Lightning talk on Java Memory Consistency Model Java Day Kiev 2014

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MANAGEMENT!!

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Posted in 2012...

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... and still not corrected

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Leslie Lamport

● Distributed system clocks

● Happens-before

● Sequential Consistency

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Bill Pugh

● FindBugs

● Java Memory Model is broken● Final - Volatile● Double-checked

locking

● New JMM

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Sarita Adve

● Java Memory Model is broken

● Number of great papers on memory (consistency) models

● Best definition of MCM I found

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Within these 15-20 minutes

Intro

Memory model means?

Hardware

Java stuff

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Memory model?

If NOT about GC then...

what's it about?

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Memory CONSISTENCY

● Allowed optimizations● Possible behaviours / executions of a (possibly

multithreaded!) program● Which cores / threads see which values● How to make it consistent for us, programmers

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Where it matters?

● Javac / Jython / ...● JIT

● Hardware of course

● So JMM is a LANGUAGE memory consistency model

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Hardware

● CISC or RISC CPU● Number of registers● Caches size or type● How many functional units per CPU● Pipeline:

● Instruction decode > address decode > memory fetch > register fetch > compute ...

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Program / code optimizations?

● Reorder ● (e.g. prescient store)

● Remove what's unnecessary ● (e.g. synchronize)

● Replace instructions / shorten machine code● Function optimizations

● (e.g. Inlining)

● ...

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Exemplary CPU

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Barriers / fences

„once memory has been pushed to the cache then a protocol of messages will occur to ensure all caches are coherent for any shared data. The techniques for making memory visible from a processor core are known as memory barriers or fences.

– Martin Thompson, Mechanical Sympathy

differs per architecture / CPU / cache type!

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Barriers / Fences

● CPU instruction● Means „Flush now!”

● Forces update● Starts cache

coherency protocols

● Read / Write / Full

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Summarizing Java Language Spec:

● Describes whether the execution trace is a legal execution of the program

● Works by examining each read and checking write observed by that read

● Write is valid when it follows certain rules● Describes possible behaviours of the program

● Implementor adhering to above can optimize code as he likes

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What rules?

● Write is valid when it follows certain rules

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Before, or after?

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Before, or after?

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JSR-133 = new JM(C)M

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Class Reordering { int x = 0, y = 0; public void writer() { x = 1; y = 2; }

public void reader() { int r1 = y; // HERE: y == 2 => x == ? int r2 = x; }}

Reordering - classic example

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What was wrong with old JM(C)M?

You took a look, read the specs entire and...

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New Java Memory Model● SC on single-core and single-thread CPU is

fine but it doesn't cut it now● To ensure visibility, JVM JMM spec ensures:

● final ● volatile ● synchronized

are done well this time

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Barriers in Java – rules

● JMM● volatile – sfence after write, lfence before read

● final – sfence after init (field visibility)

● Atomic instructions (like lock) = mfence

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Further topics

● Why MCM and not a threading library? H.Boehm

● Better MCM? Sarita Adve● Possible optimizations and their gains● Performance of Java and hardware MCMs?

Clashes?● JMCM rules in more detail