netdev - Re: [PATCH 0/3] tun: fix aio

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Message-ID: <20090427093455.GA29082@redhat.com>
Date:	Mon, 27 Apr 2009 12:34:55 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Herbert Xu <herbert@...dor.apana.org.au>, davem@...emloft.net,
	netdev@...r.kernel.org
Subject: Re: [PATCH 0/3] tun: fix aio

On Mon, Apr 27, 2009 at 10:48:37AM +0930, Rusty Russell wrote:
> On Mon, 20 Apr 2009 09:51:38 pm Michael S. Tsirkin wrote:
> > On Mon, Apr 20, 2009 at 08:09:30PM +0800, Herbert Xu wrote:
> > > Michael S. Tsirkin <mst@...hat.com> wrote:
> > > >
> > > > Note: I started out just allocating and copying the iovec rather than adding
> > > > yet another skb-iterating routine, but this turned out to add small but
> > > > measurable overhead on data path: tx time per packet jumped from 6500 to 6700 ns
> > > > (let me know if you want to see that version of the patch).
> > > 
> > > Can you please post the copying version as well so we can compare?
> > 
> > Sure. Here it is: much smaller, but slightly slower.
> 
> Which could probably be fixed by using an on-stack version for a iovec
> of less than a certain size...

I agree that for large message sizes the malloc would probably be
dwarfed by the cost of memory copy. However a large iovec might pass a
small message, might it not?

-- 
MST
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