Activity Monitor – How is the Memory Column Calculated?

I was doing some reading about inactive memory after wondering about it myself. My understanding is that inactive memory is memory containing programs and data which you have quit. OS X keeps the memory incase you decide to restart the program. Because it is still in memory it can start it much quicker. You can see this in action by quitting a large program and restarting it. It should start much faster than it originally did.

Inactive memory does not suck up your memory though because OS X regards it as still available for programs to use, jsut at a lower priority than free memory. ie. if OS X needs memory for another program, it will allocate memory from the free area, then the inactive allocation. So when thinking about how much memory you have free for applications to use, inactive and free is combined.

As for swap, if a program is using it you should easily be able to tell by the amount of I/O going on and a noticable slow down.

If you really want to see whats going on I would suggest download iStat menus which is a really nice tool for seeing the internals of your system and display various instruments in the menu bar.

Yes - the per process memory statistics and mach call counters mean the same thing as the system wide statistics.

Activity Monitor helps you break down which specific processes are making up the conglomerated statistics if you use the screens shown above. You may also find the sysdoagnose command useful in probing how the various numbers all add up if you prefer a textual output as opposed to a combined graphical and textual one.