[aclug-L] Re: Determining memory usage by a user

[John Goerzen <jgoerzen@xxxxxxxxxxxx>]

On Tue, Jun 08, 2004 at 10:42:35AM -0500, Jonathan Hall wrote:
> If it is at all possible (I'm beginning to think it's not), I need to find a
> way to determine how much memory a specific user is using.

It is, but the trick is to first define what you mean by how much memory
a specific user is using.

The place to start is /proc.  (See proc(5)).  The statm file will be
most useful.  From the manpage:

       /proc/[number]/statm
              Provides information about memory status in pages.  The columns
              are:
               size       total program size
               resident   resident set size
               share      shared pages
               trs        text (code)
               drs        data/stack
               lrs        library
               dt         dirty pages

I'm not sure exactly how those numbers tally with ps/top fields.
You can probably use a formula such as (size - share) to obtain what you
are after.  Note that units may differ; you'll have to check on that.  A
page is 4K on i386 systems.  The /proc/[number]/stat file also has
fields that will probably interest you.

If you are interested in this for enforcement purposes, the ulimit /
setrlimit() features are probably what you want.  They let the kernel do
the calculating for you.

-- John

> 
> My first thought was to add the values of the VSZ or RSS fields from ps's
> output.  This, however, is worthless:
> 
> VSZ tells the entire 'virtual memory' usage by each process, which includes
> shared memory.  This means that a memory segment used by multiple processes
> is counted multiple times.  As an example of how worthless this is, if I add
> the VSZ values for every process on my router machine, the result is 712mb
> of used memory.  That machine has only 32mb of physical RAM and 32mb of
> swap, so it is physically impossible for the used memory to ever exceed
> 64mb.
> 
> The RSS value is also nearly-worthless, as it only reports 'resident' memory
> (i.e. memory not in swap space).  I have been unable to determine how the
> RSS value treats shared memory, but if I add all values in the RSS field on
> all processes, it always seems to roughly equal the amount of used memory
> reported by 'free', so I think this value is close to accurate.
> 
> If I can find some way to determine how much swap space is used by a process
> (or all processes owned by a user), then I could possibly use the sum of
> that value and the values of RSS from ps to come up with a reasonable
> estimate for the total memory used by the user.
> 
> Can anyone think of a way to accomplish this?
> 
> -- Jonathan
> 
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