High CPU usage in thread c#

Question

Hi I'm having high CPU usage (15-16%) when running this thread, it is a thread that is supposed to keep looping unless "ssStop" is set to true (which works), however, while it is running I open task manager and I see that the program is using up 15% of the computers processing power, it drops back down to 1-2% once the thread has been exited. Even when using EventWaitHandle which I found searching for this problem online it still remains this high, does anyone know what I'm doing wrong here?:

public void Receive()
{
    try
    {              
        bool firstTimeRun = true;
        TcpListener ssTcpListener = new TcpListener(IPAddress.Any, 1500);
        TcpClient tcpReceiver;
        ssTcpListener.Start();
        while (!ssStop)
        {
            //Start listening for connection.
            //Accept any incoming connection requests on port 1500.
            tcpReceiver = new TcpClient();
            if (ssTcpListener.Pending())
            {
                tcpReceiver = ssTcpListener.AcceptTcpClient();
            }
            if (tcpReceiver.Connected)
            {
                //looped for first time; receives whole image.
                if (firstTimeRun)
                {
                    //TCP connected. Receive images from contact
                    NetworkStream receivedNs = new NetworkStream(tcpReceiver.Client);
                    Bitmap image = new Bitmap(receivedNs);
                    receivedImage = image;
                    pboScrnShare.Image = image;
                    receivedNs.Flush();
                    firstTimeRun = false;                            
                }
                //second time or higher looped; receives difference and overlays it.
                else if (!firstTimeRun)
                {
                    NetworkStream receivedNs = new NetworkStream(tcpReceiver.Client);

                    //Put image into picturebox.
                    receivedImage2 = new Bitmap(receivedNs);
                    receivedImage2.MakeTransparent(Color.Black);
                    Bitmap overlayedImage = new Bitmap(receivedImage.Width, receivedImage.Height);

                    using (Graphics gr = Graphics.FromImage(overlayedImage))
                    {
                        gr.DrawImage(receivedImage, new Point(0, 0));
                        gr.DrawImage(receivedImage2, new Point(0, 0));
                    }


                    try
                    {
                        pboScrnShare.Image = overlayedImage;
                    }
                    catch (Exception ex)
                    {
                        MessageBox.Show(ex.Message, "pbo second run");
                    }
                    receivedImage2.Dispose();
                    if (this.InvokeRequired) { this.Invoke(new MethodInvoker(delegate() { receivedImage = overlayedImage; })); } else { receivedImage = overlayedImage; }
                    receivedNs.Flush();
                }
                tcpReceiver.Close();
            }
            myEventWaitHandle.WaitOne(10, true);
        }
        ssTcpListener.Stop();
    }
    catch (Exception ex)
    {
        MessageBox.Show(ex.ToString(), "Invited ReceiveSS()", MessageBoxButtons.OK, MessageBoxIcon.Error);
    }
}

If you can afford to relinquish the thread for 20 ms or more during each loop, you can insert a sleep(1) somewhere in your loop to allow other threads to execute. There are other problems with your code, however. Since it is working code, consider posting it to codereview.stackexchange.com so that you can get some additional feedback. Even if you do use WaitEventHandle, I don't think it will have any impact on your CPU usage unless there's something to do in other threads. — Robert Harvey
– Robert Harvey, Commented Apr 9, 2014 at 16:00
Add some instrumentation to measure clock ticks. That will help narrow down where you CPU usage is. Measure the clock ticks either side of a system or function call. But note that measuring ticks will also introduce some overhead, so don't do for production. Measuring is always better than guessing .... — Tim Child
– Tim Child, Commented Apr 9, 2014 at 16:05

jaket · Accepted Answer · 2014-04-15 07:30:36Z

2

You are burning cpu cycles busy polling for a connection. Considering most of the time Pending is returning false, your loop is spinning around like this:

while (!ssStop)
{
    tcpReceiver = new TcpClient();
    myEventHandle.WaitOne(10, false);
}

Now if myEventHandle is unset then the 10ms delay in WaitOne would effectively throttle execution but my guess would be that the event is set so that WaitOne immediately returns true.

It is not necessary to poll for a connection since AcceptTcpClient will block waiting for a connection. So if you changed your code around a bit it should work as expected:

while (!ssStop)
{
    TcpClient tcpReceiver = ssTcpListener.AcceptTcpClient(); // this blocks
    ...
}

answered Apr 15, 2014 at 7:30

jaket

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