> > Signed-off-by: Paolo Abeni <pabeni@...hat.com>
> > ---
> 
> Worked in my tests, though the performance win was in the noise (+0.6%
> - +1% bandwidth).
> What tests did you see a 2-3% win on?

I'm doing net2VM tests (i.e. the traffic generator is on a different
host) with guest tuned a bit for performance. i.e. cpu pinning, no
iptables, no dhclient (yes, that makes a big difference:
http://lists.openwall.net/netdev/2013/10/29/5)

> Do you think its worth modifying add_recvbuf_small() to use
> napi_alloc_skb() when called from
> Rx NAPI (virtnet_receive)?

Oops, I missed that invocation. Probably that path is not hit in my
test. I think it should be really worthy.

I'll send a v2.

The important thing, more than the current improvement, is allowing the
virtio_net driver to leverage future napi allocation improvement (i.e.
bulk alloc: http://www.spinics.net/lists/netdev/msg367568.html) which
should be a real win.

Paolo