Charting server load

[Lennart Sorensen]

On Thu, Feb 01, 2007 at 04:03:31PM -0500, Ian Petersen wrote:
> I can see how a system with high load might still be responsive
> because I would define responsive as "responds nearly immediately to
> user input" where "user input" could be defined as key presses, mouse
> movements, ssh connection requests, etc.  This could easily be
> accomplished by running user-facing processes at high priority so that
> user input forces the relevant processes onto the CPU.
> 
> On the other hand, a non-hyperthreaded, single-core uniprocessor can
> still only do "one thing at a time", ignoring vector instructions, so
> if, for the last minute, there was an average of 50 ready processes in
> the run-queue, how is that not "overloaded"?  A load of 50, to me,
> means that at any given moment the kernel had 50 things to do but had
> to pick one (or N on an N-core system).  If I had 50 things to do at
> any given moment and was restricted to doing only one at a time, I'd
> feel pretty stressed.

If the 50 processes each needed 2.01% of the cpu to get all their work
done all the time, then you are only overloaded by 0.5%, so really you
are keeping up with the load.  If even one of the processes was to be
stopped, your load should actually drop quickly to less than 1 since it
would suddenly be able to keep up with all the work.  Load averages are
rather weird that way.  Another system could have 2 tasks running which
both need all the cpu they can get, which would mean by some definitions
that the system is overloaded by 100%, although of course if they are
really cpu bound no matter how fast your cpu, they would still want 100%
of it until the task is done.  You could have 2 tasks, each of which need
75% of the cpu to get all their work done real time, but by having two,
you get a load average of 2 and both tasks are not quite keeping up with
their work in real time.

--
Len Sorensen
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