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Memory Models (1) Xinyu Feng University of Science and Technology of China

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Why Memory Models C1 || C2Compiler Memory Result

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Why Memory Models C1 || C2Compiler Memory Result Which reads see which write?

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Sequential Consistent (SC) Model... Memory Interleaving semantics: P1 P2P3Pn [Lamport 1979]

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The need of weak memory models SC model prohibits many optimization: x = 1; r1 = y; y = 1; r2 = x; r1 = r2 = 0? Initially: x = y = 0 Impossible in SC model, but allowed in x86 or Java. r1 = y; x = 1 y = 1; r2 = x; Weak memory model allow more behaviors.

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Usability: DRF guarantee – DRF programs have the same behaviors as in SC model Not too strong – Allow common optimization techniques – In some sense hijacked by the mainstream compiler Preserve type-safety and security guarantee – Cannot be too weak Design Criteria Very challenging to satisfy all the requirements!

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Compiler Optimization Can Be Smart r1 = x; r2 = x; if (r1 == r2) y = 2; r3 = y; x = r3; Initially: x = 0, y = 1 r1 = r2 = r3 = 2? Must be allowed! y = 2; r1 = x; r2 = r1; if (true) Redundant read elim.

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Efforts for Java Memory Model (JMM) First edition in Java Language Spec – Fatally flawed, not support key optimizations [Pough 2000] Current JMM [Manson et al. 2005] – Based on 5-year discussion and many failed proposals – “very complex” [Adve & Boehm 2010] – Surprising behaviors and bugs [Aspinall & Sevcik 2007] Next generation: JEP 188, Dec. 2013

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Happens-Before Order Program execution: a set of events, and some orders between them. po: Time T1 T2 rel acqrel acq sw: Happens-before order (hb): transitive closure of po sw [Lamport 1978]

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Happens-Before Order Time T1 T2 po: rel acqrel acq sw: Happens-before order (hb): transitive closure of po sw w1 w2 r w1 hb w2w1 hb r Not: w2 hb rr w2

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Happens-Before Memory Model (HMM) Time T1 T2 rel acqrel acq w1 w2 r Read can see (1) the most recent write that happens-before it, or (2) a write that has no happens-before relation. r could see both w1 (which happens-before it) and w2 (with which there is no happens-before relation)

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HMM – Relaxed Ordering x = 1; r1 = y; y = 1; r2 = x; r1 = r2 = 0? Initially: x = y = 0 Allowed in HMM

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HMM – Examples with Global Analysis r1 = x; r2 = x; if (r1 == r2) y = 2; r3 = y; x = r3; Initially: x = 0, y = 1 r1 = r2 = r3 = 2? Speculation: r3 reads 2 Allowed in HMM! Justified!

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HMM – Out-of-Thin-Air Read r1 = x; y = r1; r2 = y; x = r2; r1 = r2 = 42? Initially: x = y = 0 Allowed in HMM! Speculation: r1 will get 42 Justified! May break the security and type-safety of Java!

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r1 = x; r2 = x; if (r1 == r2) y = 2; r3 = y; x = r3; r1 = r2 = r3 = 2? Speculation: r3 reads 2 r1 = x; y = r1; r2 = y; x = r2; r1 = r2 = 42? Speculation: r1 will get 42 Good speculation. Should allow! Bad speculation. Disallow!

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JMM Take HMM as the core, and try hard to distinguish good speculation from bad speculation! Very complex, with surprising results and bugs. Introduce 9 axioms to constrain causality.

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JMM – Surprising Results C1; lock l; C2; unlock l; C3 lock l; C1; C2; unlock l; C3 lock l; C1; unlock l; lock l; C2; unlock l; C3 Adding more synchronization may increase behaviors!

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JMM – Surprising Results (2) C1; C3 C2; C1; C2; C3; Inlining threads may increase behaviors! More: Re-ordering independent operations may change behaviors. Adding/removing redundant reads may change behaviors.

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Reading http://www.cs.umd.edu/~pugh/java/memoryModel/ http://openjdk.java.net/jeps/188

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