Wednesday, October 26, 2011

Garbage collection gives us the illusion of nearly unlimited memory for temporary storage. To pull off this illusion, we must be able to satisfy every allocation request (at least up to the point where we throw up our hands and raise an “out of memory” condition). In other words, there must be free or unused memory available whenever we need some. Since the point of automatic memory management is to free the programmer from having to think about it, we expect to be able to allocate temporary storage at any time we wish during program execution.

A data structure can be considered “in use” if modifications to it can change the future behavior or result of a program. Since we cannot in general statically determine the future behavior, we compute the transitive closure of the data structures under the data access primitives. This is a superset of the data “in use”. That is, if you can't access the data, it cannot be “in use”.

In order for garbage collection to succeed as a storage management strategy, there must be at all times (anytime we want to allocate) some storage that is either unallocated or unreachable. The size of the heap must always exceed the size of the reachable data. Or we could say that the amount of reachable data must at all times be less than the size of the heap. (By ‘size of the heap’ I mean the total amount of storage both in use and free. For example, I could have a five megabyte heap, but only have 3 words in use.)

Any allocated data structure has but one of two ultimate fates: it remains reachable for the remainder of the run of the program or it becomes unreachable at some point (and we may re-use the storage). Assuming the heap starts empty, the size of reachable storage at any point in time is the difference between the total amount of all allocation and the total amount of all storage that has become unreachable up to that point. For example, if my program allocates sixty megabytes over its lifetime and it abandons fifty-nine megabytes, then the heap must contain the remaining megabyte.

We now consider the differential case, which is to say, the rates of allocation, collection, and heap growth or shrinkage. If the rate of allocation exceeds the rate of collection, the amount of storage in use in the heap must grow to meet the demand. Conversely, if the amount of reachable storage in the heap is decreasing, then we are abandoning storage at a faster rate than we are allocating it. But there are hard boundaries on the size of the heap. We can never have a negative amount of storage in use, and we have the requirement (see above) that the heap must be larger than the reachable storage at all times. The heap size is limited, so the rate of increase in reachable storage must be quite small over the long term if we are not to run out of heap.

The rate of storage allocation must be nearly equal to the rate of deallocation (or abandonment) almost all the time.

(This is only slightly different from the “steady-state” assumption that the rates are essentially equal. We're simply admitting the possibility that the reachable storage grows indefinitely and that the program would eventually crash if it doesn't finish first.)

2 comments:

John Cowan said...

Steadily does not equal indefinitely. Consider a program that allocates storage as it reads its input, but keeps all of it in use — the simplest case is a filter that prints out its input in reverse order. Such a program will not allocate an infinite amount of storage, because its input cannot be infinite in size; however, it will necessarily fail if the input is too large.

Jrm said...

That's why I said ‘indefinite’ rather than ‘infinite’.

It is tempting to rely on steady-state arguments to make the transition from full program execution to the differential case. But this is clearly unrealistic. Even when compiling hundreds of files, the compiler never reaches a steady state, but rather keeps consuming more and more memory. Fortunately, the rate of accumulation is very, very slow (several orders of magnitude slower than the temporary allocation rate). Presumably, the compiler would eventually crash for any heap size if it were given enough files to compile. I also presume that there are many, many useful programs that never reach steady state and would eventually run out of heap if given enough time or input, but rarely or never run out in practice.

My point is that we can relax the steady state condition slightly to admit very slowly growing programs as well, provided we don't expect these programs to run indefinitely, but long enough to ‘finish’ on some realistic problems.