Concurrent garbage collector

[KCreate]

• Load / Write Barriers

  • Some kind of load barrier when loading from other heap objects
  • When does the runtime have to check the forward pointers of a cell?
  • How does this mechanism work?
  • How is it implemented?

• Phases

  • Idle | Application is running normally, GC is not running
  • Idle -> Marking | Mark roots and start background marking
  • Marking | Heap tracing, write barriers append to queue drained at next pause
  • Marking -> Evacuation | Drain SATB buffers and initialize evacuation phase
  • Evacuation | Heap objects get relocated, write barriers access forward pointer
  • Evacuation -> UpdateRef | Initialize update ref phase
  • UpdateRef | Heap references to old objects get updated, write barriers update references
  • UpdateRef -> Idle | Finalize garbage collection, prepare for idle phase

• Concurrent Marking

  • Init phase
    • Workers mark root set
    • New allocations marked grey
    • Mark all VM reachable objects
    • Arm SATB buffers
  • Concurrent phase
    • Traverse application and mark reachable objects
    • Collect statistics and information about heap regions
  • Application threads
    • Write barriers
    • Read barriers

• Evacuation

  • Init phase
    • Disable and mark values from SATB buffers
    • Finish marking
    • Build From-To evacuation mappings
    • Evacuate worker root set
  • Concurrent phase
    • Traverse From Regions and evacuate cells to To-regions
  • Application threads
    • Evacuate in barriers
  • How do evacuated cells know where they end up?
    • Build mapping of From-spaces to To-spaces
      • During the Init-Evacuate pause, allocate each alive cell in From-regions a slot in the corresponding
        To-regions
      • Evacuation can atomically check if the final copy has already been created / initialized
    • Each from-space knows in what to-space it will end up
      • Need some easy way of knowing which offset each cell ultimately ends up at
  • How are the copies that are created during a contended evacuation recycled?
    • Mechanism to un-allocate a cell, works only if no other cell was allocated since
      • Since another copy will be allocated in another region, we can just unwind the last allocation and reuse it for the next one

• UpdateRef

  • Init phase
    • Update root set
  • Concurrent phase
    • Update references to objects in from-spaces
  • Application threads
    • Update references during load barriers