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Date: Sun, 1 Apr 2018 23:23:41 -0700 (PDT)
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Subject: Re: Large number of tasks slows down my program (using debian) - any
 fix?
From: alby.gamper@gmail.com
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Xref: reader02.eternal-september.org comp.lang.ada:51291
Date: 2018-04-01T23:23:41-07:00
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On Friday, March 30, 2018 at 8:04:19 PM UTC+11, Dmitry A. Kazakov wrote:
> On 2018-03-30 00:33, Shark8 wrote:
> 
> > Hm, you shouldn't be hitting the limit with so few tasks. (500 sounds like a lot, but most OSes you'll be using use an integer-ID; meaning something on the order of 2**32 or 2**64.)
> 
> It is a simple calculation. Assuming a context switch were around 
> 1000ns. x 500 tasks / 4 cores = 125ms if all tasks are busy. 125ms is a 
> long time.
> 
> Another factor is scheduling. Assuming the time quant is 10ms (Windows 
> default) and all task are doing pure calculations uninterrupted by any 
> I/O, then, again: 10 x 500 / 4 = 1.25s [+125ms] is the time a task gets 
> back to a core. 1.3s is an eternity.
> 
> -- 
> Regards,
> Dmitry A. Kazakov
> http://www.dmitry-kazakov.de

Hi All
I've done a quick mock-up test which just shows the activation time of 500 tasks
This is the code, targeting windows (x64). Results show 40 & 45 ms for the two
cases (running on a Intel Core i7-2600 cpu running @ 3.4 GHz)

with Ada.Text_IO;                       use Ada.Text_IO;
procedure AdaTasking is

    type Bool is new Integer;
    type Large_Integer is mod 2 ** 64;

    task type Worker is
    end;

    task body Worker is
    begin
        null;
    end;

    task type WorkerWithEntry is
        entry Start;
        entry Stop;
    end;

    task body WorkerWithEntry is
    begin
        accept Start;
        null;
        accept Stop;
    end;

    function QueryPerformanceFrequency( lpFrequency : access Large_Integer) return Bool;
    pragma import (stdcall , QueryPerformanceFrequency , "QueryPerformanceFrequency");

    function QueryPerformanceCounter( lpPerformanceCount : access Large_Integer) return Bool;
    pragma import (stdcall , QueryPerformanceCounter , "QueryPerformanceCounter");

    Ok          : Bool := 0;
    Frequency   : aliased Large_Integer := 0;
    StartTime   : aliased Large_Integer := 0;
    EndTime     : aliased Large_Integer := 0;
    ElapsedTime : aliased Large_Integer := 0;
    
begin

    Ok := QueryPerformanceFrequency(Frequency'access);
    Frequency := Frequency / 1000; -- units is ms (ie milli seconds)
    
    Ok := QueryPerformanceCounter(StartTime'access);
	declare
        Worker_Pool : array (1..500) of Worker;
    begin
        for i in Worker_Pool'Range loop
            null;
        end loop;
    end;
    Ok := QueryPerformanceCounter(EndTime'access);
    ElapsedTime := (EndTime-StartTime) / Frequency;
    Put_Line("Worker          Elapsed Time = " & ElapsedTime'Image & " (ms)");

    Ok := QueryPerformanceCounter(StartTime'access);
	declare
        Worker_Pool : array (1..500) of WorkerWithEntry;
    begin
        for i in Worker_Pool'Range loop
            Worker_Pool(i).Start;
            Worker_Pool(i).Stop;
        end loop;
    end;
    Ok := QueryPerformanceCounter(EndTime'access);
    ElapsedTime := (EndTime-StartTime) / Frequency;
    Put_Line("WorkerWithEntry Elapsed Time = " & ElapsedTime'Image & " (ms)");

end;