Final Result Preview
Let’s take a look at the final result we will be working towards:Note that the “Memory Used” and “CPU Load” stats are based on all the SWFs you have open across all browser windows, including Flash banner ads and the like. This may make the SWF appear more resource intensive than it actually is.
Step 1: Understanding the Flash Movie
The Flash movie has two main elements: a particle simulation of fire, and a graph showing the animation’s resource consumption over time. The graph’s pink line tracks the total memory consumed by the movie in megabytes, and the green line plots CPU load as a percentage.ActionScript objects take up most of the memory allocated to the Flash Player, and the more ActionScript objects a movie contains, the higher its memory consumption. In order to keep a program’s memory consumption low, the Flash Player regularly does some garbage collection by sweeping through all ActionScript objects and releasing from memory those no longer in use.
A memory consumption graph normally reveals a hilly up-down pattern, dipping each time garbage collection is performed, then slowly rising as new objects are created. A graph line that’s only going up points to a problem with garbage collection, as it means new objects are being added to memory, while none are being removed. If such a trend continues, the Flash player may eventually crash as it runs out of memory.
The CPU load is calculated by tracking the movie’s frame rate. A Flash movie’s frame rate is much like its heartbeat. With each beat, the Flash Player updates and renders all on-screen elements and also runs any required ActionScript tasks.
It is the frame rate that determines how much time Flash Player should spend on each beat, so a frame rate of 10 frames per second (fps) means at least 100 milliseconds per beat. If all the required tasks are performed within that time, then Flash Player will wait for the remaining time to pass before moving on to the next beat. On the other hand, if the required tasks in a particular beat are too CPU intensive to be completed within the given time frame, then the frame rate automatically slows down to allow for some extra time. Once the load lightens, the frame rate speeds up again, back to the set rate.
(The frame rate may also be automatically throttled down to 4fps by the Flash Player when the program’s parent window looses focus or goes offscreen. This is done to conserve system resources whenever the user’s attention is focused elsewhere.)
What this all means is that there are actually two kinds of frame rates: the one you originally set and hope your movie always runs at, and the one it actually runs at. We’ll call the one set by you the target frame rate, and the one it actually runs at the actual frame rate.
The graph’s CPU load is calculated as a ratio of actual to target frame rate. The formula used to calculate this is:
CPU load = ( target frame rate - actual frame rate ) / actual frame rate * 100 For example, if the target frame rate is set to 50fps but the movie actually runs at 25fps, the CPU load will be 50% – that is,
( 50 - 25 )/ 50 * 100. Please note that this is not the actual percentage of system CPU resources used by the running movie, but rather a rough estimate of the actual value. For the optimization process outlined here, this estimate is a good enough metric for the task at hand. To get the actual CPU usage, use the tools provided by your operating system, e.g. the Task Manager in Windows. Looking at mine it right now, it shows the unoptimized movie is using 53% of CPU resources, while the movie’s graph shows a CPU load of 41.7%.
ActionScript 3.0 Optimization: A Practical Example
No comments:
Post a Comment