lightning talk on java memory consistency model java day kiev 2014

@LAFK_pl, Polish JUG, May 2014 2


MANAGEMENT!!


Posted in 2012...


... and still not corrected


Leslie Lamport

● Distributed system clocks

● Happens-before

● Sequential Consistency


Bill Pugh

● FindBugs

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

locking

● New JMM


Sarita Adve

● Java Memory Model is broken

● Number of great papers on memory (consistency) models

● Best definition of MCM I found


Within these 15-20 minutes

Intro

Memory model means?

Hardware

Java stuff


Memory model?

If NOT about GC then...

what's it about?


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


Where it matters?

● Javac / Jython / ...● JIT

● Hardware of course

● So JMM is a LANGUAGE memory consistency model


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 ...


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)

● ...


Exemplary CPU


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!


Barriers / Fences

● CPU instruction● Means „Flush now!”

● Forces update● Starts cache

coherency protocols

● Read / Write / Full


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


What rules?

● Write is valid when it follows certain rules


Before, or after?


JSR-133 = new JM(C)M


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


What was wrong with old JM(C)M?

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


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


Barriers in Java – rules

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

● final – sfence after init (field visibility)

● Atomic instructions (like lock) = mfence


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

lightning talk on java memory consistency model java day kiev 2014

Software