[gclist] Finalizers & Reference counting.
Fri, 30 Aug 2002 19:10:42 -0400
At 07:56 AM 8/30/2002 -0400, Jerrold Leichter wrote:
>This whole discussion does bring up an interesting point, however. Some
>programs - compilers are a great example - go through phases with the
> 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
In my experience, compilers tend to gave generational collectors
a hard time, especially when compiling a large number of files.
The first big file that gets compiled stuffs a lot of crud into
old space, and then it lingers for a lot longer than it ought to.
>The question is: How could you capture the same advantages in a GC'ed
As a very crude approximation, you can simply run a full GC when you know
that the live storage ought to be near a minimum. In a compiler, that
would be just before compiling a new file, and perhaps between phases.
Since GC-cost is roughly proportional to live, you get the maximum
bang (freed space) for the minimum buck (time spent in GC).
Admittedly, this is crude.
You could do better.
- If you have generational infrastructure (e.g., you are marking
cards, and typically you mark blindly because it is faster), and
- If you can keep track of creation time (in a sliding-compacting
collector, an object's address is a proxy for its creation)
then it's not too hard to request a GC of everything younger than
a certain age. This would probably be cheaper than a full GC.
Depending on how you implement your card marking, you might be able
to collect *only* that region, but you might not have the necessary
cards from younger to older.
So, your interface could be something like
GCTime gct = gc.getTimeStamp();
// Insert compiler phase here.
A more precise interface would be
Of course, an implementation is free to take these as a mere hint,
and either not collect, or collect everything.