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This happens with many different programs from the terminal. Usually accompanied with some error message about not being able to allocate memory.

When I try "free -m" this is the output:

martin@martin-ThinkPad-T410:~$ free -m
             total       used       free     shared    buffers     cached
Mem:          3823       3079        744          0         99       1159
-/+ buffers/cache:       1820       2003
Swap:         3953         60       3893

Why is my swap usage so low? Is this related?

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This was driving me crazy, especially since no one else had this problem. I've since left to Debian Squeeze, and the problem has disappeared, so it wasn't a hardware issue. – Martin Konecny Nov 4 '11 at 2:23

You must have set a limit for the maximum number of processes in /etc/security/limits.conf or perhaps some default is set in Ubuntu.

I had that set to 350 processes to prevent accidential fork-bombs. What drove me crazy - when my processes couldn't fork anymore, the number of processes I ran at the time was far from the limit.

Turns out the reason for that was, that not processes were counted, but kernel level threads.

ps -efL | grep ^$USER | wc -l 

indeed showed that I was close to the limit. The many tabs/windows I had open in chromium alone could amount to over 100 such threads - no wonder I hit that limit easily.

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up vote 0 down vote accepted

The problem ended up being this one:

fuser process started by cron job forking uncontrollably

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