How to improve the framerate in simple java game loop?

Question

I'm working on a java 2d game, using this simple game loop to cap the FPS and UpdatesPS to 60:

public void run() {

        final int MAX_FPS = 60;
        final int MAX_UPS = 60;

        final double fOPTIMAL_TIME = 1000000000 / MAX_FPS;
        final double uOPTIMAL_TIME = 1000000000 / MAX_UPS;

        double uDeltaTime = 0, fDeltaTime = 0;
        int frames = 0, updates = 0;
        long startTime = System.nanoTime();
        long timer = System.currentTimeMillis();

        // GameLOOP starts here
        while (running) {

            long currentTime = System.nanoTime();
            uDeltaTime += (currentTime - startTime);
            fDeltaTime += (currentTime - startTime);
            startTime = currentTime;

            if (uDeltaTime >= uOPTIMAL_TIME) {
                gameUpdate();
                updates++;
                uDeltaTime -= uOPTIMAL_TIME;
            }

            if (fDeltaTime >= fOPTIMAL_TIME) {
                gameRender();
                gameDraw();
                frames++;
                fDeltaTime -= fOPTIMAL_TIME;
            }

            if (System.currentTimeMillis() - timer >= 1000) {

                fps = frames;   //saves the current FPS
                ups = updates;  //saves the current UPS

                updates = 0;
                frames = 0;
                timer += 1000;
            }

        }
    }

The loop works, but I get only 30 FPS for the first ~10 Seconds after starting the game.

After I wait, the FPS raises up to the wanted 60. I don't have a problem to wait a few seconds to let the program stabilize and reach the wanted framerate. But I can't find the reason like a methode who drops the FPS because it's fetching a big file after startup.

Do you have any idea why my engine needs so long to stabilize the framerate?

Thanks for your help!

Answer 1

I think this should do the trick:

    public static void run()
    {
        final int desiredFPS = 60;
        final int desiredUPS = 60;
        
        final long updateThreshold = 1000000000 / desiredUPS;
        final long drawThreshold = 1000000000 / desiredFPS;
        
        long lastFPS = 0, lastUPS = 0, lastFPSUPSOutput = 0;
        
        int fps = 0, ups = 0;
        
        loop:
        while(true)
        {
            if((System.nanoTime() - lastFPSUPSOutput) > 1000000000)
            {
                System.out.println("FPS: " + (double)fps);
                System.out.println("UPS: " + (double)ups);
                
                fps = 0;
                ups = 0;
                
                lastFPSUPSOutput = System.nanoTime();
            }
            
            if((System.nanoTime() - lastUPS) > updateThreshold)
            {
                lastUPS = System.nanoTime();
                updateGame();
                ups++;
            }
            
            if((System.nanoTime() - lastFPS) > drawThreshold)
            {
                lastFPS = System.nanoTime();
                drawGame();
                fps++;
            }
            
            // Calculate next frame, or skip if we are running behind
            if(!((System.nanoTime() - lastUPS) > updateThreshold || (System.nanoTime() - lastFPS) > drawThreshold))
            {
                long nextScheduledUP = lastUPS + updateThreshold;
                long nextScheduledDraw = lastFPS + drawThreshold;
                
                long minScheduled = Math.min(nextScheduledUP, nextScheduledDraw);
                
                long nanosToWait = minScheduled - System.nanoTime();
                
                // Just in case
                if(nanosToWait <= 0)
                    continue loop;
                    
                try
                {
                    Thread.sleep(nanosToWait / 1000000);
                } 
                catch (InterruptedException e)
                {
                    e.printStackTrace();
                }
            }
        }
    }

Edit: I fixed the issue now! The issue was that I was saving the lastFPS/lastUPS after the scene was updated/drawn, and when I set both lastUPS/lastFPS before the scene is drawn/updated, we get the desired fps!

Another neat thing about this code is that it doesn't consume a whole cpu core(I measured the difference, your code was consuming 100%, whilest my code only consumed about 10%. If you want to measure it yourself, please note that for some reason, the core on which the code is executed regularly switches(at least this was the case when I measured the code))

By the way if you use LWJGL (or have direct access to a windowing library like GLFW) you can activate V-Sync, which would cut your fps down to 60 fps.