Firebug and the JIT
One thing we hear a lot from Firebug users is that Firebug slows down Firefox. When Firebug is active, particularly when you’ve enabled the Console/Script panels, some pages perform much more slowly. Enabling these panels turns on all of our debugging hooks, so some slowdown isn’t surprising, but what may surprise you is that, in order to get accurate debugging information, these hooks also turn off Firefox’ high-performance Javascript JIT compiler, even when Firebug is inactive. And now we have a fix for that.
First, a little terminology. Feel free to skip this paragraph if you consider yourself a master of Javascript internals or compiler run-time optimization makes your skin crawl. Tracing is the mechanism Firefox’ Javascript engine (aka, “SpiderMonkey”) uses to improve code execution performance. It provides a major speedup for code running in Firefox 3.5 and up, often an order of magnitude for certain types of operations. It is the basis of the JIT or Just-in-Time compiler. Without tracing, the JS engine can’t optimize code as well, leading to significantly slower execution.
I need to be clear here: If you have Firebug installed you are probably not getting fast Javascript. Firebug doesn’t have to be active on your current page. If you have the grey icon on your status bar, you have probably disabled the JIT. This is true if you have ever enabled the Console and consequently the Script panels and left them on. This is likely true for most recent versions of Firebug. The quick fix is to disable the Script and Console panels via the mini menu on their respective tabs.
Boris Zbarsky has been doing a lot of poking around in the belly of the JS debugger lately. Working in conjunction with John Barton, they have concocted fixes for the soon-to-be-released Firebug 1.5 and Firefox 3.6. We’re working to get testable versions out as quickly as possible, but I can say that I’ve tested development builds using the pages mentioned in the bug and can verify that these fix the problem. As a side-benefit, I got to watch Boris dissect the problem through gdb in an Xterm (yes, an Xterm) and Emacs and it was pretty impressive and fun, to boot.
When installing Firebug one of the first things most people do is enable the Console. It’s such a useful debugging tool. I myself have been using Firebug for over a year without JITted code. I probably never noticed the slowdown simply because I’m so used to running my browser this way and have pretty fast machines. After disabling the Console and Script panels there are a few pages that just load much much quicker. It’s kind of shocking.
Since the patch has landed on Firefox 3.6, I’m expecting either a beta or release candidate of that to come out shortly. I’m also hoping for another Firebug beta (or release candidate) later this week. In the meantime, if you’re not using them, I recommend disabling the Console and Script panels and find out what you’ve been missing and turn them on only as-needed. This will likely be the work-around for Firefox 3.5 users unless we decide to push this back to that branch but I have my doubts that this is something we would do.
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