netdev - Re: [PATCH] Make CUBIC Hystart more robust to RTT variations

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Date:	Wed, 9 Mar 2011 07:53:19 +0100
From:	Lucas Nussbaum <lucas.nussbaum@...ia.fr>
To:	Injong Rhee <rhee@...u.edu>
Cc:	Stephen Hemminger <shemminger@...tta.com>,
	David Miller <davem@...emloft.net>, xiyou.wangcong@...il.com,
	netdev@...r.kernel.org, sangtae.ha@...il.com
Subject: Re: [PATCH] Make CUBIC Hystart more robust to RTT variations

On 08/03/11 at 20:30 -0500, Injong Rhee wrote:
> Now, both tools can be wrong. But that is not catastrophic since
> congestion avoidance can kick in to save the day. In a pipe where no
> other flows are competing, then exiting slow start too early can
> slow things down as the window can be still too small. But that is
> in fact when delays are most reliable. So those tests that say bad
> performance with hystart are in fact, where hystart is supposed to
> perform well.

Hi,

In my setup, there is no congestion at all (except the buffer bloat).
Without Hystart, transferring 8 Gb of data takes 9s, with CUBIC exiting
slow start at ~2000 packets.
With Hystart, transferring 8 Gb of data takes 19s, with CUBIC exiting
slow start at ~20 packets.
I don't think that this is "hystart performing well". We could just as
well remove slow start completely, and only do congestion avoidance,
then.

While I see the value in Hystart, it's clear that there are some flaws
in the current implementation. It probably makes sense to disable
hystart by default until those problems are fixed.
-- 
| 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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