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tcp: switch rtt estimations to usec resolution

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Upcoming congestion controls for TCP require usec resolution for RTT
estimations. Millisecond resolution is simply not enough these days.

FQ/pacing in DC environments also require this change for finer control
and removal of bimodal behavior due to the current hack in
tcp_update_pacing_rate() for 'small rtt'

TCP_CONG_RTT_STAMP is no longer needed.

As Julian Anastasov pointed out, we need to keep user compatibility :
tcp_metrics used to export RTT and RTTVAR in msec resolution,
so we added RTT_US and RTTVAR_US. An iproute2 patch is needed
to use the new attributes if provided by the kernel.

In this example ss command displays a srtt of 32 usecs (10Gbit link)

lpk51:~# ./ss -i dst lpk52
Netid  State      Recv-Q Send-Q   Local Address:Port       Peer
Address:Port
tcp    ESTAB      0      1         10.246.11.51:42959
10.246.11.52:64614
         cubic wscale:6,6 rto:201 rtt:0.032/0.001 ato:40 mss:1448
cwnd:10 send
3620.0Mbps pacing_rate 7240.0Mbps unacked:1 rcv_rtt:993 rcv_space:29559

Updated iproute2 ip command displays :

lpk51:~# ./ip tcp_metrics | grep 10.246.11.52
10.246.11.52 age 561.914sec cwnd 10 rtt 274us rttvar 213us source
10.246.11.51

Old binary displays :

lpk51:~# ip tcp_metrics | grep 10.246.11.52
10.246.11.52 age 561.914sec cwnd 10 rtt 250us rttvar 125us source
10.246.11.51

With help from Julian Anastasov, Stephen Hemminger and Yuchung Cheng

Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Cc: Stephen Hemminger <stephen@networkplumber.org>
Cc: Yuchung Cheng <ycheng@google.com>
Cc: Larry Brakmo <brakmo@google.com>
Cc: Julian Anastasov <ja@ssi.bg>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Eric Dumazet 2014-02-26 14:02:48 -08:00 • committed by David S. Miller
commit 740b0f1841
17 changed files with 175 additions and 170 deletions
Download patch file Download diff file Expand all files Collapse all files

7
include/uapi/linux/tcp_metrics.h
View file

@ -11,12 +11,15 @@
#define TCP_METRICS_GENL_VERSION 0x1
enum tcp_metric_index {
TCP_METRIC_RTT,
TCP_METRIC_RTTVAR,
TCP_METRIC_RTT, /* in ms units */
TCP_METRIC_RTTVAR, /* in ms units */
TCP_METRIC_SSTHRESH,
TCP_METRIC_CWND,
TCP_METRIC_REORDERING,
TCP_METRIC_RTT_US, /* in usec units */
TCP_METRIC_RTTVAR_US, /* in usec units */
/* Always last. */
__TCP_METRIC_MAX,
};