netdev - Re: [PATCH v2] net: Allow no-cache copy from user on transmit

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Date:	Wed, 23 Mar 2011 11:51:23 -0700
From:	Rick Jones <rick.jones2@...com>
To:	Tom Herbert <therbert@...gle.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org
Subject: Re: [PATCH v2] net: Allow no-cache copy from user on transmit

On Wed, 2011-03-23 at 10:10 -0700, Tom Herbert 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 netdevice 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 x86.
> 
> No-cache copy disabled:
>    672703 tps, 97.13% utilization
>    50/90/99% latency:244.31 484.205 1028.41
> 
> No-cache copy enabled:
>    702113 tps, 96.16% utilization,
>    50/90/99% latency 238.56 467.56 956.955
> 
> Using 14000 byte request and response sizes demonstrate the
> effects more dramatically:
> 
> No-cache copy disabled:
>    79571 tps, 34.34 %utlization
>    50/90/95% latency 1584.46 2319.59 5001.76
> 
> No-cache copy enabled:
>    83856 tps, 34.81% utilization
>    50/90/95% latency 2508.42 2622.62 2735.88
> 
> Note especially the effect on tail latency (95th percentile).
> 
> This seems to provide a nice 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>

Having raised the question about tying it to CKO and seeing it addressed
to my satisfaction I'll go ahead and:

Acked-by: Rick Jones <rick.jones2@...com>

rick jones

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