Re: power-efficient scheduling design

From: David Lang
Date: Tue Jun 18 2013 - 14:42:23 EST

On Tue, 18 Jun 2013, Morten Rasmussen wrote:

I don't think that you are passing nearly enough information around.

A fairly simple example

take a relatively modern 4-core system with turbo mode where speed controls
affect two cores at a time (I don't know the details of the available CPUs to
know if this is an exact fit to any existing system, but I think it's a
reasonable fit)

If you are running with a loadave of 2, should you power down 2 cores and run
the other two in turbo mode, power down 2 cores and not increase the speed, or
leave all 4 cores running as is.

Depending on the mix of processes, I could see any one of the three being the
right answer.

If you have a process that's maxing out it's cpu time on one core, going to
turbo mode is the right thing as the other processes should fit on the other
core and that process will use more CPU (theoretically getting done sooner)

If no process is close to maxing out the core, then if you are in power saving
mode, you probably want to shut down two cores and run everything on the other

If you only have two processes eating almost all your CPU time, going to two
cores is probably the right thing to do.

If you have more processes, each eating a little bit of time, then continuing
to run on all four cores uses more cache, and could let all of the tasks finish

So, how is the Power Scheduler going to get this level of information?

It doesn't seem reasonable to either pass this much data around, or to try and
give two independant tools access to the same raw data (since that data is so
tied to the internal details of the scheduler). If we are talking two parts of
the same thing, then it's perfectly legitimate to have this sort of intimate
knowledge of the internal data structures.

I realize that my description is not very clear about this point. Total
load is clearly not enough information for the power scheduler to take
any reasonable decisions. By current load, I mean per-cpu load, number
of tasks, and possibly more task statistics. Enough information to
determine the best use of the system cpus.

As stated in my previous reply, this is not the ultimate design. It
expect to have many design iterations. If it turns out that it doesn't
make sense to have a separate power scheduler, then we should merge
them. I just propose to divide the design into manageable components. A
unified design covering the scheduler, two other policy frameworks, and
new policies is too complex in my opinion.

The power scheduler may be viewed as an external extension to the
periodic scheduler load balance. I don't see a major problem in
accessing raw data in the scheduler. The power scheduler will live in
sched/power.c. In a unified solution where you put everything into
sched/fair.c you would still need access to the same raw data to make
the right power scheduling decisions. By having the power scheduler
separately we just attempt to minimize the entanglement.

Why insist on this being treated as an external component that you have to pass messages to?

If you allow it to be combined, then it can lookup the info it needs rather than trying to define an API between the two that accounts for everything that you need to know (now and in the future)

This will mean that as the internals of one change it will affect the internals of the other, but it seems like this is far more likely to be successful.

If you have hundreds or thousands of processes, it's bad enough to lookup the data directly, but trying to marshal the infromation to send it to a separate component seems counterproductive.

David Lang
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