[gclist] Finalizers & Reference counting.

Andrew Shi-hwa Chen chenandr@cse.msu.edu
Fri, 30 Aug 2002 10:52:22 -0400

In Objective-C, something similar is done, except instead of GC it is
programmer-explicit RC, but the notion of "ad" as you refer to it
corresponds to an NSAutoReleasePool.

Andrew Chen

> This whole discussion does bring up an interesting point, however.  Some
> programs - compilers are a great example - go through phases with the
> property that:
> 	a)  Each phase allocates and manipulates a significant amount of
> 		memory, much of which remains live throughout the phase;
> 	b)  Much of the memory allocated in a phase goes dead when the
> 		phase ends.
> (In many cases, the "much" memory is actually identifiable at the point of
> allocation.)
> The classic method to deal with this in a non-GC environment is to support
> the notion of an allocation domain (various name have been used for this).
> Each phase creates an AD, and phase-specific memory is allocated from the
> AD.  At the end of the phase, the entire AD is deleted, normally using a
> fast mechanism that is often essentially independent of the number of
> allocated blocks in the AD.  In a very simple case, an AD is a block of
> contiguous memory, allocations just "bump the pointer", and deleting an AD
> simply requires deleting the large block, ignoring its content.  This can be
> surprisingly effective if AD-allocated memory is rarely deleted within the
> phase.  (It also helps cache/working set locality.)
> The question is:  How could you capture the same advantages in a GC'ed
> system?  It seems that AD's, in some ways, are similar to generations.
> However, generations have the wrong lifetime semantics:  Anything we would
> likely have put into an AD will probably get promoted to a generation that
> will stick around for long after the phase ends.  Still, the basic
> *techniques* used to implement generational collection could perhaps be used
> here.
> Thus, imagine that, in a GC'ed system, we retain the same three user
> visible primitives:
> 	ad <- createAD()
> 	pointer <- allocate(size,ad)
> 	destroy(ad)
> We would like destroy() to be fast, as immediate as possible, but of course
> safe.  If we maintain the invariant that no pointer to an object in an
> AD can come from outside the AD, then destroy() becomes trivial - just put
> everything in the AD - or the whole AD, if you allocate it as a big chunck -
> on the free list.  This should be implementable with write barriers.  (There
> are, of course, subtleties like allowing register and stack pointers to
> *not* force stuff out of the AD, or everything will wander out of it rapidly.)
> I'm sure a complete design and implementation would reveal all kinds of
> issues.  Has anyone ever looked at this kind of "hinted" GC?
> 							-- Jerry