Off the shelf garbage collectors such as BDW would be inappropriate because we use some weird kinds of "pointers" (such as object handles) stored in weird kinds of ways (such as a zend_hash object). I think it would be pretty inefficient, since those implementations just scan the stack, registers and heap and we're trying to do GC not for the PHP interpreter, but for the code the PHP interpreter is running.
surely it wouldn't be the top performance GC, but I think it worths a try. And it doesn't seem difficult to use that GC (after your patch specially).
For just displaying page, there wouldn't be much memory used and that's all freed at the end of a request anyway: all of that reference counting overhead would just disappear. For larger scripts that use a lot of memory, the only problem would be pause times but in most real life cases, it seems the total time would be shorter than reference counting. However, I'm not sure if that would be the case in PHP: rummaging through objects scattered all over memory would result in a lot of cache misses. The question is whether that is greater than all the misses we're currently having just managing the refcount.
exactly. for most PHP requests the GC wouldn't even run. The garbage would be collected after the request, thus reducing the latency of the request (for Gopal pleasure :)
However, answering that question would require implementing the thing, and that honestly seems like it would be a bit of a nightmare. Roots would include zvals linked to PHP variables, the stack of the running PHP code, and the stack and heap of the PHP interpreter itself. It would've been far easier if PHP had been designed from the ground up to use some sane way of managing memory, but with the current situation, with extensions depending on reference counting, it's pretty difficult.
implementing a GC from scratch is a difficult job, yes and hence my idea to try an existenting GC. But if we look to the GCs used by e.g. the Common Lisp implementations, we see that they have highly-tuned and highly-performant GC implementations that take advantage of how the internal structures are implemented. And I suspect Java does that, too.
Well, maybe we can find some crazy student next year to do it *hint* :). Or maybe I get crazy too one of these days ;) (well I still have to pick something to do for the master thesis..)
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