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From Robin Garner <robin.gar...@anu.edu.au>
Subject Re: [drlvm] vote on class unloading design (was Class unloading support - tested one approach)
Date Wed, 08 Nov 2006 10:19:47 GMT
Pavel Pervov wrote:
> Really BIG -1 from me.
> As Aleksey (Ignatenko) described in original thread, j/l/Class'es and
> j/l/ClassLoader's are always available in rootset, so even if no objects
> of a class exist, this class will be reachable.
> Actually, some sort of class unloading prototype exists in DRLVM code, 
> which
> implements the scheme, which is very close to what is currently voted. It
> was integrated with GC v4 and is not supported by other GCs. This prototype
> traces up to class loader. Robin's approach is way faster then prorotype 
> is.
> Unfortunately, that approach requires up to 3 GC cycles to complete in

In a full-heap STW collector, my proposal would require 1 GC to collect 
unused classloaders.  In a generational STW collector, 1 full-heap GC, 
and would depend on the particular invariants enforced by an 
incremental/concurrent collector, but would be 1 complete "cycle" of any 
of the standard algorithms (I guess up to 3 GCs if the sweeps happened 
at the wrong places).

> BTW, voted approach does not describe "proof-of-full-collection" algorithm
> (at least I didn't find one). The only one I think of is
> full-heap-collection, which _requires_ STW.

My approach simply requires the underlying collector to have a notion 
that periodically it can say that 'every reachable object allocated 
since time 't' is now marked reachable.  If the class-unloader can 
ensure that one full epoch of this invariant has passed, then it can 
safely perform unloading.

> Although "automatic anloading" brings some additional requirements for GC
> (weak roots (references) support and pinned allocation), it is proven to
> work (patch available) and, also, is the most natural algorithm for DRLVM.

What is the run-time cost of it ?  And where is it described ?  I was 
only aware of Etienne's proposal as a full class-unloading scheme.

> With the best regards,

Robin Garner
Dept. of Computer Science
Australian National University

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