Tuesday, May 31, 2005

Drip: IE Leak Detector

To anyone still following this site, my apologies for taking a millenium or two between posts recently. Things have been a bit crazy of late, but I have something to introduce that will hopefully make up for the radio silence:

Drip -- an Internet Explorer leak detector

Over the last few months, a number of people have written to me or left comments asking questions about their memory leak issues with DHTML (or AJAX or whatever-you-want-to-call-it-this-week) applications. Unfortunately, there's not much I could offer in the way of advice that most people don't already know. Get rid of closures, unhook your event handlers, etc. This advice just isn't all that helpful when you've got a giant mess of JavaScript (often inherited) and visually detecting leak scenarios can be maddeningly subtle.

I did, however, find it quite surprising that no one had ever built a leak detector for Internet Explorer (or apparently for any other browser with leak problems; Mozilla has some, but they seem to be more for developers working on Mozilla itself, and the browser does a pretty good job of cleaning up leaks anyway). So I built one.

What it Does

It's a pretty simple application. Basically, it lets you open an HTML page (or pages, in succession) in a dialog box, mess around with it, then check for any elements that were leaked.

The interface is currently rather spartan. Here's what the main app looks like:

[Sorry, I lost the image somewhere along the way]

On the top you'll notice what looks like a crude version of Explorer's navigation bar. You've got the standard back and forward buttons, the URL box, and the 'go' button. These behave exactly as you might expect. To the right of it, however, is a 'check leaks' button, which will be grayed out when you first run the app. In order to try it out, you will first need to go to an HTML page (preferably one that you suspect leaks). The test page at [sorry I lost this page] will work. When you load this page, the 'check leaks' button will become enabled. Click it to see the following report:

[Yet again, I lost the image somewhere along the way]

This simple page leaks two DOM elements, a DIV and a BUTTON. These two elements are displayed in the top list, along with their source documents (useful if you've loaded more than one document between leak tests, or if you have more than one frame), the number of outstanding references on them, and their ID and CLASS attributes.

If you click on one, you'll see a list of its enumerable attributes in the bottom list. A particularly useful attribute for identifying the elements is 'innerHTML'.

Blowing Memory

Back to the main dialog for a moment. You might also have noticed the interestingly-titled 'blow memory' button. Its function is simple: to constantly reload a page as fast as it can, and to report the process' memory usage in the list box below. This is a helluva lot easier than pressing F5 for hours to determine how fast a page leaks memory.

How it Works

Fortunately, Internet Explorer's architecture made this app fairly easy to build. It's basically a simple MFC app with a browser COM component in it. The strategy for catching leaked elements is as follows:

  • When a document has been downloaded, sneakily override the document.createElement() function so that the application is notified of all dynamically-created elements.
  • When the document is fully loaded, snag a reference to all static HTML elements.
  • To detect leaks: navigate to a blank HTML page (so that IE attempts to release all of the document's elements),
  • force a garbage-collection pass (by calling window.CollectGarbage()),
  • and look at each element to see if it has any outstanding references (by calling AddRef() and Release() in succession on it).

Within the leak dialog, each element's attributes are discovered and enumerated using the appropriate IDispatch/ITypeInfo methods.


This is basically an alpha release. The interface more or less blows, and I may have left glaring holes in the leak-detection strategy or in the code itself. It seems to work for me, but I would really like for anyone using it to keep an eye out for any problems so that I can fix them. And please don't hesitate to contact me, of course, if you have any ideas, praise, criticism, or even rants to offer. I really want this to help people to stop dealing with these god-awful leaks, and since Microsoft doesn't seem inclined to fix this design flaw, we can at least try to make it more bearable.

What Next?

Obviously, I would like any feedback I can get. There are definitely some interface quirks I need to iron out. And I would like to do more to help determine the actual cause of each leak. There are a few things that I would like to find out, and if anyone has any pointers, please share them:

  • Can you perform similar tricks with Safari/KHTML or Opera? (I know you can with Mozilla, but since it doesn't really leak much, that seems rather pointless)
  • Does anyone know if it's possible to enumerate variables on one of IE's JavaScript closures? (meaning the stack frame hanging off of the function reference)
  • How about enumerating expandos on IE DOM objects from C++? (I only seem to get built-in properties from ITypeInfo)

I'm sure other questions will come up in the near future. Oh, and I will be releasing the source before too long, as soon as I get a few things cleaned up.

Happy leak hunting!