assigning priority to task

assigning priority to task

[Stribor40]

I am playing with example from here..
http://www.infres.enst.fr/~pautet/Ada95/e_c26_p2.ada

and each run i get different output which is ok but what I am trying to do is somehow control running of each of those 3 tasks by assigning priority to each task by using pragma PRIORITY(some int) where higher the int higher the priority.
So my question is this ....If i assign first task priority(8) and second priority(4) and third priority(2) does that mean that first task will always run first second second and third third? How does OS time slicing fit into this when priority is assigned to each task?

Thanks

Re: assigning priority to task

[Jeffrey Carter]

On 09/23/2014 06:50 PM, Stribor40 wrote:
> I am playing with example from here..
> http://www.infres.enst.fr/~pautet/Ada95/e_c26_p2.ada
> 
> and each run i get different output which is ok but what I am trying to do is somehow control running of each of those 3 tasks by assigning priority to each task by using pragma PRIORITY(some int) where higher the int higher the priority.
> So my question is this ....If i assign first task priority(8) and second priority(4) and third priority(2) does that mean that first task will always run first second second and third third? How does OS time slicing fit into this when priority is assigned to each task?

Suppose you run the program on a system with 4 cores. Will the priorities
control the order in which the tasks run?

-- 
Jeff Carter
"All citizens will be required to change their underwear
every half hour. Underwear will be worn on the outside,
so we can check."
Bananas
29

Re: assigning priority to task

[Stribor40]

I am not sure. I would think it wouldnt matter since each task can run on one core

Re: assigning priority to task

[Stribor40]

Also my box has 2 cores. Is there way in ada to see which core is running which task?

Re: assigning priority to task

[Jeffrey Carter]

On 09/23/2014 08:36 PM, Stribor40 wrote:
> I am not sure. I would think it wouldnt matter since each task can run on one core

Probably right. That is the abstraction that Ada tasking is supposed to
represent. Trying to control when such tasks execute without having them
interact is futile.

-- 
Jeff Carter
"All citizens will be required to change their underwear
every half hour. Underwear will be worn on the outside,
so we can check."
Bananas
29

Re: assigning priority to task

[Jeffrey Carter]

On 09/23/2014 08:39 PM, Stribor40 wrote:
> Also my box has 2 cores. Is there way in ada to see which core is running which task?

On many modern processors, each core can execute 2 tasks at the same time
(effectively). So it may be possible for all 4 tasks in your example to run at
the same time.

An Ada-12 compiler that implements Annex D has pkg
System.Multiprocessors.Dispatching_Domains (ARM D.16.1) that lets you determine
the CPU that a task is running on, as well as aspect CPU to force a task to run
on a specific CPU. Compilers for earlier versions of Ada may have
vendor-specific ways to do similar things.

-- 
Jeff Carter
"All citizens will be required to change their underwear
every half hour. Underwear will be worn on the outside,
so we can check."
Bananas
29

Re: assigning priority to task

[Stribor40]

I know but i wanted to try to see if i sssign priorities output can be somewhat controled but apparently those assigned priorities didnt matter

Re: assigning priority to task

[AdaMagica]

Basically, priorities have only effect on tasks contending for the processor.

Ada 83 RM says someting like: If two tasks are runnable at the same time on a processor, it cannot be the case that the lower priority task executes while the higher one does not... Priorities should only be used for fine-tuning.

Since then, this basic rule has not changed, but has been modified by the presence of multiprocessors.

So be careful.

Re: assigning priority to task

[Stribor40]

how is calling another task at the entry point different from main thread calling another function or procedure and passing parameters to those? sorry if question is dumb but I am just failing to see the difference....
Is it because of tasks synchronization mechanism that calling tasks waits if another tasks is using things defined after accept statement?

Re: assigning priority to task

[Björn Lundin]

On 2014-09-24 15:17, Stribor40 wrote:
> how is calling another task at the entry point different from main thread calling another function or procedure and passing parameters to those? sorry if question is dumb but I am just failing to see the difference....
> Is it because of tasks synchronization mechanism that calling tasks waits if another tasks is using things defined after accept statement?
> 

(Not compiled code)

task T is
  entry T1;
  entry T2;
end T


task body T is
begin

  loop
    select
      accept T1 do
        Put_Line ("rendez-vouz start");
        Put_Line ("Do_stuff that holds caller");
        Put_Line ("rendez-vouz stop");
      done
    or
      accept T1 do
        Put_Line ("rendez-vouz start");
        Put_Line ("rendez-vouz stop");
      done
      -- parallel execution from here
      Put_Line ("Do_stuff that DOES NOT hold caller");
      -- to here
    or
      terminate;
    end select;
  end loop;
end T



if main calls T.T1, the difference
compared with calling a procedure
T1 in package T is that the work is
performed in another thread and main waits until complete.
You do not get parallel execution.
You serialize work, but could use a protected object instead

however,

if main calls T.T2, the difference
compared with calling a procedure
T2 in package T is that the work is
performed in another thread and main is released right away,
so the T2 work is carried out while main continues.
You get _parallel_ execution.



--
Björn

Re: assigning priority to task

[Anh Vo]

On Tuesday, September 23, 2014 8:39:20 PM UTC-7, Stribor40 wrote:
> Also my box has 2 cores. Is there way in ada to see which core is running which task?

Yes, there is. In addition, a given task can be assigned to run on specific core (CPU)

Anh Vo

Re: assigning priority to task

[Simon Wright]

Stribor40 <ikamzic@gmail.com> writes:

> I know but i wanted to try to see if i sssign priorities output can be
> somewhat controled but apparently those assigned priorities didnt
> matter

It used to be the case on Linux (and, I think, Solaris) that pragma
Priority only had any effect if the program was running as root:

   $ sudo ./task1

I just tried this on Mac OS X and running as root made no difference.