netdev - Re: [PATCH net-next] tcp: refine pacing rate determination

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Message-ID: <CADVnQymkAxnhA19GGQ9LkhgoQu=2vzE069766x3-3zMvp5-Rwg@mail.gmail.com>
Date:	Fri, 21 Aug 2015 21:22:46 -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>,
	Yuchung Cheng <ycheng@...gle.com>
Subject: Re: [PATCH net-next] tcp: refine pacing rate determination

On Fri, Aug 21, 2015 at 8:38 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> From: Eric Dumazet <edumazet@...gle.com>
>
> When TCP pacing was added back in linux-3.12, we chose
> to apply a fixed ratio of 200 % against current rate,
> to allow probing for optimal throughput even during
> slow start phase, where cwnd can be doubled every other gRTT.
>
> At Google, we found it was better applying a different ratio
> while in Congestion Avoidance phase.
> This ratio was set to 120 %.
>
> We've used the normal tcp_in_slow_start() helper for a while,
> then tuned the condition to select the conservative ratio
> as soon as cwnd >= ssthresh/2 :
>
> - After cwnd reduction, it is safer to ramp up more slowly,
>   as we approach optimal cwnd.
> - Initial ramp up (ssthresh == INFINITY) still allows doubling
>   cwnd every other RTT.
>
> 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>

Looks great to me. Thanks, Eric!

neal
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