Garbage collection and MM

Posted Mar 9, 2007 0:04 UTC (Fri) by njs (guest, #40338) [Link]

Python is certainly a GC language (since version 2.0). It happens to optimize that GC by using reference counting to catch the easy cases, and only doing mark/sweep type stuff occasionally to catch reference cycles, but it definitely has a full GC.

I'm told that some of the hottest Java GC techniques actually involve reference counting these days, because you can massively optimize your actual walking -- the only time a cycle can be created is when a reference count is decremented, and thus achieves a number greater than 0. This is pretty rare, and it also tells you that any cycle that was just created must involve that object in particular, so you don't have to tromp through all memory either.

None of this affects your original point, though, because reference counting already trashes caches by itself -- especially in the multiprocessor case, where supposedly read-only access to variables is suddenly triggering cache flushes...

Depending on your cache hierarchy and the characteristics of your GC, you can minimize its impact, though. E.g., in gcc, I thought I remember some trick where you only run the collector between passes, since you know already that that's when everything becomes garbage, and also where it doesn't matter if you trash the cache? Similarly, Graydon was saying something about in initial implementations of firefox's GC, they would just run it after page load, because no-one notices if the browser pauses for 400 milliseconds then, they're just looking at the page.

Long run: build garbage collection into the RAM hardware! That'll work around those pesky cache issues ;-)