[Omp] schedule(static)

[ami marowka]

I think that you are absolutely right.

Ami


--- Dieter an Mey <anmey at rz.rwth-aachen.de> wrote:

> Hi,
> 
> Before the feedback phase for the 2.5 version of the
> OpenMP specs has 
> come to an end on Jan. 31 I'd like to start
> (continue) a discussion 
> about schedule(static).
> 
> During EWOMP2004 Mark Bull pointed out that
> according to the OpenMP 
> specifications the following toy program would
> contain a data race:
> 
> program example
>          integer, parameter :: n=11
>          real, dimension(n) :: a, b
> 
> !$omp parallel num_threads(2)
> !$omp do schedule(static)
>          do i = 1, n
>                  a(i) = 1.0
>          end do
> !$omp end do nowait
> 
> !$omp do schedule(static)
>          do i = 1, n
>                  b(i) = a(i) + 1.0
>          end do
> !$omp end do
> !$omp end parallel
> 
>          write(*,*) b
> end program example
> 
> The problem is the nowait clause. The draft V2.5
> specs say on page 34 
> line 11-18:
> 
> "When schedule(static, chunk_size) is specified,
> iterations are divided
> into chunks of size chunk_size, and the chunks are
> statically assigned 
> to threads in the team in a round-robin fashion in
> the order of the 
> thread number.
> Note that the last chunk to be assigned may have a
> smaller number of
> iterations.
> When no chunk_size is specified, the iteration space
> is divided into 
> chunks which are approximately equal in size, and
> such that each thread 
> is assigned at most one chunk."
> 
> The interpretation is that if the chunk_size would
> specified, the work 
> distribution is unambiguously defined and the nowait
> clause would be ok.
> 
> In the above example
> 	!$omp do schedule(static,6)
> would fix the problem.
> But without the specification of the chunk_size it
> might happen that
> 
> Thread 0 would have to execute
> 	a(1) = 1.0
> 	...
> 	a(6) = 1.0    ! <=======
> 	b(1) = a(1) + 1.0
> 	...
> 	b(5) = a(5) + 1.0
> 
> while thread 1 would get
> 
> 	a(7) = 1.0
> 	...
> 	a(11) = 1.0
> 	b(6) = a(6) + 1.0    ! <=======
> 	...
> 	b(11) = a(11) + 1.0
> 
> If thread 1 is quicker than thread 0, it would use
> the old value of 
> a(6), which is obiously not what is intended.
> 
> I suggest that the specs should be modified and that
> for a fixed number 
> of threads and a fixed number of iterations an
> identical schedule should 
> be used, even if the chunk_size is not specified
> explicitely.
> 
> I think that this difference between a static
> schedule with and without 
> an explicitely specified chunk_size is very
> misleading and dangerous.
> 
> One argument in favour of the current definition is
> the flexibility for 
> the compiler to determine the schedule depending on
> locality of the data 
> used in the parallel loop. I think that for this
> kind of affinity 
> scheduling another kind name than static should be
> added to the OpenMP 
> specs in a future release.
> 
> In fact I have been using the above construct
> heavily without any 
> problems so far on many OpenMP compilers. Also
> Assure, the famous OpenMP 
> verification tool, does not complain.
> 
> I would be curious to here your oppinion.
> 
> Dieter
> 
> -- 
>
--------------------------------------------------------------------
> Dieter an Mey
> High Performance Computing              
> Hochleistungsrechnen
> RWTH Aachen University                   Rechen- und
> Kommunikations-
> Center for Computing and Communication   zentrum der
> RWTH Aachen
> phone: ++49-(0)241-80-24377              Seffenter
> Weg 23
> fax:   ++49-(0)241-80-22134              52074
> Aachen, Germany
> email: anmey at rz.rwth-aachen.de
>
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> 
> 
> 
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