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Jul 17

Opening up conversation on browser interrogation tools with Browser Memory Tool Prototype

Mozila, Open Source, Tech with tags: , , Add comments

Do you sometimes feel like the browser is a black box? We are building richer and richer applications on the Web platform and this means that developers are running up against new issues to debug and test.

We feel like it is a great time to develop new tools that afford you the ability to look into the runtime to hopefully help you find a bug, or allow you to keep your application as responsive as possible.

Today we want to start a conversation about some of our thinking, with the hope that you will join in.

We have been taking a hard look at the tools landscape, and here is a presentation that gives you an idea of our thinking:

We will be posting more of our thoughts, but as you can hear, our vision for these tools is that they:

  • Are able to run out-of-process. We view out of process tools as the preferred way to observe the runtime because it enables us to somewhat ignore the Heisenberg uncertainty principle. If we are profiler the browser, having to deal with NOT profiling the profiler code can be painful. Also, we want to be able to use the same tools on devices. I would much rather point my desktop tool to a Fennec device, compared to trying to use the tool on the phone itself! This leads us too…
  • Enable cross browser experiences: Our lab doesn’t have the resources to develop deep integrations with multiple browsers, but we very much want to enable that. Since we are running out-of-process, we can document the communication API and many hosts can then be wired up.


The first experiment in this vein is a stand-alone memory tool prototype that lets you poke around in the JavaScript heap. What objects are there? How many of them are there? Any dangling references due to closures or event listeners?

To kick this off we worked with awesome Mozilla colleagues such as David Barron and Atul Varma which enabled us to spike down to the bare metal of the browser.

We ended up with an architecture for the tool that contains these components:

Firefox Add-on

A special Firefox add-on installs a binary component that gives us access to the low level JavaScript heap. This gives us a simple API with methods that allow you to enable profiling, get the root objects in the heap, and get detailed information on the objects themselves.

Firefox Memory Server

The current consumer of the core API is a min server. Once activated, the browser freezes and your only option to interact with it is via this server. It exposes a simple socket API with URLs mapping to the high level APIs. For example, you can access /gc-roots to get the root object ids. Or you can ask for details on an object via /objects/XXX where XXX is the object id you are inspecting. When you are done, you access /quit and the browser is unfrozen.

All of these APIs support JSONP which is how we get the data back into our main application.

NOTE: Currently, the server lives within the add-on itself (at chrome://jetpack/content/memory-profiler-server.html) but eventually this will migrated to a Jetpack.

Memory Tool Ajax Application

The main application itself is a simple Web application that can be run in any browser (not just Firefox!) After you have installed the Firefox Add-on, and turned on profiling via the Memory Server, you can visit the tool. Currently, after you connect, the tool gets a dump of the root object for the first tab in the browser (not including the memory server tab). You will see the meta data associated with the object, and you can click on any of the data elements that have their own memory id (memory locations are integers with 9 digits). This tree view lets you poke around the heap.

If you want to aggregate the data, you can click on the “2. Dump Heap” button, which goes through the entire heap (which can be big!) and aggregates all of the objects for you. If you see a massive number of objects of a particular type, this could be a flag!

Enough talk, lets see it briefly in action:

The tool is very early stage and changing constantly. However, it is all out in the open. You can grab the open source pieces:

As is always the case with Mozilla, and Mozilla Labs, we want to get ideas out into the community as soon as possible. This tool is very much alpha, and the goal of getting it out in the wild is to start a conversation about tools like these.

What tools in this area would help your job as a Web developer? We are all ears, and would like to share our dev tools mailing list / group as a good area to share ideas.

What about Firebug? This particular tool freezes the browser, and since Firebug is in-process right now, it wasn’t a great fit. However, we very much want to take this kind of work and get it into Firebug at some stage. We just aren’t at that stage yet!

On our side, we will be engaging with the Mozillans who truly grok the JavaScript (and entire browser) internals to see what interesting data we can expose to developers. We have found that the Firefox team has already added a lot of the infrastructure there, and now the task is to work out what will be useful, and how can we best report it.

We have plenty of ideas too. A wish list could contain:

  • Short term clean up (fix the backend code that interfaces with the heap, abstract out the service into a Jetpack, and make sure we are using the correct APIs, and get these APIs added where appropriate)
  • We want to visually add the graph to the object dump, so you can really understand what you are looking at. It will probably look something like this:


  • We have wired up Bespin, and we will suck out the source code from functions and showing that inline to the tool itself. There is much more to do though, and we want to find out what you need.
  • More profiling info: break up buckets of memory (images, js, DOM, etc)
  • Great way to see who is reference whom (memory leak detection)
  • Garbage Collection: When and how long are collections occuring?
  • Granular filtered profiling: Profile this event and measure every event-of-interest from the start of navigation to the present e.g. DNS & TCP connections, page header parsing, resource fetching, DOM parsing, reflow, etc.
  • Web Worker thread monitoring
  • Have a profiling mode that gives you data without having to freaze the heap, and only when you need to do a deep dive do we get to that.

We have learned a lot as we created this prototype. Atul is going to write up his experience, and we will continue to talk in the open about how we take this prototype and your ideas to the next level with browsers.


Atul has posted on his experience with SpiderMonkey and how the JS Runtime works. Nice in depth stuff. He also created PyMonkey “a Python C extension module to expose the Mozilla SpiderMonkey engine to Python.” which is crazy cool.

9 Responses to “Opening up conversation on browser interrogation tools with Browser Memory Tool Prototype”

  1. Patrick Mueller Says:

    The movies are b0rked. The audio doesn’t match up with the video, and the audio seems to run much longer than the video.

  2. kdsfksd Says:

    This is pretty damned cool.

  3. Kim Sullivan Says:

    tldr, but: “Enable cross browser experiences: Our lab doesn’t have the resources to develop deep integrations with multiple browsers, but we very much want to enable that. Since we are running out-of-process, we can document the communication API and many hosts can then be wired up. ”

    You mean, something like Opera’s Dragonfly? Ok, maybe the fact that Opera didn’t succeed doesn’t mean that if Firefox reinvents the wheel, it will do better (it does have a much larger developer codebase), but I don’t think you can expect that Opera is going to implement your API when they have their own (as far as I understood). And since neither you nor Webkit implemented Opera’s API, what’s the chance that Webkit is going to implement your api?

    What will happen is that Webkit will have their own API, incompatible with everyone elses API.

  4. Ian Stirling Says:

    Is there a simple walkthrough for setting this up?
    I followed the links, and became lost in a maze of tiny directories all alike.

  5. Mathijs Says:

    The video’s are crappy: sound doesn’t match the video !! please repair (?)

  6. Arjen Wisse Says:

    A memory profiling tool is exactly what I was looking for. It seems currently really a topic which is very unknown and unexplored. Memory profiling is really needed in web applications, however nearly the only topic which is extensively explored are the closure issues with DOM and Javascript combination.

    Therefore, I decided to take a look and I have downloaded and installed this to see how this tool could help me. To start, I had to fix an issue in the profiler server, which was still pointing to the old ‘jsweakrefdi’ factory, which has been renamed to ‘jetpackdi’. After changing that I could start working..

    For this to work you need to different browsers (every instance of Firefox will really freeze) but the tool had some issues with other browsers (IE was giving a script error on showing the heap dump, Chrome was not able to browse through the tree). But it’s a prototype, so I did not expect full functionality.

    There are a couple of things I would like to see improve in next versions:
    -1- Browsing through the object tree will cause a loss of context (how do I get back after clicking some object and where am I?). Probably better to build up a subtree while clicking on an object reference (solves both).
    -2- It is not clear to me what I see in the quick look tab?
    -3- The heap dump shows now objects and functions mixed. I think it would be nice to distinguish between those two (as VisualVM does with Heap and NonHeap)
    -4- It is very handy when you are able to see the content of simple type fields in an object (like the content of a String, Number or Boolean field).
    -5- Besides being able to see in the heap dump which instances of a specific type are available with there referrers, it would also be great to see that on the meta level (meaning that you see which types are referring to each other with the amount of instances as this gives a quick insight in the ‘hotspots’).

    Besides that, what I currently saw in the ‘heap dump’ was nearly only a list of functions (including prototype methods), while I was expecting a list of object types? Also a lot of ‘internal’ firefox and jetpack functions where shown there (can those be filtered out?).

    I hope to see this tool grow and provide valuable information like memory profilers as VisualVM and YourKit do. When do you expect this to be ready ;)? (I can’t wait..)

  7. Kevin Says:

    Hi Arjen -

    I understand the build is not in final, but I am having some significant issues trying to deploy this application.

    Just to be clear – I downloaded and installed the Jetpack addon for FF 3.5 Windows, I navigated to the memory-profiler…html page and clicked the link – nothing happened. I navigated to c:/documents and settings/..memory-profiler-server.html and changed the reference from jsweakrefdi TO jetpackdi per your recommendation.

    Restarted FF and still no initialization of Server. I’ve been working on this for about 12 hours now because my web app is leaking memory and I’d kill to just have a glimpse of the memory allocation.

    If you could shed some light on anything I may have missed, could you let me know? I’ll throw some karma your way!


  8. Dion Almaer Says:


    We will have a new version of the app out hopefully this week that should be a lot easier for you…. sorry for the pain.



  9. Kevin Says:

    Hi Dion -

    Thanks for your reply! Well it looks like it only took one hour more to find out what was wrong. It wasn’t just the one reference to the jsweakrefdi factory that was wrong. I in fact had to replace the entire block of code that initialized that factory – it was also invoking a set method instead of a get method.

    I look forward to trying out the program more and seeing more updates come out of the pipeline! Again, I appreciate your response – its great to know that when you put yourself out there someone actually is listening :)

    Can’t wait to test future releases!


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