Verification of obstruction-free algorithm with contention management Niloufar Shafiei

2 Agenda  The algorithm  Correctness condition for shared objects  Java PathFinder  Verification challenges  Verification  Summary  The algorithm  Correctness condition for shared objects  Java PathFinder  Verification challenges  Verification  Summary

3 The algorithm  Obstruction-free deque algorithm with different contention management policies  AtomicLongArray  AtomicLong  How should the algorithm behave? (correctness)  Data structure represents the abstract deque at any time  All operations terminate  No livelock or deadlock  Obstruction-free deque algorithm with different contention management policies  AtomicLongArray  AtomicLong  How should the algorithm behave? (correctness)  Data structure represents the abstract deque at any time  All operations terminate  No livelock or deadlock

4 Correctness condition for shared objects implementations  Find the linearization point

5 Correctness condition for shared objects implementations  Find the linearization point push(v 1 ) push(v 2 ) pop time stack ?

6 Correctness condition for shared objects implementations  Find the linearization point push(v 1 ) push(v 2 ) pop time stack ? X X X empty v1v1 v2v2

7 Check the correctness of shared object implementation  Find the linearization point  Define abstract variables (abstract stack,…)  Change the abstract variables at linearization points  At all linearization points, check if the abstract variables are consistent with data structures  In java, insert assert(expression) atomically at linearization points  Synchronized block  Atomic block  Find the linearization point  Define abstract variables (abstract stack,…)  Change the abstract variables at linearization points  At all linearization points, check if the abstract variables are consistent with data structures  In java, insert assert(expression) atomically at linearization points  Synchronized block  Atomic block

8 Java PathFinder  JPF  Model checker  Deadlocks  Invariants  User-defined assertions  JPF versus Spin  JPF covers the java programming language (not more than lines)  JPF design goal is to make it as modular and understandable as possible  Spin is faster than JPF  JPF  Model checker  Deadlocks  Invariants  User-defined assertions  JPF versus Spin  JPF covers the java programming language (not more than lines)  JPF design goal is to make it as modular and understandable as possible  Spin is faster than JPF

9 Verification challenges  JPF does not support AtomicLongArray and AtomicLong  Volatile Long[] and Long  Synchronized methods to implement C&S  Warning “unprotected field access of deque”  JPF employ Partial Order Reduction to save space  For lock protection, determines if a field access is scheduling relevant (transaction boundary)  vm.por.sync_detection=false  JPF does not support AtomicLongArray and AtomicLong  Volatile Long[] and Long  Synchronized methods to implement C&S  Warning “unprotected field access of deque”  JPF employ Partial Order Reduction to save space  For lock protection, determines if a field access is scheduling relevant (transaction boundary)  vm.por.sync_detection=false

10 State search  JPF searches  DFS  With backtracking is most appropriate for checking liveness properties  BFS  Search.heuristic.class = gov.nasa.jpf.search.heuristic.BFSHeuristic  JPF searches  DFS  With backtracking is most appropriate for checking liveness properties  BFS  Search.heuristic.class = gov.nasa.jpf.search.heuristic.BFSHeuristic

11 Verification ResultNumber of paths 1 thread (DFS - BFS) No error4 2 threads (DFS - BFS) No error threads Out of memory >1200

12 Verification  How to save the memory?  More synchronized methods  Synchronized blocks and Atomic blocks (Verify class)  Local instructions  At most one shared memory instruction  No instruction prevent the program from accessing endAtomic()  Return - break - join - if statement  How to save the memory?  More synchronized methods  Synchronized blocks and Atomic blocks (Verify class)  Local instructions  At most one shared memory instruction  No instruction prevent the program from accessing endAtomic()  Return - break - join - if statement

13 Verification  Atomic blocks  Sometimes threads loop in Atomic block  Why processes killed?  Need memory more than available memory  Atomic blocks  Sometimes threads loop in Atomic block  Why processes killed?  Need memory more than available memory ResultNumber of paths 1 thread Processes killed 0 2 threads Processes killed 0 3 threads Processes killed 0

14 Verification  Synchronized blocks ResultNumber of paths 1 thread (DFS - BFS) No error4 2 threads (DFS - BFS) No error threads Out of memory >1200 Why results are not improved? Partial Order reduction

15 Summary  Correctness conditions of shared object  Java PathFinder  Verification of shared deque implementation with JPF  Correctness conditions of shared object  Java PathFinder  Verification of shared deque implementation with JPF

16 Questions?