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

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Date:	Wed, 9 Mar 2011 19:25:05 +0100
From:	Lucas Nussbaum <lucas.nussbaum@...ia.fr>
To:	Stephen Hemminger <shemminger@...tta.com>
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 09/03/11 at 09:56 -0800, Stephen Hemminger wrote:
> 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.

What kind of issues?

Host1 is connected through a gigabit ethernet LAN to Router1
Host2 is connected through a gigabit ethernet LAN to Router2
Router1 and Router2 are connected through an experimentation network at
10 Gb/s
RTT between Host1 and Host2 is 11.3ms.
The network is not congested.

(I can provide access to the testbed if someone wants to do further
testing)
-- 
| 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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