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

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

On 2016-07-02 06:21, Randy Brukardt wrote:
> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message
> news:nknteq$177m$1@gioia.aioe.org...
>> On 2016-06-26 05:09, Randy Brukardt wrote:
>>> "Hadrien Grasland" <hadrien.grasland@gmail.com> wrote in message
>>> news:f4c3e42a-1aa1-4667-83d7-de6f81a8fdd2@googlegroups.com...
>>> ....
>>> It seems to me that the problem is with the "typical" Ada implementation
>>> more than with the expressiveness of features, when it comes to highly
>>> parallel implementations. Mapping tasks directly to OS threads only works
>>> if
>>> the number of tasks is small. So if it hurts when you do that, then DON'T
>>> DO
>>> THAT!! :-)
>>>
>>> There's no reason for any particular mapping of Ada tasks to OS threads.
>>
>> Ah, but there is the reason. The OS can switch threads on I/O events. If
>> Ada RTS could do this without mapping tasks to threads, fine. Apparently
>> it cannot without imposing same or higher overhead OS threads have.
>
> Dunno; I doubt that it really has been tried. If that was really impossible,
> virtualization wouldn't work either, and there's lots of evidence that works
> fine.

But it does not, in this sense. It can handle only virtual devices. In 
order to access a physical one, you need a software layer. It could be 
quite difficult for the RTS to follow this path.

> The problem (if there is a problem at all) is fast character-at-a-time I/O -
> but that's a bad idea for lots of reasons (it doesn't work very well for a
> sequential program either) so perhaps it wouldn't matter than much.

It will make normal I/O slower and RTS larger with compatibility issues.

>> Co-routines could offer something in between.
>
> Sure, but I don't see any need to expose that. Certainly not beyond an
> aspect much like "Inline", which is just a suggestion to the compiler to do
> something. It's better to leave implementation decisions (and that's all
> this is) to implementations!

Well, the difference is that Inline can be safely ignored. Limitation on 
the task number cannot be.

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