Resolved Two nearly identical threads, big performance difference

[Henrik Kragh]

I have a big multithreaded system in C#, and I realized that performance was very different between two threads.
Now I have designed two nearly identical threads, where one performs 4-5 times faster (And it scales up linearly if you change the amount of loops they have to run).
And the difference?
One clumsy condition surrounding the actual heavy code of one of them.
It makes no sense to me, and I feel powerless in optimizing going forward, if such a minor detail can have such a huge impact.
This was tested in Unity, and as such it could be that the result is different in other environments.

ThreadA finish time: 2.8 seconds.
ThreadB finish time: 0.6 seconds.

Mind you "ThreadB" is the one that has a condition (Which will evaluate to true instantly on first while iteration).
How can such a stupid addition to the code make the actual payload (The for loops and the number crunching) perform so much faster?
Also, if I change the "delay" variable with a static "0.0" directly in the condition of ThreadB, it performs like ThreadA again.
In other words: One single double, and whether it is a hardcoded value, or references a variable, makes a difference in performance of a factor of 4-5.

Never mind the actual algorithm, which is only there to make the computer crunch some numbers.
I know I am comparing the same data again and again, that is besides the point.

I am no compilation nerd, and I have no way of probing how this differs in actual machine/assembler code.
I just know that the difference is huge, and non sensical to me.
What do I miss?
I discovered this by accident, and in the future I may not have any way of knowing a given thread performs at 20% possible speed, and that one slight change could solve it.

Please.
I need an expert to make this going from pure magic to "Oh, that's why...!
Now I know how to avoid it in the future...".
I know compilation of C# is surrounded by layers of managed stuff, but there must be a logical reason. Right?

Here is some test code with some simple structs to support it. If anyone has the time to check if they get the same results as I, I would be happy.

using System.Threading;

public class ThreadTest
{
    Thread threadA;
    Thread threadB;

    bool runThreadA = false;
    bool runThreadB = false;

    System.Diagnostics.Stopwatch stopWatch;

    double elapsedTimeA = 0;
    double elapsedTimeB = 0;
    
    public ThreadTest()
    {
        stopWatch = new System.Diagnostics.Stopwatch();
        StartThreads();
    }

    public void StartThreads ()
    {
        stopWatch.Reset();
        stopWatch.Start();
        threadA = new Thread(ThreadA);
        threadB = new Thread(ThreadB);

        runThreadA = true;
        runThreadB = true;

        elapsedTimeA = 0;
        elapsedTimeB = 0;
        threadA.Start();
        threadB.Start();
    }

    void ThreadA ()
    {
        while (runThreadA)
        {
            
            runThreadA = false;
            double preTicks = stopWatch.ElapsedTicks;

            Line3Double lineA = new Line3Double(new Vector3DoublePrecision(10, 20, 30), new Vector3DoublePrecision(100, 140, 180));
            Line3Double lineB = new Line3Double(new Vector3DoublePrecision(-10, -20, -30), new Vector3DoublePrecision(-100, -140, -180));

            int lines = 1000;

            for (int i = 0; i < 8; i++)
            {
                for (int j = 0; j < lines; j++)
                {
                    double aStartX = lineA.startX;
                    double aStartY = lineA.startY;
                    double aStartZ = lineA.startZ;

                    double aEndX = lineA.endX;
                    double aEndY = lineA.endY;
                    double aEndZ = lineA.endZ;

                    double aDirX = lineA.dirX;
                    double aDirY = lineA.dirY;
                    double aDirZ = lineA.dirZ;

                    double aDotSelf = lineA.dotSelf;

                    for (int k = 0; k < 8; k++)
                    {
                        for (int l = 0; l < lines; l++)
                        {
                            double wX = aStartX - lineB.startX;
                            double wY = aStartY - lineB.startY;
                            double wZ = aStartZ - lineB.startZ;

                            double b = aDirX * lineB.dirX + aDirY * lineB.dirY + aDirZ * lineB.dirZ;
                            double d = aDirX * wX + aDirY * wY + aDirZ * wZ;
                            double e = lineB.dirX * wX + lineB.dirY * wY + lineB.dirZ * wZ;

                            double D = aDotSelf * lineB.dotSelf - b * b;
                            double sc, tc;
                            if (D < 0.0000001)
                            {
                                // the lines are almost parallel
                                sc = 0.0f;
                                tc = (b > lineB.dotSelf ? d / b : e / lineB.dotSelf);
                            }
                            else
                            {
                                sc = (b * e - lineB.dotSelf * d) / D;
                                tc = (aDotSelf * e - b * d) / D;
                            }

                            double shortestX = wX + (sc * aDirX) - (tc * lineB.dirX);
                            double shortestY = wY + (sc * aDirY) - (tc * lineB.dirY);
                            double shortestZ = wZ + (sc * aDirZ) - (tc * lineB.dirZ);

                            double distance = shortestX * shortestX + shortestY * shortestY + shortestZ * shortestZ;
                        }
                    }
                }
            }

            double postTicks = stopWatch.ElapsedTicks;
            double time = ((postTicks - preTicks) / System.Diagnostics.Stopwatch.Frequency) * 1000;
            elapsedTimeA = time;
        }
    }

    void ThreadB()
    {
        long startTicks = stopWatch.ElapsedTicks;
        double delay = 0;

        while (runThreadB)
        {
            if ((double)(stopWatch.ElapsedTicks - startTicks) / System.Diagnostics.Stopwatch.Frequency >= delay)
            {
                runThreadB = false;
                double preTicks = stopWatch.ElapsedTicks;

                Line3Double lineA = new Line3Double(new Vector3DoublePrecision(10, 20, 30), new Vector3DoublePrecision(100, 140, 180));
                Line3Double lineB = new Line3Double(new Vector3DoublePrecision(-10, -20, -30), new Vector3DoublePrecision(-100, -140, -180));

                int lines = 1000;

                for (int i = 0; i < 8; i++)
                {
                    for (int j = 0; j < lines; j++)
                    {
                        double aStartX = lineA.startX;
                        double aStartY = lineA.startY;
                        double aStartZ = lineA.startZ;

                        double aEndX = lineA.endX;
                        double aEndY = lineA.endY;
                        double aEndZ = lineA.endZ;

                        double aDirX = lineA.dirX;
                        double aDirY = lineA.dirY;
                        double aDirZ = lineA.dirZ;

                        double aDotSelf = lineA.dotSelf;

                        for (int k = 0; k < 8; k++)
                        {
                            for (int l = 0; l < lines; l++)
                            {
                                double wX = aStartX - lineB.startX;
                                double wY = aStartY - lineB.startY;
                                double wZ = aStartZ - lineB.startZ;

                                double b = aDirX * lineB.dirX + aDirY * lineB.dirY + aDirZ * lineB.dirZ;
                                double d = aDirX * wX + aDirY * wY + aDirZ * wZ;
                                double e = lineB.dirX * wX + lineB.dirY * wY + lineB.dirZ * wZ;

                                double D = aDotSelf * lineB.dotSelf - b * b;
                                double sc, tc;
                                if (D < 0.0000001)
                                {
                                    // the lines are almost parallel
                                    sc = 0.0f;
                                    tc = (b > lineB.dotSelf ? d / b : e / lineB.dotSelf);
                                }
                                else
                                {
                                    sc = (b * e - lineB.dotSelf * d) / D;
                                    tc = (aDotSelf * e - b * d) / D;
                                }

                                double shortestX = wX + (sc * aDirX) - (tc * lineB.dirX);
                                double shortestY = wY + (sc * aDirY) - (tc * lineB.dirY);
                                double shortestZ = wZ + (sc * aDirZ) - (tc * lineB.dirZ);

                                double distance = shortestX * shortestX + shortestY * shortestY + shortestZ * shortestZ;
                            }
                        }
                    }
                }

                double postTicks = stopWatch.ElapsedTicks;
                double time = ((postTicks - preTicks) / System.Diagnostics.Stopwatch.Frequency) * 1000;
                elapsedTimeB = time;
            }
        }
    }
}
public struct Vector3DoublePrecision
{
    public double x;
    public double y;
    public double z;

    public Vector3DoublePrecision(double x, double y, double z)
    {
        this.x = x;
        this.y = y;
        this.z = z;
    }
}

public struct Line3Double
{
    public double startX;
    public double startY;
    public double startZ;
    public double endX;
    public double endY;
    public double endZ;

    public double dirX;
    public double dirY;
    public double dirZ;

    public double dotSelf;

    public Line3Double(Vector3DoublePrecision start, Vector3DoublePrecision end)
    {
        startX = start.x;
        startY = start.y;
        startZ = start.z;

        endX = end.x;
        endY = end.y;
        endZ = end.z;

        dirX = end.x - start.x;
        dirY = end.y - start.y;
        dirZ = end.z - start.z;

        dotSelf = dirX * dirX + dirY * dirY + dirZ * dirZ;
    }
}

 

[Henrik Kragh]

Thanks for all your inputs and especially thanks to Skydiver for testing it out. Your low times made me accept something was more "off" than just the difference in compilation between the two threads.
And so, I really dug into the whole compilation setup of the my Unity project, and looked into settings I normally don't touch (Never had to, never cared to), and now that it has become a Unity specific problem, I may as well talk Unity specific solutions. Because a solution HAS been found. And a major one at that.
So, in project settings>player one can choose between Mono and IL2CPP as "scripting backend". Default is Mono.
Which gives me both these high times, AND the difference between the two threads.
2.8s on A
0.6s on B

When changed to IL2CPP the times went to
0.000001s on both A and B

Now... that was too little I felt. So I took all the distance calculations, and actually put them to use by summing two global variables with the results. I feared compilation got rid of the calculation due to the result never leaving the threads.
And now?

A: 0.15s
B: 0.149s

In other words: Major improvements on times, and best of all? They are now the same!
So. Thanks again. Your feedback forced me to change stuff I normally don't touch, which made a BIG difference on both performance and reliability.
This thread can be considered solved. And not only that... I can be considered improved!
Thanks...