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I'm trying to diagnose intermittent poor wifi performance on my laptop, which has a fresh installation of 13.04.

I installed several network monitoring tools, including "slurm", which I'm now using to monitor both the wireless interface (wlan0) and the loopback interface (lo). The wireless interface, predictably, shows the same information displayed in the System Monitor; data generally flows at the rate of several hundred KB/second. The loopback interface, however, consistently shows a transaction speed of 5-40 KB/sec, maxing out at around 80-90. I don't really understand how Ubuntu's network interfaces work (plus the complexities of how they interact with Network Manager). Is it normal for the lo interface to be so slow? How does the loopback interface affect networking performance? Can I pretty much ignore it when trying to figure out my wifi problem?

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Relevant question: What is the loopback device and how do I use it? – gertvdijk Jun 23 '13 at 16:52
Your question is kind of hard to read. Could you improve it and keep it to the essential parts? The loopback interface only routes and has nothing to do with regular networking. – gertvdijk Jun 23 '13 at 16:53
Edited for brevity. @gertvdijk, I just looked at the answer to the question you linked. I am still confused. If the lo interface only involves tcp/ip communication between my computer and itself, why is there such a steady stream of data (slowly) flowing across it? Is that normal, or not? Note that right now, the only thing happening on the computer is Bitcoin Armory trying to download the blockchain. (Could communication between Amory and the bitcoin daemon be using lo?) – Sam Lichtenstein Jun 23 '13 at 17:03
What does sudo tcpdump -i lo give you? It will dump everything running on the interface and help you identifying what is happening there, hopefully. – gertvdijk Jun 23 '13 at 17:06
8332 is the usual port of bitcoin JSON remote control... Are you running a bitcoin daemon and some GUI for it? Usually some traffic on loopback is normal, because some programs use it for interprocess communication (although Sockets would be the preferred means for that...). – soulsource Jun 23 '13 at 18:11
up vote 2 down vote accepted

As soulsource already pointed out, you're probably looking at interprocess communication done over the localhost network. That isn't very efficient, but in programming this is sometimes useful to have a single piece of code whether the two components are running on the same machine or over the network. Let's have a look how to track this down.

As your tcpdump shows it's sending data to port 8332 on localhost, you can see what's listening there using netstat:

sudo netstat -nlptu | grep 8332

(-n for numeric output, -l for listening sockets, -p to show the process, -t and -u to show both TCP and UDP sockets.)

You should see something like

tcp   0      0*   LISTEN     21891/SomeProcessName

Then investigate this process ID further:

ps u --pid 21891

This will show something like

gert     21891  0.0  0.0 480768 13836 ?        Sl   13:49   0:11 /path/to/some/binary


How does the loopback interface affect networking performance? Can I pretty much ignore it when trying to figure out my wifi problem?

This has nothing to do with the performance over other network interface cards. I suggest to ask a new question for that.

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