Nick Gravelyn

Ok, so that title seemed shorter in my head.

A common theme around the XNA forums is that garbage collections are the most devastating thing on the Xbox 360. If you listened to some people you would think a single garbage collection would cause your Xbox 360 to implode, create a black hole, and be the catalyst for the end of the world. I’m afraid my experience shows somewhat of the opposite.

Let’s get something straight first. I do try to avoid garbage collections. Object management is a big part of my framework whereby I can re-use object references to avoid lots of garbage. However there are lots of places where I throw those “best practices” for avoiding garbage right out the window because it just takes too much work to remember to worry about it. I think the prime way to show this is simply to put some code on the table.

First let’s take a look at a custom text drawing routine I wrote for my tutorial screens as well as the little instruction text in the bottom left corner of the screen:

public static void DrawSpecialText(
   SpriteBatch batch,
   SpriteFont font,
   SpriteFont carotFont,
   string text,
   Vector2 position,
   Color color)
{
   string[] splitText = text.Split('@');

   float xPosition = position.X;

   float characterHeight = font.MeasureString("O").Y;

   foreach (string s in splitText)
   {
      if (s.Length == 1)
      {
         Rectangle buttonRect = new Rectangle(
            (int)xPosition,
            (int)position.Y,
            (int)characterHeight,
            (int)characterHeight);

         if (s == AButtonText)
            batch.Draw(AButton, buttonRect, Color.White);
         else if (s == BButtonText)
            batch.Draw(BButton, buttonRect, Color.White);
         else if (s == XButtonText)
            batch.Draw(XButton, buttonRect, Color.White);
         else if (s == YButtonText)
            batch.Draw(YButton, buttonRect, Color.White);
         else if (s == StartButtonText)
            batch.Draw(StartButton, buttonRect, Color.White);
         else if (s == BackButtonText)
            batch.Draw(BackButton, buttonRect, Color.White);

         xPosition += characterHeight;
      }
      else
      {
         string[] boldSplit = s.Split('^');
         bool bold = s.StartsWith("^");

         foreach (string subStr in boldSplit)
         {
            SpriteFont fontForString = (bold) ? carotFont : font;
            batch.DrawString(
                fontForString,
                subStr,
                new Vector2(xPosition, position.Y),
                color);
            xPosition += fontForString.MeasureString(subStr).X;
            bold = !bold;
         }
      }
   }
}

What’s that, you say? “Nick, you’re using string literals! And allocating an array! 60 times a second?!” Yep. This code, when something is using the method obviously, runs every frame and just throws out garbage. Could I “fix” this? Sure. You can fix anything. But why spend the time when this had no effect on my frame rates.

Let’s look at another example. This is the code I use to render the in-game HUD:

private void DrawInGameHUD(Rectangle screen)
{
   BlocGame.SpriteBatch.Begin();

   Color hudColor = Color.White;

   float alpha = 255f;

   if (coop)
   {
      if (!circle.IsDead && !circle2.IsDead)
         alpha = Math.Min(
            Math.Abs(circle.Position.Y - screen.Top),
            Math.Abs(circle2.Position.Y - screen.Top));
      else if (circle.IsDead)
         alpha = Math.Abs(circle2.Position.Y - screen.Top);
      else if (circle2.IsDead)
         alpha = Math.Abs(circle.Position.Y - screen.Top);
   }
   else if (!circle.IsDead)
   {
      alpha = Math.Abs(circle.Position.Y - screen.Top);
   }

   // clamp the alpha between 50 and 255
   alpha = MathHelper.Clamp(alpha, 50, 255);

   hudColor = new Color(hudColor.R, hudColor.G, hudColor.B, (byte)alpha);

   int totalScore = player1Score.Score + player2Score.Score;

   if (!coop)
   {
      string caption = ProfileManager.GetGamer(ProfileManager.PlayerOne).Gamertag+":";
      Vector2 captionSize = hudFont.MeasureString(caption);

      string scoreString;
      if (totalScore > 999999)
         scoreString = totalScore.ToString();
      else
         scoreString = string.Format(scoreFormat, totalScore);

      BlocGame.SpriteBatch.DrawString(
         hudFont,
         caption,
         new Vector2(screen.Left + 10f, screen.Top - 3f),
         hudColor);

      BlocGame.SpriteBatch.DrawString(
         hudFont,
         scoreString,
         new Vector2(screen.Left + 10f, screen.Top + captionSize.Y - 10f),
         hudColor);

      int placement;
      HighScore nextScore = highScoreList.GetNextHighScore(
         currentPlayTime,
         ProfileManager.GetGamer(ProfileManager.PlayerOne).Gamertag,
         player1Score.Score,
         player1Score.PlayTime,
         out placement);

      // figure out the caption
      caption = (placement > 0) ? nextScoreCaption1 : nextScoreCaption2;
      captionSize = hudFont.MeasureString(caption);

      totalScore = (placement > 0)
         ? nextScore.TotalScore
         : player1Score.Score + player2Score.Score;
      if (totalScore > 999999)
         scoreString = totalScore.ToString();
      else
         scoreString = string.Format(scoreFormat, totalScore);

      // measure some stuff
      Vector2 scoreSize = hudFont.MeasureString(scoreString);

      // draw the score itself
      BlocGame.SpriteBatch.DrawString(
         hudFont,
         caption,
         new Vector2(screen.Right - 10f - captionSize.X, screen.Top - 3f),
         hudColor);

      BlocGame.SpriteBatch.DrawString(
         hudFont,
         scoreString,
         new Vector2(
             screen.Right - 10f - scoreSize.X,
             screen.Top + captionSize.Y - 10f),
         hudColor);
   }
   else
   {
      PlayerIndex p;
      GameScore score;
      if (ProfileManager.PlayerOne < ProfileManager.PlayerTwo)
      {
         p = ProfileManager.PlayerOne;
         score = player1Score;
      }
      else
      {
         p = ProfileManager.PlayerTwo;
         score = player2Score;
      }

      string caption = ProfileManager.GetGamer(p).Gamertag + ":";
      Vector2 captionSize = hudFont.MeasureString(caption);

      string scoreString;
      if (player1Score.Score > 999999)
         scoreString = score.Score.ToString();
      else
         scoreString = string.Format(scoreFormat, score.Score);

      BlocGame.SpriteBatch.DrawString(
         hudFont,
         caption,
         new Vector2(screen.Left + 10f, screen.Top - 3f),
         hudColor);

      BlocGame.SpriteBatch.DrawString(
         hudFont,
         scoreString,
         new Vector2(screen.Left + 10f, screen.Top + captionSize.Y - 10f),
         hudColor);

      // figure out the caption
      if (ProfileManager.PlayerOne > ProfileManager.PlayerTwo)
      {
         p = ProfileManager.PlayerOne;
         score = player1Score;
      }
      else
      {
         p = ProfileManager.PlayerTwo;
         score = player2Score;
      }
      caption = ProfileManager.GetGamer(p).Gamertag + ":";
      captionSize = hudFont.MeasureString(caption);

      if (player2Score.Score > 999999)
         scoreString = score.Score.ToString();
      else
         scoreString = string.Format(scoreFormat, score.Score);

      // measure some stuff
      Vector2 scoreSize = hudFont.MeasureString(scoreString);

      // draw the score itself
      BlocGame.SpriteBatch.DrawString(
         hudFont,
         caption,
         new Vector2(screen.Right - 10f - captionSize.X, screen.Top - 3f),
         hudColor);

      BlocGame.SpriteBatch.DrawString(
         hudFont,
         scoreString,
         new Vector2(
              screen.Right - 10f - scoreSize.X,
              screen.Top + captionSize.Y - 10f),
         hudColor);
   }

   float y = screen.Top + 15f;

   if (coop)
   {
      int cumulateScore = player1Score.Score + player2Score.Score;
      string scoreString;
      if (cumulateScore > 999999)
         scoreString = cumulateScore.ToString();
      else
         scoreString = string.Format(scoreFormat, cumulateScore);

      BetterSpriteBatch.DrawCenteredText(
         BlocGame.SpriteBatch,
         hudFont,
         scoreString,
         new Vector2(screen.Left + screen.Width / 2, y),
         hudColor);

      y += hudFont.MeasureString(scoreString).Y - 10f;
   }

   string timeString = string.Format(
      (currentPlayTime.Hours == 0) ? BlocGame.TimeFormat : BlocGame.TimeFormat2,
      currentPlayTime.Hours,
      currentPlayTime.Minutes,
      currentPlayTime.Seconds);
   BetterSpriteBatch.DrawCenteredText(
      BlocGame.SpriteBatch,
      hudFont,
      timeString,
      new Vector2(
      screen.Left + screen.Width / 2,
      y),
      hudColor);

   BlocGame.SpriteBatch.End();
}

In addition to be way too long for a single method, and being quite a mess of code, you’ll notice quite a few string.Formats in there, a few ToStrings, and even a few string literals. From my quick counting this method allocates, in co-op mode, eight strings per frame. That’s equivalent to 240 string allocations per second. Should I be worried? I mean, the Xbox 360 garbage collector is definitely not going to like that! So what. The game still runs fine so why should I put any effort into fixing a problem I don’t have?

So while I once was in the camp of “avoid garbage collection at all costs”, I’ve fallen further to the side of “know where you allocate in case you need to optimize out the allocations”. Don’t start a game trying to avoid allocating arrays here and there or string literals once in a while.