[gclist] Baker Treadmill as an alternative to reference counting.

Charles Fiterman cef@geodesic.com
Tue, 05 Sep 2000 08:08:36 -0500

At 11:06 AM 9/4/00 -0700, you wrote:
>>Date: Sun, 3 Sep 100 08:47:32 -0700 (PDT)
>>From: "Henry G. Baker" <hbaker@netcom.com>
>>Subject: Re: [gclist] ref-counting performance cost
>>> I've definitely seen memory, files, and windows dealt with using all
>>> three approaches more frequently than reference counting.  With good
>>> reason:  all three have much better performance than reference counting
>>> and are usually easier to implement correctly, even if they don't apply
>       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>Yes and no.
>Reference counting falls victim to cycles. It also falls victim to
>programmers trying to "optimize" it.
>On the other hand, you can implement reference counting with a few C++
>templates and have it work with just about any compiler. (Okay. So, trying
>to make the pointers work for casting starts giving some compilers fits.)
>This is without special knowledge of the complier or the runtime
>Reference counting also works cleanly with C++ destructors.
>The performance of reference counting is lackluster (though generally
>fairly smooth).  There are cases it doesn't handle. But it's portable.

These advantages of reference counting are equally true of the Baker
Treadmill. This doesn't cascade destructors and doesn't have a bug for
cyclic structures.

However, I regard finalizers as a hopelessly flawed concept regardless of
implementation and that includes reference counting. If the collector
invokes a destructor that's a finalizer. The point of collection is that
when an object can no longer influence execution its deleted. A finalizer
says, "Now that we know this object can no longer influence execution lets
run a finalizer on it and influence execution." Another way to state that
is the collector goes to the mutator and says "Here's a dead object."

If you have finalizers you will have bugs. With reference counting those
bugs are often hidden behind a bigger bug, cyclic structures don't collect
at all. 

The answer boils down to some form of death notice, there are multiple
names and implementations of the idea. But the mutator goes to the
collector saying "When this object dies tell me in the following fashion."
And the collector does so. The mutator doesn't keep a pointer to the object
connected to the death notice because if it did the object wouldn't die.