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

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Message-ID: <AANLkTimpLWoSB2H9PObxwK1iGkFBWQZMw=1kTP9BA7qb@mail.gmail.com>
Date:	Wed, 9 Mar 2011 13:12:03 -0800
From:	Yuchung Cheng <ycheng@...gle.com>
To:	Stephen Hemminger <shemminger@...tta.com>
Cc:	Lucas Nussbaum <lucas.nussbaum@...ia.fr>,
	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, 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?

>
> I assume you haven't done anything that would disable RFC1323
> support like turn off window scaling or tcp timestamps.
>
>
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