netdev - Re: [PATCH net-next 09/11] sfc: Prefetch RX skb header area

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Message-ID: <1371683445.1956.134.camel@bwh-desktop.uk.level5networks.com>
Date:	Thu, 20 Jun 2013 00:10:45 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	David Miller <davem@...emloft.net>, <netdev@...r.kernel.org>,
	<linux-net-drivers@...arflare.com>
Subject: Re: [PATCH net-next 09/11] sfc: Prefetch RX skb header area

On Wed, 2013-06-19 at 15:56 -0700, Eric Dumazet wrote:
> On Wed, 2013-06-19 at 23:39 +0100, Ben Hutchings wrote:
> 
> > You can't use build_skb() if you put multiple RX buffers in a page.  See
> > <1365805319.2791.18.camel@...-desktop.uk.solarflarecom.com>.
> 
> Well, you use small skb (used to have 64 bytes headroom and you changed
> it to 128) and attach a frag on it, so anyway build_skb() brings
> nothing.
> 
> I suspect you have a win on workloads with flood of rx, but on moderate
> load sk_buff will be cold, so the cache adds latencies.
> 
> My feeling is that this kind of tradeoff should not be per driver.

Jon's testing showed a significant latency improvement.

In Linux 3.10 sfc has reduced UDP performance (both latency and
throughput) as we removed the support for skb pre-allocation in
preparation for the implementation for DMA scatter and recycling of
IOMMU-mapped buffers.  The RX changes in this series are intended to
recover that, and our benchmark results suggest that we succeeded.

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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