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Subject: Re: [boost] [task]
From: k-oli_at_[hidden]
Date: 2009-10-02 12:59:10

First - boost.task is the successor of boost.threadpool and uses the wait-
notifications too (but doesn't wait infinit because some co-worker-thread could
have produced additonal tasks which could be stolen).

Second - I'm finishing boost.task-0.4.0 which replaces
this_task::reschedule_until() by this_task::block().
block() suspends the current task without blocking the worker-thread. The
blocked task gets rescheduled by the internal scheduler (returning from

while ( ! condition)

Version 0.4.0 allows to synchronize or exchange messages between the tasks
without deadlocking the threadpool (that means amount of tasks >> worker-
threads in pool).


Am Freitag 02 Oktober 2009 14:06:13 schrieb Baxter Jeremy:
> I have been looking at the boost task library from the vault and
> comparing it with the threadpools library
> and have a few comments.
> Firstly I should say that I'm interested in tasks / thread pools for a
> situation rather different from the one for which they are designed.
> Namely the case in which threads (tasks) are being used to monitor some
> internal or external activities and so may block for significant periods
> of time waiting for the result of a complex planning algorithm or for
> some condition being met.
> I'm therefore worried about blocking all the available threads. I think
> what I need is to be able to dynamically add new threads to the thread
> pool, or just not use a pool at all and create and destroy a thread for
> every subtask.
> However during my review I came across a helper function which I though
> might allow re-use of the same thread but I'm concerned could actually
> lead to deadlock:
> reschedule_until
> <file:///D:\Boost\boost.task-0.3.0\doc\html\boost_task\utilities.html>
> In the function boost::this_task::reschedule_until( Pred const&) allows
> to synchronize the task with other asynchronous events without blocking
> the worker-threads (bool Pred::operator()() must not block). The current
> task will be rescheduled until the passed predicate becomes true.
> Looking at the code I don't think this is an accurate description and
> gives the potential to introduce deadlocks in tasks which otherwise
> would operate normally.
> >>From the code what reschedule_until does is repeatedly call the process_
> function to look for new tasks, testing once each task has finished to
> see if the predicate has been met. This is not quite as described since
> the task is not 're-scheduled' it is actually blocked until whatever
> task the scheduler next selects is complete.
> This introduces problems if multiple tasks on the queue use
> reschedule_until. If there are three tasks T1 T2 and T3 on the queue
> doing the following:
> T1
> {
> ...
> Reschedule_until(A)
> B.setTrue()
> ...
> }
> T2
> {
> ...
> Reschedule_until(B)
> ...
> }
> T3
> {
> ...
> A.setTrue()
> }
> What we would like to happen, and what will happen if they are run in
> different threads (or a different order on the one thread) is that T1
> does some work, waits for T3 then terminates. T2 does some work, waits
> for T1, then does some more work. T3 independently does its work and
> finishes by signalling that T1 can continue.
> Unfortunately if they are scheduled in the order T1 T2 T3 then T1 runs,
> reschedules and T2 gets to run until it reschedules and T3 gets to run.
> When T3 finishes T1 is in a position when it can run, however it will
> not get the chance to because in order for condition A to be checked T2
> has to have finished, and it can't do that until B is set true.
> Therefore although progression should be possible as the condition T1 is
> waiting for has been met it will never notice and T1 and T2 will never
> complete.
> I'm not sure that there is a general solution to this problem without
> using more threads so that any potentially runnable task will get a
> chance.
> A second comment.
> Comparing the task and thread pool libraries they use different waiting
> mechanisms. Thread pool uses a wait-notify construct on the queue to
> block worker threads if the queue is empty. Task uses active polling,
> trying to get items from the queue and then sleeping for a short period
> if nothing can be found on the queue. I wondered why the difference is
> there. Wait - notify would look like it should perform better in the
> case where there are fewer tasks than threads is polling better in the
> case where there are many more tasks than threads and so threads will
> never sit idle polling for long?
> Jeremy
> Jeremy Baxter
> Lead Researcher, UAVs & Autonomy
> Tel: 01684 894801
> email: jwbaxter_at_[hidden]
> QinetiQ - Delivering customer focused solutions
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