[Bloat] TCP vegas vs TCP cubic

[d@taht.net (Dave Täht)]

On a suggestion from one of the posters to jg's blog, I took a look at
tcp vegas. The results I got were puzzling. 

With tcp cubic, I typically get 71Mbit/sec and all the side effects of
bufferbloat with a single stream.

With vegas turned on, a single stream peaks at around 20Mbit.

10 vegas streams did about 55Mbit in total. 

Can I surmise that TCP cubic is like a dragster, able to go really fast
in one direction down a straightaway, and TCP vegas more like an 80s
model MR2, maneuverable, but underpowered?

The testbed network: http://nex-6.taht.net/images/housenet.png 
The test path: laptop->nano-m->nano-m->openrd 
    (I note that this path almost never exhibits packet loss) 

Most of the machines on the path are running with
minimal txqueues and dma buffers running as low as they can go.

The tests:


With cubic:

$ openrd: iperf -s 
$ laptop: iperf -t 60 -c openrd

With vegas (on both laptop and server)
modprobe tcp_vegas
echo vegas > /proc/sys/net/ipv4/tcp_congestion_control

openrd:$ iperf -s 
laptop:$ iperf -t 60 -c openrd &
laptop:$ ping  

On a failed hunch, I also re-ran the tests with a much larger 
congestion window:

echo /proc/sys/net/core/rmem_max  
echo /proc/sys/net/core/wmem_max  

iperf -w8m -s

To no net difference in effect.

-- 
Dave Taht
http://nex-6.taht.net