Scrubbing Movie Clip Fills Memory, Crashes Flash

Question

Hello,
We have a Flex 4/Actionscript 3 application that involves "scrubbing" through a Movie Clip that is a swf file derived from a .mov file.  This is accomplished by listenening to the change in value of an HSlider and calling gotoAndStop(n) on the Movie Clip repeatedly as the slider is dragged.  The Movie Clip is 720 pixels wide by 405 pixels high.
It works great, however, after doing this scrubbing action a number of times, which is required by our application, we notice the memory (by tracing System.totalMemory and System.privateMemory) increasing dramatically; and when it gets to around 100 MB (though it's not the same every time), the Flash plugin in the browser will crash.  Our assumption is that this happens because the scrubbing essentially loads dozens of bitmaps (each frame of the Movie Clip that is scrubbed over) very rapidly.
To try and ameliorate this, we have implemented aggressive garbage collection by calling System.pauseForGCIfCollectionImminent(0.1) on a timer every 10 seconds.  This usually keeps Flash from crashing, however, sometimes the garbage collection itself causes Flash to crash, though it's not 100% predictable.  We know this because we are tracing the garbage collection action and can see Flash crash when that action occurs.
It doesn't seem like we have a memory leak per se, as the free memory is eventually regained when forcing garbage collection.
So I have a couple of questions:
1) Is there any way to keep the memory from getting so large in the first place, given that we need this scrubbing functionality?
2) If not, is there a better way of tweaking the garbage collection so that it frees up memory while not itself causing Flash to crash?

Answer 1

OK, I created a new Flex project with just the simplest possible scrubbing code, plus the memory tracing.  Unfortunately, I'm getting the same results as I get in our big app.  If I can figure out what you're doing differently, I might be able to get somewhere. Here's the code:
<?xml version="1.0" encoding="utf-8"?>
<s:Application xmlns:fx="http://ns.adobe.com/mxml/2009"
               xmlns:s="library://ns.adobe.com/flex/spark"
               xmlns:mx="library://ns.adobe.com/flex/mx" minWidth="955" minHeight="600" addedToStage="onAddedToStage()">
    <fx:Declarations>
        <!-- Place non-visual elements (e.g., services, value objects) here -->
    </fx:Declarations>
    <fx:Script>
        <![CDATA[
            import flash.display.DisplayObject;
            import flash.display.MovieClip;
            import flash.events.Event;
            import flash.events.EventDispatcher;
            import flash.display.Loader;
            import flash.system.LoaderContext;
            import flash.net.URLLoader;
            import flash.net.URLLoaderDataFormat;
            import flash.net.URLRequest;
            private var swf:MovieClip;
            private var loader:Loader;
            private var memoryReportTimer:Timer = new Timer( 15000 );
            private var initialMemory:Number;
            private var currentMemory:Number;
            private function onAddedToStage():void {
                var req:URLRequest = new URLRequest( "sampleAsset/AlfredCiminoDemoReel.mov.swf" );
                var loaderContext:LoaderContext = new LoaderContext(false, ApplicationDomain.currentDomain, null);
                loader = new Loader();
                //loader.dataFormat = URLLoaderDataFormat.BINARY;
                swfContainer.addChild( loader );
                loader.contentLoaderInfo.addEventListener( Event.COMPLETE, swfLoadComplete );
                loader.load( req, loaderContext );
                //(loader.content as MovieClip).gotoAndStop(1);
                //swf.gotoAndStop(1);
                trace( "initial free memory allocation: ", (System.freeMemory / 1024).toFixed(2));
                initialMemory = Number((System.totalMemory/1024).toFixed(2));
                currentMemory = initialMemory;
                memoryReportTimer.addEventListener( TimerEvent.TIMER, reportMemoryDelta );
                memoryReportTimer.start();
            private function swfLoadComplete( e:Event ):void {
                trace( "loading complete." );
                swf = loader.content as MovieClip;
                swf.gotoAndStop(1);
                //swfContainer.addChild(swf);
                scrubber.maximum = swf.totalFrames;
            private function reportMemoryDelta( evt:TimerEvent ):void {
                var currentMemory_temp:Number = Number((System.totalMemory/1024).toFixed(2));
                var tenSecDelta:Number = currentMemory_temp - currentMemory;
                currentMemory = currentMemory_temp;
                var overallDelta:Number = currentMemory - initialMemory;
                trace( "10 second change: ", tenSecDelta, ", overall change: ", overallDelta  );
                trace( "free memory: ", (System.freeMemory / 1024).toFixed(2));
            public function setCurrentFrame():void {
                swf.gotoAndStop(scrubber.value);
        ]]>
    </fx:Script>
    <s:VGroup>
        <mx:UIComponent id="swfContainer" width="720" height="405">
        </mx:UIComponent>
        <s:HSlider id="scrubber" change="setCurrentFrame()" width="100%" />
    </s:VGroup>
</s:Application>
And here's the tracing output: