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author | Eric Dumazet <edumazet@google.com> | 2014-04-30 11:58:13 -0700 |
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committer | David S. Miller <davem@davemloft.net> | 2014-05-02 17:54:35 -0400 |
commit | e114a710aa5058c0ba4aa1dfb105132aefeb5e04 (patch) | |
tree | 3d7c656358bbc5cd37f7c2a973923e6be6ced1d9 /net/ipv4/tcp_highspeed.c | |
parent | 4e8bbb819d1594a01f91b1de83321f68d3e6e245 (diff) | |
download | op-kernel-dev-e114a710aa5058c0ba4aa1dfb105132aefeb5e04.zip op-kernel-dev-e114a710aa5058c0ba4aa1dfb105132aefeb5e04.tar.gz |
tcp: fix cwnd limited checking to improve congestion control
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: Eric Dumazet <edumazet@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Diffstat (limited to 'net/ipv4/tcp_highspeed.c')
0 files changed, 0 insertions, 0 deletions