Feedback Control for Computer Systems

Chapter 18. Case Study: Controlling Memory Consumption in a Game Engine

This final case study allows us to see how a nonproportional controller can be used in a feedback loop to good effect.

The Situation

Imagine a game engine managing the movements of various game objects (widgets, sprites, space ships, whatever). During the course of a game, the number of game objects changes. Our job is to make sure that the memory consumption of the game engine stays within acceptable limits even as the number of game objects grows large.

The way to control memory consumption in the game engine is via the graphics resolution: high resolution translates into high memory consumption. To make things concrete, let’s say that we can choose from five different resolution levels and that each subsequent level requires twice as much memory per game object. (For instance, the levels may set aside 100, 200, 400, 800, and 1,600 memory units per game object.) The problem of choosing the correct resolution therefore appears quite simple: divide the maximum amount of memory available by the number of game objects to find the greatest number of memory units available to each object, and then choose the greatest resolution that respects this limit. We don’t need feedback for that.

Remember that feedback is a method to achieve robust control in the face of uncertainty. Feedforward works great as long as the system is totally deterministic and all relevant information is available. So in this case, if the amount ...