• Eric Dumazet's avatar
    tcp: switch rtt estimations to usec resolution · 740b0f18
    Eric Dumazet authored
    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
    tcp    ESTAB      0      1
             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 age 561.914sec cwnd 10 rtt 274us rttvar 213us source
    Old binary displays :
    lpk51:~# ip tcp_metrics | grep age 561.914sec cwnd 10 rtt 250us rttvar 125us source
    With help from Julian Anastasov, Stephen Hemminger and Yuchung Cheng
    Signed-off-by: default avatarEric Dumazet <edumazet@google.com>
    Acked-by: default avatarNeal 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: default avatarDavid S. Miller <davem@davemloft.net>
tcp_yeah.c 6.91 KB