[Omp] schedule(static)

[Bronis R. de Supinski]

Dieter:

Re:
>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).

I agree that the example illustrates a less-than-perfect
aspect of the specification. However, I feel this issue 
should be deferred until the 3.0 specification.

The problem is that the specification is not really broken
for the example. If you do not specify the chunksize in the
two loops and you use the nowait clause then you are stating 
that you do not care which thread executes which chunks. In
the example, a dependence between the two loops makes that
statement incorrect. That Assure does not detect it indicates
a limitation of that (generally excellent) tool.

I think what you are really looking for here is the ability to
"name" schedules and then reuse them on multiple loops. I can
envision 2 or 3 ways to provide this capability. This is a general 
capability that would be a fairly simple and useful extension to 
the existing specifications (essentially a fairly straightforward
way to provide data affinity). However, the charter of the 2.5 
specification precludes extensions so the issue should be deferred.

At least that's my opinion.

Bronis


>
>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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