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
DRLVM.
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
