Using iPerf to Troubleshoot Speed/Throughput Issues

December 29, 2011

Two of the most common network characteristics we look at when investigating network-related concerns in the NOC are speed and throughput. You may have experienced the following scenario yourself: You just provisioned a new bad-boy server with a gigabit connection in a data center on the opposite side of the globe. You begin to upload your data and to your shock, you see "Time Remaining: 10 Hours." "What's wrong with the network?" you wonder. The traceroute and MTR look fine, but where's the performance and bandwidth I'm paying for?

This issue is all too common and it has nothing to do with the network, but in fact, the culprits are none other than TCP and the laws of physics.

In data transmission, TCP sends a certain amount of data then pauses. To ensure proper delivery of data, it doesn't send more until it receives an acknowledgement from the remote host that all data was received. This is called the "TCP Window." Data travels at the speed of light, and typically, most hosts are fairly close together. This "windowing" happens so fast we don't even notice it. But as the distance between two hosts increases, the speed of light remains constant. Thus, the further away the two hosts, the longer it takes for the sender to receive the acknowledgement from the remote host, reducing overall throughput. This effect is called "Bandwidth Delay Product," or BDP.

We can overcome BDP to some degree by sending more data at a time. We do this by adjusting the "TCP Window" – telling TCP to send more data per flow than the default parameters. Each OS is different and the default values will vary, but most all operating systems allow tweaking of the TCP stack and/or using parallel data streams. So what is iPerf and how does it fit into all of this?

What is iPerf?

iPerf is simple, open-source, command-line, network diagnostic tool that can run on Linux, BSD, or Windows platforms which you install on two endpoints. One side runs in a 'server' mode listening for requests; the other end runs 'client' mode that sends data. When activated, it tries to send as much data down your pipe as it can, spitting out transfer statistics as it does. What's so cool about iPerf is you can test in real time any number of TCP window settings, even using parallel streams. There's even a Java based GUI you can install that runs on top of it called, JPerf (JPerf is beyond the scope of this article, but I recommend looking into it). What's even cooler is that because iPerf resides in memory, there are no files to clean up.

How do I use iPerf?

iPerf can be quickly downloaded from SourceForge to be installed. It uses port 5001 by default, and the bandwidth it displays is from the client to the server. Each test runs for 10 seconds by default, but virtually every setting is adjustable. Once installed, simply bring up the command line on both of the hosts and run these commands.

On the server side:
iperf -s

On the client side:
iperf -c [server_ip]

The output on the client side will look like this:

#iperf -c 10.10.10.5
------------------------------------------------------------
Client connecting to 10.10.10.5, TCP port 5001
TCP window size: 16.0 KByte (default)
------------------------------------------------------------
[  3] local 0.0.0.0 port 46956 connected with 168.192.1.10 port 5001
[ ID] Interval       Transfer     Bandwidth
[  3]  0.0- 10.0 sec  10.0 MBytes  1.00 Mbits/sec

There are a lot of things we can do to make this output better with more meaningful data. For example, let's say we want the test to run for 20 seconds instead of 10 (-t 20), and we want to display transfer data every 2 seconds instead of the default of 10 (-i 2), and we want to test on port 8000 instead of 5001 (-p 8000). For the purposes of this exercise, let's use those customization as our baseline. This is what the command string would look like on both ends:

Client Side:

#iperf -c 10.10.10.5 -p 8000 -t 20 -i 2
------------------------------------------------------------
Client connecting to 10.10.10.5, TCP port 8000
TCP window size: 16.0 KByte (default)
------------------------------------------------------------
[  3] local 10.10.10.10 port 46956 connected with 10.10.10.5 port 8000
[ ID] Interval       Transfer     Bandwidth
[  3]  0.0- 2.0 sec  6.00 MBytes  25.2 Mbits/sec
[  3]  2.0- 4.0 sec  7.12 MBytes  29.9 Mbits/sec
[  3]  4.0- 6.0 sec  7.00 MBytes  29.4 Mbits/sec
[  3]  6.0- 8.0 sec  7.12 MBytes  29.9 Mbits/sec
[  3]  8.0-10.0 sec  7.25 MBytes  30.4 Mbits/sec
[  3] 10.0-12.0 sec  7.00 MBytes  29.4 Mbits/sec
[  3] 12.0-14.0 sec  7.12 MBytes  29.9 Mbits/sec
[  3] 14.0-16.0 sec  7.25 MBytes  30.4 Mbits/sec
[  3] 16.0-18.0 sec  6.88 MBytes  28.8 Mbits/sec
[  3] 18.0-20.0 sec  7.25 MBytes  30.4 Mbits/sec
[  3]  0.0-20.0 sec  70.1 MBytes  29.4 Mbits/sec

Server Side:

#iperf -s -p 8000 -i 2
------------------------------------------------------------
Server listening on TCP port 8000
TCP window size: 8.00 KByte (default)
------------------------------------------------------------
[852] local 10.10.10.5 port 8000 connected with 10.10.10.10 port 58316
[ ID] Interval Transfer Bandwidth
[  4]  0.0- 2.0 sec  6.05 MBytes  25.4 Mbits/sec
[  4]  2.0- 4.0 sec  7.19 MBytes  30.1 Mbits/sec
[  4]  4.0- 6.0 sec  6.94 MBytes  29.1 Mbits/sec
[  4]  6.0- 8.0 sec  7.19 MBytes  30.2 Mbits/sec
[  4]  8.0-10.0 sec  7.19 MBytes  30.1 Mbits/sec
[  4] 10.0-12.0 sec  6.95 MBytes  29.1 Mbits/sec
[  4] 12.0-14.0 sec  7.19 MBytes  30.2 Mbits/sec
[  4] 14.0-16.0 sec  7.19 MBytes  30.2 Mbits/sec
[  4] 16.0-18.0 sec  6.95 MBytes  29.1 Mbits/sec
[  4] 18.0-20.0 sec  7.19 MBytes  30.1 Mbits/sec
[  4]  0.0-20.0 sec  70.1 MBytes  29.4 Mbits/sec

There are many, many other parameters you can set that are beyond the scope of this article, but for our purposes, the main use is to prove out our bandwidth. This is where we'll use the TCP window options and parallel streams. To set a new TCP window you use the -w switch and you can set the parallel streams by using -P.

Increased TCP window commands:

Server side:
#iperf -s -w 1024k -i 2

Client side:
#iperf -i 2 -t 20 -c 10.10.10.5 -w 1024k

And here are the iperf results from two Softlayer file servers – one in Washington, D.C., acting as Client, the other in Seattle acting as Server:

Client Side:

# iperf -i 2 -t 20 -c 10.10.10.5 -p 8000 -w 1024k
------------------------------------------------------------
Client connecting to 10.10.10.5, TCP port 8000
TCP window size: 1.00 MByte (WARNING: requested 1.00 MByte)
------------------------------------------------------------
[  3] local 10.10.10.10 port 53903 connected with 10.10.10.5 port 5001
[ ID] Interval       Transfer     Bandwidth
[  3]  0.0- 2.0 sec  25.9 MBytes   109 Mbits/sec
[  3]  2.0- 4.0 sec  28.5 MBytes   120 Mbits/sec
[  3]  4.0- 6.0 sec  28.4 MBytes   119 Mbits/sec
[  3]  6.0- 8.0 sec  28.9 MBytes   121 Mbits/sec
[  3]  8.0-10.0 sec  28.0 MBytes   117 Mbits/sec
[  3] 10.0-12.0 sec  29.0 MBytes   122 Mbits/sec
[  3] 12.0-14.0 sec  28.0 MBytes   117 Mbits/sec
[  3] 14.0-16.0 sec  29.0 MBytes   122 Mbits/sec
[  3] 16.0-18.0 sec  27.9 MBytes   117 Mbits/sec
[  3] 18.0-20.0 sec  29.0 MBytes   122 Mbits/sec
[  3]  0.0-20.0 sec   283 MBytes   118 Mbits/sec

Server Side:

#iperf -s -w 1024k -i 2 -p 8000
------------------------------------------------------------
Server listening on TCP port 8000
TCP window size: 1.00 MByte
------------------------------------------------------------
[  4] local 10.10.10.5 port 8000 connected with 10.10.10.10 port 53903
[ ID] Interval       Transfer     Bandwidth
[  4]  0.0- 2.0 sec  25.9 MBytes   109 Mbits/sec
[  4]  2.0- 4.0 sec  28.6 MBytes   120 Mbits/sec
[  4]  4.0- 6.0 sec  28.3 MBytes   119 Mbits/sec
[  4]  6.0- 8.0 sec  28.9 MBytes   121 Mbits/sec
[  4]  8.0-10.0 sec  28.0 MBytes   117 Mbits/sec
[  4] 10.0-12.0 sec  29.0 MBytes   121 Mbits/sec
[  4] 12.0-14.0 sec  28.0 MBytes   117 Mbits/sec
[  4] 14.0-16.0 sec  29.0 MBytes   122 Mbits/sec
[  4] 16.0-18.0 sec  28.0 MBytes   117 Mbits/sec
[  4] 18.0-20.0 sec  29.0 MBytes   121 Mbits/sec
[  4]  0.0-20.0 sec   283 MBytes   118 Mbits/sec

We can see here, that by increasing the TCP window from the default value to 1MB (1024k) we achieved around a 400% increase in throughput over our baseline. Unfortunately, this is the limit of this OS in terms of Window size. So what more can we do? Parallel streams! With multiple simultaneous streams we can fill the pipe close to its maximum usable amount.

Parallel Stream Command:
#iperf -i 2 -t 20 -c -p 8000 10.10.10.5 -w 1024k -P 7

Client Side:

#iperf -i 2 -t 20 -c -p 10.10.10.5 -w 1024k -P 7
------------------------------------------------------------
Client connecting to 10.10.10.5, TCP port 8000
TCP window size: 1.00 MByte (WARNING: requested 1.00 MByte)
------------------------------------------------------------
 [ ID] Interval       Transfer     Bandwidth
[  9]  0.0- 2.0 sec  24.9 MBytes   104 Mbits/sec
[  4]  0.0- 2.0 sec  24.9 MBytes   104 Mbits/sec
[  7]  0.0- 2.0 sec  25.6 MBytes   107 Mbits/sec
[  8]  0.0- 2.0 sec  24.9 MBytes   104 Mbits/sec
[  5]  0.0- 2.0 sec  25.8 MBytes   108 Mbits/sec
[  3]  0.0- 2.0 sec  25.9 MBytes   109 Mbits/sec
[  6]  0.0- 2.0 sec  25.9 MBytes   109 Mbits/sec
[SUM]  0.0- 2.0 sec   178 MBytes   746 Mbits/sec
 
(output omitted for brevity on server & client)
 
[  7] 18.0-20.0 sec  28.2 MBytes   118 Mbits/sec
[  8] 18.0-20.0 sec  28.8 MBytes   121 Mbits/sec
[  5] 18.0-20.0 sec  28.0 MBytes   117 Mbits/sec
[  4] 18.0-20.0 sec  28.0 MBytes   117 Mbits/sec
[  3] 18.0-20.0 sec  28.9 MBytes   121 Mbits/sec
[  9] 18.0-20.0 sec  28.8 MBytes   121 Mbits/sec
[  6] 18.0-20.0 sec  28.9 MBytes   121 Mbits/sec
[SUM] 18.0-20.0 sec   200 MBytes   837 Mbits/sec
[SUM]  0.0-20.0 sec  1.93 GBytes   826 Mbits/sec 

Server Side:

#iperf -s -w 1024k -i 2 -p 8000
------------------------------------------------------------
Server listening on TCP port 8000
TCP window size: 1.00 MByte
------------------------------------------------------------
[  4] local 10.10.10.10 port 8000 connected with 10.10.10.5 port 53903
[ ID] Interval       Transfer     Bandwidth
[  5]  0.0- 2.0 sec  25.7 MBytes   108 Mbits/sec
[  8]  0.0- 2.0 sec  24.9 MBytes   104 Mbits/sec
[  4]  0.0- 2.0 sec  24.9 MBytes   104 Mbits/sec
[  9]  0.0- 2.0 sec  24.9 MBytes   104 Mbits/sec
[ 10]  0.0- 2.0 sec  25.9 MBytes   108 Mbits/sec
[  7]  0.0- 2.0 sec  25.9 MBytes   109 Mbits/sec
[  6]  0.0- 2.0 sec  25.9 MBytes   109 Mbits/sec
[SUM]  0.0- 2.0 sec   178 MBytes   747 Mbits/sec
 
[  4] 18.0-20.0 sec  28.8 MBytes   121 Mbits/sec
[  5] 18.0-20.0 sec  28.3 MBytes   119 Mbits/sec
[  7] 18.0-20.0 sec  28.8 MBytes   121 Mbits/sec
[ 10] 18.0-20.0 sec  28.1 MBytes   118 Mbits/sec
[  9] 18.0-20.0 sec  28.0 MBytes   118 Mbits/sec
[  8] 18.0-20.0 sec  28.8 MBytes   121 Mbits/sec
[  6] 18.0-20.0 sec  29.0 MBytes   121 Mbits/sec
[SUM] 18.0-20.0 sec   200 MBytes   838 Mbits/sec
[SUM]  0.0-20.1 sec  1.93 GBytes   825 Mbits/sec

As you can see from the tests above, we were able to increase throughput from 29Mb/s with a single stream and the default TCP Window to 824Mb/s using a higher window and parallel streams. On a Gigabit link, this about the maximum throughput one could hope to achieve before saturating the link and causing packet loss. The bottom line is, I was able to prove out the network and verify bandwidth capacity was not an issue. From that conclusion, I could focus on tweaking TCP to get the most out of my network.

I'd like to point out that we will never get 100% out of any link. Typically, 90% utilization is about the real world maximum anyone will achieve. If you get any more, you'll begin to saturate the link and incur packet loss. I should also point out that Softlayer doesn't directly support iPerf, so it's up to you install and play around with. It's such a versatile and easy to use little piece of software that it's become invaluable to me, and I think it will become invaluable to you as well!

-Andrew

Comments

December 29th, 2011 at 10:50am

Nice thanks. I was using NetPCS but it only runs on Windows.

January 5th, 2012 at 1:59pm

We were using iperf for a while. We switched to pathtest - it's still command line and still free, but more customizable - TCP, UDP and ICMP and results have been consistent. www.testmypath.com

October 4th, 2012 at 9:39pm

Hi,

how you calculate window size to achieve a particular throughput for a stream?
in iperf log single user peak throughput represents which portion?

in parallel tcp stream single user peak throughput means what??

how can i achive single user peak throughput 25 Mbps for multiple tcp streams in perf....

can you please let me know the answers of the above questions....
i need it urgently...

Thanks for your help..
Rajib

July 30th, 2013 at 9:01pm

You're using your units wrong!

You keep referring to MB/s, when the iperf output is in Mbit/s. One is 8 times the other -- you're not getting 824MB/s on a gigabit network, that's for sure!

July 31st, 2013 at 12:00pm

You're right about that, Chris. Thanks for pointing out that typo! The code output reflected the correct units but the summary of the results was incorrect. We've edited the content to show that the numbers are megabits per second rather than megabytes per second.

September 3rd, 2013 at 3:56am

We are testing Fast Ethernet circuit using iperf, mainly looking for through put of TCP & UDP.
We are using laptop both side. UDP have no issue for both side.
TCP result is strange. Because I made A side as server other as client, the result was far better (like 70~80M). Then I closed iperf, restarted both laptops.

This time I made B end as server other as client. The result is less than 5M.
Anyone have any idea? please help

October 14th, 2013 at 5:18am

hi

plz help me m dion my project in linux xenomai and rtnet so i need to find iprf and jperf i dnt knpw how to work with that so can u help me plz....

December 3rd, 2013 at 6:39pm

928Mb/s here on Gbit LAN.
JPerf client on windows, iperf server on linux.
TCP, 4 streams, 2Mbit TCP Windows size, 16 sec transmit time
CAT5 FTP cable (foiled twisted pair), not even CAT5e!

So 824Mb/s and 90% are NOT the practical maximums ;)

December 10th, 2013 at 5:45pm

Could anyone tell me what I am doing wrong here?
Is anyone experiencing the same results as me?

first run:
machine1: iperf -s
machine2: iperf -c 10.0.0.20
provides 304Mbit/sec saying default window size 64KByte

second run:
machine1: iperf -s -w 64K
machine2: iperf -c 10.0.0.20 -w 64K
provides 3.80Gbit/sec with provided 64KByte window size...

Proof: http://puu.sh/5IJy4.jpg

January 16th, 2014 at 2:47pm

I am seeing the same behavior as @Stranger
First run after rebooting the machines: 9.41Gbitsper second on a 10Gb link. The two machines are connected by one optical cable sitting side by side.
Subsequent runs yield ~300Mbps.

I did not see any difference in TCP handshake on wireshark.

1st run 9.4 Gbps 99.4% CPU utilization
2nd run 300 Mbps 4% CPU utilization.

So, why is this different?
BTW both machines are running ubuntu 12.04 and identical blade server hardware with massive CPU/memory resources and iperf is the only "application" running.

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Comments

December 29th, 2011 at 10:50am

Nice thanks. I was using NetPCS but it only runs on Windows.

January 5th, 2012 at 1:59pm

We were using iperf for a while. We switched to pathtest - it's still command line and still free, but more customizable - TCP, UDP and ICMP and results have been consistent. www.testmypath.com

October 4th, 2012 at 9:39pm

Hi,

how you calculate window size to achieve a particular throughput for a stream?
in iperf log single user peak throughput represents which portion?

in parallel tcp stream single user peak throughput means what??

how can i achive single user peak throughput 25 Mbps for multiple tcp streams in perf....

can you please let me know the answers of the above questions....
i need it urgently...

Thanks for your help..
Rajib

July 30th, 2013 at 9:01pm

You're using your units wrong!

You keep referring to MB/s, when the iperf output is in Mbit/s. One is 8 times the other -- you're not getting 824MB/s on a gigabit network, that's for sure!

July 31st, 2013 at 12:00pm

You're right about that, Chris. Thanks for pointing out that typo! The code output reflected the correct units but the summary of the results was incorrect. We've edited the content to show that the numbers are megabits per second rather than megabytes per second.

September 3rd, 2013 at 3:56am

We are testing Fast Ethernet circuit using iperf, mainly looking for through put of TCP & UDP.
We are using laptop both side. UDP have no issue for both side.
TCP result is strange. Because I made A side as server other as client, the result was far better (like 70~80M). Then I closed iperf, restarted both laptops.

This time I made B end as server other as client. The result is less than 5M.
Anyone have any idea? please help

October 14th, 2013 at 5:18am

hi

plz help me m dion my project in linux xenomai and rtnet so i need to find iprf and jperf i dnt knpw how to work with that so can u help me plz....

December 3rd, 2013 at 6:39pm

928Mb/s here on Gbit LAN.
JPerf client on windows, iperf server on linux.
TCP, 4 streams, 2Mbit TCP Windows size, 16 sec transmit time
CAT5 FTP cable (foiled twisted pair), not even CAT5e!

So 824Mb/s and 90% are NOT the practical maximums ;)

December 10th, 2013 at 5:45pm

Could anyone tell me what I am doing wrong here?
Is anyone experiencing the same results as me?

first run:
machine1: iperf -s
machine2: iperf -c 10.0.0.20
provides 304Mbit/sec saying default window size 64KByte

second run:
machine1: iperf -s -w 64K
machine2: iperf -c 10.0.0.20 -w 64K
provides 3.80Gbit/sec with provided 64KByte window size...

Proof: http://puu.sh/5IJy4.jpg

January 16th, 2014 at 2:47pm

I am seeing the same behavior as @Stranger
First run after rebooting the machines: 9.41Gbitsper second on a 10Gb link. The two machines are connected by one optical cable sitting side by side.
Subsequent runs yield ~300Mbps.

I did not see any difference in TCP handshake on wireshark.

1st run 9.4 Gbps 99.4% CPU utilization
2nd run 300 Mbps 4% CPU utilization.

So, why is this different?
BTW both machines are running ubuntu 12.04 and identical blade server hardware with massive CPU/memory resources and iperf is the only "application" running.

Leave a Reply

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