Re: [Boost-Users] Re: Newbie needs pointer to currently executing thread.

From: dick.bridges_at_[hidden]
Date: 2002-04-10 10:00:14

I'm implementing the Leader/Followers thread pool pattern - reference
"Pattern-Oriented Software Architecture Volume 2" (Schmidt, et al).

When a thread is released via notify_one(), it is [assumed to be] in the
"leader" role and stores it's id.  A sanity check should be performed to
verify that there is no other thread in the leader role (i.e.,
current_leader is "none") before the thread stores it's id there.

When one of the event sources is signaled and the leader is unblocked, it
must, among other tasks, demote itself (i.e., set the current_leader value
to "none")
and promote one of the followers by notify_one() before continuing to
service the request.  Because of the potential for race conditions,
(especially in the context of future program updates, modifications, etc.)
I should enforce the rule that only the leader can promote a follower.
Something along the lines of:  current_leader == Thread::this().

                                                                                                                                        
                      "dmoore99atwork"                                                                                                  
                      <dmoore_at_ashtontec         To:      Boost-Users_at_[hidden]                                                    
                      hgroup.com>               cc:                                                                                     
                                                Subject: [Boost-Users] Re: Newbie needs pointer to currently executing thread.          
                      04/09/2002 06:53                                                                                                  
                      PM                                                                                                                
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                      Boost-Users                                                                                                       
                                                                                                                                        
                                                                                                                                        

--- In Boost-Users_at_y..., dick.bridges_at_t... wrote:
> Threads in a thread_group - derived pool are waiting on a condition.
> Condition gets notify_one().  How do I get id/pointer for the
thread that
> was released?  I've got a feeling I'm gonna be *REAL* embarrassed
when I
> find out, but I can't figure it out.
>
> Thanks for any help.

Dick,

In general, condition variables (both the Boost.Threads version and
the POSIX version) don't tell the "notifier" which thread woke up.

That's not necessarily a problem, though.  A thread pool usually
implies a set of threads that are equivalent in some way, even
running the same thread function.

Referring to the example at:
http://www.boost.org/libs/thread/doc/condition.html#Example,

Take a look the the receive() function.

while (buffered == 0)
   buffer_not_empty.wait(lk);

You might have three threads which reach this point when buffered==0
(meaning the buffer is empty).   When a new sender in a fourth thread
adds something to the buffer and calls notify_one(), exactly one of
the three threads waiting on the condition will wake up and do some
work.  You won't care which thread wakes up, because any one of them
will be prepared to do the necessary work.

If this doesn't get you unstuck, please post more details, in
particular why you feel it is important to know which thread "woke up"

Regards,
Dave

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