1. 18 4月, 2011 1 次提交
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    • T
      net: Allow no-cache copy from user on transmit · c6e1a0d1
      Tom Herbert 提交于
      This patch uses __copy_from_user_nocache on transmit to bypass data
      cache for a performance improvement.  skb_add_data_nocache and
      skb_copy_to_page_nocache can be called by sendmsg functions to use
      this feature, initial support is in tcp_sendmsg.  This functionality is
      configurable per device using ethtool.
      
      Presumably, this feature would only be useful when the driver does
      not touch the data.  The feature is turned on by default if a device
      indicates that it does some form of checksum offload; it is off by
      default for devices that do no checksum offload or indicate no checksum
      is necessary.  For the former case copy-checksum is probably done
      anyway, in the latter case the device is likely loopback in which case
      the no cache copy is probably not beneficial.
      
      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 latency tail (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.
      Signed-off-by: NTom Herbert <therbert@google.com>
      Signed-off-by: NDavid S. Miller <davem@davemloft.net>
      c6e1a0d1
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