tcp: sk_add_backlog() is too agressive for TCP
While investigating TCP performance problems on 10Gb+ links, we found a tcp sender was dropping lot of incoming ACKS because of sk_rcvbuf limit in sk_add_backlog(), especially if receiver doesnt use GRO/LRO and sends one ACK every two MSS segments. A sender usually tweaks sk_sndbuf, but sk_rcvbuf stays at its default value (87380), allowing a too small backlog. A TCP ACK, even being small, can consume nearly same truesize space than outgoing packets. Using sk_rcvbuf + sk_sndbuf as a limit makes sense and is fast to compute. Performance results on netperf, single flow, receiver with disabled GRO/LRO : 7500 Mbits instead of 6050 Mbits, no more TCPBacklogDrop increments at sender. Signed-off-by: NEric Dumazet <edumazet@google.com> Cc: Neal Cardwell <ncardwell@google.com> Cc: Tom Herbert <therbert@google.com> Cc: Maciej Żenczykowski <maze@google.com> Cc: Yuchung Cheng <ycheng@google.com> Cc: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi> Cc: Rick Jones <rick.jones2@hp.com> Signed-off-by: NDavid S. Miller <davem@davemloft.net>
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