netdev - Bad performance when CUBIC HyStart enabled

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Date:	Thu, 3 Mar 2011 15:22:29 +0100
From:	Lucas Nussbaum <lucas.nussbaum@...ia.fr>
To:	netdev@...r.kernel.org
Cc:	Sangtae Ha <sha2@...u.edu>
Subject: Bad performance when CUBIC HyStart enabled

Hi,

I'm experiencing bad performance transferring data over a large BDP
network with CUBIC when HyStart is enabled. Disabling HyStart fixes the
problem.

Test setup:
I'm transfering a 2-GB file between two machines using netcat. On both
machines, /dev/shm is used to store the data.
On the sending side, I use:
/dev/shm# while true; do dd if=foo bs=10M | nc -l -p 2222 -q 0; done
On the receiving side, I use:
/dev/shm# /usr/bin/time nc 131.254.203.6 2222 > foo
Both machines are on Gigabit ethernet networks, and the RTT between them
is 11.3ms.
The default sysctl values are used.
I use nload -i 1000000 -o 1000000 -u m eth0 to visualize the transfer
rate.

With a 2.6.26 kernel, everything is fine. The bandwidth quickly gets to
~940 Mbps, and I get the following in nload:
      ..................................
     .##################################
     ###################################
     ###################################
     ###################################
     ####################################
     ####################################
     ####################################
     ####################################
     ####################################
     ####################################
     ####################################
     ####################################
     ####################################
The transfer time is 18.2s, which is reasonable for a 2-GB file.

With a 2.6.32 kernel (Debian's) or with a 2.6.37 kernel (on the sending
side only), with the default HyStart parameters, I get the following:
                           ..........................
                          |##########################.
                         |############################
                        |#############################
                       |##############################
                      |###############################
                    .#################################
                   |##################################| 
                 |##################################### 
            ..||####################################### 
    ....|############################################## 
 .##################################################### 
The transfer time is 26.8s. (There's one character every 0.5s on the
graph above)

Disabling hystart on the sending machine with:
echo 0 > /sys/module/tcp_cubic/parameters/hystart
Fixes the problem.

It seems that waiting for some time (or rebooting) fixes the problem for
some time, so I'm suspecting that the problem is caused by a cache of
some sort. However, doing:
echo 1 > /proc/sys/net/ipv4/route/flush
doesn't fix the problem.

Ideas? I could arrange to give access to the testbed if needed.
(https://www.grid5000.fr/)
-- 
| 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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