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

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Message-ID: <20110309095659.1373c296@s6510>
Date:	Wed, 9 Mar 2011 09:56:59 -0800
From:	Stephen Hemminger <shemminger@...tta.com>
To:	Lucas Nussbaum <lucas.nussbaum@...ia.fr>
Cc:	Injong Rhee <rhee@...u.edu>, 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 Wed, 9 Mar 2011 07:53:19 +0100
Lucas Nussbaum <lucas.nussbaum@...ia.fr> wrote:

> 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.


What is the speed and RTT time of your network?
I think you maybe blaming hystart for other issues in the network.
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