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

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Date:	Wed, 9 Mar 2011 22:33:56 +0100
From:	Lucas Nussbaum <lucas.nussbaum@...ia.fr>
To:	Yuchung Cheng <ycheng@...gle.com>
Cc:	Stephen Hemminger <shemminger@...tta.com>,
	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 13:12 -0800, Yuchung Cheng wrote:
> On Wed, Mar 9, 2011 at 12:01 PM, Stephen Hemminger
> <shemminger@...tta.com> wrote:
> > On Wed, 9 Mar 2011 19:25:05 +0100
> > Lucas Nussbaum <lucas.nussbaum@...ia.fr> wrote:
> >
> >> 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.
> >
> > By my calculations (1G * 11.3ms) gives BDP of 941 packets which means
> > CUBIC would ideally exit slow start at 900 or so packets. Old CUBIC
> > slowstrart of 2000 packets means there is huge overshoot which means
> > large packet loss burst which would cause a large CPU load on receiver
> > processing SACK.
> It's not clear from Lucas's report that the hystart is exiting when
> cwnd=2000 or when sender has sent 2000 packets.
> Lucas could you clarify?

When cwnd is around 2000.
-- 
| 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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