Re: [EXPERIMENT] Emacs with the SpiderMonkey garbage collector

[Steve Fink <sphink@gmail.com>]

> - during GC you need *double* the heap space.  If you only GC a sub-part
>    of your heap (e.g. just one generation), then you only need double
>    that sub-part.  So if you only use a copy-GC for the youngest generation
>    it's not a big deal, but if you use stop&copy over your whole heap,
>    we're talking about a 100% overhead on memory usage.

Yes. The Spidermonkey GC uses moving collection for the nursery 
generation and nonmoving collection for the tenured heap, though it will 
also periodically do a compacting GC of the tenured heap that moves 
things. But the compacting GC breaks the heap into small chunks, so the 
overhead is at most one of those chunks.

And it doesn't do semispace collection; it moves everything from the 
nursery to the tenured heap, so you don't need double the nursery size 
ever. (Not that the nursery is fixed size; it'll double itself if you're 
filling it up fast, up to a maximum size.)