Fork/Join for Fun and Profit!
@Sander_Mak

What is the problem anyway?

What is the problem anyway?

What is the problem anyway?

What is the problem anyway?
‣ So let the Compiler figure out the hard stuff!
if (n < 2) n else fib(n - 1) + fib(n - 2)
n < 2
n
‣ Or maybe not ...

Fork/Join in pictures
Fork:
Recursively decompose
Result
large task into subtasks
Task 1
Join: Task 2 Task 3
Await results of
recursive tasks Task 4 Task 5 Task 6 Task 7
and combine

Fork/Join in pseudocode
compute(problem) {
if (problem.size < threshold)
directlySolve(problem)
else {
do-forked {
leftResult = compute(left(problem))
rightResult = compute(right(problem))
}
join(leftResult, rightResult)
return combine(leftResult, rightResult)
}
}

ForkJoinPool
Introducing:
java.uCl.concurrent.ForkJoinPool
ForkJoinTask
java.uCl.concurrent.ForkJoinTask
RecursiveAction RecursiveTask
ForkJoinPool:
void execute (ForkJoinTask<?>)
T invoke (ForkJoinTask<T>)
ForkJoinTask<T> submit (ForkJoinTask<T>)

ForkJoinPool
Introducing:
java.uCl.concurrent.ForkJoinPool Worker 1
‣ Implements ExecutorService
‣ Autosizing workers Worker 2
‣ Double‐ended queue
‣ Workstealing algorithm
Worker 3

Sorting demo
Mergesort

ForkJoinTasks
‣ 100 < ‘basic computaConal steps’ < 10.000
‣ Acyclic, typically decreasing in size
‣ Join doesn’t block thread!
‣ Do:
‣ OpCmize sequenCal threshold
‣ Share, don’t copy input (task locality)
‣ Don’t:
‣ Synchronize/lock (but use: Phaser)
‣ Do blocking I/O

Work stealing
Worker 1 Worker 2 Worker 3
ForkJoinPool

Speedups

What about threads?
‣ Heavyweight (try starCng a million)
‣ Implicit dependencies between tasks
‣ Manual synchronizaCon
‣ Deadlock/livelock/race condiCons
‣ Hard to scale to available parallelism

Pooling/ExecutorService then?
‣ ForkJoinPool implements ExecutorService
‣ Coarse‐grained independent tasks
‣ Recursively decomposed tasks spend most Cme
waiCng
‣ In normal threadpool: starvaCon
‣ Task‐queue of threadpool‐backed ExecutorService not
opCmized for many small tasks
‣ No workstealing

Map/Reduce?
Environment Single JVM Cluster
Model Recursive forking O^en single map
Scales with Cores/CPUs Nodes
Worker Workstealing No inter‐node
interacDon communicaCon

Demo
Sony’s been hacked...
Are we compromised...?

Fork/Join and
‣ ForkJoinPool starts threads
‣ Illegal in EJBs
‣ Fair game in servlets/CDI beans
‣ Don’t create ForkJoinPool for each request
‣ Idea: WorkManager to create poolthreads
‣ Single pool, async submit(ForkJoinTask)
‣ Don’t Ce up request thread: Servlet 3.0

Alternatives outside Java?
‣ Actors
‣ So^ware transacConal memory
‣ Dataflow concurrency
‣ Agents
‣ ... Some of this built on F/J
Akka GPars

Criticism
‣ ImplementaCon too complex (uses Unsafe)
‣ Some assumpCons quesConable
‣ 1‐1 mapping workerthread/OS thread
‣ Workstealing best opCon?
‣ Scalability 100+ cores?

Future
GPU?
Java 8 OpenCL?
Java 7
Parallel
Java 5 Collections
Fork/Join
Java
1.0 - 1.4 - j.u.concurrent:
high-level locks,
concurrent coll.
- threads
- synchronized
- volatile

Future
int scanLog(List<String> lines, String query) {
Pattern p = ...
return lines.parallel()
.filter(s => p.matcher(s).matches())
.count();
}

ForkJoinPool.shutDownNow()
Questions?
Code @ bit.ly/jfall-forkjoin