netdev - Re: [PATCH v7 net-next 2/2] tcp: switch rtt estimations to usec resolution

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Message-ID: <20140226170908.5f028c88@nehalam.linuxnetplumber.net>
Date:	Wed, 26 Feb 2014 17:09:08 -0800
From:	Stephen Hemminger <stephen@...workplumber.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>, Julian Anastasov <ja@....bg>,
	Yuchung Cheng <ycheng@...gle.com>,
	netdev <netdev@...r.kernel.org>,
	Neal Cardwell <ncardwell@...gle.com>,
	Larry Brakmo <brakmo@...gle.com>
Subject: Re: [PATCH v7 net-next 2/2] tcp: switch rtt estimations to usec
 resolution

On Wed, 26 Feb 2014 14:02:48 -0800
Eric Dumazet <eric.dumazet@...il.com> wrote:

> From: Eric Dumazet <edumazet@...gle.com>
> 
> 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@...gle.com>
> Acked-by: Neal Cardwell <ncardwell@...gle.com>
> Cc: Stephen Hemminger <stephen@...workplumber.org>
> Cc: Yuchung Cheng <ycheng@...gle.com>
> Cc: Larry Brakmo <brakmo@...gle.com>
> Cc: Julian Anastasov <ja@....bg>
> ---

Since srtt is scaled by 8 and is 32 bits, This does reduce the maximum
possible RTT to 8 minutes or so. Which is fine, just wanted it to be
noted.
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