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I have spend 2 hours reading questions about this matter, and still there is some misunderstanding.

I have this process:

USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND    
root 1452 0.4 1.8 1397012 19308 ? Sl 04:23 3:48 ./transaction_server

This shows it uses 19.3Mb of system resident memory (I have no swap file), around 1.8% of whole 1GB system memory. Virtual size is 1.39GB?!?. I have read that ulimit -m doesn't work. People use ulimit -v e.g. setting virtual memory for the process. Is this virtual memory is the one VSZ listed with ps? What value I should set if I want to restrict this process to use 100MB system memory at most. I have read documentation for setrlimit and this seems legit:

RLIMIT_AS
        This  is  the maximum size of a process' total available memory, 
        in bytes. If this limit is exceeded, the malloc() and mmap() 
        functions shall fail with errno set to [ENOMEM]. In addition, 
        the automatic stack growth fails with the effects outlined above.

But on other versions of the documentation this RLIMIT_AS parameter sets virtual memory size. What is the truth?

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Please, pay some attention to post formatting. –  rozcietrzewiacz Feb 22 '12 at 11:57
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1 Answer 1

up vote 3 down vote accepted

Yes, VSZ is virtual memory. As to RLIMIT_AS, where did you find the paragraph quoted above? Since setrlimit(2) is a Linux system call, I do not see how it could possibly monitor malloc(3), a library function. Instead, it can only work with brk(2), sbrk(2), and mmap(2) -- this is also what its manpage (checked of Scientific Linux) suggests. However, the total amount of memory requested via these functions is virtual memory, so RLIMIT_AS indeed limits virtual memory. (This is, again, in accordance with the setrlimit(2) manpage.)

Unfortunately, you cannot limit RSS under Linux (this would be ulimit -m). You can try ulimit -d (RLIMIT_DATA), but this will ignore mmap(2), typically used for large allocations. Another possibility would be to limit virtual memory, but with such a large difference between RSS and VSZ, this might be difficult.

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Thank you for your answer. The paragraph is from setrlimit man page IEEE/The Open Group 2003 GETRLIMIT(3P) How come ps can show me the RSS, but the kernel can't enforce limit on it? –  Dragomir Ivanov Feb 22 '12 at 10:26
3  
According to Alan Cox, this has historical reasons: calculating RSS used to be expensive, so enforcing the limit would put a large load on the kernel; source: linux.derkeiler.com/Mailing-Lists/Kernel/2006-01/msg03306.html. See also stackoverflow.com/questions/3043709/… –  Ansgar Esztermann Feb 22 '12 at 11:08
    
I see. I guess I would go with ulimit -d then. The application is written by me, and I didn't use mmap(). –  Dragomir Ivanov Feb 22 '12 at 11:16
1  
No, you usually don't (nor sbrk()), but malloc() might. –  Ansgar Esztermann Feb 22 '12 at 11:32
    
I see. That is very unfortunate. So the solution to this question then, is either cgroups, or memory usage poll with some script language. –  Dragomir Ivanov Feb 22 '12 at 15:35
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