diff options
| author |   Yuchung Cheng <ycheng@google.com> | 2012-05-02 13:30:03 +0000 |
|---|---|---|
| committer |   David S. Miller <davem@davemloft.net> | 2012-05-02 20:56:10 -0400 |
| commit | eed530b6c67624db3f2cf477bac7c4d005d8f7ba (patch) | |
| tree | c07096807ead2adb9d85e85d1a9cd1ada85755ac /include | |
| parent | tcp: early retransmit: tcp_enter_recovery() (diff) | |
| download | linux-dev-eed530b6c67624db3f2cf477bac7c4d005d8f7ba.tar.xz linux-dev-eed530b6c67624db3f2cf477bac7c4d005d8f7ba.zip | |
tcp: early retransmit
This patch implements RFC 5827 early retransmit (ER) for TCP.
It reduces DUPACK threshold (dupthresh) if outstanding packets are
less than 4 to recover losses by fast recovery instead of timeout.
While the algorithm is simple, small but frequent network reordering
makes this feature dangerous: the connection repeatedly enter
false recovery and degrade performance. Therefore we implement
a mitigation suggested in the appendix of the RFC that delays
entering fast recovery by a small interval, i.e., RTT/4. Currently
ER is conservative and is disabled for the rest of the connection
after the first reordering event. A large scale web server
experiment on the performance impact of ER is summarized in
section 6 of the paper "Proportional Rate Reduction for TCP”,
IMC 2011. http://conferences.sigcomm.org/imc/2011/docs/p155.pdf
Note that Linux has a similar feature called THIN_DUPACK. The
differences are THIN_DUPACK do not mitigate reorderings and is only
used after slow start. Currently ER is disabled if THIN_DUPACK is
enabled. I would be happy to merge THIN_DUPACK feature with ER if
people think it's a good idea.
ER is enabled by sysctl_tcp_early_retrans:
0: Disables ER
1: Reduce dupthresh to packets_out - 1 when outstanding packets < 4.
2: (Default) reduce dupthresh like mode 1. In addition, delay
entering fast recovery by RTT/4.
Note: mode 2 is implemented in the third part of this patch series.
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Diffstat (limited to 'include')
| -rw-r--r-- | include/linux/tcp.h | 1 | ||||
| -rw-r--r-- | include/net/tcp.h | 15 |
2 files changed, 16 insertions, 0 deletions
diff --git a/include/linux/tcp.h b/include/linux/tcp.h index 278af9ea42d4..7859b416d46e 100644 --- a/include/linux/tcp.h +++ b/include/linux/tcp.h @@ -372,6 +372,7 @@ struct tcp_sock { repair : 1, unused : 1; u8 repair_queue; + u8 do_early_retrans:1;/* Enable RFC5827 early-retransmit */ /* RTT measurement */ u32 srtt; /* smoothed round trip time << 3 */ diff --git a/include/net/tcp.h b/include/net/tcp.h index 0fb84de6da36..685437a16c97 100644 --- a/include/net/tcp.h +++ b/include/net/tcp.h @@ -252,6 +252,7 @@ extern int sysctl_tcp_max_ssthresh; extern int sysctl_tcp_cookie_size; extern int sysctl_tcp_thin_linear_timeouts; extern int sysctl_tcp_thin_dupack; +extern int sysctl_tcp_early_retrans; extern atomic_long_t tcp_memory_allocated; extern struct percpu_counter tcp_sockets_allocated; @@ -797,6 +798,20 @@ static inline void tcp_enable_fack(struct tcp_sock *tp) tp->rx_opt.sack_ok |= TCP_FACK_ENABLED; } +/* TCP early-retransmit (ER) is similar to but more conservative than + * the thin-dupack feature. Enable ER only if thin-dupack is disabled. + */ +static inline void tcp_enable_early_retrans(struct tcp_sock *tp) +{ + tp->do_early_retrans = sysctl_tcp_early_retrans && + !sysctl_tcp_thin_dupack && sysctl_tcp_reordering == 3; +} + +static inline void tcp_disable_early_retrans(struct tcp_sock *tp) +{ + tp->do_early_retrans = 0; +} + static inline unsigned int tcp_left_out(const struct tcp_sock *tp) { return tp->sacked_out + tp->lost_out; |