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Iperf output question

Posted on 2014-10-20
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Last Modified: 2014-11-13
I am running iperf across a few different type of links. I have a server setup on the other end and I am getting varying values depending on the line. I'm finding the output very up and down. e.g iperf -c myserver -I 1 can produce speeds of anywhere from 30Mbps - 85mbps.
I am a little confused on the transfer figure. Why does the amount being transferred keep changing. I have looked at a number of videos but they don't seem to give specifics on the actual output. Sorry for the stupid question but I'm guessing that the bandwidth figure being presented is relative to the amount of data being transferred e.g I can't say my network has a 44Mbit/sec throughput to server X, I have to say when transferring 7.25MBytes per second my network has a throughput of 44Mbit/sec to the server..
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Question by:Sid_F
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by:aleghart
ID: 40393580
Speed can vary based on connection type (wireless, wired), congestion, resource contention, and network (Layer 2 or routed) + the number of hops.

If both end points are not loaded with any other services, and the network is stable, then the reading should be close.

If it's wireless in a busy environment, then all bets are off.  In a wireless environment I can get 5Mbps, then 15Mbps only a few seconds later.  Or by taking one step around a corner, or by standing instead of sitting...many variables.

What kind of environment are you testing in?
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by:Aaron Tomosky
ID: 40393588
Instead of -I 1 do -I 60
That will give you the average bandwidth over 60 seconds
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by:eeRoot
ID: 40393623
Network congestion, QOS, server utilization...  There are many things that can affect performance.  If you are testing across a network that others are using, and testing a server that others are using, then you have to share resources with them.  If this is a local network, and a dedicated test server, something else may be going on.  What is the server's NIC, CPU, & Mem utilization when you are NOT running iperf?
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by:aleghart
ID: 40393636
Good catch, Aaron Tomosky.  Why are you changing the sampling time?  The default is 10 seconds, and should be good enough for a snapshot of a network without tying up the line for other people.
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by:Sid_F
ID: 40393789
Thanks all, It's a wired network, I'm using iperf to get familiar with my network benchmarks but with such a huge difference in speed its difficult to know what through put I have. If for a minute I suppose that a third party has indicated my network is the reason there application is slow, if I ran the throughput test I couldn't say hey I'm getting 72Mbits throughput as some of the below indicates as low as 26Mbits?.
As mentioned in my OP I am still confused on the transfer amounts, why does it transfer 8MBytes and then jump to transferring 72MBytes?

[ ID] Interval       Transfer     Bandwidth
[  3]  0.0- 1.0 sec  8.62 MBytes  72.4 Mbits/sec
[  3]  1.0- 2.0 sec  8.88 MBytes  74.4 Mbits/sec
[  3]  2.0- 3.0 sec  8.12 MBytes  68.2 Mbits/sec
[  3]  3.0- 4.0 sec  8.62 MBytes  72.4 Mbits/sec
[  3]  4.0- 5.0 sec  8.50 MBytes  71.3 Mbits/sec
[  3]  5.0- 6.0 sec  6.75 MBytes  56.6 Mbits/sec
[  3]  6.0- 7.0 sec  8.50 MBytes  71.3 Mbits/sec
[  3]  7.0- 8.0 sec  3.12 MBytes  26.2 Mbits/sec
[  3]  8.0- 9.0 sec  5.25 MBytes  44.0 Mbits/sec
[  3]  9.0-10.0 sec  5.75 MBytes  48.2 Mbits/sec
[  3]  0.0-10.0 sec  72.2 MBytes  60.5 Mbits/sec
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aleghart earned 500 total points
ID: 40394288
Look at the interval.  You're seeing 1 second intervals, then the total size and average bandwidth for all 0-10 seconds.

The last figure is for the entire 10 second test.  It should be the total of the size transferred for all of the preceding intervals.

The drops in speed are congestion at a choke point (uplink or router) and/or resource contention at either the server or client doing the iperf test.

It could be a random file write, anti-virus kicking off, update downloading, Outlook syncing because you forgot to close it...etc.  Try to kill as many programs and services as possible while you're using iperf.
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by:Sid_F
ID: 40441345
Thanks sorry for delay
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