An Introduction to JVM Internals and Garbage Collection in Java
Lp seminar
1. On-the-Fly Garbage Collection Using Sliding Views Erez Petrank Technion – Israel Institute of Technology Joint work with Yossi Levanoni, Hezi Azatchi, and Harel Paz
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3. Garbage Collection Two Classic Approaches Reference counting [Collins 1960]: keep a reference count for each object, reclaim objects with count 0. Tracing [McCarthy 1960]: trace reachable objects, reclaim objects not traced. Traditional Wisdom Good Problematic
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8. Terminology (stop the world, parallel, concurrent, …) Stop-the-World Parallel (STW) Concurrent On-the-Fly program GC
9. Benefits & Costs Informal Pause times 200ms 2ms 20ms Throughput Loss: 10-20% Stop-the-World Parallel (STW) Concurrent On-the-Fly program GC
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24. Elimination of RC Updates Mpegaudio 5,517,795 51 1/108192 Jess 26,258,107 27,333 1/961 Javac 22,042,028 535,296 1/41 Jack 135,174,775 1,546 1/87435 Db 33,124,780 30,696 1/1079 Compress 64,905 51 1/1273 jbb 71,011,357 264,115 1/269 Benchmark No of stores No of “first” stores Ratio of “first” stores
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28. Running Write Barrier Concurrently Thread 1: Update(Object **slot, Object *new){ Object *old = *slot if (!IsDirty(slot)) { /* if we got here, Thread 2 has */ /* yet set the dirty bit, thus, has */ /* not yet modified the slot. */ log( slot, old ) SetDirty(slot) } *slot = new } Thread 2: Update(Object **slot, Object *new){ Object *old = *slot if (!IsDirty(slot)) { /* if we got here, Thread 1 has */ /* yet set the dirty bit, thus, has */ /* not yet modified the slot. */ log( slot, old ) SetDirty(slot) } *slot = new }
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30. Timeline Stop threads. Scan roots; Get buffers; erase dirty bits; Resume threads. Decrement values in read buffers; Increment “current” values; Collect dead objects
31. Timeline Stop threads. Scan roots; Get buffers; erase dirty bits; Resume threads. Decrement values in read buffers; Increment “current” values; Collect dead objects Unmodified current values are in the heap. Modified are in new buffers.
![Garbage Collection ,[object Object],[object Object],[object Object],[object Object]](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)