Incremental GC in Firefox 16!
Firefox 16 will be the first version to support incremental garbage collection. This is a major feature, over a year in the making, that makes Firefox smoother and less laggy. With incremental GC, Firefox responds more quickly to mouse clicks and key presses. Animations and games will also draw more smoothly.
The basic purpose of the garbage collector is to collect memory that JavaScript programs are no longer using. The space that is reclaimed can then be reused for new JavaScript objects. Garbage collections usually happen every five seconds or so. Prior to incremental GC landing, Firefox was unable to do anything else during a collection: it couldn’t respond to mouse clicks or draw animations or run JavaScript code. Most collections were quick, but some took hundreds of milliseconds. This downtime can cause a jerky, frustrating user experience. (On Macs, it causes the dreaded spinning beachball.)
Incremental garbage collection fixes the problem by dividing the work of a GC into smaller pieces. Rather than do a 500 millisecond garbage collection, an incremental collector might divide the work into fifty slices, each taking 10ms to complete. In between the slices, Firefox is free to respond to mouse clicks and draw animations.
At this point, we’re still working heavily on incremental collection. There are still some phases of collection that have not been incrementalized. Most of the time, these phases don’t take very long. But users with many tabs open may still see unacceptable pauses. Firefox 17 and 18 will have additional improvements that will decrease pause times even more.
If you want to explore further, you can install MemChaser, an addon for Firefox that shows garbage collection pauses as they happen. For each collection, the worst pause is displayed in the addon bar at the bottom of the window. It’s important to realize that not all pauses in Firefox are caused by garbage collection. You can use MemChaser to correlate the bumps in the chart with garbage collections reported by MemChaser.
If there is a bump when no garbage collection happened, then something else must have caused the pause. The Snappy project is a larger effort aimed at reducing pauses in Firefox. They have developed tools to figure out the sources of pauses (often called “jank”) in the browser. Probably the most important tool is the SPS profiler. If you can reliably reproduce a pause, then you can profile it and figure out what Firefox code was running that made us slow. Then file a bug!
Keep hitting..
-- Jidheesh Rajan --
