CODEWITHSUNDEEP
delphidelphi-10.1-berlinomnithreadlibrary
justyy
justyy

Reputation: 6041

Does Delphi Parallel.For have a MaxParallism

In C#, you can limit the number of threads, like this:

Parallel.For(0, 10, new ParallelOptions { MaxDegreeOfParallelism = 4 }, count =>
{
    Console.WriteLine(count);
});

Does Delphi have this in latest compiler e.g. Berlin. Or does the omnithreadlibrary have this?

Upvotes: 2

Views: 1621

Answers (3)

zeus
zeus

Reputation: 12955

this question have more implication that you think. first Tparallel is designed to work only on specifical case. for exemple

this is good to do with tparallel

 TParallel.For(1, Max, procedure (I: Integer)
   begin
     Do_processor_intensive_work
   end);

this must be avoiding :

 TParallel.For(1, Max, procedure (I: Integer)
   begin
     do_waiting_proc_like_downloading_url
   end);

why? and this is where i answer to your question : it's because Tparallel create the number of thread that match the number of processor (virtual or physical) available on the system. so if you have 32 processors then it's will create max 32 threads, if you have only 1 processor then it's will create only one thread. this is also global to all the app, if you have 2 thread that each do Tparalell, you will not have more than one thread by processor

so the idea behing Tparallel is that you don't need to worry about the number of thread, the system choose the most optimal number for you. but as you see in my sample, if your tparallel is not processor intensive, then you will probably need more thread than the number of available processor and in that case i strongly sugest to avoid tparallel and use instead TanonymousThread

you can override this number by doing SetMaxWorkerThreads (that by default is the number of processor) but if you need to do this their is great luck that you must avoid at all to use Tparallel and need to use instead TanonymousThread

Upvotes: 1

Arioch 'The
Arioch 'The

Reputation: 16065

Or does the omnithreadlibrary have this?

It does have it too.

If you would start with Parallel.For constructor, then you obtain IOmniParallelSimpleLoop Interface which has .NumTasks method, defaulting to CPU cores count. It might be adjusted if you detach the loop from the current execution thread using .NoWait call.

Whether you start with Parallel.ForEach generator you obtain the IOmniParallelLoop Interface and it has NumTasks as well.

See chapters 2.12.4 and 2.11.1 at http://otl.17slon.com/book/chap04.html#highlevel-for

Also see http://www.thedelphigeek.com/2011/01/parallel-for-implementation-1-overview.html for the generic idea of the intended call syntax.

It should be something like

Parallel.ForEach(0, 10 (* ,1 *) )
  .NumTasks( 4 )
  .Execute(
    procedure (const elem: integer)
    begin
      WriteLn( elem );
    end
  );

Upvotes: 2

David Heffernan
David Heffernan

Reputation: 613451

Use the Parallel.For overload that accepts a TThreadPool, and supply a thread pool that limits the number of threads. That is done with the SetMaxWorkerThreads method.

Note that I obtained this information by reading the documentation.

Upvotes: 3

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