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

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Message-ID: <CADVnQympGLkaYVAfac5QkALYCHO2D=h_+yVe3MYwirg9EhapHQ@mail.gmail.com>
Date:	Tue, 6 Aug 2013 09:49:44 -0400
From:	Neal Cardwell <ncardwell@...gle.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>, Van Jacobson <vanj@...gle.com>,
	Yuchung Cheng <ycheng@...gle.com>
Subject: Re: [PATCH] tcp: cubic: fix bug in bictcp_acked()

On Mon, Aug 5, 2013 at 11:05 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> 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>
> Cc: Neal Cardwell <ncardwell@...gle.com>
> Cc: Yuchung Cheng <ycheng@...gle.com>

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

Nice catch!

neal
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