Support for Shenandoah GC

[rkennke]

The Graal compiler currently does not support the Hotspot Shenandoah GC. Shenandoah GC is relevant in situations where latency is a concern, because it drastically reduces GC pause times. This is not only relevant in large-heap settings where GC tends to take a long time due to the size of live data that needs to be marked and relocated, but also in resource-constrained environment where even relatively little STW-work can balloon.

A first step in supporting Shenandoah GC in Graal compiler would be to implement the basic Shenandoah barriers:

• Load-reference barrier to be employed after loading references
• SATB-barrier to be employed before reference writes (very similar to G1's SATB barrier)
• CAS-obj barrier that handle Unsafe.compareAndSwapReference() and related methods
• Arraycopy barrier to deal with arraycopies of Object[]
• Reference.get() barrier

This would be enough to support Graal compiler in JDK 11. However, additional features have been added between JDK 11 and JDK 17, which require extra work:

• Concurrent class-unloading requires so-called nmethod-barriers: little pieces of GC code to be called when calling into 'armed' nmethods (and a mechanism to arm nmethods).
• Concurrent thread-scanning which requires stack-watermark support.

Both these features are not Shenandoah specific. For example, ZGC would also make use of them.

Much of the first step (JDK 11 support) is implemented in #2426.

[munishchouhan]

@dougxc please advise

[rkennke]

Notice that this is mostly informational for now. We recently had a discussion with @thomaswue were we agreed that I file this issue to collect TODOs and a rough outline, also with an eye on possibly supporting Shenandoah in native-image.

[dougxc]

I'm closing this issue due to lack of activity. Please re-open it (and reassign) when/if there are plans to make progress on adding Shenandoah to Native Image.

[bric3]

@dougxc I don't think this issue is related to Shenandoah on the native image but on the Graal Compiler.

[dougxc]

Ah yes, thanks for pointing that out. Still, the issue will remain closed until further plans are made for supporting Shenandoah in Graal (in any config).

[reneleonhardt]

It's sad that there is no progress after 3 years, switching from G1 to Shenandoah massively reduced our spring-web latency.

[JohnTortugo]

Please, re-open this issue. I'm resuming @rkennke work on this patch.

[rkennke]

I cannot seem to assign this back to me. I'm currently working on it, WIP PR is here: #10904