netdev - Re: [PATCH net] tcp: avoid multiple ssthresh reductions in on retransmit window

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Date:	Wed, 18 Jun 2014 00:17:32 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Michal Kubecek <mkubecek@...e.cz>
Cc:	Yuchung Cheng <ycheng@...gle.com>,
	Neal Cardwell <ncardwell@...gle.com>,
	"David S. Miller" <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	James Morris <jmorris@...ei.org>,
	Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
	Patrick McHardy <kaber@...sh.net>
Subject: Re: [PATCH net] tcp: avoid multiple ssthresh reductions in on
 retransmit window

On Wed, 2014-06-18 at 00:42 +0200, Michal Kubecek wrote:

> BtW, this is exactly the problem our customer is facing: they have
> relatively fast line (15 Mb/s) but with big buffers so that the
> roundtrip times can rise from unloaded 35 ms up to something like 1.5 s
> under full load.

It looks like some cwnd limiting would be nice ;)

BDP is about 45 packets for a 35ms rtt and 15 Mb/s link.

> 
> What happens is this: cwnd initally rises to ~2100 then first drops
> are encountered, cwnd is set to 1 and ssthresh to ~1050. The slow start
> lets cwnd reach ssthresh but after that, a slow linear growth follows.
> In this state, all in-flight packets are dropped (simulation of what
> happens on router switchover) so that cwnd is reset to 1 again and
> ssthresh to something like 530-550 (cwnd was a bit higher than ssthresh).
> If a packet loss comes shortly after that, cwnd is still very low and
> ssthresh is reduced to half of that cwnd (i.e. much lower than to half
> of ssthresh). If unlucky, one can even end up with ssthresh reduced to 2
> which takes really long to recover from.


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