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Michael Bond Kathryn McKinley The University of Texas at Austin.

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1 Michael Bond Kathryn McKinley The University of Texas at Austin

2 http://www.codeproject.com/KB/showcase/IfOnlyWedUsedANTSProfiler.aspx  Driverless truck  10,000 lines of C#  Leak: past obstacles remained reachable  No immediate symptoms “This problem was pernicious because it only showed up after 40 minutes to an hour of driving around and collecting obstacles.”  Quick “fix”: restart after 40 minutes  Environment sensitive  More obstacles in deployed setting  Unresponsive after 28 minutes

3  Driverless truck  10,000 lines of C#  Leak: past obstacles remained reachable  No immediate symptoms “This problem was pernicious because it only showed up after 40 minutes to an hour of driving around and collecting obstacles.”  Quick “fix”: restart after 40 minutes  Environment sensitive  More obstacles in deployed setting  Unresponsive after 28 minutes Different environments & inputs  different behavior Uncertainty in deployed systems Hard to fix before deployment

4  Deployed systems have leaks  Critical systems need immediate help  Leak pruning tolerates bad effects  Reclaims memory automatically  High precision & low overhead  Bounds resources  Preserves semantics

5  Why tolerate leaks  Why leaks are so bad  How leak pruning works  How leak pruning predicts leaked objects  Evaluation

6  Memory leaks are a real problem  Managed languages do not eliminate them

7  Memory leaks are a real problem  Managed languages do not eliminate them Reachable Unreachable

8  Memory leaks are a real problem  Managed languages do not eliminate them Live ReachableDead

9  Memory leaks are a real problem  Managed languages do not eliminate them Live Reachable Dead

10  Memory leaks are a real problem  Managed languages do not eliminate them Live Reachable Dead

11  Memory leaks are a real problem  Managed languages do not eliminate them  Slow & crash real programs Live Dead

12  Memory leaks are a real problem  Managed languages do not eliminate them  Slow & crash real programs  Unacceptable for some applications  Fixing leaks is hard  Leaks take time to materialize  Failure far from cause

13  Is it a memory leak or a case of stale memory ?  Is it correct to always consider stale memory as equivalent to unreachable memory ?

14  Why tolerate leaks  Why leaks are so bad  How leak pruning works  How leak pruning predicts leaked objects  Evaluation

15  Garbage collection based on liveness Live Reachable Dead

16  Garbage collection based on liveness  Take action when memory exhausted Live Reachable Dead

17  Garbage collection based on liveness  Take action when memory exhausted Live Reachable Dead

18  Garbage collection based on liveness  Take action when memory exhausted Live Dead Throw OOM error Out of memory!

19  Garbage collection based on liveness  Take action when memory exhausted Live Dead Out of memory! Throw OOM error Reclaim some objects

20  Garbage collection based on liveness  Reclaim predicted dead objects Reclaimed Live

21  Garbage collection based on liveness  Reclaim predicted dead objects Reclaimed Live A A B B

22  Garbage collection based on liveness  Reclaim predicted dead objects Live A A ?

23  Garbage collection based on liveness  Reclaim predicted dead objects  Poison references to reclaimed objects Live A A X

24  Garbage collection based on liveness  Reclaim predicted dead objects  Poison references to reclaimed objects Live A A X Throw InternalError with OOMError attached

25  Garbage collection based on liveness  Reclaim predicted dead objects  Poison references to reclaimed objects Live A A X Throw InternalError with OOMError attached Worst case: defers fatal errors Best case: keeps leaky programs running much longer or indefinitely

26  Instead of returning the error can we return some other object, to keep program going ? Eg DieHard  What about semantics in that case ?

27 INACTI VE OBSER VE SELECTPRUNE Heap not full <50% Heap filled >50% Heap not full Heap full

28  Why tolerate leaks  Why leaks are so bad  How leak pruning works  How leak pruning predicts leaked objects  Evaluation

29  Predicting the future  Leaked objects  not used again  Highly stale objects  likely leaked  Alternative: offload to disk [Melt, Bond & McKinley ’08] [LeakSurvivor, Tang et al. ’08] [Panacea, Goldstein et al. ’07, Breitgand et al. ’07]  Tolerates mispredictions  Eventually exhausts disk (if disk at all)

30  Predicting the future  Leaked objects  not used again  Highly stale objects  likely leaked  Alternative: offload to disk [Melt, Bond & McKinley ’08] [LeakSurvivor, Tang et al. ’08] [Panacea, Goldstein et al. ’07, Breitgand et al. ’07]  Tolerates mispredictions  Eventually exhausts disk (if disk at all) Need high precision One misprediction: program terminates Need high precision One misprediction: program terminates

31  Identify references to prune  Roots of leaked data structures  Categorize by reference type ParserInfo PreparedStatement

32  Identify references to prune  Roots of leaked data structures  Categorize by reference type  Criteria  Highly stale references ▪ More stale than most stale instance accessed previously ParserInfo PreparedStatement MaxS&U 2-4 GCs

33  Identify references to prune  Roots of leaked data structures  Categorize by reference type  Criteria  Highly stale references ▪ More stale than most stale instance accessed previously  Reference type keeping most bytes reachable ParserInfo PreparedStatement MaxS&U 2-4 GCs Size 132MB

34  Identify references to prune  Roots of leaked data structures  Categorize by reference type  Criteria  Highly stale references ▪ More stale than most stale instance accessed previously  Reference type keeping most bytes reachable Piggyback on GC: two-phase transitive closure ParserInfo PreparedStatement MaxS&U 2-4 GCs Size 132MB

35  Based on colleague’s JDBC application  Leak: SQL statements remain in set

36 HashSet Parser Info Parser Info Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] byte[][] byte[]

37 HashSet … …… Parser Info Parser Info Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] byte[][] byte[] Parser Info Parser Info Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] byte[][] byte[] Parser Info Parser Info Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] byte[][] byte[]

38 HashSet … …… Parser Info Parser Info Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] byte[][] byte[] Parser Info Parser Info Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] byte[][] byte[] Parser Info Parser Info Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] byte[][] byte[]

39 HashSet … …… Parser Info Parser Info Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] byte[][] byte[] Parser Info Parser Info Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] byte[][] byte[] Parser Info Parser Info Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] byte[][] byte[] HashEntry PreparedStatement ParserInfo PreparedStatement ResultSet MaxS&U 0-1 16-32

40 HashSet … …… Parser Info Parser Info Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] byte[][] byte[] Parser Info Parser Info Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] byte[][] byte[] Parser Info Parser Info Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] byte[][] byte[] HashEntry PreparedStatement ParserInfo PreparedStatement ResultSet MaxS&U 0-1 16-32

41 HashSet … …… Parser Info Parser Info Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] byte[][] byte[] Parser Info Parser Info Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] byte[][] byte[] Parser Info Parser Info Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] byte[][] byte[] HashEntry PreparedStatement ParserInfo PreparedStatement ResultSet MaxS&U 0-1 16-32

42 HashSet … …… Parser Info Parser Info Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] byte[][] byte[] Parser Info Parser Info Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] byte[][] byte[] Parser Info Parser Info Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] byte[][] byte[] enqueued HashEntry PreparedStatement ParserInfo PreparedStatement ResultSet MaxS&U 0-1 16-32

43 HashSet … …… Parser Info Parser Info Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] byte[][] byte[] Parser Info Parser Info Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] byte[][] byte[] Parser Info Parser Info Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] byte[][] byte[] enqueued HashEntry PreparedStatement ParserInfo PreparedStatement ResultSet MaxS&U 0-1 16-32 enqueued

44 HashSet … …… Parser Info Parser Info Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] byte[][] byte[] Parser Info Parser Info Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] byte[][] byte[] Parser Info Parser Info Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] byte[][] byte[] enqueued HashEntry PreparedStatement ParserInfo PreparedStatement ResultSet MaxS&U 0-1 16-32 Bytes 0 0 0 enqueued

45 HashSet … …… Parser Info Parser Info Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] byte[][] byte[] Parser Info Parser Info Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] byte[][] byte[] Parser Info Parser Info Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] byte[][] byte[] enqueued HashEntry PreparedStatement ParserInfo PreparedStatement ResultSet MaxS&U 0-1 16-32 Bytes enqueued 20 0 0

46 HashSet … …… Parser Info Parser Info Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] byte[][] byte[] Parser Info Parser Info Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] byte[][] byte[] Parser Info Parser Info Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] byte[][] byte[] enqueued HashEntry PreparedStatement ParserInfo PreparedStatement ResultSet MaxS&U 0-1 16-32 Bytes 60 enqueued 0 0

47 HashSet … …… Parser Info Parser Info Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] byte[][] byte[] Parser Info Parser Info Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] byte[][] byte[] Parser Info Parser Info Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] byte[][] byte[] enqueued HashEntry PreparedStatement ParserInfo PreparedStatement ResultSet MaxS&U 0-1 16-32 Bytes 572 enqueued 0 0

48 HashSet … …… Parser Info Parser Info Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] byte[][] byte[] Parser Info Parser Info Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] byte[][] byte[] Parser Info Parser Info Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] byte[][] byte[] HashEntry PreparedStatement ParserInfo PreparedStatement ResultSet MaxS&U 0-1 16-32 Bytes 572 enqueued 0 0

49 HashSet … …… Parser Info Parser Info Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] byte[][] byte[] Parser Info Parser Info Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] byte[][] byte[] Parser Info Parser Info Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] byte[][] byte[] HashEntry PreparedStatement ParserInfo PreparedStatement ResultSet MaxS&U 0-1 16-32 Bytes 572 enqueued 244 0

50 HashSet … …… Parser Info Parser Info Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] byte[][] byte[] Parser Info Parser Info Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] byte[][] byte[] Parser Info Parser Info Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] byte[][] byte[] HashEntry PreparedStatement ParserInfo PreparedStatement ResultSet MaxS&U 0-1 16-32 Bytes 420M 180M 0

51 HashSet … …… Parser Info Parser Info Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] byte[][] byte[] Parser Info Parser Info Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] byte[][] byte[] Parser Info Parser Info Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] byte[][] byte[] HashEntry PreparedStatement ParserInfo PreparedStatement ResultSet MaxS&U 0-1 16-32 Bytes 420M 180M 0

52 HashSet … …… Parser Info Parser Info Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] byte[][] byte[] Parser Info Parser Info Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] byte[][] byte[] Parser Info Parser Info Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] byte[][] byte[] HashEntry PreparedStatement ParserInfo PreparedStatement ResultSet MaxS&U 0-1 16-32 Bytes 420M 180M 0

53 HashSet … …… Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] HashEntry PreparedStatement ParserInfo PreparedStatement ResultSet MaxS&U 0-1 16-32 Bytes 420M 180M 0

54 HashSet … …… Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] HashEntry PreparedStatement ParserInfo PreparedStatement ResultSet MaxS&U 0-1 16-32 Bytes 0 0 0

55 HashSet … …… Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] HashEntry PreparedStatement ParserInfo PreparedStatement ResultSet MaxS&U 0-1 16-32 Bytes 0 0 0 enqueued

56 HashSet … …… Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] HashEntry PreparedStatement ParserInfo PreparedStatement ResultSet MaxS&U 0-1 16-32 Bytes 50M 220M 0

57 HashSet … …… Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] HashEntry PreparedStatement ParserInfo PreparedStatement ResultSet MaxS&U 0-1 16-32 Bytes 50M 220M 0

58 HashSet … …… Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] Result Set Result Set Hash Entry Hash Entry Prepared Statement Prepared Statement Field[] HashEntry PreparedStatement ParserInfo PreparedStatement ResultSet MaxS&U 0-1 16-32 Bytes 50M 220M 0

59 HashSet … … Hash Entry Hash Entry Prepared Statement Prepared Statement Hash Entry Hash Entry Prepared Statement Prepared Statement Hash Entry Hash Entry Prepared Statement Prepared Statement HashEntry PreparedStatement ParserInfo PreparedStatement ResultSet MaxS&U 0-1 16-32 Bytes 50M 220M 0

60 HashSet … … Hash Entry Hash Entry Prepared Statement Prepared Statement Hash Entry Hash Entry Prepared Statement Prepared Statement Hash Entry Hash Entry Prepared Statement Prepared Statement Algorithm efficiently selects references to consistently unused data structures based on type

61  Why tolerate leaks  Why leaks are so bad  How leak pruning works  How leak pruning predicts leaked objects  Evaluation

62  Leak pruning added to Jikes RVM 2.9.2 http://www.jikesrvm.org/Research+Archive  Performance stress test  Non-leaking programs: DaCapo & SPEC benchmarks  Low overhead ▪ 3% (Core 2) or 5% (Pentium 4) ▪ Primarily read barriers

63

64

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67 Low footprint & often helps a lot  all-the-time deployed use Low footprint & often helps a lot  all-the-time deployed use

68 INACTI VE OBSER VE SELECTPRUNE Heap not full <50% Heap filled >50% Heap not full Heap full

69  What about bursty leaks ?  Can we have dynamically adjustable limit for switching on Leak Pruning ?  Can we dynamically switch off Leak Pruning ?

70  What if stale objects interleaved with active objects ?  Is it useful to find source of these leaks ? If yes then how ?

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