----- Original Message ----- From: "Anthony Williams" <anthony.ajw@gmail.com> To: <boost@lists.boost.org> Sent: Monday, November 03, 2008 12:09 PM Subject: Re: [boost] [threadpool] new version v12
"Oliver Kowalke" <k-oli@gmx.de> writes:
But what about executing fibered tasks only in the worker-thread which has created the fiber?
There is still a problem (just different).
Then a new task is dequeued from the local worker-queue, then from the global -queue and then from worker-queues of other threads.
Here's the problem: if you take a task from the global queue or the worker queue of another thread and *don't* do thread migration, then you risk delaying a task and potentially deadlocking the pool.
Consider the sequence above: at step 5, task A is blocked, and the only task it is waiting for (B) has been picked up by an idle worker thread. The thread running task A now takes a new task from the global queue (task N). Suppose this is a really long-running task (calculate PI to 3 million digits, defrag a hard disk, etc.). Without thread migration, task A cannot resume until task N is complete.
Obviously, if task N blocks on a future you can then reschedule task A, but if it doesn't then you don't get that opportunity. If task N waits for another event to be triggered from task A (e.g. a notify on a condition variable) then it will never get it because task A is suspended, and so the pool thread will deadlock *even when task A is ready to run*.
Obiously this sequence justify that a task could need to migrate to another thread. It seems that task scheduling is not as easy as I thought. Thanks for draw forth this issue. Vicente