Re: [Announce] [patch] Modular Scheduler Core and Completely FairScheduler [CFS]

[Davide Libenzi]

On Wed, 18 Apr 2007, Ingo Molnar wrote:

> That's one reason why i dont think it's necessarily a good idea to 
> group-schedule threads, we dont really want to do a per thread group 
> percpu_alloc().

I still do not have clear how much overhead this will bring into the 
table, but I think (like Linus was pointing out) the hierarchy should look 
like:

Top (VCPU maybe?)
    User
        Process
            Thread

The "run_queue" concept (and data) that now is bound to a CPU, need to be 
replicated in:

ROOT <- VCPUs add themselves here
    VCPU <- USERs add themselves here
        USER <- PROCs add themselves here
            PROC <- THREADs add themselves here
                THREAD (ultimate fine grained scheduling unit)

So ROOT, VCPU, USER and PROC will have their own "run_queue". Picking up a 
new task would mean:

VCPU = ROOT->lookup();
USER = VCPU->lookup();
PROC = USER->lookup();
THREAD = PROC->lookup();

Run-time statistics should propagate back the other way around.


> In fact for threads the _reverse_ problem exists, threaded apps tend to 
> _strive_ for more performance - hence their desperation of using the 
> threaded programming model to begin with ;) (just think of media 
> playback apps which are typically multithreaded)

The same user nicing two different multi-threaded processes would expect a 
predictable CPU distribution too. Doing that efficently (the old per-cpu 
run-queue is pretty nice from many POVs) is the real challenge.



- Davide


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