netdev - Re: [PATCH 1/2] net: Allow no-cache copy from user on transmit

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Date:	Tue, 8 Mar 2011 22:12:52 -0800
From:	Stephen Hemminger <shemminger@...tta.com>
To:	Tom Herbert <therbert@...gle.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org
Subject: Re: [PATCH 1/2] net: Allow no-cache copy from user on transmit

On Tue, 8 Mar 2011 21:05:06 -0800 (PST)
Tom Herbert <therbert@...gle.com> wrote:

> This patch uses __copy_from_user_nocache (from skb_copy_to_page)
> on transmit to bypass data cache for a performance improvement.
> This functionality is configurable per device using ethtool, the
> device must also be doing TX csum offload to enable.  It seems
> reasonable to set this when the device does not copy or
> otherwise touch the data.
> 
> This patch was tested using 200 instances of netperf TCP_RR with
> 1400 byte request and one byte reply.  Platform is 16 core AMD.
> 
> No-cache copy disabled:
>    672703 tps, 97.13% client utilization
>    50/90/99% latency 244.31 484.205 1028.41
> 
> No-cache copy enabled:
>    702113 tps, 96.16% client utilization,
>    50/90/99% latency 238.56 467.56 956.955
> 
> This seems to provide a nice little performance improvement and is
> consistent in the tests I ran.  Presumably, this would provide
> the greatest  benfits in the presence of an application workload
> stressing the cache and a lot of transmit data happening.  I don't
> yet see a downside to using this.
> 
> Signed-off-by: Tom Herbert <therbert@...gle.com>

Since this is not an ethernet only optimization or device
specific, I would prefer it be a property of device not
something in ethtool.

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