Re: RFC: Prototype for a user threading library in Ada

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On 21/06/2016 04:40, rieachus@comcast.net wrote:
> On Friday, June 17, 2016 at 12:46:46 PM UTC-4, Dmitry A. Kazakov wrote:
>> On 2016-06-17 18:18, Niklas Holsti wrote:
>>
>> My take on this problematic is that there cannot exist a solution
>> implemented at the library level. All these frameworks maybe fun (for
>> the developer) but useless (horror for the end user) when the key
>> problem is not solved. That is, the control-flow state (as you said) and
>> the stack of the local objects both preserved between scheduling points.
>> This can be done IMO only at the language level as co-routines,
>> non-preemptive, cooperative, user-scheduled tasks, call it as you wish.
>
> I've been trying to understand not just the code, but the goal. I
> decided to start from a different perspective. What if I had a problem
> and wanted to distribute the solution across thousands of processors?
> Since I tend to bang my head against NP-hard or NP-complete problems, I
> would want a program structure that allowed me to start up at least one
> (Ada) task per processor, with enough data to complete, and for the job
> creation software to use a very wide tree. A similar reverse tree could
> be used to collect results if needed.

I have a similar problem. I implement network protocols. A classic 
implementation is to start one task per client-server connection, or two 
tasks when the exchange is full duplex. An OS can typically handle a few 
hundred tasks before it runs out of juice. So the solution is to share a 
large number of sockets between some few tasks. And so it runs into the 
problem of an I/O-events-driven design and thus to co-routines.

> But to do any of this, I head for the distributed systems annex. It
> might be nice to have a simple example of how to do that on top of MPI:
> https://computing.llnl.gov/tutorials/mpi/#MPI2-3 (To be honest, using
> the C or Fortran bindings is what I have done...)

Distributed annex is unusable for massively parallel systems because it 
is based on RPCs. It is the same problem in its core. Such a system 
requires asynchronous communication driven by I/O events. Yet the 
application logic is incoherent with the logic of I/O. In order to bring 
them together, again, [distributed] co-routines are needed to restore 
the application view of control flow.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de