Memory management

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 :)

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

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

See also

• http://linux-mm.org
• http://linux-mm.org/Low_On_Memory
• http://www.linux.com/learn/tutorials/42048-uncover-the-meaning-of-tops-statistics
• http://en.wikipedia.org/wiki/Resident_set_size
• http://en.wikipedia.org/wiki/Mmap
• http://lwn.net/Articles/230975/
• https://fedorahosted.org/libpagemap/