Hello, ----- Original Message ----- From: "Kowalke Oliver (QD IT PA AS)" <Oliver.Kowalke@qimonda.com> To: <boost@lists.boost.org> Sent: Monday, June 09, 2008 1:53 PM Subject: Re: [boost] [thread-pool/futures] Explicit yielding
For what would be 'task-stealing' be usefull? Wouldn't be one task-queue on which worker threads are waiting be sufficient?
It may be sufficient, but it can be ineficient.
Polling of one thread in task-queues of other threads seams to be expensive.
You are right, the main raison this is expensive is that we need to synchronize the access to this shared resource. But this is already the case when we have only one task-queue. I think that the quiz is that we need to manage the root-task(without parent) and the sub-task(with a parent task) differently. root-task must be handle in fifo order, but sub-tsaks are needed to complete other tasks, so they need to be handled in lifo order. What happens if we have only one queue? We need to synchronize the access to the single queue for every task, and as you say this is expensive. So we need to avoid to synchronize with other threads as much as possible. What happens if a task needs to wait on futures provided by other subtasks? We can make the worker thread wait directly on this future blocking one thread, or we can try to execute other tasks. As very often a task is waiting for futures provided by task launched by the task itself, it will be better that these subtasks run on the same worker thread (this avoid thread switching). In order to do that we need a stack of subtasks. But even in this case the worker thread would have is stack of task empty, and here he can just take a task from the queue of other threads
Johan Torp:
Does existing threadpools (such as java.util.concurrency.ExecutorService) offer thread re-use and if so is it considered an success?
Doug Lea says that ThreadPoolExecutor doesn't steal threads, but the upcoming ForkJoin framework does. ForkJoin is described in
http://gee.cs.oswego.edu/dl/papers/fj.pdf
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