Let’s Talk About The Flex Profiler

Yeah, I know. This is a compiler blog, not a profiler blog. But I can explain…

The performance and memory profiling feature in Flex Builder Professional Edition wasn’t done by the FB development team. It was done by the Flex compiler team. Yes. I mean everything: the Eclipse plugin, user interface, profiler agent, data collection and processing!

So now you know that the compiler team was not only busy with the compiler, but also the profiler, during the Flex 3 development cycle. And that explains why I’m “eligible” to talk about the profiler here…

Those who have been closely following Flex since version 1 should notice that profiling was present in version 1 and 1.5 but absent in version 2 and 2.0.1. Why no profiler in version 2? It’s a long story. But it was a combination of poor scheduling, coordination and… well… lack of technological advances.

You may ask, what technological advances? Well, if you had used Flex 1.0 and 1.5 in the past, you should know that there was a performance profiler (http://www.adobe.com/devnet/flex/articles/profiler.html). The performance profiler in Flex 1.x was trace-based, i.e. the Player records timing data for everything that happens in the old AVM. The upside is that it captures everything. The downside is that it makes the Player run slowly and the amount of data for a short period of time (say 10 seconds) could be huge and take time to process. I’m not suggesting that the Flex 1.x performance profiler is not useful. It needs you to know roughly where to optimize. In fact, thanks to the profiler, we were able to improve Flex 1.5 framework startup performance by over 30%. Yahoo! was able to speed up their Flash/Flex-based Yahoo! Maps before the debut. But the breakthrough came when Tom Reilly added support for sample-based profiling to the latest Player…

I’m making an assumption here that you all studied statistics when you were in college and know what sampling is. If not, ask the people around you about the purpose of national presidential polls you hear these days. Same idea. Of course, I’m very sure that Tom’s stuff for identifying performance bottlenecks in your code is a lot more accurate than those national polls for predicting the next president of the United States! 🙂

Unlike traced-based, with sample-based profiling, you don’t get everything. But your application runs just as fast (or as slow) when sampling is enabled. Because it’s sampling, you don’t get a notification every time your application enters or exits a function. But you still get a pretty accurate performance profile.

So far so good, but there are some negatives. For instance, it is pretty tricky to find, given a function that is getting called excessively, the time distribution from the list of callers of that function.

Other than performance profiling, the Player now also supports memory profiling. It reports object instantiations, object deletions and where objects were instantiated.

So far, I’ve talked quite a bit about the Player side of the Flex profiler story. Let’s turn to the profiler architecture. It is actually pretty simple. There are 4 pieces:

1. The flash.sampler.* API support and the PreloadSwf support in the Player
2. ProfilerAgent.swf as the PreloadSwf
3. Java-based profiler client library
4. Eclipse plugin + the graphical user interface

For performance profiling, the flash.sampler API returns sampling data as a stream of stack traces. So, it’s pretty easy to process. For memory profiling, the API returns object instantiation and deletion events and also stack traces indicating where objects were instantiated.

The PreloadSwf support is new in the latest Player. The idea is that, if you specify a local SWF in your mm.cfg, the Player will load/run the specified SWF in parallel to the target SWF. These two crucial Player features together provide the one-two punch we desperately needed against slow-running Flash movies.

Now anyone could write some AS3 codes to collect performance and memory data. For the Flex Builder profiling feature, we implemented a non-GUI-based AS3 program (ProfilerAgent.swf) that collects and sends performance/memory data to Flex Builder via socket connections.

On the client side, there are two pieces. The first piece is the Eclipse plugin and the graphical user interface. The “Flex Profiling” perspective and the associated views (a total of 11) are a major addition to Flex Builder 3. The user interface is aimed at:

1. identifying performance bottlenecks
2. exposing excessive object allocations
3. finding loitering objects

The second piece is the profiler client library. Obviously, it’s for maintaining server socket connections; taking performance/memory data; processing them and archiving them. We need this library because we need a demarcation between the Eclipse code and the non-Eclipse code so that QA can use the Eclipse-free client library to write automated test cases for ProfilerAgent.swf.

If you want to know how to use the Flex Profiler, please check out the livedocs (http://livedocs.adobe.com/flex/3/html/help.html?content=profiler_3.html).

To Be Continued…