netdev - Re: poor network loopback performance and scalability

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Message-ID: <20080421101908.GC9295@gondor.apana.org.au>
Date:	Mon, 21 Apr 2008 18:19:08 +0800
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	David Miller <davem@...emloft.net>
Cc:	mingo@...e.hu, dada1@...mosbay.com, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, a.p.zijlstra@...llo.nl
Subject: Re: poor network loopback performance and scalability

On Mon, Apr 21, 2008 at 01:16:23AM -0700, David Miller wrote:
>
> What's being discussed is having the top of the transmit call path
> getting a socket "buffer" pointer, that it can feed back into the
> packet input path directly.  Loopback would return buffer pointers
> from ->hard_start_xmit() instead of passing them netif_rx().  The top
> of the transmit call path, upon getting a non-NULL buffer returned,
> would pass it to netif_receive_skb().

Yes this will definitely reduce the per-packet cost.  The other
low-hanging fruit is to raise the loopback MTU to just below 64K.
I belive the current value is a legacy from the days when we didn't
support skb page frags so everything had to be physically contiguous.

Longer term we could look at generating packets > 64K on lo, for
IPv6 anyway.

Cheers,
-- 
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Email: Herbert Xu ~{PmV>HI~} <herbert@...dor.apana.org.au>
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