Fork/Join for Fun and Profit!

Fork/Join for Fun and Profit!

• 1.
  Fork/Join for Funand Profit! @Sander_Mak
• 2.
  What is theproblem anyway?
• 3.
  What is theproblem anyway?
• 4.
  What is theproblem anyway?
• 5.
  What is theproblem 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 ...
• 6.
  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
• 7.
  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)   } }
• 8.
  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>)   
• 9.
  ForkJoinPool Introducing:   java.uCl.concurrent.ForkJoinPool Worker 1 ‣ Implements ExecutorService ‣ Autosizing workers Worker 2 ‣ Double‐ended queue ‣ Workstealing algorithm Worker 3   
• 10.
  Sorting demo Mergesort
• 11.
  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
• 12.
  Work stealing Worker 1 Worker 2 Worker 3 ForkJoinPool
• 13.
  Speedups
• 14.
  What about threads? ‣Heavyweight (try starCng a million) ‣ Implicit dependencies between tasks ‣ Manual synchronizaCon ‣ Deadlock/livelock/race condiCons ‣ Hard to scale to available parallelism
• 15.
  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
• 16.
  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
• 17.
  Demo Sony’s been hacked... Are we compromised...?
• 18.
  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
• 19.
  Alternatives outside Java? ‣ Actors ‣ So^ware transacConal memory ‣ Dataflow concurrency ‣ Agents ‣ ... Some of this built on F/J Akka GPars
• 20.
  Criticism ‣ ImplementaCon too complex (uses Unsafe) ‣ Some assumpCons quesConable ‣ 1‐1 mapping workerthread/OS thread ‣ Workstealing best opCon? ‣ Scalability 100+ cores?
• 21.
  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
• 22.
  Future int scanLog(List<String> lines,String query) { Pattern p = ... return lines.parallel() .filter(s => p.matcher(s).matches()) .count(); }
• 23.
  ForkJoinPool.shutDownNow() Questions? Code @ bit.ly/jfall-forkjoin