Memory Usage – Actual Memory Usage of a Process

memoryvirtual-memory

The following are the memory usage of mysql and apache respectively on my server. As per the output of pmap say, mysql is using about 379M and apache is using 277M. 

[root@server ~]# pmap 10436 | grep total
 total           379564K

[root@server ~]# pmap 10515 | grep total
 total           277588K

Comparing this with the output of top, I see the values are almost matching.

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
10515 apache    20   0  271m  32m 3132 S  0.0  6.6   0:00.73 /usr/sbin/httpd
10436 mysql     20   0  370m  21m 6188 S  0.0  4.3   0:06.07 /usr/libexec/mysqld --basedir=....

Now these values definitely are not the current memory usage of those two processes, since if it were it would've exceeded the 512M ram on my system and I understand the fact that these are the size of the pages assigned to these two processes and not really the size of the memory actively used by them. Now, when we use pmap -x, I see an extra coloumn Dirty which shows far less memory usage for the process. As seen in the example show below, the Dirty coloumn shows 15M as opposed to 379M in the first coloumn. My question is: Is the value under coloumn Dirty is the 'real' amount of memory actively used by that process? If its not, then how can we find out the real memory usage of a process? Not ps and top for the same reasons above. Do we have anything under /proc that will give this info?

[root@server ~]# pmap -x 10436 | grep total
total kB          379564   21528   15340
[root@server ~]#


[root@server ~]# free -m
             total       used       free     shared    buffers     cached
Mem:           489        447         41          0         52        214
-/+ buffers/cache:        180        308
Swap:         1023          0       1023
[root@server ~]#

Best Answer

There is no command that gives the “actual memory usage of a process” because there is no such thing as the actual memory usage of a process.

Each memory page of a process could be (among other distinctions):

  • Transient storage used by that process alone.
  • Shared with other processes using a variety of mechanisms.
  • Backed up by a disk file.
  • In physical memory or swap.

I think the “dirty” figure adds up everything that is in RAM (not swap) and not backed by a file. This includes both shared and non-shared memory (though in most cases other than forking servers, shared memory consists of memory-mapped files only).

The information displayed by pmap comes from /proc/PID/maps and /proc/PID/smaps. That is the real memory usage of the process — it can't be summarized by a single number.

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