Posted Aug 25, 2011 11:41 UTC (Thu) by pboddie
In reply to: Puhlease...
Parent article: HP dropping webOS devices
1. The language speed is due to their interpreting loop rather than the fact they use GC. As a rule of thumb, an interpreter evaluating an opcode stream appears to be 10 times slower than compiler that translates it into native form.
Indeed. One can switch off GC and just let programs allocate memory until they exit to see the performance impact of GC, if one is really interested in knowing what that is. This is something people seem to try only infrequently and in pathological cases - it's not a quick speed-up trick.
2. Probably true. Missing from this discussion is the observation that even malloc/free can be unpredictable with respect to latency because they must maintain the free list and occasionally optimize it to maintain performance of allocations. No doubt advances to malloc technology have happened and will happen, and there are multiple implementations to pick from that expose different tradeoffs.
Quite right. This is not so different from any discussions of latency around garbage collectors.
3. Python, I've been told, also contains a true GC in addition to the refcounter.
CPython uses reference counting and a cycle detector. PyPy uses a generational GC by default, if I remember correctly. The PyPy people did spend time evaluating different GCs and found that performance was significantly improved for some over others.
I think the largest single advantage of a refcounter is that it gives a very predictable lifecycle for an object, often removing it as soon as the code exits a block.
This advantage is almost sacred to the CPython developers, but I don't think it is entirely without its own complications. Since we're apparently obsessed with latency now, I would also note that a reference counting GC is also unlikely to be unproblematic with regard to latency purely because you can have a cascade of unwanted objects and the GC would then need to be interruptable so that deallocation work could be scheduled at convenient times. This could be done (and most likely is done) in many different kinds of GC.
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