Re: Multitasking and containers

[Maciej Sobczak <no.spam@no.spam.com>]

Dmitry A. Kazakov wrote:

>> Paragraph 3 in Annex A says that it's OK to call any standard subprogram 
>> from concurrent tasks as long as the parameters do not overlap. John 
>> Barnes ("Progamming in Ada 2005") suggests that in order to (for 
>> example) read from the same container, the operations need to be 
>> protected "by using the normal techniques such as protected objects".
>>
>> But reading from the protected object is not mutually exclusive (many 
>> readers are allowed) - so where's the gain? What's the difference 
>> between concurrent reads of, say, a Vector via protected object vs. 
>> direct access?
> 
> I think he didn't mean container being a protected object.

No, but you might want to encapsulate a container within a protected 
object and have some protected procedures and functions forwarding to 
relevant ones in the container.

> [ I don't know if ARM 2005 permits mutable implementations of read
> operations on the containers. I suppose it does, so the need to lock upon
> read. ]

That would be interesting, but would break apart when encapsulated 
within a protected object, because there multiple readers would be allowed.

Having a mutex for readers sounds like a concurrency killer and relying 
on protected wrappers seems to be fragile because of this possible 
mutability. So - what is The Solution (tm) for multiple tasks reading 
from the same container?

Let's say you want to have N worker tasks consulting a shared dictionary 
(map) that was initialized before the tasks started their work. How 
would you solve this?

-- 
Maciej Sobczak : http://www.msobczak.com/
Programming    : http://www.msobczak.com/prog/