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Subjectdifferent speed cores in one system (aks arm big.LITTLE)
Following an article on lwn about more arm systems with different speed 
cores in them ( subscribers: http://lwn.net/Articles/481055
non-subscribers: http://lwn.net/SubscriberLink/481055/cc3426371b328030 )

It seems to me that the special case approach of pairing a 'fast' and
'slow' core together is a hack that will work with this particular part,
but not work more generally. The approaches discussed seem to be more
complicated than they needed to be. I've outlined my thoughts in the
comments there, but figured I'd post here to get the attention of the
scheduler folks.

First off, even on Intel/AMD x86 systems we have (or soon will have) the
potential for different cores to be run at different speeds, including
thermal/current limitations that make it so that if you turn off some
cores you can run others at higher speeds. This means that the problem is
not an ARM specific problem.

As I understand the current scheduler to work, there are two 'layers' to
the scheduler.

The first 'layer' runs independantly on each core (using cpu local
variables for performance) and schedules the next task to run from the
tasks assigned to that core.

The second 'layer' moves tasks from one core to another. Ideally it runs
when a core is otherwise idle, but it looks at the load on all the cores
and can choose to 'pull' work from another core to itself. Part of the
logic in deciding if it should pull a job could be considering the NUMA
positioning of the old and new core to decide if it's a benefit to pull it
or not.


I believe that unless one tasks (i.e. thread) is using more CPU than the
slowest core can provide the current scheduler will 'just work' in the
presense of cores of differing speeds, a slower core will get less work
done, but that will just mean that it's utilization is higher for the same
amount of work, so work will migrate around until the utilization is
rougly the same.

I think it may be worth adding a check to the 'slow path' rebalancing
algorithm, probably in a similar place to where the NUMA checks are made
that will scale the tasks utilization to see if there is an advantage in
pulling a task that's maxing out one core onto the new core (if the new
core is faster, it can be a win), possibly adding a second check to make
sure that you aren't migrating a task to a core that's not fast enough.

With this additional type of check, I think that the current scheduler
will work well on systems with different speed cores, including drastic
differences.

At that point, the remaining question is the policy question of what cores
should be powered up/down, clockspeeds changed, etc. Since that sort of
thing is very machine and workload specific, it seems to me that the
obvious answer is that a userspace daemon working completely independantly
to the kernel should watch the system and make the policy decisions to
reconfigure the CPUs (very similar to how userspace power management tools
work today), and 'just' extend the ability to change clock speeds to the
ability to power down particular cores entirely.

The lwn article positions this as a super complex thing to figure out and
is talking about doing hacks like pairing a fast and slow core togeather
to only use one of the two at a time and consider moving work from one to
the other 'just' a clockspeed change. This seems to me to be a far more
complex approach than just adding the extra check to the scheduler slow
path and doing the power management in userspace.

Thoughts?

Am I completely misunderstanding how the scheduler works? (I
know I'm _drastically_ simplifying it)

Am I completely off base here? or am I seeing something that the ARM folks
have been missing because they are too close to the problem?

David Lang


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