Affinity and Scheduler Instance Interaction

[Joel Sherrill]

Hi

We have an SMP behavioral decision to make and it isn't getting
enough discussion.

With cluster scheduling, there are potentially multiple scheduler
instances associated with non-overlapping subsets of cores.

With affinity, a thread can be restricted to execute on a subset
of the cores associated with a scheduler instance.

There are operations to change the scheduler associated with
a thread and the affinity of a thread.

The question is whether changing affinity should be able to
implicitly change the scheduler instance?

I lean to no because affinity and scheduler changes should
should be so rare in practical use that nothing should be
implicit. 

Consider this scenario:

Scheduler A: cores 0-1
Scheduler B: cores 2-3

Thread 1 is associated with Scheduler B and with affinity 2-3
can run on either processor scheduled by B.

Thread 1 changes affinity 1-3. Should this change the scheduler,
be an error, or just have an affinity for a core in the system
that is not scheduled by this scheduler instance?

If you look at the current code for _Scheduler_Set_affinity(),
it looks like the current behavior is none of the above and
appears to just be broken. Scheduler A's set affinity operation
is invoked and Scheduler B is not informed that it no longer
has control of Thread 1.

-- 
Joel Sherrill, Ph.D.             Director of Research & Development
joel.sherrill at OARcorp.com        On-Line Applications Research
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