what is Java Memory Consistency Model, why you should not think of GC when you hear it and how it's done in general and in Java

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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; // TUTAJ: 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 thistime

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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