Memory management, virtual and residential memory

Memory management is a complex topic and most can be left for the kernel to handle. But having a fundamental idea about where memory is allocated greatly helps in understanding top(1) and the memory footprint of applications.

Process memory address space (page)

When a process starts up, the kernel assigns is a so called memory page. The page size depends on the architecture. On amd64 it’s 2⁶⁴ - 1 bytes.

Every memory allocation this process performs, returns a pointer to some place within this page. Forcing a pointer outside this page, will cause a SEGFAULT.

char *w = 1; // segfault
char *w = malloc(12); // returns pointer within page

Memory allocation (virtual memory)

Let’s say we allocatate 2G of memory:

char *m = malloc(2*1073741824); // 2*1G in bytes

This will grab 2G of consecutive address space within the process memory. At this point, the memory is likely available but not guaranteed. The allocation shows up in top(1) as “SIZE” or on linux as “VIRT"ual memory. This memory is not actually used. So nothing has been written to the physical RAM chip in your computer.

Using memory (residential memory)

Once memory gets used, it will actually use up space on your RAM chip.

memset(m, 'u', 1073741824);

Now we’ve written the character “u” to the first 1G of our allocated memory. If we look at top(), we’ll see something like this:

  PID      TID PRI NICE  SIZE   RES STATE     WAIT      TIME    CPU COMMAND
96621   569318   3    0 2048M 1027M sleep/12  ttyin     0:01  1.66% ./a.out
                          ^     ^
        allocated memory -'     `- used (written) memory

Note 1: When memory is swapped to disk, it leaves the residential bucket and can be seen as swap->used.

Note 2: Stack memory will also show up as residential when used. Unused stack memory will not show up as virtual memory.