netdev - Re: [PATCH] tcp: remove 1ms offset in srtt computation

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Date:	Thu, 6 Feb 2014 20:08:02 -0500
From:	Neal Cardwell <ncardwell@...gle.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	Yuchung Cheng <ycheng@...gle.com>,
	Vytautas Valancius <valas@...gle.com>
Subject: Re: [PATCH] tcp: remove 1ms offset in srtt computation

On Thu, Feb 6, 2014 at 6:57 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> From: Eric Dumazet <edumazet@...gle.com>
>
> TCP pacing depends on an accurate srtt estimation.
>
> Current srtt estimation is using jiffie resolution,
> and has an artificial offset of at least 1 ms, which can produce
> slowdowns when FQ/pacing is used, especially in DC world,
> where typical rtt is below 1 ms.
>
> We are planning a switch to usec resolution for linux-3.15,
> but in the meantime, this patch removes the 1 ms offset.
>
> All we need is to have tp->srtt minimal value of 1 to differentiate
> the case of srtt being initialized or not, not 8.
>
> The problematic behavior was observed on a 40Gbit testbed,
> where 32 concurrent netperf were reaching 12Gbps of aggregate
> speed, instead of line speed.
>
> This patch also has the effect of reporting more accurate srtt and send
> rates to iproute2 ss command as in :
>
> $ ss -i dst cca2
> Netid  State      Recv-Q Send-Q          Local Address:Port
> Peer Address:Port
> tcp    ESTAB      0      0                10.244.129.1:56984
> 10.244.129.2:12865
>          cubic wscale:6,6 rto:200 rtt:0.25/0.25 ato:40 mss:1448 cwnd:10 send
> 463.4Mbps rcv_rtt:1 rcv_space:29200
> tcp    ESTAB      0      390960           10.244.129.1:60247
> 10.244.129.2:50204
>          cubic wscale:6,6 rto:200 rtt:0.875/0.75 mss:1448 cwnd:73 ssthresh:51
> send 966.4Mbps unacked:73 retrans:0/121 rcv_space:29200
>
> Reported-by: Vytautas Valancius <valas@...gle.com>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Cc: Yuchung Cheng <ycheng@...gle.com>
> Cc: Neal Cardwell <ncardwell@...gle.com>
> ---
>  net/ipv4/tcp_input.c  |   18 ++++++++++--------
>  net/ipv4/tcp_output.c |    2 +-
>  2 files changed, 11 insertions(+), 9 deletions(-)

Acked-by: Neal Cardwell <ncardwell@...gle.com>

Thanks, Eric!

neal
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