netdev - Re: [PATCH v2] tcp: reduce the bloat caused by tcp_is_cwnd

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Message-Id: <20140224.191423.1857998272406050896.davem@davemloft.net>
Date:	Mon, 24 Feb 2014 19:14:23 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	netdev@...r.kernel.org, ycheng@...gle.com, ncardwell@...gle.com,
	nanditad@...gle.com, vanj@...gle.com
Subject: Re: [PATCH v2] tcp: reduce the bloat caused by
 tcp_is_cwnd_limited()

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Sat, 22 Feb 2014 22:25:57 -0800

> From: Eric Dumazet <edumazet@...gle.com>
> 
> tcp_is_cwnd_limited() allows GSO/TSO enabled flows to increase
> their cwnd to allow a full size (64KB) TSO packet to be sent.
> 
> Non GSO flows only allow an extra room of 3 MSS.
> 
> For most flows with a BDP below 10 MSS, this results in a bloat
> of cwnd reaching 90, and an inflate of RTT.
> 
> Thanks to TSO auto sizing, we can restrict the bloat to the number
> of MSS contained in a TSO packet (tp->xmit_size_goal_segs), to keep
> original intent without performance impact.
> 
> Because we keep cwnd small, it helps to keep TSO packet size to their
> optimal value.
> 
> Example for a 10Mbit flow, with low TCP Small queue limits (no more than
> 2 skb in qdisc/device tx ring)
> 
> Before patch :
> 
> lpk51:~# ./ss -i dst lpk52:44862 | grep cwnd
>          cubic wscale:6,6 rto:215 rtt:15.875/2.5 mss:1448 cwnd:96
> ssthresh:96
> send 70.1Mbps unacked:14 rcv_space:29200
> 
> After patch :
> 
> lpk51:~# ./ss -i dst lpk52:52916 | grep cwnd
>          cubic wscale:6,6 rto:206 rtt:5.206/0.036 mss:1448 cwnd:15
> ssthresh:14
> send 33.4Mbps unacked:4 rcv_space:29200
> 
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>

Applied, thanks Eric.
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