How to determine the real memory usage of a single process?

Question

How can I calculate the real memory usage of a single process? I am not talking about the virtual memory, because it just keeps growing. For instance, there are proc files like smaps, where you can get the mappings of a process. But this is virtual memory and the values of that file just keeps growing for running process. But I would like to reflect the real memory usage of a process. E.g. if you plot the memory usage of a process it should represent the allocations of memory and also the freeing of memory. So the plot should be like an up and down movement instead of a linear function, that just keeps growing for a running process.

So, how could I calculate the real memory usage? I would appreciate any helpful answer.

Answer 1

It's actually kind of a complicated question. The two most common metrics for a program's memory usage at the OS level are virtual size and resident set size. (These show in the output of ps -u as the VSZ and RSS columns.) Roughly speaking, these tell the total memory the program has assigned to it, versus how much it is currently actively using.

Further complicating the question is that when you use malloc (or the C++ new operator) to allocate memory, memory is allocated from a pool in your process which is built by occasionally requesting an allocation of memory from the operating system. But when you free memory, the memory goes back into this pool, but it is typically not returned to the OS. So as your program allocates and frees memory, you typically will not see its memory footprint go up and down. (However, if it frees a lot of memory and then doesn't allocate it any more, eventually you may see its rss go down.)