[Python-Dev] Garbage collector problem

[Jeremy Hylton]

I had a different ideas to solve this performance problem and perhaps
others.  It's only half baked, but I thought it was at least worth
mentioning in an e-mail.  The premise is that the garbage collector
tracks a lot of objects that will never participate in cycles and can
never participate in cycles.  The idea is to avoid tracking objects
until it becomes possible for them to participate in a collectible
cycle.

For example, an object referenced from a local variable will never be
collected until after the frame releases its reference.  So what if we
did not track objects that were stored in local variables?  To make
this work, we would need to change the SETLOCAL macro in ceval to
track the object that it was DECREFing.  There are a lot of little
details that would make this complicated unfortunately.  All new
container objects are tracked, so we would need to untrack ones that
are stored in local variables.  To track objects on DECREF, we would
also need to ask if the object type was GC-enabled.

Another kind of object that is never going to participate in a cycle,
I think, is an object that lives only temporarily on the ceval stack.
For example, a bound method object created on the stack in order to be
called.  If it's never bound to another object as an attribute or
stored in local variable, it can never participate in the cycle.

How hard would it be to add logic that avoided tracking objects until
it was plausible that they would participate in a cycle?

Jeremy