Re: [Boost-users] [Boost.thread] Simultaneously starting thread execution

From: Ruediger Berlich (ruediger.berlich_at_[hidden])
Date: 2008-08-06 04:53:43

Hi there,

o.k., being a physicist, I hope I can find the right words in this
computer-scientist-rich environment ;-)

Imagine an environment where m entities produce items ("producers") which
are processed by n workers ("consumers"), running as threads. The results
are then returned back to the original entities, which themselves run in
threads. Both m and n are not known in advance, as it is the user (or an
automated procedure) that determines how many producers and consumers there
will be. In the middle of this sits a broker, which takes care of all
communication between producers and consumers. Both need to register with
the broker before being able to communicate with each other.

I want to have as few restrictions for my users as possible, as I want my
library to be easy to use. Hence there is no rule when producers and
consumers may or may not be registered. However, my current setup makes it
desirable that consumer threads do not *start* before at least one producer
has been registered (they are started by the broker). Hence I need to have
a mechanism that blocks all consumers that have been registered before the
first producer, until such time that the first items to be processed are
available.

If the number of consumer threads would be known in advance, Boost's
Thread-Barrier might be a possible choice. As I do not know this number,
however, I'll probably have to do my own locking (see my sample
implementation).

Best,
Ruediger

Zeljko Vrba wrote:
> On Wed, Aug 06, 2008 at 01:36:19AM +0200, Ruediger Berlich wrote:
>> 
>> I'm trying to find a way to let a number of threads start execution
>> simultaneously, and in any case not before a certain condition has come
>> true.
>>
> Define "simultaneously".  If you have more threads than CPUs,  it should
> be obvious that, due to time-slicing, it is physically impossible that
> they start
> simultaneously.  Even with more CPUs than threads, this is achievable only
> in architecture-specific way and with instructions that are accessible
> only to supervisor-level code (e.g. Pentium's MONITOR instruction or
> broadcasted
> interprocessor interrupt -- if the latter exists at all).  Every other
> mechanism will incur some delay between start of different threads.
> 
>>
>> I also believe I do not have to use a condition
>> variable, as the different threads need to synchronize their access to a
>> data ressource anyway, and I can use the same mutex to block them in the
>> beginning.
>> 
> While one thread is in the critical section protected by the mutex, all
> others
> will wait until that thread exits the critical section.  This is hardly
> "simultaneous" start.
> 
>> 
>> One of the problems (which seems to be unrelated to the code below) is
>> that the three threads do not get called equally often. In one run, one
>> thread was called 14564 times, another 1607 times and the third one 8676
>> times.
>> 
> This is perfectly normal, and is not a "problem".  Threaded code that
> depends on relative speeds of threads (on a general-purpose OS) is calling
> for trouble.
> 
> Can you describe what are you trying to achieve?  Threads might not be the
> best solution..

-- 
Kind Regards / Mit freundlichen Gruessen,
Ruediger Berlich
------------------------------------------------------------------------
Dr. Ruediger Berlich               | E-mail: ruediger.berlich_at_[hidden]
Forschungszentrum Karlsruhe        | Web: http://ruediger.berlich.com
Steinbuch Centre for Computing     | Phone:  [phone]
Hermann-von-Helmholtz-Platz 1      | Fax:    [phone]
D-76344 Eggenstein-Leopoldshafen   | Mobile: [phone]
------------------------------------------------------------------------
/*************************************************************************/
The Steinbuch Centre for Computing of Karlsruhe Institute of Technology
(the Cooperation of Forschungszentrum Karlsruhe GmbH and Universitaet 
Karlsruhe TH) will run its annual GridKa School from September 8th to 
12th, 2008. Please find further information at http://www.fzk.de/gks08 .
/*************************************************************************/
Forschungszentrum Karlsruhe GmbH
Sitz: Weberstraße 5
76133 Karlsruhe
Amtsgericht Mannheim HRB 100302
Vorsitzende des Aufsichtsrats: Baerbel Brumme-Bothe
Vors. der Geschäftsführung: Prof. Dr. Eberhard Umbach