lecture21-concurrency-4.md

Lecture 21 Introduction to concurrency, part4: In the trenches of parallelism

Policies for Thread Safety

• Thread-confined state - mutate but don't share
  • Stack-confined
    • Primitive local variables are never shared between threads
    • Fast and cheap
  • Unshared object references
    • The thread that creates an object must take action to share (“publish”)
  • Thread-local variables
    • Shared object with a separate value for each thread
    • Rarely needed but invaluable (e.g., for user ID or transaction ID)
• Shared read-only state - share but don't mutate
  • Immutable data is always safe to share
• Shared thread-safe - object synchronizes itself internally
  • Thread-safe objects that perform internal synchronization
  • Better use ones from java.util.concurrent
• Shared guarded - client synchronizes objects externally
  • Shared objects that must be locked by user
  • Can be error prone: burden is on user
  • High concurrency can be difficult to achieve
    • Lock granularity is the entire object
  • You’re generally better off avoiding guarded objects

Parallel Prefix Sums Algorithm

void iterativePrefixSums(long[] a) {
	int gap = 1;
	for (; gap < a.length; gap *= 2) {
	  parfor(int i = gap-1; i+gap < a.length; i += 2*gap) {
 			a[i+gap] = a[i] + a[i+gap];
	  }
	}
  for (; gap > 0; gap /= 2) {
    parfor(int i = gap-1; i < a.length; i += 2*gap) {
      a[i] = a[i] + ((i-gap >= 0) ? a[i-gap] : 0);
    }
  }
}

• Work: O(n)
• Span: O(ln n)

Fork/join in Java

• For a thread or more to do some work
• Join the threads to obtain the result of the work
• java.util.concurrent.ForkJoinPool class
  • implements ExecutorService
  • Executes java.util.concurrent.ForkJoinTask<V> or java.util.concurrent.RecursiveTask<V> or java.util.concurrent.RecursiveAction

The RecursiveAction Abstract Class

public class MyActionFoo extends RecursiveAction {
  public MyActionFoo(...) {
    store the data fields we need
  }
  @Override
  public void compute() {
    if (the task is small) {
      do he work here;
      return;
    }
    invokeAll(new MyActionFoo(...), // smaller
              new MyActionFoo(...). // subtasks
              ...)
  }
}