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tcp-tso: do not split TSO packets at retransmit time

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Linux TCP stack painfully segments all TSO/GSO packets before retransmits.

This was fine back in the days when TSO/GSO were emerging, with their
bugs, but we believe the dark age is over.

Keeping big packets in write queues, but also in stack traversal
has a lot of benefits.
 - Less memory overhead, because write queues have less skbs
 - Less cpu overhead at ACK processing.
 - Better SACK processing, as lot of studies mentioned how
   awful linux was at this ;)
 - Less cpu overhead to send the rtx packets
   (IP stack traversal, netfilter traversal, drivers...)
 - Better latencies in presence of losses.
 - Smaller spikes in fq like packet schedulers, as retransmits
   are not constrained by TCP Small Queues.

1 % packet losses are common today, and at 100Gbit speeds, this
translates to ~80,000 losses per second.
Losses are often correlated, and we see many retransmit events
leading to 1-MSS train of packets, at the time hosts are already
under stress.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Eric Dumazet
2016-04-21 10:55:23 -07:00
committed by David S. Miller
parent 8cee83dd29
commit 10d3be5692
4 changed files with 34 additions and 40 deletions
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4
net/ipv4/tcp_timer.c
View File

@@ -404,7 +404,7 @@ void tcp_retransmit_timer(struct sock *sk)
goto out;
}
tcp_enter_loss(sk);
tcp_retransmit_skb(sk, tcp_write_queue_head(sk));
tcp_retransmit_skb(sk, tcp_write_queue_head(sk), 1);
__sk_dst_reset(sk);
goto out_reset_timer;
}
@@ -436,7 +436,7 @@ void tcp_retransmit_timer(struct sock *sk)
tcp_enter_loss(sk);
if (tcp_retransmit_skb(sk, tcp_write_queue_head(sk)) > 0) {
if (tcp_retransmit_skb(sk, tcp_write_queue_head(sk), 1) > 0) {
/* Retransmission failed because of local congestion,
* do not backoff.
*/