How do I speed up the gwt compiler?

We're starting to make heavier use of GWT in our projects, and the performance of the GWT compiler is becoming increasingly annoying.

We're going to start altering our working practices to mitigate the problem, including a greater emphasis on the hosted-mode browser, which defers the need to run the GWT compiler until a later time, but that brings its own risks, particularly that of not catching issues with real browsers until much later than we'd like.

Ideally, we'd like to make the GWT compiler itself quicker - a minute to compile a fairly small application is taking the piss. However, we are using the compile if a fairly naive fashion, so I'm hoping we can make some quick and easy gains.

We're currently invoking com.google.gwt.dev.Compiler as a java application from ant Ant target, with 256m max heap and lots of stack space. The compiler is launched by Ant using fork=true and the latest Java 6 JRE, to try and take advantage of Java6's improved performance. We pass our main controller class to the compiler along with the application classpath, and off it goes.

What else can we do to get some extra speed? Can we give it more information so it spends less time doing discovery of what to do?

I know we can tell it to only compile for one browser, but we need to do multi-browser testing, so that's not really practical.

All suggestions welcome at this point.

Solution 1:

Let's start with the uncomfortable truth: GWT compiler performance is really lousy. You can use some hacks here and there, but you're not going to get significantly better performance.

A nice performance hack you can do is to compile for only specific browsers, by inserting the following line in your gwt.xml:

<define-property name="user.agent" values="ie6,gecko,gecko1_8"></define-property>

or in gwt 2.x syntax, and for one browser only:

<set-property name="user.agent" value="gecko1_8"/>

This, for example, will compile your application for IE and FF only. If you know you are using only a specific browser for testing, you can use this little hack.

Another option: if you are using several locales, and again using only one for testing, you can comment them all out so that GWT will use the default locale, this shaves off some additional overhead from compile time.

Bottom line: you're not going to get order-of-magnitude increase in compiler performance, but taking several relaxations, you can shave off a few minutes here and there.

Solution 2:

If you run the GWT compiler with the -localWorkers flag, the compiler will compile multiple permutations in parallel. This lets you use all the cores of a multi-core machine, for example -localWorkers 2 will tell the compiler to do compile two permutations in parallel. You won't get order of magnitudes differences (not everything in the compiler is parallelizable) but it is still a noticable speedup if you are compiling multiple permutations.

If you're willing to use the trunk version of GWT, you'll be able to use hosted mode for any browser (out of process hosted mode), which alleviates most of the current issues with hosted mode. That seems to be where the GWT is going - always develop with hosted mode, since compiles aren't likely to get magnitudes faster.

Solution 3:

Although this entry is quite old and most of you probably already know, I think it's worth mention that GWT 2.x includes a new compile flag which speeds up compiles by skipping optimizations. You definitely shouldn't deploy JavaScript compiled that way, but it can be a time saver during non-production continuous builds.

Just include the flag: -draftCompile to your GWT compiler line.