The java memory model and the mutability matrix of pain - Jamie Allen
- 2. @jamie_allen Mutability Matrix of Pain 2
- 3. Agenda • The Java Memory Model • What do concurrent and parallel mean? • How does the heap work? • How do we protect shared state on the heap? Mutability Matrix of Pain 3
- 4. Scaling in the Multicore Era • We have more more cores than ever (Intel Xeon E5-2699-v3 has 18 cores!) • Shared mutable state, and the contention between threads of execution vying to update them simultaneously, is a performance killer • Blocking • Locking Mutability Matrix of Pain 4
- 5. The JVM is awesome! • The power to leverage all cores on a box through a single, unified process • The first formal memory model, with well-defined semantics for visibility Mutability Matrix of Pain 5 Diagram by Juan Pablo Francisconi http://www.researchbeta.com/?p=334
- 6. The Java Memory Model • Two primary concepts: • Happens-Before – if A happens before B, the A's result should be visible to B • Synchronize-With – action results in write-through to RAM before next action Mutability Matrix of Pain 6
- 7. The Java Memory Model • There are no visibility guarantees between threads in a JVM without locks or memory barriers (volatile) • Something may end up being visible to another thread by pure happenstance • Contended locks are arbitrated at the kernel level, leading to expensive context switches Mutability Matrix of Pain 7
- 8. Concurrent and Parallel Mutability Matrix of Pain 8
- 9. Name Versus Value Mutability Matrix of Pain 9 • The name is the handle you (or the compiler) bind to a reference • The value is the actual state stored in the heap • Therefore, the name is a reference to a location on the heap Name Value val me = new Person("Jamie", "Allen”)
- 10. Name Versus Value Mutability Matrix of Pain 10 • For instance variables, a heap location for the class instance holds the reference via the name field • The value itself is located elsewhere (possibly contiguous, no guarantee)class Customer { val me = new Person("Jamie", "Allen") ... } me Person( , , ) String("Jamie") String("Allen")
- 11. Collections Mutability Matrix of Pain 11 • Collections are an aggregation of references to values • Can be lots of “pointer chasing” val me = new Person("Jamie", "Allen") val friend = new Person("Jane", "Doe") val us = List(me, friend) Person( , , ) Person( , ) us me friend String("Jamie") String("Allen") String("Jane") String("Doe")
- 12. Method-Scoped Name Versus Value Mutability Matrix of Pain 12 • For variables defined in a method, the reference to the name is on the stack frame • The value itself is still located elsewhere (possibly contiguous, no guarantee) def myMethod() = { val me = new Person("Jamie", "Allen") ... } me Person( , , ) String("Jamie") String("Allen")
- 13. Final Mutability Matrix of Pain 13 • If we make a reference final in Java, it cannot be reassigned to a new value • Easy to forget to use the keyword • A mutable value remains mutable despite the usage of the final keyword final Person me = new Person("Jamie", "Allen"); val me: Person = Person("Jamie", "Allen")
- 14. Mutability Matrix of Pain 14 The Mutability Matrix of Pain val var ImmutableMutable NAME VALUE @volatile
- 15. Implications of Immutability Mutability Matrix of Pain 15 • Uses more Eden/NewGen heap space • Resizing Eden/NewGen larger means longer stop the world pauses, may cause Survivor spillover directly into Tenured/OldGen • Measure the effects of attempts to tune GC at varying loads • Only optimize where you must, over 99% of Typesafe customers don't even try and are still wildly successful
- 16. Volatile is Your Friend Mutability Matrix of Pain 16 • Use snapshots when mutability must be attained, and publish updates via volatile with the last write • Readers should first read the one volatile field of multiple writes (reverse your order of access)
- 17. Caveat • Just because you're using volatile doesn't mean that publishing cannot happen before you expect • For greater consistency guarantees, volatile may not be appropriate Mutability Matrix of Pain 17
- 18. Use volatile! • Don't be mutable, but if you have to, stay in the snapshot world with volatile and avoid locks • Understand the visibility guarantees you need for your application, and consider the impact when you leave visibility to chance • You can be more specific with read and write barriers if you go with sun.misc.Unsafe Mutability Matrix of Pain 18
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