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Complexity of Weak Consistency Models

Complexity of Weak Consistency Models. Madan Musuvathi Research in Software Engineering Microsoft Research. Weak Consistency Models. Hard to program Hard to test Hard to debug General consensus: trade consistency for Availability Performance Partition-tolerance ….

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Complexity of Weak Consistency Models

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  1. Complexity of Weak Consistency Models Madan Musuvathi Research in Software Engineering Microsoft Research

  2. Weak Consistency Models • Hard to program • Hard to test • Hard to debug • General consensus: trade consistency for • Availability • Performance • Partition-tolerance • …

  3. Quantifying the Cost of Weak Consistency • How hard are weak consistency models to program/test/debug? • Study the hardness of associated verification problems • Assumption: • Programming models that are hard for computers to reason are also hard for humans

  4. Memory Consistency Models • A memory consistency model is an abstraction of the memory subsystem • Defines when and in what order memory operations from one thread are visible to others • Sequential consistency is what programmers want, but is expensive

  5. Quiz: Can this assert fail ? Answer: depends on the memory model ! Init: x = 0; y = 0 x = 1; t = y; y = 1; u = x; assert( ! (t == 0&& u == 0) )

  6. Sequential Consistency Can the assert fail? No Init: x = 0; y = 0 x = 1; t = y; y = 1; u = x; assert( ! (t == 0 && u == 0) ) SC

  7. x86 Memory Model Can the assert fail? Yes Init: x = 0; y = 0 x = 1; t = y; y = 1; u = x; assert( ! (t == 0 && u == 0) ) x86

  8. Memory Model Verification Problem • Safety verification • Does this system reach a bad state • Liveness verification • Does this system repeatedly reach a good state Finite State Machine Finite State Machine Finite State Machine … Memory Model Memory (finite)

  9. Complexity of Memory Model Verification [POPL ‘10] • Formalizes the intuition that SC is easier to program than relaxed memory models

  10. Shared Memory vs Message Passing Decidable Undecidable Finite State Machine Finite State Machine Finite State Machine … Shared Memory Finite State Machine Finite State Machine Finite State Machine … Message Passing Network

  11. Conclusion • Weak consistency comes at a cost of programmability/testability/debuggability • Studying the associated verification problem can quantify this cost • Future: Formalize weak consistency models for the cloud

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