netdev - Re: [Qemu-devel] [PATCH] net: add raw backend

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Date:	Tue, 21 Jul 2009 13:27:33 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	Or Gerlitz <ogerlitz@...taire.com>,
	Jamie Lokier <jamie@...reable.org>,
	Anthony Liguori <anthony@...emonkey.ws>, qemu-devel@...gnu.org,
	Jan Kiszka <jan.kiszka@....de>,
	Mark McLoughlin <markmc@...hat.com>,
	Dor Laor <dlaor@...hat.com>, netdev@...r.kernel.org
Subject: Re: [Qemu-devel] [PATCH] net: add raw backend  - some performance
	measurements

On Tue, Jul 21, 2009 at 03:25:46PM +0800, Herbert Xu wrote:
> On Tue, Jul 21, 2009 at 10:03:00AM +0300, Or Gerlitz wrote:
> > 
> > okay, when setting net.bridge.bridge-nf-call-iptables to zero, the VM TX / tap+bridge packet rate climbs from 170K to 195K but it still way beyond the 240K rate achieved by the raw mode --> we have now a clear sign on the performance gain this approach provides. 
> 
> I find this hard to believe this bridge sans netfilter does a
> single lookup based on the MAC address and then just passes the
> packet to the underlying driver.

One advantage that raw sockets have over tap+bridge, is that they do not
do their own TX buffering, but use the TX queue for the device directly.
With raw sockets, send will block or fail if the TX queue for device is
full. With tap+bridge, the buffer in tap has to fill up instead, which
is not the same. I'm not sure this is the issue here, but could be: the
benchmark is UDP, isn't it?


> Can you do an oprofile run to see if something else is chewing
> up CPU time under the guise of bridging?
> 
> Thanks,
> -- 
> Visit Openswan at http://www.openswan.org/
> Email: Herbert Xu ~{PmV>HI~} <herbert@...dor.apana.org.au>
> Home Page: http://gondor.apana.org.au/~herbert/
> PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
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