> Linux tries to measure times spend in
> processes... Somehow. Unfortunately, it is optimistic: it will show
> less than real values under some pretty common
> circumstances. (Application sleeping for short times and doing small
> ammount of processing is marked as eating 0% of system time. There are
> some applications which do this - for example ksame, qweb (and I think
> that everything using qt) - which do evil busy waiting.)
on SMP this is different. User-space process time accounting uses a
different clock source on most architectures. (the local APIC clock on
Intel).
btw, we could do _exact_ process accounting if we want to, at the price of
~10-15 cycles per schedule(). we can just read the cycle counter (industry
standard on most leading CPUs ;), add the delta to the process and do the
math later, whenever someone (top) tries to access the values. This way we
could even differentiate between IRQ cycles, kernel cycles, kernel thread
cycles, idle cycles and user-space cycles. Is this an important and
fundamental enough feature to justify those 10-15 cycles?
-- mingo
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