Wednesday 29th of October 20141

Incinerator

Koutheir ATTOUCHI1,2, Gaël THOMAS1, André BOTTARO2, Gilles MULLER1

1 Laboratoire d’Informatique de Paris 6, France.2 Orange Labs, Grenoble, France.

Detecting & ResolvingStale Referencesin Java

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The Smart HomeMultiple Application Domains

Security

Energy

Healthcare

Comfort

Well being

Multimedia

Content sharing

Games

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The Smart HomeConnected 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 GatewayOffered 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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HypothesisUnreliable 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

O1Object in App1 Object in App2O2

Reference

Stale ReferenceO2

App2 uninstalled O2 stale

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Smart Home Alarm ApplicationMotivation by an example

Alarm Application

GasSensor DriverSiren Driver

1.0

Siren Configuration(Format: Simple Text)

Gas SensorAlarm Siren

Siren Configuration(Format: XML)

StaleReference

Siren Driver 2.0

3

1

21

2

3

Update Siren Driver version 2.0

Convert Siren configuration format 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 LeakCaused by Stale References1

O1

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

S

C2 C4

C3Class(S)

ClassLoader(S)

Stale reference

Stale object

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Significant Memory LeakCaused by Stale References

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

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IncineratorApproach

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 resourcesin 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 Issueswhen 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 Handlingwhen Eliminating Stale References

T1 T2 T3

S1

Stale reference Stale object Active thread Blocked thread

Incinerator

Wake up!

Monitor(S1) StaleMonitor(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 EvaluationMicro 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

Service Coroner1

Incinerator

Incinerator also eliminates all detected stale references

1 Gama and Donsez (SEAA’08)

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Performance EvaluationDaCapo1 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% Tolerable in:– Constrained production environments– Test environments

Implementation is mostly independent of:– The OSGi framework implementation– The garbage collection algorithm Thank you!