RSS is how much memory this process currently has in main memory (RAM). VSZ is how much virtual memory the process has in total. This includes all types of memory, both in RAM and swapped out. These numbers can get skewed because they also include shared libraries and other types of memory. You can have five hundred instances of bash running, and the total size of their memory footprint won't be the sum of their RSS or VSZ values.
If you need to get a more detailed idea about the memory footprint of a process, you have some options. You can go through /proc/$PID/map and weed out the stuff you don't like. If it's shared libraries, the calculation could get complex depending on your needs (which I think I remember).
If you only care about the heap size of the process, you can always just parse the [heap] entry in the map file. The size the kernel has allocated for the process heap may or may not reflect the exact number of bytes the process has asked to be allocated. There are minute details, kernel internals and optimisations which can throw this off. In an ideal world, it'll be as much as your process needs, rounded up to the nearest multiple of the system page size (getconf PAGESIZE will tell you what it is — on PCs, it's probably 4,096 bytes).
If you want to see how much memory a process has allocated, one of the best ways is to forgo the kernel-side metrics. Instead, you instrument the C library's heap memory (de)allocation functions with the LD_PRELOAD mechanism. Personally, I slightly abuse valgrind to get information about this sort of thing. (Note that applying the instrumentation will require restarting the process.)
Please note, since you may also be benchmarking runtimes, that valgrind will make your programs very slightly slower (but probably within your tolerances).
The man page you refer to comes from the procps version of top.
But you're on an embedded system, so you have the busybox version of top.
It looks like busybox top calculates %MEM as VSZ/MemTotal instead of RSS/MemTotal.
The latest version of busybox calls that column %VSZ to avoid some confusion. commit log
Best Answer
On busybox, "ps" doesn't have a "-o" option, but "ps l" includes the RSS column.
If the underlying O/S is Linux, you can also get more specific details for a given process from:
The output looks like this:
In this list, VmRSS is the current resident set, and VmHWM is the resident set high-water-mark.