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

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Message-ID: <4D764AAC.30302@ncsu.edu>
Date:	Tue, 08 Mar 2011 10:26:36 -0500
From:	Injong Rhee <rhee@...u.edu>
To:	Lucas Nussbaum <lucas.nussbaum@...ia.fr>
CC:	WANG Cong <xiyou.wangcong@...il.com>, netdev@...r.kernel.org
Subject: Re: [PATCH] Make CUBIC Hystart more robust to RTT variations

Thanks for updating CUBIC hystart. You might want to test the
cases with more background traffic and verify whether this
threshold is too conservative.

On 3/8/11 6:10 AM, Lucas Nussbaum wrote:
> CUBIC Hystart uses two heuristics to exit slow start earlier, before
> losses start to occur. Unfortunately, it tends to exit slow start far
> too early, causing poor performance since convergence to the optimal
> cwnd is then very slow. This was reported in
> http://permalink.gmane.org/gmane.linux.network/188169 and
> https://partner-bugzilla.redhat.com/show_bug.cgi?id=616985
>
> I am using an experimental testbed (http://www.grid5000.fr/) with two
> machines connected using Gigabit ethernet to a dedicated 10-Gb backbone.
> RTT between both machines is 11.3ms. Using TCP CUBIC without Hystart,
> cwnd grows to ~2200.  With Hystart enabled, CUBIC exits slow start with
> cwnd lower than 100, and often lower than 20, which leads to the poor
> performance that I reported.
>
> After instrumenting TCP CUBIC, I found out that the segment-to-ack RTT
> tends to vary quite a lot even when the network is not congested, due to
> several factors including the fact that TCP sends packet in burst (so
> the packets are queued locally before being sent, increasing their RTT),
> and delayed ACKs on the destination host.
>
> The patch below increases the thresholds used by the two Hystart
> heuristics. First, the length of an ACK train needs to reach 2*minRTT.
> Second, the max RTT of a group of packets also needs to reach 2*minRTT.
> In my setup, this causes Hystart to exit slow start when cwnd is in the
> 1900-2000 range using the ACK train heuristics, and sometimes to exit in
> the 700-900 range using the delay increase heuristic, dramatically
> improving performance.
>
> I could provide access to my testbed if someone wants to do further
> experiments.
>
> Signed-off-by: Lucas Nussbaum<lucas.nussbaum@...ia.fr>

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