Hacker News Reader

Comment by amelius | Hacker News Reader

The most difficult part (I think) of writing a JVM is the concurrent garbage collector. Is this making use of Javascript's garbage collector to do the heavy lifting? Or do they implement their own GC in Javascript? Also, I'm wondering how they are handling concurrency.

View on HN

Doppio uses JavaScript's garbage collector. (As a result, it cannot support weak references.) As for concurrency, thread quanta are mapped to JavaScript events, so Doppio can emulate preemptive multithreading. Doppio potentially preempts a "thread" at each function call.

The nitty gritty details are in the PLDI 2014 paper [0]. Some details have slightly changed, though (e.g. DoppioJVM supports JDK8 now).

[0] https://plasma-umass.github.io/doppio-demo/paper.pdf

Once ES6 support is everywhere, you should be able to use WeakMap and WeakSet.

[0] https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...

Actually, those are not sufficient to implement weak references, and have a much different use case! With WeakMap and WeakSet, the keys are weakly referenced, hence the names. If you have a WeakMap, you can't produce a value stored in the map without a strong reference to a key.

You can actually polyfill WeakMap and WeakSet. You can't do the same for weak references.

I misspoke -- you can polyfill WeakSet, but you cannot polyfill WeakMap, since you do not know when you can shrink the map. (But you still cannot use it to emulate weak references, since you need a strong reference to get data out of the map.)

Reply on news.ycombinator.com