netdev - Re: [PATCH 0/6] TCP CUBIC and Hystart

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Date:	Fri, 11 Mar 2011 17:08:09 +0100
From:	Lucas Nussbaum <lucas.nussbaum@...ia.fr>
To:	Sangtae Ha <sangtae.ha@...il.com>
Cc:	Stephen Hemminger <shemminger@...tta.com>, davem@...emloft.net,
	rhee@...u.edu, netdev@...r.kernel.org
Subject: Re: [PATCH 0/6] TCP CUBIC and Hystart

On 11/03/11 at 10:58 -0500, Sangtae Ha wrote:
> Hi Lucas,
> 
> From your setup, ca->delay_min is 90 and this means the one-way delay
> is 90 >> 4 (5ms).
> And our gap detection threshold is 2ms, which means that if the gap is
> loosely spread over 5ms with delayed ACKs, it can early terminate the
> slow start. But, given the optimal cwnd is 941 in your setup, exiting
> slow start one RTT before the loss (half of the optimal cwnd) is what
> hystart does.
> 
> Since the resolution is now ms, can you change the gap detection to
> 1ms and run it again?
> Also, the following change you did doesn't hurt (1ms more train to
> detect the ACK train).

Hi,

Changing it to 1ms only improves the situation marginally.
[25271.861481] found ACK TRAIN: cwnd=835 now=2779244747 ca->last_ack=2779244747 ca->round_start=2779244741 ca->delay_min=90 delay_min>>4=5 nbacks=261
[25291.507340] found ACK TRAIN: cwnd=585 now=2779264393 ca->last_ack=2779264393 ca->round_start=2779264387 ca->delay_min=90 delay_min>>4=5 nbacks=221
[25327.585396] found ACK TRAIN: cwnd=1034 now=2779300471 ca->last_ack=2779300471 ca->round_start=2779300465 ca->delay_min=90 delay_min>>4=5 nbacks=245
[25347.300351] found ACK TRAIN: cwnd=1463 now=2779320186 ca->last_ack=2779320186 ca->round_start=2779320180 ca->delay_min=90 delay_min>>4=5 nbacks=427
[25390.702328] found ACK TRAIN: cwnd=587 now=2779363588 ca->last_ack=2779363588 ca->round_start=2779363582 ca->delay_min=90 delay_min>>4=5 nbacks=211
[25394.775396] found ACK TRAIN: cwnd=588 now=2779367661 ca->last_ack=2779367661 ca->round_start=2779367655 ca->delay_min=90 delay_min>>4=5 nbacks=242
[25402.061328] found ACK TRAIN: cwnd=1282 now=2779374947 ca->last_ack=2779374947 ca->round_start=2779374941 ca->delay_min=90 delay_min>>4=5 nbacks=335
[25404.894336] found ACK TRAIN: cwnd=585 now=2779377780 ca->last_ack=2779377780 ca->round_start=2779377774 ca->delay_min=90 delay_min>>4=5 nbacks=205
[25408.584337] found ACK TRAIN: cwnd=587 now=2779381470 ca->last_ack=2779381470 ca->round_start=2779381464 ca->delay_min=90 delay_min>>4=5 nbacks=209
[25421.699331] found ACK TRAIN: cwnd=856 now=2779394585 ca->last_ack=2779394585 ca->round_start=2779394579 ca->delay_min=90 delay_min>>4=5 nbacks=239

There are still some cases when ack trains are detected too early.
-- 
| Lucas Nussbaum             MCF Université Nancy 2 |
| lucas.nussbaum@...ia.fr         LORIA / AlGorille |
| http://www.loria.fr/~lnussbau/  +33 3 54 95 86 19 |
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