As for the shared APC cache, I think hope I counted that correctly. The ps_mem.py script I used to measure memory does a pretty good job of measuring shared memory usuage in linux.

(Of course one could also expect good results with an nginx front end serving the static stuff and proxying the PHP stuff to Apache + mod_php + APC. Just another way of reaching the same goal)

I think that extra memory usage without APC is real – PHP doesn’t return memory to the operating system until it exits. And PHP needs the memory for compiling purposes immediately before it needs yet more memory to run the actual PHP code, so there isn’t much of an opportunity for pages to swap out and make room for other processes.

On the other hand, it’s possible your stats aren’t counting the APC cache shared by all of the processes at least once… some OSes won’t show the shared memory at all, others will count it against every process, neither is really entirely fair for benchmarking but we know the size of the APC cache is fixed and we can take that into account when we read stats as long s we know which behavior the OS exhibits in its stats