SpiderMonkey is Firefox’s JavaScript engine.\u00a0 In Firefox 3.0 and earlier versions, it was just an interpreter.\u00a0 In Firefox 3.5, a tracing JIT called TraceMonkey was added.\u00a0 TraceMonkey was able to massively speed up certain parts of programs, such as tight loops;\u00a0 parts of programs it couldn’t speed up continued to be interpreted.\u00a0 TraceMonkey provided large speed improvements, but JavaScript performance overall still didn’t compare that well against that of Safari and Chrome, both of which used method JITs that had worse best-case performance than TraceMonkey, but much better worst-case performance.<\/p>\n
If you look at the numbers, this isn’t so surprising.\u00a0 If you’re 10x faster than the competition on half the cases, and 10x slower on half the cases, you end up being 5.05x slower overall.\u00a0 Firefox 4.0 added a method JIT, JaegerMonkey, which avoided those really slow cases, and Firefox’s JavaScript performance is now very competitive with other browsers.<\/p>\n
The take-away message:\u00a0 you lose more when slow than you gain when fast.<\/em> Your performance is determined primarily by your slowest operations.\u00a0 This is true for two reasons.\u00a0 First, in software you can easily get such large differences in performance: 10x, 100x, 1000x and more aren’t uncommon.\u00a0 Second, in complex programs like a web browser, overall performance (i.e. what a user feels when browsing day-to-day) is determined by a huge range of different operations, some of which will be relatively fast and some of which will be relatively slow.<\/p>\n Once you realize this, you start to look for the really slow cases. You know, the ones where the browser slows to a crawl and user starts cursing and clicking wildly and holy crap if this happens again I’m switching to another browser.\u00a0 Those are the performance effects that most users care about, not whether their browser is 2x slower on some benchmark.\u00a0 When they say “it’s really fast”, the probably actually mean “it’s never really slow”.<\/p>\n That’s why memory leaks are so bad — because they lead to paging, which utterly destroys performance, probably more than anything else.<\/p>\n It also makes me think that the single most important metric when considering browser performance is page fault counts.\u00a0 Hmm, I think it’s time to look again at Julian Seward’s VM profiler<\/a> and the Linux perf<\/a> tools.<\/p>\n <\/p>\n","protected":false},"excerpt":{"rendered":" SpiderMonkey is Firefox’s JavaScript engine.\u00a0 In Firefox 3.0 and earlier versions, it was just an interpreter.\u00a0 In Firefox 3.5, a tracing JIT called TraceMonkey was added.\u00a0 TraceMonkey was able to massively speed up certain parts of programs, such as tight loops;\u00a0 parts of programs it couldn’t speed up continued to be interpreted.\u00a0 TraceMonkey provided large […]<\/p>\n","protected":false},"author":139,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[30,4544,311,467],"tags":[],"_links":{"self":[{"href":"https:\/\/blog.mozilla.org\/nnethercote\/wp-json\/wp\/v2\/posts\/848"}],"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=848"}],"version-history":[{"count":0,"href":"https:\/\/blog.mozilla.org\/nnethercote\/wp-json\/wp\/v2\/posts\/848\/revisions"}],"wp:attachment":[{"href":"https:\/\/blog.mozilla.org\/nnethercote\/wp-json\/wp\/v2\/media?parent=848"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/blog.mozilla.org\/nnethercote\/wp-json\/wp\/v2\/categories?post=848"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/blog.mozilla.org\/nnethercote\/wp-json\/wp\/v2\/tags?post=848"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}