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Maybe this will clear it up... You have a bunch of processes, some running, and some not. If a processes is not running, it isn't using CPU.
The reason some are not running is that they are waiting for an I/O to complete (iowait), or in hardware interrupt processing (hi), or software interrupt processing (si), or they can't get the CPU because the hypervisor is running (stealing CPU) at the moment (st). The last reason, st, is different because it actually shrinks your available CPU for a brief interval.
Those are the currently defined reasons for not running on the CPU, and every other reason (voluntary wait, system call wait, file event wait, etc.) are dumped into the remaining bucket, idle.
Tomorrow, the kernel developers could decide to break out 20 more reasons to be idle. Seriously, this could happen (there's a new scheduler under consideration). So attempting to get available CPU by accumulating the subcategories of idle will be different on different *NIX platforms, kernel releases, measurement tool releases.
That's why a more stable way of getting it is "100-(user+system+steal)". It's independent of the reasons something isn't running, and just looks at what's missing from the total.
Last edited by macemoneta; 04-19-2007 at 07:02 PM.