A lightening talk on using Shared Memory in Java is a Thread Safe manner and why you would want to.

1. Thread Safe Interprocess
Shared Memory in Java
(in 7 minutes)
Peter Lawrey
Principal Consultant
Higher Frequency Trading
January 2014
Thread Safe Interprocess Shared Memory in
Java
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2. Shared Memory
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Java naturally uses memory on the heap
(and stack)
Java can also access memory mapped files
These files can be accessed and shared by
multiple processes at once
They can be persisted, or stored on a tmpfs or
RAM drive
January 2014
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3. Thread Safe
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Java's thread safe constructs are all on the
heap
You can use Unsafe and create off heap
thread safe constructs such as memory
barriers, volatile access and CAS operations
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4. What is CAS?
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CAS is Compare And Swap also known as
Compare And Set in some Java libraries
It is a thread safe operation which only
succeeds for one thread at a time and can be
used to build a lock
Is a single machine code instruction → fast
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5. Why not use the heap?
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Can support 100s of GB with no GC impact
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Can share data structures between processes
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Can persist data without extra overhead
This allows you to implement a simple
embedded database with sub micro-second
read/write latency
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6. sun.misc.Unsafe
public native compareAndSetInt(
Object obj,
// can be null
long offset,
int expected,
int value);
Also compareAndSetLong
Likely to go away in Java 9
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7. Bytes
OpenHFT/Java-Lang has a wrapper

64-bit replacement for ByteBuffer
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Thread safe operations
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Supports binary data, text parsing and object
serialization. e.g. ObjectInput/Ouput
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8. Demo - LockingViaMMapMain
Two processes toggle flags in many locked records
One process flips false → true
The other flips true → false
Toggled 100,000,000 times with
an average delay of 49 ns
January 2014
Thread Safe Interprocess Shared Memory in
Java
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9. Questions?
@PeterLawrey
github.com/OpenHFT
Thank you to
SkillsMatter – the host
RecWorks – the organisers
LJC – London Java Community
January 2014
Thread Safe Interprocess Shared Memory in Java
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10. Questions?
@PeterLawrey
github.com/OpenHFT
Thank you to
SkillsMatter – the host
RecWorks – the organisers
LJC – London Java Community
January 2014
Thread Safe Interprocess Shared Memory in Java
9