I have been working with Delphi for some time, and am now trying to implement multi-threading in my code with the following requirements:
Only 2 threads should run at a time.
Other tasks should wait in a queue and execute once a thread becomes available.
I tried using TParallel.For to achieve this, but I couldn't implement the desired behavior. Here's my sample code:
type
TMyRecord = record
msg: String;
sleep: Integer;
end;
var
Form2: TForm2;
implementation
{$R *.dfm}
procedure TForm2.Button1Click(Sender: TObject);
var
record1, record2, record3, record4, record5: TMyRecord;
MyList: TList<TMyRecord>;
begin
// Initialize records
record1.msg := 'Item1';
record1.sleep := 10;
record2.msg := 'Item2';
record2.sleep := 10;
record3.msg := 'Item3';
record3.sleep := 3000;
record4.msg := 'Item4';
record4.sleep := 3000;
record5.msg := 'Item5';
record5.sleep := 100;
MyList := TList<TMyRecord>.Create;
try
// Add records to the list
MyList.Add(record1);
MyList.Add(record2);
MyList.Add(record3);
MyList.Add(record4);
MyList.Add(record5);
// Use TParallel.For
TParallel.For(0, MyList.Count - 1,
procedure (i: Integer)
var
ListItem: TMyRecord;
begin
ListItem := MyList[i];
TThread.Synchronize(nil,
procedure
begin
Memo1.Lines.Add(ListItem.msg);
end);
Sleep(ListItem.sleep);
end);
finally
MyList.Free;
end;
end;
I also tried using TTask.Run and ThreadPool, but was unable to achieve the desired behavior.
Updated - here is what i tried to do using Threadpool
procedure TForm2.Button1Click(Sender: TObject);
var
record1, record2, record3, record4, record5: TMyRecord;
MyList: TList<TMyRecord>;
ThreadPool: TThreadPool;
begin
// Initialize records
record1.msg := 'Item1';
record1.sleep := 10;
record2.msg := 'Item2';
record2.sleep := 10;
record3.msg := 'Item3';
record3.sleep := 3000;
record4.msg := 'Item4';
record4.sleep := 3000;
record5.msg := 'Item5';
record5.sleep := 100;
MyList := TList<TMyRecord>.Create;
try
// Add records to the list
MyList.Add(record1);
MyList.Add(record2);
MyList.Add(record3);
MyList.Add(record4);
MyList.Add(record5);
ThreadPool := TThreadPool.Create;
ThreadPool.SetMaxWorkerThreads(2);
// Iterate through the list and queue each work item with a copy of the record
for var i := 0 to MyList.Count - 1 do
begin
var ListItem := MyList[i];
ThreadPool.QueueWorkItem(
procedure
var
LocalItem: TMyRecord;
begin
LocalItem := ListItem;
TThread.Queue(nil,
procedure
begin
Memo1.Lines.Add(LocalItem.msg);
Sleep(LocalItem.sleep); // Simulate the delay
end);
end);
end;
finally
MyList.Free;
end;
end;
there is problem with this code -
How can I modify this code to limit the number of threads running simultaneously to 2? And queue other tasks to execute as threads become available?
it prints only 'item5' 5 times.
That implies that your queued work items are capturing the same TMyRecord, and all of your prints are happening after that variable has been assigned the last item in the list.
Even though your loop is using a local inline variable, given the way that variable capture in anonymous procedures works, it seems to be ignoring the inlining.
The common solution to having an anonymous procedure capture a variable inside a loop is to move the anonymous procedure to a separate function with an input parameter that it can capture.
waiting time(sleep) is not working.
That is likely because you are doing the sleeps in the context of the main UI thread, not in the worker threads, so they are running at full speed.
Try something more like this:
function MakeWorkItem(Item: TMyRecord): TProc;
begin
Result := procedure
begin
TThread.Queue(nil,
procedure
begin
Form2.Memo1.Lines.Add(Item.msg);
end
);
Sleep(Item.sleep); // Simulate the delay
end;
end;
procedure TForm2.Button1Click(Sender: TObject);
var
record1, record2, record3, record4, record5: TMyRecord;
MyList: TList<TMyRecord>;
ThreadPool: TThreadPool;
begin
// Initialize records
record1.msg := 'Item1';
record1.sleep := 10;
record2.msg := 'Item2';
record2.sleep := 10;
record3.msg := 'Item3';
record3.sleep := 3000;
record4.msg := 'Item4';
record4.sleep := 3000;
record5.msg := 'Item5';
record5.sleep := 100;
MyList := TList<TMyRecord>.Create;
try
// Add records to the list
MyList.Add(record1);
MyList.Add(record2);
MyList.Add(record3);
MyList.Add(record4);
MyList.Add(record5);
ThreadPool := TThreadPool.Create;
ThreadPool.SetMaxWorkerThreads(2);
// Iterate through the list and queue each work item with a copy of the record
for var i := 0 to MyList.Count - 1 do
begin
ThreadPool.QueueWorkItem(MakeWorkItem(MyList[i]));
end;
finally
MyList.Free;
end;
end;
Alternatively, using TParallel.For, simply pass your desired TThreadPool to it as an extra parameter, eg:
procedure TForm2.Button1Click(Sender: TObject);
var
record1, record2, record3, record4, record5: TMyRecord;
MyList: TList<TMyRecord>;
ThreadPool: TThreadPool;
begin
// Initialize records
record1.msg := 'Item1';
record1.sleep := 10;
record2.msg := 'Item2';
record2.sleep := 10;
record3.msg := 'Item3';
record3.sleep := 3000;
record4.msg := 'Item4';
record4.sleep := 3000;
record5.msg := 'Item5';
record5.sleep := 100;
MyList := TList<TMyRecord>.Create;
try
// Add records to the list
MyList.Add(record1);
MyList.Add(record2);
MyList.Add(record3);
MyList.Add(record4);
MyList.Add(record5);
ThreadPool := TThreadPool.Create;
ThreadPool.SetMaxWorkerThreads(2);
// Use TParallel.For
TParallel.For(0, MyList.Count - 1,
procedure (i: Integer)
var
ListItem: TMyRecord;
begin
ListItem := MyList[i];
TThread.Queue(nil,
procedure
begin
Memo1.Lines.Add(ListItem.msg);
end
);
Sleep(ListItem.sleep);
end,
TThreadPool // <-- HERE
);
finally
MyList.Free;
end;
end;