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David F. Bacon T.J. Watson Research Center

Parallel and Concurrent Real-time Garbage Collection Part III: Tracing, Snapshot, and Defragmentation. David F. Bacon T.J. Watson Research Center. Part 2: Trace (aka Mark). Clearing Monitor Table clearing JNI Weak Global clearing Debugger Reference clearing JVMTI Table clearing

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David F. Bacon T.J. Watson Research Center

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  1. Parallel and ConcurrentReal-time Garbage CollectionPart III:Tracing, Snapshot, and Defragmentation David F. Bacon T.J. Watson Research Center

  2. Part 2: Trace (aka Mark) • Clearing • Monitor Table clearing • JNI Weak Global clearing • Debugger Reference clearing • JVMTI Table clearing • Phantom Reference clearing • Re-Trace 2 • Trace Master • (Trace*) • (Trace Terminate***) • Class Unloading • Completion • Finalizer Wakeup • Class Unloading Flush • Clearable Compaction** • Book-keeping • Initiation • Setup • turn double barrier on • Root Scan • Active Finalizer scan • Class scan • Thread scan** • switch to single barrier, color to black • Debugger, JNI, Class Loader scan • Trace • Trace* • Trace Terminate*** • Re-materialization 1 • Weak/Soft/Phantom Reference List Transfer • Weak Reference clearing** (snapshot) • Re-Trace 1 • Trace Master • (Trace*) • (Trace Terminate***) • Re-materialization 2 • Finalizable Processing • Flip • Move Available Lists to Full List* (contention) • turn write barrier off • Flush Per-thread Allocation Pages** • switch allocation color to white • switch to temp full list • Sweeping • Sweep* • Switch to regular Full List** • Move Temp Full List to regular Full List* (contention) * Parallel ** Callback *** Single actor symmetric

  3. U Y T W X Z a a a a a a b b b b b b Let’s Assume a Stack Snapshot Stack p r

  4. r U Y T X W Z a a a a a a b b b b b b Yuasa Algorithm Review:2(a): Copy Over-written Pointers Stack p

  5. U T W X Z Y Z Y W X a a a a a a a a a a b b b b b b b b b b Yuasa Algorithm Review: 2(b): Trace Stack p * Color is per-object mark bit

  6. s W Z U X Y T V X Z Y W a a a a a a a a a a a b b b b b b b b b b b Yuasa Algorithm Review: 2(c): Allocate “Black” Stack p

  7. Non-monotonicity in Tracing 1 GB

  8.   Which Design Pattern is This? Shared Monotonic Work Pool pool of work Per-Thread State Update

  9. Trace is Non-Monotonic…and requires thread-local data GC Master Thread GC Worker Threads Application Threads pool of non-monotonic work

  10. Basic Solution • Check if there are more work packets • If some found, trace is not done yet • If none found, “probably done” • Pause all threads • Re-scan for non-empty buffers • Resume all threads • If none, done • Otherwise, try again later

  11. Ragged Barriers:How to Stop without Stopping Epoch 1 Epoch 2 Epoch 3 Epoch 4 • Local Epoch • Global Min • Global Max A B C

  12. “Trace” Phase

  13. thread list pthread pthread_t Epoch agreed newest PhaseInfo phase workers writebuf 43 35 trace 3 43 color color BarrierOn BarrierOn true true pthread pthread_t writebuf 41 color color epoch epoch 37 43 The Thread’s Full Monty 16 64 256 sweep  dblb  Thread 1 next inVM  16 64 256 sweep  dblb  Thread 2 next inVM 

  14. WBufEpoch 39 39 37 37 Work Packet Data Structures wbuf.epoch < Epoch.agreed WBufCount 3 totrace wbuf trace wbuf fill free

  15. trace() { thread->epoch = Epoch.newest; bool canTerminate = true; if (WBufCount > 0) getWriteBuffers(); canTerminate = false; while (b = wbuf-trace.pop()) if (! moreTime()) return; int traceCount = traceBufferContents(b); canTerminate &= (traceCount == 0); while (b = totrace.pop()) if (! moreTime()) return; int TraceCount = traceBufferContents(b); canTerminate &= (traceCount == 0); if (canTerminate) traceTerminate(); }

  16. Getting Write Buffer Roots getWriteBuffers() { thread->epoch = fetchAndAdd(Epoch.newest, 1); WBufEpoch = thread->epoch; // mutators will dump wbufs LOCK(wbuf-fill); LOCK(wbuf-trace); for each (wbuf in wbuf-fill) if (wbuf.epoch < Epoch.agreed) remove wbuf from wbuf-fill; add wbuf to wbuf-trace; UNLOCK(wbuf-trace); UNLOCK(wbuf-fill); }

  17. Write Barrier writeBarrier(Object object, Field field, Object new) { if (BarrierOn) Object old = object[field]; if (old != null && ! old.marked) outOfLineBarrier(old); if (thread->dblb) // double barrier outOfLineBarrier(new); }

  18. Write Barrier Slow Path outOfLineBarrier(Object obj) { if (obj == null || obj.marked) return; obj.marked = true; bool epochOK = thread->wbuf->epoch == WBufEpoch; bool haveRoom = thread->wbuf->data < thread->wbuf->end; if (! (epochOK && enoughSpace)) thread->wbuf = flushWBufAndAllocNew(thread->wbuf); // Updates WBufEpoch, Epoch.newest *thread->wbuf->data++ = obj; }

  19. “Trace Terminate” Phase

  20. WBufEpoch 39 Trace Termination WBufCount 0 totrace wbuf trace wbuf fill free

  21. Epoch agreed newest PhaseInfo phase workers PhaseInfo phase workers 42 37 trace 1 terminate 1 BarrierOn BarrierOn true true Asynchronous Agreement desiredEpoch = Epooch.newest; … WAIT FOR Epoch.agreed == desiredEpoch if (WBufCount == 0) DONE else RESUME TRACING

  22. Ragged Barrier bool raggedBarrier(desiredEpoch, urgent) { if (Epoch.agreed >= desiredEpoch) return true; LOCK(threadlist); int latest = MAXINT; for each (Thread thread in threadlist) latest = min(latest, thread.epoch); Epoch.agreed = latest; UNLOCK(threadlist); if (epoch.agreed >= desiredEpoch) return true; else doCallbacks(RAGGED_BARRIER, true, urgent); return false; } * Non-locking implementation?

  23. Part 1: Scan Roots • Clearing • Monitor Table clearing • JNI Weak Global clearing • Debugger Reference clearing • JVMTI Table clearing • Phantom Reference clearing • Re-Trace 2 • Trace Master • (Trace*) • (Trace Terminate***) • Class Unloading • Completion • Finalizer Wakeup • Class Unloading Flush • Clearable Compaction** • Book-keeping • Initiation • Setup • turn double barrier on • Root Scan • Active Finalizer scan • Class scan • Thread scan** • switch to single barrier, color to black • Debugger, JNI, Class Loader scan • Trace • Trace* • Trace Terminate*** • Re-materialization 1 • Weak/Soft/Phantom Reference List Transfer • Weak Reference clearing** (snapshot) • Re-Trace 1 • Trace Master • (Trace*) • (Trace Terminate***) • Re-materialization 2 • Finalizable Processing • Flip • Move Available Lists to Full List* (contention) • turn write barrier off • Flush Per-thread Allocation Pages** • switch allocation color to white • switch to temp full list • Sweeping • Sweep* • Switch to regular Full List** • Move Temp Full List to regular Full List* (contention) * Parallel ** Callback *** Single actor symmetric

  24. Fuzzy Snapshot • Finally, we assume no magic • Initiate Collection

  25. “Initiate Collection” Phase

  26. thread list pthread pthread_t Epoch agreed newest PhaseInfo phase workers writebuf 42 37 initiate 3 43 color color BarrierOn BarrierOn true true pthread pthread_t writebuf 41 color color epoch epoch 37 43 Initiate: Color Black, Double Barrier 16 64 256 sweep  dblb  dblb  Thread 1 next inVM  16 64 256 sweep  dblb  dblb  Thread 2 next inVM 

  27. r U Y T X W Z a a a a a a b b b b b b What is a Double Barrier?Store both Old and New Pointers Stack p

  28. X Z W V a a a a b b b b Why Double Barrier? T2: m.b = n (writes X.b = W) T3: j.b = k (writes X.b = V) T1: q = p.b (reads X.b: V, W, or Z??) T1 Stack T2 Stack T3 Stack m j p q n k “Snapshot” = { V, W, X, Z }

  29. T2 T3 X Z W V a a a a b b b b Yuasa (Single) Barrier with 2 Writers T2: m.b = n (X.b = W) T3: j.b = k (X.b = V) T1 Stack T2 Stack T3 Stack m j p q n k

  30. T2 T3 X Z W V a a a a b b b b Yuasa Barrier Lost Update T2: m.b = n (X.b = W) T3: j.b = k (X.b = V) T1 Stack T2 Stack T3 Stack m j p q n k

  31. T1 T2 T3 n X Z W V a a a a b b b b Hosed! T1: Scan Stack T2: m.b = n (X.b = W) T3: j.b = k (X.b = V) T1: q = p.b (q <- W) T2: n = null T2: Scan Stack T1 Stack T2 Stack T3 Stack m j p q k

  32. “Thread Stack Scan” Phase

  33. thread list pthread pthread_t Epoch agreed newest PhaseInfo phase workers writebuf 37 42 initiate 3 43 color color BarrierOn BarrierOn true true pthread pthread_t writebuf 41 color color epoch epoch 37 43 Scan Stacks (double barrier off) 16 64 256 sweep  dblb  dblb  Scan Stack 1 Thread 1 next inVM  16 64 256 sweep  dblb  dblb  Scan Stack 2 Thread 2 next inVM 

  34. All Done!

  35. Boosting: Ensuring Progress Boost to master priority GC Master Thread GC Worker Threads Application Threads (may do GC work) Revert(?)

  36. Part 4: Defragmentation • Clearing • Monitor Table clearing • JNI Weak Global clearing • Debugger Reference clearing • JVMTI Table clearing • Phantom Reference clearing • Re-Trace 2 • Trace Master • (Trace*) • (Trace Terminate***) • Class Unloading • Completion • Finalizer Wakeup • Class Unloading Flush • Clearable Compaction** • Book-keeping • Initiation • Setup • turn double barrier on • Root Scan • Active Finalizer scan • Class scan • Thread scan** • switch to single barrier, color to black • Debugger, JNI, Class Loader scan • Trace • Trace* • Trace Terminate*** • Re-materialization 1 • Weak/Soft/Phantom Reference List Transfer • Weak Reference clearing** (snapshot) • Re-Trace 1 • Trace Master • (Trace*) • (Trace Terminate***) • Re-materialization 2 • Finalizable Processing • Flip • Move Available Lists to Full List* (contention) • turn write barrier off • Flush Per-thread Allocation Pages** • switch allocation color to white • switch to temp full list • Sweeping • Sweep* • Switch to regular Full List** • Move Temp Full List to regular Full List* (contention) • Defragmentation * Parallel ** Callback *** Single actor symmetric

  37. free 16 free 64 free 256 alloc’d sweep 256 16 64 ? free

  38. pointers have changed objects have moved Two-way Communication GC Master Thread GC Worker Threads Application Threads (may do GC work)

  39. T’ C D X Z B U E T a a a a a a a a a b b b b b b b b b Defragmentation

  40. T’ T T T T T’ a a a a a a b b b b b b Staccato Algorithm create store FREE NORMAL access load allocate nop defragment cas reap store access (abort) cas MOVED COPYING access load  commit cas

  41. Scheduling

  42. Guaranteeing Real Time • Guaranteeing usability without realtime: • Must know maximum live memory • If fragmentation & metadata overhead bounded • We also require: • Maximum allocation rate (MB/s) • How does the user figure this out??? • Very simple programming style • Empirical measurement • (Research) Static analysis

  43. Conclusions • Systems are made of concurrent components • Basic building blocks: • Locks • Try-locks • Compare-and-Swap • Non-locking stacks, lists, … • Monotonic phases • Logical clocks and asynchronous agreement • Encapsulate so others won’t suffer!

  44. http://www.research.ibm.com/metronomehttps://sourceforge.net/projects/tuningforkvphttp://www.research.ibm.com/metronomehttps://sourceforge.net/projects/tuningforkvp

  45. GC Phases … … … APP APP APP APP APP APP APP APP … … APP APP APP APP APP APP APP … … APP APP APP APP APP APP APP APP APP … APP APP APP APP APP APP APP Legend APP SNAPSHOT TRACE FLIP SWEEP TERMINATE

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