RFR: Heuristics should do concurrent cycles

[Zhengyu Gu]

Looks good.

-Zhengyu

On 11/02/2018 03:20 PM, Aleksey Shipilev wrote:
> http://cr.openjdk.java.net/~shade/shenandoah/heuristics-concurrent/webrev.01/
> 
> This is the pack of 4 changes that make heuristics do concurrent cycles consistently. It consists of
> the following changes:
> 
>   *) Make heuristics tell if we can process references or unload classes
> 
>   This adds can_* methods to heuristics, so it can say if it possibly can, but not necessarily should
> either process the references or unload classes right now. This simplifies the dependencies all over
> the GC code that either polls flags directly, or otherwise guesses on different things.
> 
>   *) Factor out implicit/explicit GC requests
> 
>   This rewrites the handling for "requested" and "explicit" GCs. Before, "explicit" was the subtype
> of "requested" GC, which is confusing both conceptually and code-wise. Instead, there is now the
> split between "explicit" (e.g. forced by user action) and "implicit" (e.g. coming from some other VM
> facility) GCs. Recent metadata-induced GCs are the examples of implicit GCs.
> 
> Additionally, both implicit and explicit GCs always process references and unload classes -- and
> they know that they can by polling new can_* methods. And they can do that without diving into Full GC.
> 
>   *) Print special cause for Metadata GC
> 
>   This moves the special "cause" message to control thread. This makes sure cause is printed only
> when GC cycle is actually happening, which makes vmTestbase/metaspace tests happy.
> 
>   *) Concurrent cycle by default on any external GC request
> 
>   This makes Shenandoah react with concurrent GC on any GC request by default. This makes sure that
> users would not get accidentally exposed to either Degen or Full GC in all normal uses, and it would
> only happen if something goes really wrong. This is accomplished by defaulting to new values for
> *GCInvokesConcurrent, which are also overridable. This is possible after all three things above are
> done.
> 
> 
> Testing: tier3_gc_shenandoah, vmTestbase/metaspace/ -XX:+UseShenandoahGC
> 
> Thanks,
> -Aleksey
> 
>