2003/12/30 Understanding ulimit

© December 2003 Tony Lawrence

This sets limits on processes. It's well known, but I suspect many people only know it for setting the size of core files. It can actually control much more :

$ ulimit -a
core file size        (blocks, -c) unlimited
data seg size         (kbytes, -d) 524288
file size             (blocks, -f) unlimited
max locked memory     (kbytes, -l) unlimited
max memory size       (kbytes, -m) unlimited
open files                    (-n) 16424
pipe size          (512 bytes, -p) 1
stack size            (kbytes, -s) 65536
cpu time             (seconds, -t) unlimited
max user processes            (-u) 8211
virtual memory        (kbytes, -v) 589824
 

You can use it to control processes that you expect to end but might "run away" or even to put a stop to things you deliberately have put in a "forever" loop. For example, this script would run forever if it were not for the beginning line:

ulimit -t 1
while :
do
ps -lp $$
done
 

But in fact it will be stopped:

  UID   PID  PPID CPU PRI NI      VSZ    RSS WCHAN  STAT  TT       TIME COMMAND
  501  2714   661   0  28  0    18644    396 -      R+   std    0:01.30 /bin/ba
  UID   PID  PPID CPU PRI NI      VSZ    RSS WCHAN  STAT  TT       TIME COMMAND
  501  2714   661   0  28  0    18644    396 -      R+   std    0:01.30 /bin/ba
  UID   PID  PPID CPU PRI NI      VSZ    RSS WCHAN  STAT  TT       TIME COMMAND
  501  2714   661   0  29  0    18644    396 -      R+   std    0:01.30 /bin/ba
  UID   PID  PPID CPU PRI NI      VSZ    RSS WCHAN  STAT  TT       TIME COMMAND
  501  2714   661   0  29  0    18644    396 -      R+   std    0:01.30 /bin/ba
Cputime limit exceeded
$ 
 

Things to note: it doesn't stop after exactly 1 second of time, and that is cpu time, not wall time. And, this is a function of your shell, so you need to do "man bash" etc. to find out what its capabilities are.

You can do similar things to control badly written processes that get confused and start sucking down too much memory or have other bad behavior. Just start them from a script that sets appropriate limits.

The system call is "setrlimit".

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