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  4. 22 Mar, 2008 1 commit
    • Herbert Xu's avatar
      [TCP]: Let skbs grow over a page on fast peers · 69d15067
      Herbert Xu authored
      While testing the virtio-net driver on KVM with TSO I noticed
      that TSO performance with a 1500 MTU is significantly worse
      compared to the performance of non-TSO with a 16436 MTU.  The
      packet dump shows that most of the packets sent are smaller
      than a page.
      
      Looking at the code this actually is quite obvious as it always
      stop extending the packet if it's the first packet yet to be
      sent and if it's larger than the MSS.  Since each extension is
      bound by the page size, this means that (given a 1500 MTU) we're
      very unlikely to construct packets greater than a page, provided
      that the receiver and the path is fast enough so that packets can
      always be sent immediately.
      
      The fix is also quite obvious.  The push calls inside the loop
      is just an optimisation so that we don't end up doing all the
      sending at the end of the loop.  Therefore there is no specific
      reason why it has to do so at MSS boundaries.  For TSO, the
      most natural extension of this optimisation is to do the pushing
      once the skb exceeds the TSO size goal.
      
      This is what the patch does and testing with KVM shows that the
      TSO performance with a 1500 MTU easily surpasses that of a 16436
      MTU and indeed the packet sizes sent are generally larger than
      16436.
      
      I don't see any obvious downsides for slower peers or connections,
      but it would be prudent to test this extensively to ensure that
      those cases don't regress.
      Signed-off-by: default avatarHerbert Xu <herbert@gondor.apana.org.au>
      Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
      69d15067