Use cases for WeakMap

Erik Corry erik.corry at gmail.com
Mon May 16 11:30:25 PDT 2011


2011/5/16 Andreas Gal <gal at mozilla.com>:
>
> 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.

>
> 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
>
>


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