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John · Guest

Hello,

I have to apply a graphic filter on a Bitmap.

I have a corde 2 duo since a few weeks so I decided to port this filter
to multithreading, and I was very surprised that it takes more time in
multithreading than normal.
After spending some times on my code I have discovered that the
bottleneck was the random function.

So i have made a small example that do only random, and i was a lot
surpised.

here's the result for doing 48000000 randoms on my core 2 duo :

Normal : 204 ms
Multithread (2 threads) : 1109 ms.

so My questtions are :
- Am I missing something ?
- Is there any alternative to RTL random that support better and
faster random ?

here's the code :

Type
TRandom = class(TThread)
private
FCount: Integer;
protected
procedure Execute; override;
public
constructor Create(Count: integer; Suspended: Boolean);
end;

//The threaded random Class
constructor TRandom.Create(Count: integer; Suspended: Boolean);
begin
FreeOnTerminate := False;
inherited Create(Suspended);
FCount := Count;
end;

procedure TRandom.Execute;
var
i, j: integer;
begin
for i := 0 to FCount - 1 do
j := random(255);
end;

//the way it is called
procedure DoRandom(Count: integer; NbThreads: integer);
var
threads: array of THandle;
ft: array of TRandom;
i, j, NBCountThread, Lastthread, NBC: integer;
begin
try

//Number of count per thread
NBCountThread := Count div NbThreads;
Lastthread := NbThreads - 1;

SetLength(threads, NbThreads);
SetLength(ft, NbThreads);
j := 0;
// start all threads
for i := 0 to high(threads) {Lastthread} do
begin
if i < Lastthread then
begin
//Number of count for this thread
NBC := NBCountThread;
inc(j, nbc);
end
else
begin
//Number of count for last thread
NBC := Count - j;
end;
ft[i] := TRandom.Create(NBC, True);
// store handle of the thread so we know what to wait for
threads[i] := ft[i].Handle;
ft[i].Resume;
end;

// wait until all threads are done
WaitForMultipleObjects(NbThreads, @threads[0], true, INFINITE);
finally
for i := 0 to high(ft) do
if Ft[i] <> nil then
FreeAndNil(Ft[i]);
SetLength(ft, 0);
SetLength(Threads, 0);
end;
end;

//the same thing without multithreading
procedure NormalRandom(FCount: integer);
var
i, j: integer;
begin
for i := 0 to FCount - 1 do
j := random(255);
end;

//the test
procedure TForm1.Button2Click(Sender: TObject);
var
TC: integer;
begin
tc := Gettickcount;
NormalRandom(48000000);
tc := Gettickcount - tc;
Showmessage(inttostr(tc));

tc := Gettickcount;
DoRandom(48000000,2); //2 threads
tc := Gettickcount - tc;
Showmessage(inttostr(tc));
end;

thanks

John

Les Pawelczyk · Guest

John · Guest

Thanks Les for your reply.

Is there a way to avoid this by using another fast ranndom routines ?
or any other solution ?

Thanks

Dan Downs · Guest

Les Pawelczyk · Guest

John · Guest

So one solution would be to "clone" the random function and puting it
directly tin the thread class and using RandSeed as a local variable ?

but I don't see any random function in system.pas

John Herbster · Guest

"John" <NoSpam (AT) NoSpam (DOT) com> wrote

John · Guest

Bob Gonder · Guest

John Herbster wrote:

John · Guest

Any usefull sample ?

John Herbster · Guest

"John" <NoSpam (AT) NoSpam (DOT) com> wrote

Sven Pran · Guest

"John" <NoSpam (AT) NoSpam (DOT) com> wrote in message
news:461e4f99$1 (AT) newsgroups (DOT) borland.com...

Bob Gonder · Guest

John wrote:

John · Guest

I have seen Randseen not Random....

John · Guest