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Umbranocturna

•Created by Umbranocturna on 9/17/2023 in #help

How to efficiently use Threads and locks

I'm writing a short program that computes the mandelbrot set. I have implemented threads, but ts my first time using them and i'm a little confused.

            Thread thread1 = new Thread(() => bitmapRenderer.UpdateBitmap(0,height/8));
            Thread thread2 = new Thread(() => bitmapRenderer.UpdateBitmap(height/8,2*height/8));
            Thread thread3 = new Thread(() => bitmapRenderer.UpdateBitmap(2*height/8,3*height/8));
            Thread thread4 = new Thread(() => bitmapRenderer.UpdateBitmap(3*height/8,4*height/8));
            Thread thread5 = new Thread(() => bitmapRenderer.UpdateBitmap(4*height/8,5*height/8));
            Thread thread6 = new Thread(() => bitmapRenderer.UpdateBitmap(5*height/8,6*height/8));
            Thread thread7 = new Thread(() => bitmapRenderer.UpdateBitmap(6*height/8,7*height/8));
            Thread thread8 = new Thread(() => bitmapRenderer.UpdateBitmap(7*height/8,height));

            thread1.Start();
            thread2.Start();
            thread3.Start();
            thread4.Start();
            thread5.Start();
            thread6.Start();
            thread7.Start();
            thread8.Start();
            thread1.Join();
              .
              .
            thread8.Join();

            Thread thread1 = new Thread(() => bitmapRenderer.UpdateBitmap(0,height/8));
            Thread thread2 = new Thread(() => bitmapRenderer.UpdateBitmap(height/8,2*height/8));
            Thread thread3 = new Thread(() => bitmapRenderer.UpdateBitmap(2*height/8,3*height/8));
            Thread thread4 = new Thread(() => bitmapRenderer.UpdateBitmap(3*height/8,4*height/8));
            Thread thread5 = new Thread(() => bitmapRenderer.UpdateBitmap(4*height/8,5*height/8));
            Thread thread6 = new Thread(() => bitmapRenderer.UpdateBitmap(5*height/8,6*height/8));
            Thread thread7 = new Thread(() => bitmapRenderer.UpdateBitmap(6*height/8,7*height/8));
            Thread thread8 = new Thread(() => bitmapRenderer.UpdateBitmap(7*height/8,height));

            thread1.Start();
            thread2.Start();
            thread3.Start();
            thread4.Start();
            thread5.Start();
            thread6.Start();
            thread7.Start();
            thread8.Start();
            thread1.Join();
              .
              .
            thread8.Join();

There probably is a better way to create, start and join threads, but i wasn't able to find anything better. Its not clear to me how threads work overall. I have a 16 thread CPU and using 8 threads bumped the usage from 10% to 30%. How can i find out what a good number of threads to use is?

  lock (bmp) 
        {
              bmp.SetPixel(j,i,iterToColor(iterCount_temp));
        }

  lock (bmp) 
        {
              bmp.SetPixel(j,i,iterToColor(iterCount_temp));
        }

I had to lock the bitmap so that the threads wouldn't try to write at the same time. But i feel like this isn't the best practise as the threads will wait on one another fairly often. I wanted to save the X Y Color values in some structure and write all at once after joining, but was not yet able to come up with anything. Any ideas?

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