simplify-lxr

name	simplify-lxr
description	Simplify LXR GC for upstreaming
disable-model-invocation	true

Simplify LXR GC for upstreaming

lxr-x/simplified branch in mmtk-core and lxr-x/jdk21-simplified branch in mmtk-openjdk contain the implementation of the LXR garbage collector.

lxr-x/simplified in mmtk-core is based on tag v0.32.0 lxr-x/jdk21-simplified in mmtk-openjdk is based on tag v0.32.0-jdk-21

Both branches contains:

• The core LXR GC algorithm
• Additional performance optimizations
• Changes to the MMTk interface and other components of MMTk
• Changes to support additional fetures, e.g. weak refs and class unloading
• Custom instrumentation and logging code
• Changes to the other GCs in MMTk
• Other stale dead code

You need to work on the lxr-x/simplified and lxr-x/jdk21-simplified.

You need to simplify the two branches so that the user can easily upstream a simplified version of LXR GC easily, although the performance may not be as good as the original one.

Assume the upstream is at v0.32.0 and v0.32.0-jdk-21.

Compare and understand git diff

Generate git diff for both mmtk-core and mmtk-openjdk, and understand the changes in the two branches.

Start a small refactoring/simplification task

The diff may be over 8K LOC, You can't do all the changes in one go. Only pick a few components/aspects of the codebase that can be simplified/refactored, and perform this task.

Enter plan mode, and preset a plan before proceeding with the task.

What can be simplified/removed

• Additional performance optimizations
  • e.g., Fast primitive array scanning, vm_map_32
• Changes to the other GCs in MMTk
  • Unless they are part of the necessary MMTk interface change
• Changes to support additional fetures, e.g. weak refs and class unloading
• Custom instrumentation and logging code
• Other stale dead code or comments that is not in upstream

What can be refactored

• Move LXR-related code, e.g. work packets and counters, to mmtk-core/src/plan/lxr
• Refactor MMTk interface changes to make it more reasonable

What cannot change

• Necessary changes to the MMTk interface
• The core LXR algorithm
• Keep the following features: lxr_no_evac, lxr_no_cm, lxr_no_lazy, lxr_stw, lxr_no_nursery_evac, lxr_no_mature_evac
• panic = "abort" in mmtk-openjdk/mmtk/Cargo.toml, otherwise pmd may fail. Leave this bug for now.

Testing

Use test-simplified-lxr skill to test and verify LXR GC.

Debugging

Refer to the test-simplified-lxr skill on how to run the benchmarks and verify the correctness of the simplified LXR GC.

To fix a crashing benchmark, you have the following options:

1. Use debug build (without --release) to get more debug info
2. Use GDB: add --gdb --no-interactive to the command, and it will automatically run the program in gdb, and print the backtrace if it crashes.
3. Inspect crash logs: hs_err_pid*.log

Current progress

• Class unload and weak refs are reverted
• BlockPageResournce and ImmixAllocator changes are mostly reverted (may need more cleanups)

PRECAUTIONS

• Don't commit, push, revert, or create/edit PRs or issues, unless the user explicitly ask you to do so.
• Do simplifications/refactorings one piece at a time. Don't aim to solve everything in one session.
• Benchmarks can run in parallel, but don't build jdk when other benchmarks or build task are still running.
• Do a cargo fmt under either mmtk-core/ or mmtk-openjdk/mmtk before finish.

Additional User Messages

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