Parallel Programming in Fortran 95 using OpenMP - People

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26 2. OpenMP constructs thread 0 thread 1 thread N p ❄ E xecution parallel region !$OMP BARRIER ❄ ✞ ☎ ❄✝ ❝ ✆ ... ❄ ❄ ❄ ❄ Figure 2.6: Graphical representation of the effect of the !$OMP BARRIER directive on a team of threads. 2.3.4 !$OMP ATOMIC When a variable in use can be modified from all threads in a team, it is necessary to ensure that only one thread at a time is writing/updating the memory location of the considered variable, otherwise unpredictable results will occur. In the following example !$OMP DO do i = 1, 1000 a = a + i enddo !$OMP END DO the desired result would be that a contains the sum of all values of the do-loop variable i, but this will not be the case. The present directive targets to ensure that a specific memory location is updated atomically, rather than exposing it to the possibility of multiple, simultaneously writing threads. Its syntaxis is very simple: !$OMP ATOMIC This directive only affects the immediately following statement. Not all possible statements are allowed, since most cannot be treated in an atomic way. Only the following ones can be used together with the !$OMP ATOMIC directive:
2.3. Synchronization constructs 27 x = x operator expr x = intrinsic_procedure(x, expr_list) where again not all existing operators and intrinsic procedures are allowed, only: operator = +, *, -, /, .AND., .OR., .EQV. or .NEQV. intrincic_procedure = MAX, MIN, IAND, IOR or IEOR The variable x, affected by the !$OMP ATOMIC directive, must be of scalar nature and of intrinsic type. Obviously, the expression expr must also be a scalar expression. All these limitations (and a few more appearing in the following explanations) are necessary to ensure, that the statement can be treated in an atomic way and that this is done in an efficient manner. Later on, a few additional words are going to be said about the latter aspect. Only the load and store of the variable x are atomic. This means that the evaluation of expr can take place simultaneously in all threads. This feature, which leads to faster executing codes, limits the nature of expr, since it cannot make use of the value of x! It is possible to emulate the effect of the !$OMP ATOMIC directive using the !$OMP CRITICAL/!$OMP END CRITICAL directive-pair as follows: xtmp = expr !$OMP CRITICAL x x = x operator xtmp !$OMP END CRITICAL x The reason for existence of the present directive is to allow optimizations beyond those possible with the !$OMP CRITICAL/!$OMP END CRITICAL directive-pair. It is left to the OpenMP-implementation to exploit this. Therefore, the gain achieved by using the !$OMP ATOMIC directive will be OpenMP-implementation dependent. 2.3.5 !$OMP FLUSH This directive, whether explicit or implied, identifies a sequence point at which the implementation is required to ensure that each thread in the team has a consistent view of certain variables in memory: the same correct value has to be seen by all threads. This directive must appear at the precise point in the code at which the data synchronization is required. At a first glance it seems that this directive is not necessary, since in general the writing/updating of shared variables has been done in such a way, that only one thread at a time is allowed to do that. But this is only true in theory, because what the OpenMPimplementation does to simulate the ”one thread at a time” feature is not specified by the OpenMP specification. This is not a fault, moreover it is a door left open to the OpenMP-implementations so that they can try to optimize the resulting code. As an example of this, in the following do-loop
Page 1: Parallel Programming in Fortran 95
Page 4 and 5: ii CONTENTS 3.1.4 FIRSTPRIVATE(list
Page 6 and 7: 2 1. OpenMP Fortran Application Pro
Page 14 and 15: 10 2. OpenMP constructs • Work-sh
Page 16 and 17: 12 2. OpenMP constructs real(8) ::
Page 18 and 19: 14 2. OpenMP constructs do k = 1, 1
Page 20 and 21: 16 2. OpenMP constructs !$OMP SECTI
Page 22 and 23: 18 2. OpenMP constructs thread 0 th
Page 24 and 25: 20 2. OpenMP constructs • If the
Page 26 and 27: 22 2. OpenMP constructs 2.3 Synchro
Page 28 and 29: 24 2. OpenMP constructs thread 0 th
Page 32 and 33: 28 2. OpenMP constructs !$OMP DO do
Page 34 and 35: 30 2. OpenMP constructs 2.3.6 !$OMP
Page 36 and 37: 32 2. OpenMP constructs !$OMP END D
Page 38 and 39: 34 2. OpenMP constructs Shared memo
Page 40 and 41: 36 2. OpenMP constructs
Page 42 and 43: 38 3. PRIVATE, SHARED & Co. Shared
Page 46 and 47: 42 3. PRIVATE, SHARED & Co. In this
Page 50 and 51: 46 3. PRIVATE, SHARED & Co. Operato
Page 52 and 53: 48 3. PRIVATE, SHARED & Co. it is n
Page 54 and 55: 50 3. PRIVATE, SHARED & Co. DYNAMIC
Page 56 and 57: 52 3. PRIVATE, SHARED & Co. Pieces
Page 58 and 59: 54 3. PRIVATE, SHARED & Co.
Page 60 and 61: 56 4. The OpenMP run-time library T
Page 62 and 63: 58 4. The OpenMP run-time library d
Page 64 and 65: 60 4. The OpenMP run-time library 4
Page 66 and 67: 62 4. The OpenMP run-time library T
Page 68 and 69: 64 4. The OpenMP run-time library I
Page 70 and 71: 66 4. The OpenMP run-time library 4
Page 72 and 73: 68 4. The OpenMP run-time library i
Page 74 and 75: 70 5. The environment variables 5.1

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