Synchronize with MTProcs -> selfmade multithreading
lazarus / 5-Lazarus Extra Components

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Ocye · 2009-10-28 08:15

I'm using the great tool Multithreadproc. It scales perfectly and
boost the calculation time significantly. My problem is that MTProcs
has no Synchronize. During the operation no feedback can be given to
the user and the program seems to hang. Any idea how to implement
interaction into ProcThreadPool.DoParallel()?

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Ocye · 2010-01-18 12:22

For those who are interested in my solution:

Code: [Select]procedure TBruteforce.RunParallel(Index: PtrInt; Data:
Pointer; Item: TMultiThreadProcItem);
begin
  ProcThreadPool.EnterPoolCriticalSection;
  try
    Application.ProcessMessages; //e.g.
  finally
    ProcThreadPool.LeavePoolCriticalSection;
  end;
  TSingleOperation(FProcessing[index]).Run;
end;

FProcessing is a TList filled with items of TSingleOperation=class
with parameters and a run method. Enter- and LeavePoolCriticalSection
did the "trick" for interaction.

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Ocye · 2010-01-19 22:45

Rejoyed too soon! With Linux no synch error occours. Under Windows I
get an exception "CheckSynchronization called from non-main thread" or
program hangs. Now I remember what the problem is . There is still no
way for processing messages in the main thread.

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José Mejuto · 2010-01-21 00:17

Quote from: Ocye on January 19, 2010, 11:45:58 pmRejoyed too soon!
With Linux no synch error occours. Under Windows I get an exception
"CheckSynchronization called from non-main thread" or program hangs.
Now I remember what the problem is . There is still no way for
processing messages in the main thread.

Hello,

You can not in anyway call ProcessMessages from something different
than main thread, that's a task for Synchronize. I had not used it in
MTProc but you should be able to call Synchronize or take the current
thread object from "Item" parameter and synchronize using it.

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Ocye · 2010-01-21 11:03

As I said, with Linux ProcessMessages can be called without problems,
but, of course, it's not correct. I tried Item.Thread.Synchronize()
yesterday at different positions but the application freezes
completely. I created a callback method in mtprocs.pas (OnBeforeWork)
and invoke it with Synchronize. Code: [Select]procedure
TMT...SynchMethod;
begin
  if assigned(FOnBeforeWork) then FOnBeforeWork;
end;

  AThread.Sychronize(AThread.SynchMethod);
  AThread.Resume;
It works partially if I place it just before (or was it after?)
Resume() but not if call it within a CriticalSection (any other
position). Maybe it's not possible to Synchronize a locked thread,
which would be reason for freeze at Item.Thread.Synchronize as well.
On the other hand, a beta tester reported no success with that
implementation (Vista).

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José Mejuto · 2010-01-21 16:19

Quote from: Ocye on January 21, 2010, 12:03:52 pmAs I said, with Linux
ProcessMessages can be called without problems, but, of course, it's
not correct. I tried Item.Thread.Synchronize() yesterday at different
positions but the application freezes completely. I
[...]
On the other hand, a beta tester reported no success with that
implementation (Vista).

Hello,

You are right, I was only able to perform some kind of
"synchronization" using PostMessage as even SendMessage finally hangs.

I was calling it:

Code: [Select][....]
GetMem(DataPointer,1024);
pchar(DataPointer):=TimeToStr(Now);
Postmessage(frmMain.Handle,WM_USER,DataPointer,0);
[....]

and in frmMain:

Code: [Select]Procedure CustomMessage(var MSG: TMessage); message
WM_USER;
begin
  Self.Caption:=pchar(Msg.WParam);
  FreeMem(Msg.WParam);
end;

It is a dirty hack, it works but its far away from confortable. Only
tested in WinXP Sp3.

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Ocye · 2010-01-21 16:45

Thanks for your help, Joshy.
WParam is a PtrInt, no GetMem needed. If I send WM_USER with
WPARAM=Now I get the message after calculation not during - Linux
64bit.
I don't understand why a user message should be preferred if even
mouse events aren't evaluated. I need to show the calculation progress
and want to give a chance to abort since calculation could take some
minutes. Perhaps I should implement multithreading by myself with
Waitfor().

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cdbc · 2010-01-21 18:17

Hi Ocye

Have a look at Primoz' blog/pages...
http://17slon.com/blogs/gabr/2009/02/omnithreadlibrary-103.html

Omnithread library. Primoz is a very skilled programmer... :-)

HtH
Regards Benny

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José Mejuto · 2010-01-21 23:24

Quote from: Ocye on January 21, 2010, 05:45:04 pmThanks for your help,
Joshy.
WParam is a PtrInt, no GetMem needed. If I send WM_USER with
WPARAM=Now I get the message after calculation not during - Linux
64bit.
I don't understand why a user message should be preferred if even
mouse events aren't evaluated. I need to show the calculation progress
and want to give a chance to abort since calculation could take some
minutes. Perhaps I should implement multithreading by myself with
Waitfor().

Hello,

It should be processed when the mainform queue is empty, which should
happend almost of the time when running a process in a different
thread, but for some reason seems that the main thread is not
respawned after posting message, and sendmessage hangs waiting for
completation (Windows).

I'm not 100% sure but I think that Syncronize and family is broken in
FPC 2.5.1 I'll take a closer look.

If you have only one extra thread running it is better to use the
TThread class.

For process notification and cancel threads I'm using always a pooling
method, this means that the thread updates some variables every large
round, or every X milliseconds and the main thread reads that values
every 500 ms. or so.

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Ocye · 2010-01-22 12:37

I have between 2 and 3375 single operations to run. Each takes some
seconds depending on hardware and task. MTProcs blocks the application
completely and no message is processed. Synchronize works since all
network stuff is okay. I'm wondering why Item.Thread.Synchonize hangs.
Maybe OmniThread fits better, I'll have a look.

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Ocye · 2010-01-26 12:22

OmniThread seems to be a toolkit for Delphi only. Finally, I realized
multithreading myself - quick and dirty but it works.
Code: [Select]type
  TOnAddBest = procedure(...) of Object;

  TSingleOperation=class(...)
    public
       procedure Run;
    end;

  TBruteForce=class
      FProcessing   : TList; //list of TSingleOperation
      FMaxThreads : integer;//assigned by GetSystemThreadCount() of
MTProcs ->MTCPU
      procedure SetupProcessing;//fills FProcessing
      procedure RunThreaded(aStart:word=0);
      procedure AddBest(...);
    public
      function Compu]"]>Blockedve:boolean
    end;

  TSingleThread=class(TThread)
    private
      FOperation : TSingleOperation;
    protected
      procedure Execute; override;
    public
      constructor Create(aOperation:TSingleOperation);
    end;

implementation

var
  {$ifdef CPU32}
    aCriticalSection:LongWord;
  {$else}
    aCriticalSection:QWord;
  {$endif}
  ThreadsRunning : integer;

{ --- TBruteForce --- }

procedure TBruteForce.RunThreaded(aStart: word);
begin
  if aStart=0 then ThreadsRunning:=0;
  if aStart<FProcessing.Count then
  begin
    with TSingleThread.Create(TSingleOperation(FProcessing[aStart]))
do
    try
      Resume;
      if (ThreadsRunning>FMaxThreads) or "no operation left = last
thread" then Waitfor;
      OnProgress(nil,round((aStart/FProcessing.Count)*100));
      Application.ProcessMessages;
      RunThreaded(aStart+1);
    finally
      Free;
    end;
  end;
end;

function TBruteForce.Compu]"]>Blockedve:boolean;
begin
  FMaxThreads:=GetSystemThreadCount;
  SetupProcessing;
  ...
  if fmGameOptions.cbParallel.Checked then          //run parallel
  begin
    InitializeCriticalSection(aCriticalSection);
    try
      RunThreaded;
    finally
      DeleteCriticalSection(aCriticalSection);
    end;
  end else
  begin                                                          
//run single threaded
    for i:=0 to FProcessing.Count-1 do
    begin
      OnProgress(self,round((i/FProcessing.Count)*100));
      Application.ProcessMessages;
      TSingleOperation(FProcessing[i]).Run(nil);
    end;
  end;
  ...
end;

{ --- TSingleOperation --- }

procedure TSingleOperation.Run(aThread:TThread);
begin
  ... //loops and calculations
  if aThread<>nil then EnterCriticalsection(aCriticalSection);
//  if aThread<>nil then aThread.Synchronize(aThread,@SynchMethod);
//fails
  try
    if assigned(FOnAddBest) then FOnAddBest(...); //data exchange
  finally
    if aThread<>nil then LeaveCriticalSection(aCriticalSection);
  end;
  ...
end;

{ --- TSingleThread --- }

constructor TSingleThread.Create(aOperation: TSingleOperation);
begin
  inherited Create(true);
  FOperation:=aOperation;
end;

procedure TSingleThread.Execute;
begin
  inc(ThreadsRunning);
  FOperation.Run(self);
  dec(ThreadsRunning);
end;

If I call Synchronize program blocks; debugging, which should not be
trusted, reports an runerror after Thread.Execute. Strange...

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mas steindorff · 2010-02-05 21:16

It sounds like your far more advanced than I am with threading but my
fix to a similar problem was to place a sleep(1) in the Execute
procedure loop of the object.  that would release the CPU so it would
check other processes including the main code. 
It did however slow down the thread.  I added a check to my port input
thread to only sleep if the thread was not too far behind on it's work
load and that worked for me.  In my case, I needed as much CPU clocks
as I could get to handle the streeming data.
good luck.

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mattias · 2010-02-06 01:46

Synchronize works for MTProcs just like for any other TThread.
Synchronize requires that the main thread still runs.
Solution:
Create a worker thread and execute the DoParallel there.

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Ocye · 2010-02-06 10:04

Synchronize doesn't work as expected, see Joshy's posting. Example:
Code: [Select]procedure TForm1.Button1Click(Sender: TObject);
begin
  test:=0;
  ProcThreadPool.DoParallel(@MyParallelProc,0,MaxInt,nil);
end;

procedure TForm1.MySynchMethod;
begin
  label1.caption:=inttostr(test);
end;

procedure TForm1.MyParallelProc(Index: PtrInt; Data: Pointer; Item:
TMultiThreadProcItem);
begin
  inc(test);
//  Item.Thread.Synchronize(Item.Thread,@MySynchMethod); -> SIGSEV
end;

Maybe it's not a question of MTProcs rather than threading itself, I
don't know.
BTW: Please add a short sentence to the wiki how Synchronize can be
used with MTProcs (per Item.Thread). I was looking for that a long
time 
Thanks for your work, Matthias!

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José Mejuto · 2010-02-24 18:25

Quote from: Ocye on February 06, 2010, 11:04:05 amSynchronize doesn't
work as expected, see Joshy's posting. Example:
[...]
Maybe it's not a question of MTProcs rather than threading itself, I
don't know.
BTW: Please add a short sentence to the wiki how Synchronize can be
used with MTProcs (per Item.Thread). I was looking for that a long
time 
Thanks for your work, Matthias!

Hello,

I had understand the explanation, MTProcs dispatcher runs in main
thread, well, in fact, in the current thread, so when you run
"DoParallel" the main thread is blocked and syncronize will not
happend. Solution run the DoParallel inside a Execute of a TThread.
Something like:

MyParallelThread=class(TThread)
...
procedure MyParallelThread.Execute;
begin
  DoParallel(....)
end;
[......]
procedure Form1.Button1Click();
var
  Worker: TMyParallelThread;
begin
  Worker:=TMyParallelThread.Create(true,false,...); //Start paused
  Worker.FreeOnTerminate:=true;
  Worker.Resume;
end;

This way when synchronize should work.

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Ocye · 2010-02-25 09:20

As far as I remember I tried something like that. Maybe my code was
grubby implemented but it didn't unblock the main thread. At least, it
works with the simple Waitfor() construct.