netdev - Re: [PATCH] tcp: cubic: fix bug in bictcp

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Message-Id: <20130807.103702.2279717727976902224.davem@davemloft.net>
Date:	Wed, 07 Aug 2013 10:37:02 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	ncardwell@...gle.com, netdev@...r.kernel.org, vanj@...gle.com,
	ycheng@...gle.com
Subject: Re: [PATCH] tcp: cubic: fix bug in bictcp_acked()

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Mon, 05 Aug 2013 20:05:12 -0700

> From: Eric Dumazet <edumazet@...gle.com>
> 
> While investigating about strange increase of retransmit rates
> on hosts ~24 days after boot, Van found hystart was disabled
> if ca->epoch_start was 0, as following condition is true
> when tcp_time_stamp high order bit is set.
> 
> (s32)(tcp_time_stamp - ca->epoch_start) < HZ
> 
> Quoting Van :
> 
>  At initialization & after every loss ca->epoch_start is set to zero so
>  I believe that the above line will turn off hystart as soon as the 2^31
>  bit is set in tcp_time_stamp & hystart will stay off for 24 days.
>  I think we've observed that cubic's restart is too aggressive without
>  hystart so this might account for the higher drop rate we observe.
> 
> Diagnosed-by: Van Jacobson <vanj@...gle.com>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>

Applied and queued up for -stable, thanks.
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