Wednesday 29th of October 2014 1

Incinerator

Koutheir ATTOUCHI1,2, Gaël THOMAS1, André BOTTARO2, Gilles MULLER1 1 Laboratoire d’Informatique de Paris 6, France. 2 Orange Labs, Grenoble, France.

Detecting & Resolving Stale References in Java

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The Smart Home Multiple Application Domains

§  Security

§  Energy

§  Healthcare

§  Comfort

§  Well being

§  Multimedia

§  Content sharing

§  Games

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The Smart Home Connected devices

Devices are §  Connected §  Easy to install §  Affordable

§  Fridge §  Thermostat §  Fork §  Oven §  etc.

Connected devices §  Toothbrush §  Washing machine §  Scale §  Locks §  Dishwasher

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Home Automation Gateway Offered by a Smart Home Operator

One common embedded platform §  Hosting applications delivered by various tiers1

1 Condry et al. (IECON’99); Royon and Frénot (2007)

Objectives §  Reduce service

deployment costs

§  Share connected devices between tiers

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Hypothesis Unreliable and long-running applications

§  Applications can be –  Buggy –  Malicious

§  The platform runs for long duration §  Abrupt restarts of the platform can be dangerous for

–  Devices –  Inhabitants

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Addressed Issues

Resolve stale references in Java applications

Stale reference §  A reference to a stale object: an object that became unusable

–  Many objects become unusable after updates and uninstallation of applications

O1 Object in App1 Object in App2 O2 Reference Stale Reference

O2

App2 uninstalled ! O2 stale

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Smart Home Alarm Application Motivation by an example

Alarm Application

GasSensor Driver Siren Driver 1.0

Siren Configuration (Format: Simple Text)

Gas Sensor Alarm Siren

Siren Configuration (Format: XML)

Stale Reference Siren Driver 2.0

3

1

2 1

2

3

Update Siren Driver è version 2.0 Convert Siren configuration format è XML Forget to update the reference to Siren Driver 1.0

1

3

2

1

3

2

Memory leak Data corruption Physical hazard

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Significant Memory Leak Caused by Stale References1

O1

1 Lindholm and Yellin (1999); Johnson and Dawson (2014)

S

C2 C4

C3 Class(S)

ClassLoader(S)

Stale reference

Stale object

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Significant Memory Leak Caused by Stale References

Memory leaks caused by stale references appearing after multiple updates of a buggy application

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Incinerator Approach

§  Detect and eliminate* stale references

§  Incinerator transforms stale references to null references during garbage collection cycle

* The Devil is in the Details…

Incinerator

O1

O2

S2

O3

S1

O4

Stale reference Stale object

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Cleaning up resources in Object.finalize()

Objective: §  Allow objects to cleanup their

resources –  Execute Object.finalize()

without risking exceptions due to stale references

§  Incinerator allows unreachable finalizable objects to hold stale references

–  Defer stale references elimination to the following garbage collection cycle

Incinerator

O3

S1

O4

Stale reference Stale object

F1

Defer

Unreachable finalizable object

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Synchronization Issues when Eliminating Stale References

Incinerator

T1 T2 T3

S1

T1, T2 and T3 synchronize on the monitor of S1

Stale reference Stale object Active thread Blocked thread

T3 and T2 blocked forever

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Synchronization Handling when Eliminating Stale References

T1 T2 T3

S1

Stale reference Stale object Active thread Blocked thread

Incinerator

Wake up!

Monitor(S1) " Stale Monitor(S1) = Stale

! NPE

! NPE

Wake up! ! NPE

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Implementation

§  1000 lines of C++ in the J3/VMKit1 JVM §  Tested on Knopflerfish2, an open source recognized

implementation of OSGi –  10 lines added to Knopflerfish

§  Independent of the garbage collector algorithm

§  Open source –  http://llvm.org/svn/llvm-project/vmkit/branches/incinerator/

1 Geoffray et al. (VEE’10) 2 Makewave AB Corp. (2014)

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Functional Evaluation Micro benchmarks

§  10 scenarios of stale references, divided by: –  Visibility from the OSGi framework –  Scope of stale references –  Synchronization on stale references –  Finalization of objects holding stale references

§  Stale references detection results

Scenario 1 2 3 4 5 6 7 8 9 10 J3/Hotspot û û û û û û û û û û

Service Coroner1 ü û û û û û û û û û Incinerator ü ü ü ü ü ü ü ü ü ü

§  Incinerator also eliminates all detected stale references 1 Gama and Donsez (SEAA’08)

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Performance Evaluation DaCapo1 benchmark suite

Low-end computer High-end computer

Average cost in execution time of DaCapo test suites on J3 and on Incinerator

Execution cost < 4%

1 Blackburn et al. (OOPSLA’06)

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Conclusion

Incinerator §  Incinerator detects and eliminates stale references, avoiding:

–  Memory leaks –  Data corruption –  Physical hazards

§  And makes stale references visible to Java/OSGi developers

§  Execution cost < 4% è Tolerable in: –  Constrained production environments –  Test environments

§  Implementation is mostly independent of: –  The OSGi framework implementation –  The garbage collection algorithm

Thank you!