HotSpot is Hot

I've been doing a lot of Java lately, and (at least at work) moving away from ColdFusion.  I've got mixed feelings about the transition, but that's a different post.  Today I was doing some VERY primitive profiling on the application, and was amazed.

Not surprisingly, the majory of the request processing time went to data access.  We're using Hibernate, and it brings it's own overhead on top of the actual DB.  However, even with some gnarly HQL, it runs reasonably fast.  And as an aside, the Hibernate guys are friggin' nuts.  Turn on debugging some time and trace through the code.  If you don't nearly soil yourself, you're a braver man than I.

Also not surprising was confirmation that ClearSilver is ludicriously fast.  Even doing some pretty complicated multi-part renderings I couldn't get an entire request to spend more than 2-3 milliseconds running the ClearSilver engine.

What was surprising was how good of a job HotSpot does.  I don't claim to understand it's voodoo, but in a nutshell it monitors your Java bytecode as it's executing and optimizes it's execution, including converting the bytecode to native code, compiling it, and then using the compiled version if it thinks it ncessary.  There's a whole pile of things that it does, but that's how it got it's name (it finds the "hot spots" and cools them down).
I was quite demoralized after getting the first full implementation of our rendering engine working when even simple pages were costing 300-400ms each.  Note that "simple" means "a GET for a dynamic, multi-object page", so not really that simple.  But simpler than some gnarly form post.  I went back and made some tweaks to some of the problem areas, restarted Tomcat, and didn't see much improvement.  On a whim, I hit refresh four of five more times, and suddenly the execution times dropped to less than half of what they were.  A smile crossed my face as I thought "I love when technology just magically does the right thing."

I've got to do some checking, but those seven or eight requests that it took for HotSpot to kick in probably comprised several hundred executions of a particular method, so it doesn't take effect right away.  I also suspect that Hibernate will have some hot spots that will eventually be optimized as well, so I'll get even better performance once they're found.

If you're doing profiling (at least on Java), make sure you give your JVM some time to "warm up" and figure out how to best optimize itself.  I know I haven't done that in the past, but I sure will from now on.

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