netdev - Re: [PATCH] net: Always allocate at least 16 skb frags regardless of page size

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Message-Id: <20110328.222702.39186706.davem@davemloft.net>
Date:	Mon, 28 Mar 2011 22:27:02 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	anton@...ba.org
Cc:	eric.dumazet@...il.com, herbert@...dor.apana.org.au,
	netdev@...r.kernel.org
Subject: Re: [PATCH] net: Always allocate at least 16 skb frags regardless
 of page size

From: Anton Blanchard <anton@...ba.org>
Date: Mon, 28 Mar 2011 11:57:26 +1100

> 
> When analysing performance of the cxgb3 on a ppc64 box I noticed that
> we weren't doing much GRO merging. It turns out we are limited by the
> number of SKB frags:
> 
> #define MAX_SKB_FRAGS (65536/PAGE_SIZE + 2)
> 
> With a 4kB page size we have 18 frags, but with a 64kB page size we
> only have 3 frags.
> 
> I ran a single stream TCP bandwidth test to compare the performance of
> different values of MAX_SKB_FRAGS on the receiver:
> 
> MAX_SKB_FRAGS   Mbps
> 3               7080	
> 8               7931    (+12%)
> 16              8335	(+17%)
> 32              8349	(+17%)
> 
> Performance continues to increase up to 16 frags then levels off so
> the patch below puts a lower bound of 16 on MAX_SKB_FRAGS.
> 
> Signed-off-by: Anton Blanchard <anton@...ba.org>

Thanks for investigating this and fixing the problem.

Applied.
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