Memory management

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Memory management is a fun thing. Think about the following:

You have a program running on your computer and you want to know how big its memory footprint is. Half of the program has not yet been loaded from disk, a quarter has been swapped out of main memory. The rest is using 200k of shared libraries and has 1 megabyte in the file system cache that can be freed up with no serious consequences. If you are interested in this kind of things, this article is for you :)

Contents

File system cache

Linux tends to fill up all your computer's memory over time. This is not bad, but works as designed. Whenever Linux reads a file from disk, it keeps the content in memory so it does not have to wait for the disk next time. This is called the file system cache, you can display its size with the command

# free                                                                                          
             total       used       free     shared    buffers     cached                                     
Mem:       6083904    1175880    4908024          0       7556     230460                                     
-/+ buffers/cache:     937864    5146040                                                                      
Swap:            0          0          0   

If you really want to clean the file system cache, you will probably want to clean dentries and inodes as well, you can do it with the commands:

sync
echo 3 > /proc/sys/vm/drop_caches

memory consumption of all programs

# ps aux|awk '{print $6"  " $11;s=s+$6} END {print s}' | sort -n

will give you a sorted list which program consumes how much, about like this:

[...]
4028  dovecot/auth                                                                                                                                                    
4528  /sbin/haveged                                                                                                                                                   
8348  /usr/sbin/httpd2-prefork                                                                                                                                        
38764  /usr/sbin/httpd2-prefork                                                                                                                                                                                                                                 
43628  /usr/sbin/httpd2-prefork                                                                                                                                       
279148

memory consumption of a program

killall gedit
# sync;echo 3 > /proc/sys/vm/drop_caches
[1]+  Terminated              gedit
# free
total       used       free     shared    buffers     cached
Mem:       6083904    1164036    4919868          0        508     210916
-/+ buffers/cache:     952612    5131292
Swap:            0          0          0
# free -k
             total       used       free     shared    buffers     cached
Mem:       6083904    1163732    4920172          0        516     210888
-/+ buffers/cache:     952328    5131576
Swap:            0          0          0
# gedit &
[1] 5859
# free -k
             total       used       free     shared    buffers     cached
Mem:       6083904    1221472    4862432          0      11252     237796
-/+ buffers/cache:     972424    5111480
Swap:            0          0          0
# sync;echo 3 > /proc/sys/vm/drop_caches
# free -k
             total       used       free     shared    buffers     cached
Mem:       6083904    1184668    4899236          0        516     217036
-/+ buffers/cache:     967116    5116788
Swap:            0          0          0
# echo $((1184668-1163732))
20936
# ps -auxf |  grep -E "(VSZ|gedit)"
warning: bad ps syntax, perhaps a bogus '-'?
See http://gitorious.org/procps/procps/blobs/master/Documentation/FAQ
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root      5859  0.7  0.4 767088 28760 pts/5    Sl   11:25   0:00      \_ gedit
root      5871  0.0  0.0   7004   872 pts/5    S+   11:26   0:00      \_ grep --color=auto -E (VSZ|gedit)

system memory map

To get a complete memory map of your system use the command

pmap $(ps -A | awk '{print $1}'|grep -v PID) | sort | grep \^0

The result will look like this:

...
00007fd6dbf45000      4K rw---  /lib/libnss_compat-2.11.1.so 
00007fd6dbf46000   1524K r-x--  /lib/libc-2.11.1.so
00007fd6dc0c3000   2044K -----  /lib/libc-2.11.1.so  
...

provoke a memory leak

Here is how to provoke a memory leak, my program ram_gourmet:

main.cpp

class t

{
public:
t(){};
};

void pollute()
{
  t* polluter=new t();
}

int main()
{
  while (true) pollute();
}

Attention: It rendered my compi useless within 10 seconds.

Compile, link and run this using:

g++ -o ram_gourmet main.cpp && ./ram_gourmet

This program can be developed further to be a cache condenser

page size

Ok, first I want to find out my system's page size so I write a program:

#include <unistd.h>
#include <stdio.h>
int main()
{
             int sz = getpagesize();
             printf("%d",sz);
}

And compile it:

# gcc main.c

And execute it:

# ./a.out
4096

Ok, so my system's page size is 4K.

Now let's write the next program and check how much memory it consumes. Here is the code:

int main()
{
  while (1);
}

Compile it

# gcc main.c

And execute it in the background:

# ./a.out &
[5] 24058

Ok, it's process ID is 24058. Let's look at its memory consumption:

# pmap 24058
24058: a.out
START               SIZE     RSS     PSS   DIRTY    SWAP PERM MAPPING
0000000000400000      4K      4K      4K      0K      0K r-xp /home/tstaerk/test/a.out
0000000000600000      4K      4K      4K      4K      0K r--p /home/tstaerk/test/a.out
0000000000601000      4K      4K      4K      4K      0K rw-p /home/tstaerk/test/a.out
00007fda6f01e000   1364K    160K      1K      0K      0K r-xp /lib64/libc-2.11.1.so
00007fda6f173000   2044K      0K      0K      0K      0K ---p /lib64/libc-2.11.1.so
00007fda6f372000     16K     16K     16K     16K      0K r--p /lib64/libc-2.11.1.so
00007fda6f376000      4K      4K      4K      4K      0K rw-p /lib64/libc-2.11.1.so
00007fda6f377000     20K     12K     12K     12K      0K rw-p [anon]
00007fda6f37c000    124K    104K      1K      0K      0K r-xp /lib64/ld-2.11.1.so
00007fda6f55a000     12K     12K     12K     12K      0K rw-p [anon]
00007fda6f599000      4K      4K      4K      4K      0K rw-p [anon]
00007fda6f59a000      4K      4K      4K      4K      0K r--p /lib64/ld-2.11.1.so
00007fda6f59b000      4K      4K      4K      4K      0K rw-p /lib64/ld-2.11.1.so
00007fda6f59c000      4K      4K      4K      4K      0K rw-p [anon]
00007fff625e5000     84K     12K     12K     12K      0K rw-p [stack]
00007fff625ff000      4K      4K      0K      0K      0K r-xp [vdso]
ffffffffff600000      4K      0K      0K      0K      0K r-xp [vsyscall]
Total:             3704K    352K     86K     80K      0K

136K writable-private, 3568K readonly-private, 0K shared, and 352K referenced

Ok, it looks like the binary has a lot of shared libraries that it makes use of. Let's see if this is true:

# ldd a.out
        linux-vdso.so.1 =>  (0x00007fffc4bee000)
        libc.so.6 => /lib64/libc.so.6 (0x00007f2c6cc78000)
        /lib64/ld-linux-x86-64.so.2 (0x00007f2c6d013000)

It is true, it is a dynamically-linked executable. Let's make it a static one:

# gcc --static main.c
# ll a.out
-rwxr-xr-x 1 tstaerk users 2947494 2012-06-06 10:14 a.out

And run it again:

# ./a.out &
[7] 21850

And look at its memory map again:

# pmap 21850
21850: a.out
START               SIZE     RSS     PSS   DIRTY    SWAP PERM MAPPING
0000000000400000    572K     84K     84K      0K      0K r-xp /home/tstaerk/test/a.out
000000000068e000      8K      8K      8K      8K      0K rw-p /home/tstaerk/test/a.out
0000000000690000    152K     20K     20K     20K      0K rw-p [heap]
00007fff34f2f000     84K      8K      8K      8K      0K rw-p [stack]
00007fff34f56000      4K      0K      0K      0K      0K r-xp [vdso]
ffffffffff600000      4K      0K      0K      0K      0K r-xp [vsyscall]
Total:              824K    120K    120K     36K      0K

244K writable-private, 580K readonly-private, 0K shared, and 120K referenced

Well then on another distro let's give it something to think about:

root@euve31070:~# cat main.c 
#include <stdlib.h> 
int main ()
{
  char * buffer;
  buffer = (char*) malloc (800000);
  buffer = "hi";
  while (1) {};
}
root@euve31070:~# gcc --static main.c 
root@euve31070:~# ./a.out &
[1] 28793
root@euve31070:~# pmap 28793
28793:   ./a.out
0000000000400000    652K r-x--  /root/a.out
00000000006a2000      8K rw---  /root/a.out
00000000006a4000     12K rw---    [ anon ]
0000000000c2b000    140K rw---    [ anon ]
00007fcd87007000    784K rw---    [ anon ]
00007fff93aab000     84K rw---    [ stack ]
00007fff93bfe000      8K r-x--    [ anon ]
ffffffffff600000      4K r-x--    [ anon ]
 total             1692K
root@euve31070:~# vi main.c 
root@euve31070:~# cat main.c 
#include <stdlib.h> 
int main ()
{
//  char * buffer;
//  buffer = (char*) malloc (800000);
//  buffer = "hi";
  while (1) {};
}
root@euve31070:~# gcc --static main.c 
root@euve31070:~# ./a.out &                                                                                                                                                                               
[2] 28802                                                                                                                                                                                                            
root@euve31070:~# pmap 28802                                                                                                                                                                                         
28802:   ./a.out                                                                                                                                                                                                     
0000000000400000    652K r-x--  /root/a.out                                                                                                                                                                          
00000000006a2000      8K rw---  /root/a.out                                                                                                                                                                          
00000000006a4000     12K rw---    [ anon ]                                                                                                                                                                           
000000000262b000    140K rw---    [ anon ]                                                                                                                                                                           
00007fff6d699000     84K rw---    [ stack ]                                                                                                                                                                          
00007fff6d79e000      8K r-x--    [ anon ]                                                                                                                                                                           
ffffffffff600000      4K r-x--    [ anon ]                                                                                                                                                                           
 total              908K 

Terms

  • heap vs stack
  • pool vs swap
  • shared mem, extended mem, buffer, cache, heap, stack
  • resident mem vs virtual mem, allocated, mapped, free, physical, used

See also