{"id":1086,"date":"2011-07-27T16:02:15","date_gmt":"2011-07-27T05:02:15","guid":{"rendered":"http:\/\/blog.mozilla.org\/nnethercote\/?p=1086"},"modified":"2011-07-27T16:02:15","modified_gmt":"2011-07-27T05:02:15","slug":"memshrink-progress-week-6","status":"publish","type":"post","link":"https:\/\/blog.mozilla.org\/nnethercote\/2011\/07\/27\/memshrink-progress-week-6\/","title":{"rendered":"MemShrink progress, week 6"},"content":{"rendered":"

Dave Hunt wrote this week about some endurance tests comparing Firefox 4, 5, 6, 7, and 8<\/a>, which show how Firefox’s memory usage is improving.\u00a0 Pay most attention to the ‘resident’ numbers for Firefox 6, 7 and 8, which indicate how much physical memory is being used.\u00a0 The ‘explicit’ numbers show a big drop between 6 and 7, and then a rise between 7 and 8, but they are suspect<\/a>, so don’t panic yet.<\/p>\n

about:memory improved again this week.\u00a0 I added a reporter for each compartment’s property table<\/a>, and two reporters js-compartments-system<\/code> and js-compartments-user<\/code><\/a> which count how many compartments are present. These latter two will be added to telemetry<\/a> soon.<\/p>\n

I also added a reporter that computes the fraction of the JS heap that is unused<\/a>, called js-gc-heap-unused-fraction<\/code>.\u00a0 The results are surprising — numbers like 30% and 50% are common.\u00a0 (Some preliminary JS heap visualization<\/a> helps explain why.)\u00a0 There are some suggestions for short-term improvements to mitigate this (e.g. smaller GC chunks<\/a>).\u00a0 However, I suspect that to fix it properly will require a compacting garbage collector<\/a> — the current mark and sweep algorithm unavoidably leaves lots of holes all over the heap each time it runs.\u00a0 But I could be wrong!\u00a0 GCs have a large design space and there may be other solutions.\u00a0 A non-compacting generational GC<\/a> would help in its own way too — fewer objects would reach the general heap and so each full heap collection would likely collect less garbage, and thus leave fewer holes.<\/p>\n

Here’s this week’s bug count:<\/p>\n