{"id":914,"date":"2011-06-21T15:36:35","date_gmt":"2011-06-21T04:36:35","guid":{"rendered":"http:\/\/blog.mozilla.org\/nnethercote\/?p=914"},"modified":"2011-06-21T18:06:19","modified_gmt":"2011-06-21T07:06:19","slug":"you-make-what-you-measure","status":"publish","type":"post","link":"https:\/\/blog.mozilla.org\/nnethercote\/2011\/06\/21\/you-make-what-you-measure\/","title":{"rendered":"You make what you measure"},"content":{"rendered":"

Inspired by Shaver’s patch<\/a>, I implemented some per-compartment stats<\/a> in about:memory.\u00a0 I then visited TechCrunch<\/a> because I know it stresses the JS engine.\u00a0 Wow, there are over 70 compartments!\u00a0 There are 20 stories on the front page.\u00a0 Every story has a Facebook “like” button, a Facebook “send” button, and a Google “+1” button, and every button gets its own compartment.<\/p>\n

That sounds like a bug, but it’s probably not<\/a>.\u00a0 Nonetheless, every one of those buttons had an entry in about:memory that looked like this:<\/p>\n

\"Old<\/a><\/p>\n

(The ‘object-slots’ and ‘scripts’ and ‘string-chars’ measurements are also new, courtesy of bug 571249<\/a>.)<\/p>\n

Ugh, 255,099 bytes for a compartment that has only 971 bytes (i.e. not much) of scripts?\u00a0 Even worse, this is actually an underestimate because there is another 68KB of tjit-data memory that isn’t being measured for each compartment.\u00a0 That gives a total of about 323KB per compartment.\u00a0 And it turns out that no JIT compilation is happening for these compartments, so all that tjit-data and mjit-code space is allocated but unused.<\/p>\n

Fortunately, it’s not hard to avoid this wasted space, by lazily initializing each compartment’s TraceMonitor<\/a> and JaegerCompartment<\/a>.\u00a0 With that done, the entry in about:memory looks like this:<\/p>\n

\"New<\/a><\/p>\n

That’s an easy memory saving of over 20MB for a single webpage.\u00a0 The per-compartment memory reporters haven’t landed yet, and may not even land in their current form, but they’ve already demonstrated their worth.\u00a0 You make what you measure.<\/p>\n","protected":false},"excerpt":{"rendered":"

Inspired by Shaver’s patch, I implemented some per-compartment stats in about:memory.\u00a0 I then visited TechCrunch because I know it stresses the JS engine.\u00a0 Wow, there are over 70 compartments!\u00a0 There are 20 stories on the front page.\u00a0 Every story has a Facebook “like” button, a Facebook “send” button, and a Google “+1” button, and every […]<\/p>\n","protected":false},"author":139,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[4550,4543,4544,467],"tags":[],"_links":{"self":[{"href":"https:\/\/blog.mozilla.org\/nnethercote\/wp-json\/wp\/v2\/posts\/914"}],"collection":[{"href":"https:\/\/blog.mozilla.org\/nnethercote\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/blog.mozilla.org\/nnethercote\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/blog.mozilla.org\/nnethercote\/wp-json\/wp\/v2\/users\/139"}],"replies":[{"embeddable":true,"href":"https:\/\/blog.mozilla.org\/nnethercote\/wp-json\/wp\/v2\/comments?post=914"}],"version-history":[{"count":0,"href":"https:\/\/blog.mozilla.org\/nnethercote\/wp-json\/wp\/v2\/posts\/914\/revisions"}],"wp:attachment":[{"href":"https:\/\/blog.mozilla.org\/nnethercote\/wp-json\/wp\/v2\/media?parent=914"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/blog.mozilla.org\/nnethercote\/wp-json\/wp\/v2\/categories?post=914"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/blog.mozilla.org\/nnethercote\/wp-json\/wp\/v2\/tags?post=914"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}