Re: decaying average for %CPU

[Albert Cahalan]

On Thu, 2003-10-16 at 22:56, Nick Piggin wrote:
> Albert Cahalan wrote:
> 
> >The UNIX standard requires that Linux provide
> >some measure of a process's "recent" CPU usage.
> >Right now, it isn't provided. You might run a
> >CPU hog for a year, stop it ("kill -STOP 42")
> >for a few hours, and see that "ps" is still
> >reporting 99.9% CPU usage. This is because the
> >kernel does not provide a decaying average.
> 
> I think the kernel provides enough info for userspace to do
> the job, doesn't it?

I'm pretty sure not. Linux provides:

per-process start time
current time
per-process total (lifetime) CPU usage
units of time measurement (awkwardly)
boot time

>From that you can compute %CPU over the whole
life of the process. This does not meet the
requirements of the UNIX standard.

What we do for load average is about right,
except that per-process values can't all get
updated at the same time. So the algorithm
needs to be adjusted a bit to allow for that


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