How to measure the memory usage of a process without calling an external program

Question

The memory usage of a process can be displayed by running:

$ ps -C processname -o size
 SIZE
 3808

Is there any way to retrieve this information without executing ps (or any external program), or reading /proc?

@mpe - I think OP wants to find a process' memory usage programmatically (and without using `exec` to spawn a separate process and executing `ps -C processname h -o size`). — ArjunShankar, Aug 16 '12 at 09:10
@mpe - Your edit was mostly good, but why did you add 'or reading `/proc`'? That seems like a reasonable solution... — Kevin Vermeer, Aug 16 '12 at 14:45
@Alexandre - Why don't you want to use an external program? Whether it's because you're worried about security concerns, because you don't want to add that complexity to your code, because launching and running the program takes too long, because they're not available on your custom system, because you don't want to worry about knowing where they're installed, or because you want to do this for educational purposes will result in very different answers. — Kevin Vermeer, Aug 16 '12 at 15:00
I removed the bit about not reading `/proc` because the original question, although quite difficult to understand, never said anything about not wanting to read it. It just required avoiding executing an external program. — ArjunShankar, Aug 16 '12 at 17:47
The original mentioned proc_open, which I took to mean "without using /proc", but I might have misinterpreted it. — mpe, Aug 16 '12 at 23:25

score 7 · Answer 1 · answered Aug 16 '12 at 09:09

On a Linux system, a process' memory usage can be queried by reading /proc/[pid]/statm. Where [pid] is the PID of the process. If a process wants to query its own data, it can do so by reading /proc/self/statm instead. man 5 proc says:

/proc/[pid]/statm

          Provides information about memory usage, measured in pages.  The
          columns are:

              size       total program size
                         (same as VmSize in /proc/[pid]/status)
              resident   resident set size
                         (same as VmRSS in /proc/[pid]/status)
              share      shared pages (from shared mappings)
              text       text (code)
              lib        library (unused in Linux 2.6)
              data       data + stack
              dt         dirty pages (unused in Linux 2.6)

You could just open the file with: fopen("/proc/self/statm", "r") and read the contents.

Since the file returns results in 'pages', you will want to find the page size also. getpagesize () returns the size of a page, in bytes.

score 0 · Answer 2 · edited May 23 '17 at 10:34

You have a few options to do find the memory usage of a program:

Run it within a profiler like Valgrind or memprof.
exec/proc_open/fork a new process to use ps, top, or pmap as you would from the command line
bundle the ps into your app and use it directly (it's open source, of course!)
Use the /proc system (which is all that ps does, anyways...)
Create a report the kernel, which watches over process memory operations. The /proc filesystem is just a view into the kernel's internal data structures, so this is really already done for you.
Develop your own mechanism to compute memory usage without kernel assistance.

The former are all educational from a system administration perspective, and would be the best options in a real-life situation, but the last bullet point is probably the most interesting. You'd probably want to read the source of Valgrind or memprof to see how it works, but essentially what you'd need to do is insert your mechanism between the app and the kernel, and intercept any requests for memory allocation. Additionally, when the process started, you would want to initialize its memory space with a preset value like 0xDEADBEEF. Then, after the process finished, you could read through the memory space and count the occurrences of words other than your preset value, giving you an estimate of memory usage.

Of course, things are always more complicated than they seem. What about memory used by shared libraries? Pipes? Shared memory between your processes and another? System calls? Virtual memory allocated but not used? Data buffered to the disk? There's a lot of calls to be made beyond your question 'memory of process', see this post for some additional concerns.

How to measure the memory usage of a process without calling an external program

2 Answers2