TCP Throughput

felix001 · 10-21-2011, 07:33 AM

Hi,

Ive been playing around and reading up on TCP throughput.
Theres a number of article on the web that refer to the following to calculate the maximum TCP throughput per flow.

RCV buffer size / RTT = Max TCP throughput = ? bps

The example I have is whilst downloading a file from speedtest.
Based on the example above I get a total of 5.68Mbps per tcp flow.

But when i perform a download of a file I get 11.23Mb/s.

Code:

[root@webserver1 ~]# echo "scale=2; `curl  --progress-bar -w "%{speed_download}" http://speedtest.wdc01.softlayer.com/downloads/test10.zip -o test.zip` / 131072" | bc | xargs -I {} echo {}Mb\/s
######################################################################## 100.0%
11.23Mb/s

I was thinking that it was opening multiple connections but I can see only 1 TCP socket open for this connection. Has anyone got any ideas ?

Thanks,

baldy3105 · 10-24-2011, 10:26 AM

Obviously the meaasured value is the correct one so the problem must be with your calculations. If you show us how you worked it out we could probably help you.

P.S. how did you find out what rxbuff size was? This may well be different dependant on all sorts of factors, type of app, method of accessing the network card driver, linux distro etc . I would be using iperf to investigate this sort of thing because you can set all of the transmission variables yourself and tinker with them one at a time.

felix001 · 10-24-2011, 03:36 PM

I think you were right about the rx buffer size but the results still do not complete match,
Ive shown all the steps below used to calculate the total.....

Code:

[root@webserver1 ~]# ping -c 2 speedtest.wdc01.softlayer.com
PING speedtest.wdc01.softlayer.com (208.43.102.250) 56(84) bytes of data.
64 bytes from speedtest.wdc01.softlayer.com (208.43.102.250): icmp_seq=1 ttl=42 time=102 ms
64 bytes from speedtest.wdc01.softlayer.com (208.43.102.250): icmp_seq=2 ttl=42 time=102 ms

--- speedtest.wdc01.softlayer.com ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 999ms
rtt min/avg/max/mdev = 102.134/102.161/102.188/0.027 ms
[root@webserver1 ~]# cat /proc/sys/net/core/rmem_max
129024
[root@webserver1 ~]#  speed=$(echo "(( 129024 * 8 ) / 0.102 ) / 1000000" | bc) ; echo "${speed}"Mbp\/s
10Mbp/s

Code:

[root@webserver1 ~]# echo "scale=2; `curl  --progress-bar -w "%{speed_download}" http://speedtest.wdc01.softlayer.com/downloads/test10.zip -o test.zip` / 131072" | bc | xargs -I {} echo {}Mb\/s
######################################################################## 100.0%
14.27Mb/s

Thanks for all your help....

baldy3105 · 10-30-2011, 09:05 AM

Its possible that your RTT as measured by ping is not entirely representative of the latency for TCP. For instance if the transfer is coming via a DMA transfer from HD controller to NIC card with all the relevant TCP offloading, RTT may be lower than for a ping which has to be dealt with by a busy processor, as an example.

If you wireshark the transfer you can get it to tell you what the average round-trip time for window/ack is which may be a bit lower than your ping response.