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MetadataReclamation.md

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Metadata Reclamation Design {#MetadataReclamation}

When an ordinary java method aJavaMethod gets recompiled, this is the sequence of operations:

  1. The first instruction of the old body of aJavaMethod gets patched to branch to a runtime helper routine which:
  2. Patches the caller to call the new body
  3. Tranfers control the new body in such a way that when it returns, it returns to the caller and not the helper routine
  4. All new invocations of aJavaMethod call the new body either directly or because it got patched to do so
  5. The old body continues to exist until GC, at which point a JIT hook checks the stacks of all threads to verify that it is no longer in use
  6. Once the JIT determines that the old body is safe to reclaim, at any Global GC it:
  7. Reclaims the majority of the body for code cache reuse leaving behind a stub
  8. Updates the J9JITExceptionTable to describe just the stub

The J9JITExceptionTable (also referred to as the metadata) is a struct that is used to describe the code body. It also has a variable length section that contains data specific to the body like exception ranges, GC Maps, etc. This table continues to persist even when all it's describing is the stub. This can be a big waste of memory since the variable length section can get quite large. This design aims to reclaim the J9JITExceptionTable when a body gets reclaimed and replace it with a "stub" J9JITExceptionTable to describe a code stub.

J9JITExeptionTable reclamation occurs when jitReleaseCodeCollectMetaData is called, the JIT tries to allocate a new J9JITExeptionTable without any variable length section

  • If it fails, it reuses the existing J9JITExceptionTable and does nothing else.
  • If it succeeds, it:
    1. memcpys the non-variable lenth section from the existing J9JITExceptionTable
    2. Updates the various prevMethod and nextMethod pointers
    3. Sets the following fields to NULL:
      • inlinedCalls - This is pointer to the list of inlined calls in the variable length section
      • gcStackAtlas - This is a pointer to the J9JITStackAtlas struct in the variable length section
      • gpuCode - This is a pointer to the GPU Data in the variable length section
      • riData - This is a pointer to the RI Data in the variable length section
      • osrInfo - This is a pointer to OSR info tn the variable length section
      • runtimeAssumptionList - This is a pointer to the list of Runtime Assumptions that gets reclaimed along with the old body
    4. Frees the Fast Walk Cache
    5. Sets the JIT_METADATA_IS_STUB flag so anyone with a PC in the stub knows as much
    6. Adds the new J9JITExceptionTable to the AVL Trees
    7. Frees the old J9JITExceptionTable

The options to enable/disable this are enableMetadataReclamation/ disableMetadataReclamation.