netdev - Re: [PATCH net 2/2] tcp_cubic: better follow cubic curve after idle period

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Message-Id: <20150910.105906.1350790432465932740.davem@davemloft.net>
Date:	Thu, 10 Sep 2015 10:59:06 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	netdev@...r.kernel.org, ycheng@...gle.com, ncardwell@...gle.com,
	jri@...gle.com, stephen@...workplumber.org, sangtae.ha@...il.com,
	lawrence@...kmo.org
Subject: Re: [PATCH net 2/2] tcp_cubic: better follow cubic curve after
 idle period

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Wed, 09 Sep 2015 21:55:07 -0700

> From: Eric Dumazet <edumazet@...gle.com>
> 
> Jana Iyengar found an interesting issue on CUBIC :
> 
> The epoch is only updated/reset initially and when experiencing losses.
> The delta "t" of now - epoch_start can be arbitrary large after app idle
> as well as the bic_target. Consequentially the slope (inverse of
> ca->cnt) would be really large, and eventually ca->cnt would be
> lower-bounded in the end to 2 to have delayed-ACK slow-start behavior.
> 
> This particularly shows up when slow_start_after_idle is disabled
> as a dangerous cwnd inflation (1.5 x RTT) after few seconds of idle
> time.
> 
> Jana initial fix was to reset epoch_start if app limited,
> but Neal pointed out it would ask the CUBIC algorithm to recalculate the
> curve so that we again start growing steeply upward from where cwnd is
> now (as CUBIC does just after a loss). Ideally we'd want the cwnd growth
> curve to be the same shape, just shifted later in time by the amount of
> the idle period.
> 
> Reported-by: Jana Iyengar <jri@...gle.com>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Signed-off-by: Yuchung Cheng <ycheng@...gle.com>
> Signed-off-by: Neal Cardwell <ncardwell@...gle.com>

Applied.
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