Correctly determining memory usage in Linux

A really good tool is pmap which list the current usage of memory for a certain process:

pmap -d PID

For more information about it see the man page man pmap and also have a look at 20 Linux System Monitoring Tools Every SysAdmin Should Know, which list great tools I always use to get information about my Linux box.

Run top, hit h for help then f to add fields. you can add the following fields:

• RSS amount of physical memory the application is using
• CODE total amount of memory the process's executable code is using
• DATA - total amount of memory (kb) dedicated to a process's data and stack

Between these 3 you should have pretty accurate results. You can also use more detailed replacements for top I recommend htop or atop.

Edit: Almost forgot if you want really detailed information. Find the PID and cat the following file.

PID=123

cat /proc/123/status

Edit 2: If you can find it or have it the book:

Optimizing Linux Performance: A Hands-On Guide to Linux Performance Tools

-has a section Chapter 5: Performance Tools: Process-Specific Memory -it has way more information than you would ever want.

ps gives you the amount of memory used by each process. Some of that memory is mmapped files, which counts under cache. Some of that memory (especially code) is shared with other processes, so if you add up the RSS values it's counted multiple times.

There's no right answer to “how much memory does this process use?”, because it doesn't depend on the process alone, it also depends on the environment. There are many different values you might call the “memory usage” of the process, and they don't match or add up because they're counting different things.

If you're looking for memory numbers that add up have a look at smem:

smem is a tool that can give numerous reports on memory usage on Linux systems. Unlike existing tools, smem can report proportional set size (PSS), which is a more meaningful representation of the amount of memory used by libraries and applications in a virtual memory system.

Because large portions of physical memory are typically shared among multiple applications, the standard measure of memory usage known as resident set size (RSS) will significantly overestimate memory usage. PSS instead measures each application's "fair share" of each shared area to give a realistic measure.

For example here:

# smem -t
  PID User     Command                         Swap      USS      PSS      RSS
...
10593 root     /usr/lib/chromium-browser/c        0    22868    26439    49364 
11500 root     /usr/lib/chromium-browser/c        0    22612    26486    49732 
10474 browser  /usr/lib/chromium-browser/c        0    39232    43806    61560 
 7777 user     /usr/lib/thunderbird/thunde        0    89652    91118   102756 
-------------------------------------------------------------------------------
  118 4                                       40364   594228   653873  1153092 

So PSS is the interesting column here because it takes shared memory into account.
Unlike RSS it's meaningful to add it up. We get 654Mb total for userland processes here.

System-wide output tells about the rest:

# smem -tw
Area                           Used      Cache   Noncache 
firmware/hardware                 0          0          0 
kernel image                      0          0          0 
kernel dynamic memory        345784     297092      48692 
userspace memory             654056     181076     472980 
free memory                   15828      15828          0 
----------------------------------------------------------
                            1015668     493996     521672 

So 1Gb _{RAM total} = 654Mb _{userland processes} + 346Mb _{kernel mem} + 16Mb _free
(give or take a few Mb)

Overall about half of memory is used for cache (494Mb).

Bonus question: what is userland cache vs kernel cache here ?

btw for something visual try:

# smem  --pie=name

Try this: it will give you the total RAM actually used by all the process running in MB

ps -eo size,pid,user,command --sort -size | awk '
  { hr=$1/1024 ; printf("%13.2f Mb ",hr) } 
  { for ( x=4 ; x<=NF ; x++ ) { printf("%s ",$x) } print "" }
  ' | awk '{total=total + $1} END {print total}'

It ll show you how much memory user by users..

#!/bin/bash
total_mem=0

printf "%-10s%-10s\n" User MemUsage

while read u m
do
        [[ $old_user != $u ]] && {  printf "%-10s%-0.1f\n" $old_user $total_mem;
                                    total_mem=0; }
        total_mem="$(echo $m + $total_mem | bc)"
        old_user=$u

done < <(ps --no-headers -eo user,%mem| sort -k1)

#EOF