Re: [boost] [threadpool] sub_tasks and active waiting

Subject: Re: [boost] [threadpool] sub_tasks and active waiting
From: Anthony Williams (anthony.ajw_at_[hidden])
Date: 2008-09-26 16:41:57

k-oli_at_[hidden] writes:

> Am Freitag, 12. September 2008 08:17:55 schrieb vicente.botet:
>> Well, we can see that the number worker threads depends on the fib_par
>> parameter. As soon as the fib_par function waits on its sub_tasks t1 and
>> t2, the worker thread cannot handle other tasks, so we need a thread
>> available for fib_par(9) and fib_par(8). When this worker thread call
>> fib_par(9) after launching fib_par(8) and fib_par(7) will be blocked to
>> manage other task, ... Evidently this do not scale very well, so we can not
>> use the threadpool library to address this kind of problems, in which the
>> tasks is split on sub-task launched to be run in parallel, and the the
>> parennt task waits on its child sub-task to do somthing.
>
> The problem is caused by this code:
>  return t1.get_future().get() + t2.get_future().get();
> Because it blocks until t1 and t2 are ready -> dead lock possible.

Not if the thread pool can detect this scenario and handle it. For
example, my prototype can handle

void recursive_task(jss::thread_pool* pool,int level,int count)
{
    {
        std::lock_guard<std::mutex> lk(iom);
        std::cout<<"level "<<level<<", count"<<count<<std::endl;
    }
    
    if(level)
    {
        jss::unique_future<void> f1(pool->submit_task_returning<void>(boost::bind(boost::type<void>(),&recursive_task,pool,level-1,1)));
        jss::unique_future<void> f2(pool->submit_task_returning<void>(boost::bind(boost::type<void>(),&recursive_task,pool,level-1,2)));
        jss::unique_future<void> f3(pool->submit_task_returning<void>(boost::bind(boost::type<void>(),&recursive_task,pool,level-1,3)));

        f1.get();
        f2.get();
        f3.get();
    }
}

just fine, even with only one worker thread. Of course, if you nest
too deeply it runs out of stack space :-(

Anthony
-- 
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