Different between memory usage in WHM/Cpanel and Linux

Question

If I go to WHM and see my server's memory usage, it says that only 16% of memory is in use. But when I connect to server using SSH and run command "free -m" then it shows that 80% is in use. Why is that? I want to know exact memory usage of all applications running like MySQL, Apache e.t.c. How do I view that?

Thanks

Answer 1

As they say, "It's Complicated".

Linux uses unused memory for disk buffering and caching. It speeds things up. But you may need to look at the -/+ buffers/cache line of free.

'ps' can show you, for any given process, or for all processes, the %cpu, %mem, cumulative cpu-time, rss (resident set size, the non-swapped physical memory that a process is using), size (very approximate amount of swap space that would be required if the process were to dirty all writable pages and then be swapped out), vsize (virtual memory usage of entire process (vm_lib + vm_exe + vm_data + vm_stack)), and much much more.

For any given process, you can cat /proc/$PID/status -- it's human readable -- and check out the VmSize, VmLck, VmRSS, VmData, VmStk, VmExe, VmLib, and VmPTE values, along with others...

But that's just for starters... Processes can allocate memory but not use it. (Memory can be allocated, but the memory pages are not created/issued until they're actually used. That whole on-demand thing.)

Processes can map in hardware space, showing up as using a large quantity of memory that's not actually coming from system RAM. (X-servers are known to sometimes do this. It's some wonky stuff involved kernel drivers...)

There's the executable, which is usually a memory-mapped file. Meaning that parts that are swapped-in are taking up RAM, but when swapped out it never takes up swapfile space.

Processes can have other memory-mapped files...

There's shared-memory libraries, where the same RAM pages are used by multiple programs concurrently.

So we have to ask, as far as memory goes, what exactly counts and what doesn't?