MMTk 0.18.0

0.18.0 (2023-04-03)

Plan

• Add a new plan, StickyImmix. This is an variant of immix using a sticky mark bit. This plan allows generational behaviors without using a compulsory copying nursery.
• Side log bits in generational plans are now set using relaxed store for the entire byte as an optimization.

Policy

• Add constants STRESS_DEFRAG and DEFRAG_EVERY_BLOCK to allow immix to stress defrag copying for debugging.

API

• Add a new write barrier method object_probable_write. The method can be called before the fields of an object may get updated without a normal write barrier.
• Add a feature immix_non_moving to replace the current immix_no_defrag feature. This is only intended for debugging uses to rule out issues from copying.

Misc

• Refactor Mmapper and VMMap. Their implementation is now chosen dynamically, rather than statically based on the pointer size of the architecture. This allows
  us to use a more appropriate implementation to support compressed pointers.
• Revert changes in d341506 about forwarding bits, and bulk zero side forward bits for defrag source blocks.
• Fix a bug in the scheduler where a worker may spuriously wake up and attempt to open new buckets while the coordinator is executing a coordinator work packet,
  which would result in an assertion failure.
• Fix a bug in the mem balancer where subtraction may silently overflow and cause unexpected statistics to be used for computing new heap sizes.
• Fix a bug where ScanObjectsWork does not properly set the worker when it internally uses ProcessEdgesWork.
• Fix a bug where nodes pushed during root scanning were not cached for sanity GC.
• Fix a bug where harness_begin does not force a full heap GC for generational plans.
• Add info-level logging for each GC to show the memory usage before and after the GC, and elapsed time for the GC.