Re: [RFC] (How to) Let idle CPUs sleep

[Tony Lindgren <tony@atomide.com>]

* Martin Schwidefsky <schwidefsky@de.ibm.com> [050512 11:15]:
> > > Seems like we could schedule timer interrupts based solely on add_timer
> 
> > > type stuff; the scheduler could use it if necessary for load balancing
> > > (along with fork/exec based balancing perhaps) on large machines where
> > > load imbalances hurt throughput a lot.  But on small systems if all
> >
> > Even if we were to go for this tickless design, the fundamental question
> > remains: who wakes up the (sleeping) idle CPU upon a imbalance? Does some
> other
> > (busy) CPU wake it up (which makes the implementation complex) or the
> idle CPU
> > checks imbalance itself at periodic intervals (which restricts the amount
> of
> > time a idle CPU may sleep).
> 
> I would prefer a solution where the busy CPU wakes up an idle CPU if the
> imbalance is too large. Any scheme that requires an idle CPU to poll at
> some intervals will have one of two problem: either the poll intervall
> is large then the imbalance will stay around for a long time, or the
> poll intervall is small then this will behave badly in a heavily
> virtualized environment with many images.
> 
> > > your processes were blocked, you'd just go to sleep indefinitely and
> > > save a bit of power and avoid unnecessary overhead.
> > >
> > > I haven't looked at the lastest tickless patches, so I'm not sure if my
> > > claims of simplicity are overblown, but especially as multiprocessor
> > > systems become more and more common it just seems wasteful to wakeup
> > > all the CPUs every so often only to have them find that they have
> > > nothing to do.
> >
> > I guess George's experience in implementing tickless systems is that
> > it is more of an overhead for a general purpose OS like Linux. George?
> 
> The original implementation of a tickless system introduced an overhead
> in the system call path. The recent solution is tickless while in idle
> that does not have that overhead any longer. The second source of
> overhead is the need to reprogram the timer interrupt source. That
> can be expensive (see i386) or cheap (see s390). So it depends, as usual.
> In our experience on s390 tickless systems is a huge win.

In the dyn-tick patch for ARM and x86, timer reprogramming is only
done during idle. When the system is busy, there is continuous tick
and timer reprogramming is skipped. So the second source of overhead
should be minimal.

Tony
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