Firefox JavaScript hiccups

While I was testing the speed of some simple JavaScripts on my recently invented JavaScript speed testing ground I noticed a weird thing going on in Firefox. But let’s start at the beginning.

Mozilla FirefoxImage via WikipediaThe JavaScript speed testing I do is very simple. I take a user specified piece of code and eval it, then I take the name of the function to call and eval that which means I get a pointer to a function I need to call. I set the number of times I want the function to be executed and then I prepare the interval.

The testing takes place in a function that is called every second which should give functions enough time to do what they’re supposed to do the specified number of times. The time is measured by setting a variable to the current date (and time of course) and then the function is executed in a for loop specified number of times. Immediately after the loop the time is measured again and the difference is the time spent by the loop. To be able to validate the output of the function I assign whatever the function returns to a variable (all the variables are set up before any of this happens). So the code looks like this:

t0 = new Date();
for (i=0;i<times;i++) {
	r = fn();
t1 = new Date();

The first thing when doing such speed test is to run an empty test. What we want to know is how much the whole time measuring takes. It’s got something to do with the fact that as soon as you measure the time you change what’s going on and also the time it takes to do that. And the fact that we’ve got a loop, a compare and an assignement going on.

So I ran this function test(){} a 100000 times. Since it doesn’t actually do anything you’d expect to get small and very similar times. And you do. So the next thing was to try something that actually does something, like function test(){var a={b:1};}. I expected bigger times but still quite similar. And I got such times in all browser I tested except Firefox. At first I thought that it might be something with the operating system. Or the extensions. Or any other number of things that could delay a JavaScript. But after quite a lot of tests on other browsers and platforms I’m quite sure that Firefox is the one to blame.

I tested Firefox 2 and Firefox 3 and both have the same problems. The only difference being that Firefox 3 has bigger problems — the times go up by 3-4 times the normal time, while in Firefox 2 I only saw a 2 times increase. I should mention that the biggest time in Firefox 3 (even with the increase) was still smaller than all the Firefox 2 times. What I did find out is that Firefox is a completely unreliable browser for speed testing. I have no idea what’s causing this but I’d really like to know. Anyone?

And while am at it — with Firebug 1.2 and all the panels on the times were about 3 times slower on Firefox 3.

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One Response to “Firefox JavaScript hiccups”

  1. Boky says:

    Garbage collection.

    You recreate objects and FF seems to be more agressive with garbage collection than other browsers.

    Try something like:
    var a = Array();
    function test(){ a.pus({b:1});}.

    This should not invoke garbage collection, IMHO.

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