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The Design, Implementation and Evaluation of a Pluggable Type - - PowerPoint PPT Presentation

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The Design, Implementation and Evaluation of a Pluggable Type Checker for Thread-Locality in Java By: Amanj Sherwany 2011 http://www.amanj.me Background Loci is a static checker for thread- Informationsteknologi locality for Java-like

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The Design, Implementation and Evaluation of a Pluggable Type Checker for Thread-Locality in Java

By: Amanj Sherwany 2011

http://www.amanj.me

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Informationsteknologi

Institutionen för informationsteknologi | www.it.uu.se

Background

 Loci is a static checker for thread-

locality for Java-like languages.

 Programmers express thread-locality

through annotations in the source code.

 Preservation of thread-locality is

checked statically.

 Proposed by Wrigstad et al. in 2009

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Informationsteknologi

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Why Thread-Locality?

 Simplifying concurrent and parallel

programming.

 Accesses to thread-local data are sequential

and easy to reason about.

 There will never be data races or dead

locks on thread-local data.

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Side-Effects of Thread- Locality

 In real-time systems, thread-locality

avoids lock inflation which is important to calculate worst-case run-times/paths.

 Thread-local data can be collected

without pausing other threads.

 No need to synchronise local data with

main memory.

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Thread-Locality in Java

 Java does not have support for

programming with thread-local data.

 The little support it provides with

ThreadLocal API is not enough, because:

 Allows defining fields for which each

accessing thread has its own copy.

 But, nothing prevents the contents of the field

to be shared across threads.

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Informationsteknologi

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Pluggable Type Checkers

 First proposed by Bracha.  Allow static checking of different program

properties at different stages.

 In Bracha’s terms:

 Have no effect on the run-time semantics of

the programming language.

 Do not mandate type annotations in the

syntax.

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Pluggable Type Checkers, Cont'd

 Since version 5, Java has basic support

for pluggable type checkers.

 Java 8 will have:

 A more expressive annotation system.  A framework for designing custom type

checkers, called “the Checker framework”.

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Loci Semantics

Memory-partitioning in Loci (Logical) Subheap Objects Reference

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Loci Semantics

Thread Shared Allowed: Disallowed:

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Loci Semantics

Thread Shared Allowed:

  • Intra-thread &

Intra-shared Disallowed:

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Loci Semantics

Thread Shared Allowed:

  • Intra-thread &

Intra-shared Disallowed:

  • Inter-thread
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Loci Semantics

Thread Shared Allowed:

  • Intra-thread &

Intra-shared

  • Thread to Shared

Disallowed:

  • Inter-thread
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Loci Semantics

Thread Shared Allowed:

  • Intra-thread &

Intra-shared

  • Thread to Shared

Disallowed:

  • Inter-thread
  • Shared to thread
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Loci Semantics

Thread Shared Allowed:

  • Intra-thread &

Intra-shared

  • Thread to Shared

Disallowed:

  • Inter-thread
  • Shared to thread

(unless guarded by thread local fields)

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Informationsteknologi

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Loci 1.0

 The old (initial) version of Loci uses the

standard Java annotation system.

 Is available as an Eclipse plugin only.  Does not support generics (due to the

limitations in Java annotation system in JDK 6).

 Does not cover all the features in Java.

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What the Thesis is About

 Extending Loci into Loci 2.0:

 Support for generics.  Support for locality-polymorphic methods.  More flexible annotations and support for

corner cases.

  • Equality test between objects from different

thread-localities.

  • Static utility methods, like sorting (more later).
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What the Thesis is About - Cont'd

 Re-implementing Loci using the Checker

framework in Java 8, instead of the standard Java annotation system.

 Allows more flexible annotations.  Fully annotated Java API.

 Evaluating our extended Loci system,

and comparing the results with the previous version.

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Loci Annotations

 @Local, which denotes a thread-local

value.

 @Shared, which denotes a value that can

be arbitrarily shared between threads.

 @ThreadSafe, which denotes a value that

must be treated in such a way that thread- locality is preserved, but the value may not be thread-local in practice.

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Data Flow Constraints

@ThreadSafe @Shared @Local ┴

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Basics of Loci

@Local class A{...} //A thread-local class @Shared class B{...} //A shared class class D extends A{...} //An implicit thread-local class class E extends B{...} //An implicit shared class @Shared F extends A{...} //Invalid @Local G extends B{...} //Invalid A a; //A thread-local data B b; //A shared data @Shared A bad1; @Local B bad2; //Invalid

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Basics of Loci, Cont'd

class A{...} //A flexible class @Local class B extends A{...} //A thread-local class @Shared class D extends A{...} //A shared class F extends A{...} //A flexible class @Local A a; //A thread-local data @Shared A b; //A shared data @ThreadSafe A c; //A thread-safe reference A d; //The same thread-locality as the enclosing object (next slide)

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Basics of Loci, Cont'd

 The golden rule:

“Unless they are explicitly

annotated, the thread-locality of instances of flexible classes follow the thread-locality of their enclosing

  • bjects.”
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The “Object” Class

public class Object { public final native @Shared Class getClass(); public boolean equals(@ThreadSafe Object obj); protected native Object clone(); }

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The “Object” Class

public class Object { public final native @Shared Class getClass(); public boolean equals(@ThreadSafe Object obj); protected native Object clone(); } Object is a flexible class

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The “Object” Class

public class Object { public final native @Shared Class getClass(); public boolean equals(@ThreadSafe Object obj); protected native Object clone(); } Object is a flexible class Inter-thread-locality equality test

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The “Object” Class

public class Object { public final native @Shared Class getClass(); public boolean equals(@ThreadSafe Object obj); protected native Object clone(); } Object is a flexible class Inter-thread-locality equality test The thread-locality of the cloned instance follows the original instance (the golden rule)

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The “ThreadLocal” Class

@Shared public class ThreadLocal<T extends @Local Object>{ protected T initialValue(); public T get(); public void set(T value); }

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The “ThreadLocal” Class

@Shared public class ThreadLocal<T extends @Local Object>{ protected T initialValue(); public T get(); public void set(T value); } ThreadLocal is a shared class

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The “ThreadLocal” Class

@Shared public class ThreadLocal<T extends @Local Object>{ protected T initialValue(); public T get(); public void set(T value); } ThreadLocal is a shared class Holds thread-local fields

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Standard Java Classes

 We have annotated the standard Java

classes.

 In JDK 6:

  • 541 @Shared classes (~15.5%).
  • 2936 flexible classes (~84.5%).

 Runnable is annotated @Shared  Throwable is annotated @Local

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The Loci Tool

 Is a command line tool.  Implemented as a plugin for the javac.  On top of the Checker framework.  Works with Java 5 and up!  Works with ANT, Maven and different

IDEs.

 Works on any OS that is supported by

Java.

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The Loci Tool, Cont'd

 Is open source, GPLv3.  Can be freely downloaded and used.  Has a production quality.  Its design allows further enhancements

(thanks to the flexibility of the Checker framework).

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How to Use Loci?

 Install JSR 308 javac.  Put Loci in your CLASSPATH.  Annotate your program, and import Loci

annotations:

import loci.quals.*;

 Run Loci, and fix the bugs:

javac -processor loci.LociChecker *.java

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Demo

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Demo 1

class Example { private Foo foo = null; // Should be a thread-local value private Foo bar = null; // Possibly shared value void frob() { bar = foo; // Leak! } }

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Demo 1, Cont'd

class Example { ThreadLocal<Foo> foo = new ThreadLocal<Foo>(); private Foo bar = null; // Possibly shared value void frob() { bar = foo.get(); // Reading foo requires indirection } }

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Demo 1, Cont'd

class Example { ThreadLocal<Foo> foo = new ThreadLocal<Foo>(); private Foo bar = null; // Possibly shared value void frob() { bar = foo.get(); // Leak! } }

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Demo 1, Cont'd

class Example { private @Local Foo foo = null; private @Shared Foo bar = null; void frob() { bar = foo; // Not Allowed! } }

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Demo 2

class Example { private @Local Foo[] foo = null; private @Shared Foo[] bar = null; void frob() { sort(foo); //Invalid, sort accepts shared arrays sort(bar); } public static Foo[] sort(Foo[] array){...} }

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Demo 2, Cont'd

class Example { private @Local Foo[] foo = null; private @Shared Foo[] bar = null; void frob() { sort(foo); //OK, but we lost type information! sort(bar); } public static @ThreadSafe Foo[] sort (@ThreadSafe Foo[] array){...} }

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Demo 2, Cont'd

class Example { private @Local Foo[] foo = null; private @Shared Foo[] bar = null; void frob() { foo = sort(foo); //Not OK, thread-safe return type sort(bar); } public static @ThreadSafe Foo[] sort (@ThreadSafe Foo[] array){...} }

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Demo 2, Cont'd

class Example { private @Local Foo[] foo = null; private @Shared Foo[] bar = null; void frob() { foo = sort(foo); //OK! bar = sort(bar); //OK! } public static <@X T extends Foo> T[] sort(T[] array){...} }

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Evaluating Loci

 We annotated over 50000 LOC.  262 classes and 13 interfaces  We chose heavily multi-threaded Java

benchmarks.

 Programs from DaCapo and JavaGrande

Benchmark suite.

 Less than 15 annotations/KLOC

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Evaluating Loci, Cont'd

Class Parameter Variable Return 20 40 60 80 100 120 @Local @Shared @ThreadSafe

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Conclusions

 Loci:

 Is a useful aid for programmers.  Is compatible with existing Java programs.  Eliminates thread-locality violations.  Requires a low annotation overhead.  Is a bit slower than normal javac (6 sec vs 45

  • n my machine) when compiling ~46 KLOC.

 Has five known bugs!

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Future Work

 Supporting object transferring across

threads without the need of deep cloning.

 Having cross thread-locality cloning.  Fixing the bugs that we have.  Speeding up the tool.

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Related Links

 Homepage: http://www.it.uu.se/research/upmarc/loci  Wiki: http://java.net/projects/loci/pages/Home  Forum: http://java.net/projects/loci/forums  Mailing List: http://java.net/projects/loci/lists/  Repository: http://java.net/projects/loci/sources  Bugzilla: http://java.net/bugzilla/buglist.cgi?product=loci  Manual: http://loci.java.net/manual  Download: http://java.net/projects/loci/downloads

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References

 The Java Grande Forum Multi-threaded

Benchmarks.

 S. M. Blackburn et 1l. “The DaCapo

benchmarks: Java benchmarking development and analysis”, OOPSLA ’06.

 G. Bracha, “Pluggable type systems”,

OOPSLA04.

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References, Cont'd

 T. Wrigstad et al. “Loci: Simple thread-

locality for java”, ECOOP 2009.

 W. Dietl et al. “Building and using

pluggable type-checkers” , ICSE’11.

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Thank You, Questions?