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From "Robin Garner" <robin.gar...@anu.edu.au>
Subject Re: Some questions about the architecture
Date Sun, 23 Oct 2005 04:44:05 GMT
> Robin Garner wrote:
>> GC is triggered in two cases: 1) the user code calls System.gc().  2)
>> the
>> heap fills up (for some suitable definition of 'fills up').  There is
>> never any need for the VM code to call the garbage collector.
> Off tanget, but IMHO this is not strictly true: it depends on whether
> VM native code relies on finalize() to free up its (non-heap) memory.
> Here's an exmaple of this:
>    http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=4797189

Ahem.  I don't think this contradicts what I said.  One of the standard
use cases for finalizers is to free up externally allocated resources when
they are no longer required.  Unfortunately the Java standard provides no
timeliness requirements for finalizers.

Finalization is triggered by the GC.  The most sensible implementation
appears to be that the GC maintains a set of objects that require
finalization.  At the end of each collection, the GC examines this list,
and if any of them is dead, it notifies the finalizer thread that the
finalizer should be executed, and revives the object so that the finalizer
can access its state.

In a generational collector, maximum efficiency is achieved by executing
as few major collections as possible.  Any object with a finalizer that
manages to be promoted to the mature space will probably live way past the
point where it becomes unreachable, and hang onto any non-java-heap
resources it has acquired for quite a while.

One solution might be to maintain a 'finalized' space that is scanned
during every minor collection.  It is probably the kind of thing you would
want run-time configurable, because it could reduce the efficiency of the

IMO the question of how to best do reference type processing and
finalization is still open.  This is one of the reasons.  But it's still
the GC that decides when an object dies!


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