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# Thursday, November 20, 2008

Yesterday I've read of a new Garbage Collection implementation G1. To be honest I was not impressed.

I think Garbage Collection is an evil, or at least its present implementations. I do not believe in algorithms that in their very core assume a centralized execution. On the other hand it's clear it's not in my power to change the status quo. My lot is to give advices mostly incompetent and ignorable.

I'm waiting for the time when someone will reach the idea to bring some parts of GC logic out of runtime scope. This will require more VM  intelligence, however will bear its fruits.

JIT or compiler during a static analysis may prove that some objects being collected may make some of their referring objects unreachable, provided it can prove that referring objects are not reachable through the other means (e.g. private field which is not stored in other places). This is close to the ideas expressed in Muse on value types in java. It's possible to prepare a garbage graph in advance before runtime.

In many cases it's also possible to prove that when method's variable goes out of scope it's not reachable through the other means and may be collected. This allows to implement a stage of automatic garbage collection when objects that are proven to be a garbage be immedeately added to a free memory set.

As an example I'm thinking of java's ArrayList object which stores private array. When ArrayList is reclaimed or resized a reference to the private array is getting lost and memory can be added to the free set immediately.

This mechanics being integrated as the first stage of GC will make it less centralized, as I believe many objects will be collected this way.

Thursday, November 20, 2008 7:54:47 AM UTC  #    Comments [0] -
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