Physics Computing

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Computing Tips and Frequently Asked Questions

This page hopes to be a nice list of computing tidbits that many will find useful.

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AFS space, and authentication tokens explained

When you login to a realm machine, you are given 'tokens' to access parts of AFS, mainly your home directory, but also software lockers. These tokens expire after about 10 hours, which means you will no longer be able to access anything in AFS. It's important not to let your tokens expire while you're logged in, you will find access to your home directory cut off, and the computer generally unusable. Fortunately, there are a couple commands to help out the situation.
A couple of helpful commands are: The kreset command is especially useful when you want to run something overnight. Without running kreset, your tokens will expire and you will be denied from your home directory, most likely killing your job. To make sure this doesn't happen, run kreset -l before you leave for the night, to make sure nothing expires.

While I haven't tested this, I have had success reports. If you are running a job overnight or over the weekend such that the tokens will expire, it should run fine as long as it doesn't need to access anything in AFS. For example, if the job is running out of /physics, and the output is going to somewhere local such as /tmp,

If you sit back down at a workstation in which you're logged in, but tokens have expired, make sure you immediately run kreset, or logout and log back in to avoid running into any problems. Also make sure you leave a terminal open to run kreset in, because launching a new terminal will most likely fail.

Login Issues

Check out the link above if you're having trouble logging in to your workstation.

Spam Filtering

NCSU runs a spam filter on the mail servers, all you have to do is set your client to toss the messages that the servers have flagged as spam. Below are instructions on setting up the filter server-side, as well as for evolution and thunderbird.

Server Side (perfered method)
This will work regardless of the email client you use because it filters spam on the server as soon as it arrives in your inbox. To do this, login to webmail at webmail.ncsu.edu. Once logged in, go to "Options" at the top, and then choose "Server-Side Message Filters". You'll want to add a new rule. Specify the type as "Header Match". Under the header to match find "X-Spam-Flag" and for what to match enter "YES". Then for the action, specify either to move it to a junk/spam folder, or to delete it, your choice. Press "Add New Rule" and you're done.

Evolution
Go under Tools to Filters. Press the Add button. Make a name for the rule, such as "spam". Under the rule "If" section, set it to "Specific header" "X-Spam-Flag" "contains" "YES". Under the "Then" section, set it to move it to a folder of your choice, or to delete it.

Thunderbird
Go under Tools to Message Filters. Click the New button. Give the filter a name, such as "spam". Under the first condition box, go to "Customize". Type "X-Spam-Flag" in the box and hit add, then hit OK. Now select "X-Spam-Flag" from that same menu. Second box is "Contains". Third box, type "YES".
Then choose an appropriate action for the message, such as moving it to the spam folder.

My computer, upon booting, says it cannot start the X server

A few computers around BOM (and possibly elsewhere) have nVidia graphics cards, which require a special binary driver to be compiled each time the kernel is updated. When this happens, the graphics driver is supposed to rebuild itself automatically, however this often fails resulting in the blue error screen you see.

The fix is relatively simple, delete the trigger file "/usr/share/nvidia/run". This will cause the nvidia drivers to get rebuilt the next time the computer boots.

If you have sudo access to your machine, log in on one of the virtual terminals (Ctrl-Alt-F1) and type "sudo rm /usr/share/nvidia/run". Then type "sudo /sbin/shutdown -r now" to restart the computer.
If you don't have sudo, send an email to the help desk, or directly to me (or come and find me) and I can fix it, usually immediately.

If this isn't the cause of the X server refusing to start, send in a help call to pyhelp at pams dot ncsu dot edu