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 :)

• 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/

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