[Omp] Threadprivate variables and nested parallelism

[Federico Massaioli]

I'm confused too.
Shared and private attributes are local to a parallel region. Thus, a 
variable which is private on a parallel region can be made shared on a 
nested one.
OTOH, "threadprivateness" is a global attribute. How could a threadprivate 
variable made shared if is globally known to be threadprivate?

Federico


On Wed, 13 Dec 2006, James Beyer wrote:

> Ok, I am confused.  If you want all of the threads to share the same data why 
> is it in the threadprivate list?  Or are you talking about nested parallelism 
> where the outer parallel region has the data in threadprivate and the inner 
> parallel region has the data shared?  If this is the case I don't see why 
> there would be a problem with this.  Of course I may change my opinion as I 
> begin bringing full support for nested parallelism on line in our compiler.
>
> james
>
> Dieter an Mey wrote:
>> Well, you now, Larry, that we are happily profiting from nested 
>> parallelism, where tasks would not be the right solution.
>> 
>> And providing copies of large chunks of data for all threads of the inner 
>> teams can be a waste of memory...
>> 
>> But I understand that implementing the feature to share threadprivate data 
>> may be painful - but I am just a stupid user ...
>> 
>> Dieter
>> 
>> 
>> Marina Kraeva schrieb:
>>> Dieter and Lawrence,
>>> 
>>> Thank you for the clarification.
>>> 
>>> Lawrence, I feel your pain :)
>>> 
>>> Marina.
>>> 
>>> Meadows, Lawrence F wrote:
>>> 
>>>>  Dieter, I'm sure that your interpretation is correct. Every
>>>> thread gets its own copy.
>>>> 
>>>> So if it is "unfortunate" then you would want to be able to
>>>> make a threadprivate variable shared in an inner team (using
>>>> an explicit shared clause on the parallel directive, I
>>>> assume). I have no idea how hard this would be to implement
>>>> and I don't like it.
>>>> 
>>>> I wish we could just drop nested parallelism... and threads...
>>>> 
>>>> -----Original Message-----
>>>> From: omp-bounces at openmp.org [mailto:omp-bounces at openmp.org] On Behalf
>>>> Of Dieter an Mey
>>>> Sent: Tuesday, December 12, 2006 9:31 AM
>>>> To: Marina Kraeva
>>>> Cc: omp at openmp.org
>>>> Subject: Re: [Omp] Threadprivate variables and nested parallelism
>>>> 
>>>> My understanding is that all threads will get there own copy - even 
>>>> threads of the inner team (unfortunately).
>>>> 
>>>> Dieter
>>>> 
>>>> Marina Kraeva schrieb:
>>>> 
>>>>> Hello,
>>>>> 
>>>>> I could not find in API 2.5 what will happen if a threadprivate
>>>> 
>>>> variable
>>>>> is referenced within a nested parallel region, other than this:
>>>>> 
>>>>> The values of data in the threadprivate objects of threads other than 
>>>>> the initial thread are guaranteed to persist between two consecutive 
>>>>> active parallel regions only if all the following conditions hold:
>>>>> * Neither parallel region is nested inside another parallel region.
>>>>> 
>>>>> 
>>>>> I would think that each thread of the initial team (created for the 
>>>>> nesting parallel region) will create a new team, and the threadprivate
>>>> 
>>>> 
>>>>> copy of this thread will become shared among threads in the new team
>>>> 
>>>> in
>>>>> the nested parallel region (so there will be NT shared variables,
>>>> 
>>>> where
>>>>> NT is the number of threads in the nesting parallel region).
>>>>> 
>>>>> Am I correct assuming this? Can I count on this behavior or it's
>>>> 
>>>> undefined?
>>>> 
>>>>> Thank you,
>>>>> Marina Kraeva.
>>>>> _______________________________________________
>>>>> Omp mailing list
>>>>> Omp at openmp.org
>>>>> http://openmp.org/mailman/listinfo/omp
>>>>> 
>>>> 
>>>> 
>>> 
>> 
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Federico Massaioli                          CASPUR
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