6. Recursive, Pools

Recursive

Ex.: Sum calculation

• Split recursively, starting two threads from one
• Stop at some cutoff
• Join and sum

Avoids synchronous summing of thread results.

Total work: $\mathcal{O}(\log n)$

More efficient: reuse one thread to sum one half, use other to sum other.

Executor Service

java.util.concurrent.ExecutorService

• Executors.newFixedThreadPool(n)
• ExecutorService.submit(Runnable)
  • Returns Future<Value>. Get with wait thru Future.get()
• ExecutorService.shutdown()

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map and reduction