netdev - Re: [PATCH 3/3] tcp_cubic: fix low utilization of CUBIC with HyStart

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Message-ID: <AANLkTinDWY2ZWE-y=OW1_-tbZFkuF73JP2fTfoj0icBt@mail.gmail.com>
Date:	Mon, 14 Mar 2011 03:36:24 -0400
From:	Sangtae Ha <sangtae.ha@...il.com>
To:	Lucas Nussbaum <lucas.nussbaum@...ia.fr>
Cc:	Stephen Hemminger <stephen.hemminger@...tta.com>,
	David Miller <davem@...emloft.net>,
	Injong Rhee <rhee@...u.edu>,
	Bill Fink <billfink@...dspring.com>, netdev@...r.kernel.org
Subject: Re: [PATCH 3/3] tcp_cubic: fix low utilization of CUBIC with HyStart

On Mon, Mar 14, 2011 at 2:28 AM, Lucas Nussbaum <lucas.nussbaum@...ia.fr> wrote:
> On 13/03/11 at 19:07 -0500, Sangtae Ha wrote:
>> Hi Lucas,
>>
>> Thank you for reporting this problem and the results below show what
>> happened with low resolution clocks.
>> The detailed testing setup and results at
>> http://netsrv.csc.ncsu.edu/wiki/index.php/HyStart_testing_between_Princeton_and_NCSU
>> I will add the results in long rtt paths soon.
>
> Thanks. What do you use to generate those graphs?

I used net/ipv4/tcp_probe.c and gnuplot to generate the graphs.

>
> The graphs from HyStart(fixed) are a bit strange. Did HyStart really
> detect something? It seems that it exited slow start due to losses, not
> due to detection, since cwnd is decreased.

You can see green points which indicate that ssthresh is set before the loss,
and right after that, the cwnd increases 20% per RTT since the
available bandwidth is still unknown.
This logic incurs far less number of losses compared to slow start and
hence more throughput.

> --
> | Lucas Nussbaum             MCF Université Nancy 2 |
> | lucas.nussbaum@...ia.fr         LORIA / AlGorille |
> | http://www.loria.fr/~lnussbau/  +33 3 54 95 86 19 |
>

Sangtae
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