[jdk19] RFR: 8290250: Shenandoah: disable Loom for iu mode

[Roman Kennke]

On Thu, 14 Jul 2022 17:40:27 GMT, Erik Österlund <eosterlund at openjdk.org> wrote:

> > > Speaking of IU mode, how does Shenandoah IU mode deal with barrier elision on newly allocated objects? It was only valid in CMS because the entire young generation was traversed when terminating the concurrent marking, so that unvisited pointers could be found.
> > 
> > 
> > Shenandoah SATB/IU both elide barrier on newly allocated objects. SATB barrier should guarantee they can only refer marked objects. Do I miss anything?
> 
> Yes, I believe so. For a SATB collector, it is okay to elide the barrier on newly allocated objects, because they were not part of the snapshot-of-the-beginning, and all new objects are implicitly alive and don't need to be visited.
> 
> However, with an IU scheme, that property does not translate.
> 
> Consider that you have a particular object graph when marking starts, then one mutator loads an object from the graph, and clears the field such that it is only reachable from the roots of said murator thread. Then the mutator writes the object to a newly allocated object without barriers and discards the root. Now, the reference is only reachable from the newly allocated object.
> 
> So for this elision to be valid with IU, the GC has to visit newly allocated objects again before terminating, which e.g. CMS indeed did, by doing a young collection in the safepoint that terminated old marking, but I don't know that Shenandoah does anything like that.

IIRC, in Shenandoah's IU mode we treat new objects as implicitely alive, much like we do with SATB mode. Therefore it should be good to elide the barriers there. I know that this is not the classical I-U scheme that I believe CMS followed. There has been a paper (I believe it is that paper [1]) that showed that it's orthogonal issues whether we follow new or old references and whether or not we treat new objects alive or not. Treating new objects as live has the advantage that it solves the termination problem.

[1] https://www.cs.technion.ac.il/~yahave/papers/pldi06-cgc.pdf

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PR: https://git.openjdk.org/jdk19/pull/140