On May 16, 2011, at 11:30 AM, Erik Corry wrote: > 2011/5/16 Andreas Gal <[email protected]>: >> >> Even if you want to store weak-map specific meta data per object, nobody >> would store that directly in the object. Thats needlessly cruel on the cache >> since >>99.9% of objects never end up in a weak map. Instead one would >> locate that meta data outside the object in a direct mapped dense area (like >> mark bitmaps), which is on its own page that is not write protected. > > More than 99.9% of objects don't have a property called "fish". > Nevertheless if someone adds a "fish" property to an object V8 will > try to (and usually succeed in) storing it in the object and it won't > be cruel on the cache. Quite the opposite.
I agree. The same logic applies to object hashcodes -- an object must always produce the same hashcode which means it will need to store it -- having a secondary map doesn't help you, because that map will itself require a hascode. However making every object always have space to store a hashcode is wasteful of memory, so essentially some form of "private name" must be used. I think i saw someone (Erik?) say that this is what v8 does. --Oliver > >> >> Andreas >> >> On May 16, 2011, at 10:39 AM, Brendan Eich wrote: >> >>> On May 16, 2011, at 12:47 AM, Erik Corry wrote: >>> >>>> I think the objects used as keys in weak maps need to be somehow >>>> annotated with this information so that the GC can clean up the weak >>>> maps when the keys die. This means that if you take an object that is >>>> frozen and use it as a key in a weak map then it will need to be >>>> mutated in some way and can't be on a read-only page. >>> >>> That's already false in Firefox nightlies. We support Object.freeze. We >>> have a WeakMap implementation. We do not mutate the frozen object. Its GC >>> metadata does not reside in a header for it, or even in the same OS page. >>> >>> >>>> Perhaps you have a different, efficient, implementation. I can't see >>>> us gaining much from putting frozen objects on read-only pages, thus I >>>> can't accept it as a very strong argument about the way that frozen >>>> objects should work together with a new feature. >>> >>> This is a bit too subjective an argument, sorry. >>> >>> My point about 50+ years of OS and MMU firewalling is important. Chrome >>> (recently hacked by French spook-types, but also hacked over a year ago >>> with a two-step attack) is a convincing example. >>> >>> Sure, we have user-code isolation tools in our belts, including fancy >>> compiler/runtime pairs. But it's hard to beat processes if you want to be >>> sure. No silver bullet, simply "stronger isolation". >>> >>> >>>>> Weak maps are in Firefox nightlies. We're playing with page protection >>>>> too (not for freezing, yet). This seems like a dare, but it also seems to >>>>> be dodging my point in replying again: that private names cannot be used >>>>> to extend frozen objects in the "[[Extensible]] = true" sense of the spec. >>>> >>>> Is there a description anywhere about how you have implemented GC of weak >>>> maps? >>> >>> http://hg.mozilla.org/tracemonkey/rev/7dcd0d16cc08 >>> >>> Look for WeakMap::mark... names. There's no need to mutate a key object. >>> There should not be, either. >>> >>> Yes, this GC can iterate. A lot, but a "fix" doesn't obviously require >>> mutating (possibly frozen) key objects. Also, since POITROAE we are going >>> to measure twice, Optimize once. >>> >>> /be >> >> > _______________________________________________ > es-discuss mailing list > [email protected] > https://mail.mozilla.org/listinfo/es-discuss _______________________________________________ es-discuss mailing list [email protected] https://mail.mozilla.org/listinfo/es-discuss
