Ubuntu Linux: Process swap memory and memory usage
The linux virtual memory system isn't quite so simple. You can't just add up all the RSS fields and get the value reported used by free. There many reasons for this, but I'll hit a couple of the biggest ones.
When a process forks, both the parent and the child will show with the same RSS. However linux employs copy-on-write so that both processes are really using the same memory. Only when one of the processes modifies the memory will it actually be duplicated.
This will cause thefreenumber to be smaller than thetopRSS sum.The RSS value doesn't include shared memory. Because shared memory isn't owned by any one process,
topdoesn't include it in RSS.
This will cause thefreenumber to be larger than thetopRSS sum.