Comments on the KSE option

[Robert Watson]

On Sun, 29 Oct 2006, Lucas James wrote:

> I read what Paul said was that system scope threads have a different 
> "fairness" than processes. ie:
>
> If your application requires 1000 threads of execution, you can write it 
> three ways, with 1000 processes, 1000 system scope threads or 1000 process 
> scope threads (or a mix of the three).
>
> This whole "fairness" argument is about making system scope threads have the 
> same priority as process scope threads.  It leaves out the process model.
>
> The real question here is: are we going to make system scope thread model 
> fair compared to process scope threaded model, or fair compared to the 
> separate processes model?
>
> Yes, the process scope threads are allways going to be the poor man with 
> regard to priority, but as the kernel doesn't see the threads you can't do 
> much about it.

I think there are at least two core questions being discussed here:

(1) Does the "fairness" model currently implemented in the KSE code mean well,
     but cause significant performance problems in practice for real-world
     applications?

(2) Are the cost and complexity impacts of KSE in kernel architecture
     outweighed by the flexibility and performance benefits of M:N threading?

Now is definitely the time for us to be discussing, measuring, experimenting, 
etc, because addressing the issues of higher concurrency for 7.0 will depend 
on having decided on a strategy for our scheduler.

Robert N M Watson
Computer Laboratory
University of Cambridge