Re: GNAT and Tasklets

[Hubert <herrdoktor@fumanchu.com>]

>
> So my question is : does GNAT create a kernel thread for each local task or is it able to compile local tasks as jobs sent to a pool of tasks created in the runtime ?

I asked a similar question about 2 years ago when I was busy 
implementing a request system on a server (albeit in C++ but I was 
already ogling at Ada) and I got different answers, depending on the 
compiler.
For me there was an additional requirement though, I had to deal with 
several
1000s of "Jobs" in parallel, not in hard realtime, but they had to be 
served as soon as possible. the result of my research was that depending 
on the OS the Ada program was running on you could get several 100 OS 
threads or maybe 1-2K on Linux but there is an upper limit because every 
OS thread that runs a task will have a stack associated with it, so 
mostly the available memory is the limit, I think.
Then when your Jobs use only very limited stack resources, this can be a 
great waste, especially when thy have different stack requirements so 
you will ahve to go with the greatest common value which means lots of 
waste.
My solution was to implement my own pre-emptive Job system on top of the 
OS threads. I allocate as many threads (or Tasks in Ada) as there are 
processor cores and then assign a number of Jobs to each.
The Jobs work internally with a state machine. Whenever they perform a 
blocking operatin (mostly communicating with other Jobs), they send a 
message and return control and when the answer arrives, my system calls 
them again with the next state whee they fetch the answer.

Depending on what your requirements are (great number of parallel Jobs), 
this may very well be your only reliable solution.







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