1. 08 5月, 2014 1 次提交
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  3. 05 5月, 2014 5 次提交
  4. 04 5月, 2014 1 次提交
  5. 03 5月, 2014 1 次提交
    • E
      tcp: fix cwnd limited checking to improve congestion control · e114a710
      Eric Dumazet 提交于
      Yuchung discovered tcp_is_cwnd_limited() was returning false in
      slow start phase even if the application filled the socket write queue.
      
      All congestion modules take into account tcp_is_cwnd_limited()
      before increasing cwnd, so this behavior limits slow start from
      probing the bandwidth at full speed.
      
      The problem is that even if write queue is full (aka we are _not_
      application limited), cwnd can be under utilized if TSO should auto
      defer or TCP Small queues decided to hold packets.
      
      So the in_flight can be kept to smaller value, and we can get to the
      point tcp_is_cwnd_limited() returns false.
      
      With TCP Small Queues and FQ/pacing, this issue is more visible.
      
      We fix this by having tcp_cwnd_validate(), which is supposed to track
      such things, take into account unsent_segs, the number of segs that we
      are not sending at the moment due to TSO or TSQ, but intend to send
      real soon. Then when we are cwnd-limited, remember this fact while we
      are processing the window of ACKs that comes back.
      
      For example, suppose we have a brand new connection with cwnd=10; we
      are in slow start, and we send a flight of 9 packets. By the time we
      have received ACKs for all 9 packets we want our cwnd to be 18.
      We implement this by setting tp->lsnd_pending to 9, and
      considering ourselves to be cwnd-limited while cwnd is less than
      twice tp->lsnd_pending (2*9 -> 18).
      
      This makes tcp_is_cwnd_limited() more understandable, by removing
      the GSO/TSO kludge, that tried to work around the issue.
      
      Note the in_flight parameter can be removed in a followup cleanup
      patch.
      Signed-off-by: NEric Dumazet <edumazet@google.com>
      Signed-off-by: NNeal Cardwell <ncardwell@google.com>
      Signed-off-by: NYuchung Cheng <ycheng@google.com>
      Signed-off-by: NDavid S. Miller <davem@davemloft.net>
      e114a710