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76 Cray T3E User’s Guide

7.4.5 Other important routines

There are two very important routines which a parallel program on T3E

will almost certainly call, namely shmem_my_pe and shmem_n_pes.The

former reveals the calling PE its identity, while the latter returns the

total number of PEs in use. Their syntaxes are as follows:

int shmem_my_pe(void);

int shmem_n_pes(void);

For Fortran 90 the syntax is

INTEGER :: mype, npes

INTEGER, EXTERNAL :: SHMEM_MY_PE, SHMEM_N_PES

mype = SHMEM_MY_PE()

npes = SHMEM_N_PES()

In some cases it may be necessary to stop the execution of a program

and wait until all other PEs have performed some critical tasks. For

these situations, there is a routine called shmem_barrier. However, on

the T3E there is a simpler (and faster!) routine for this purpose called

simply barrier:

void barrier()

In Fortran 90 the call is

CALL BARRIER()

The most important diﬀerence between shmem_barrier and barrier

is that with shmem_barrier it is possible to interrupt temporarily the

action of just some of the PEs in use. Because of its speed, we suggest

that the barrier routine be preferred whenever all PEs are halted. See

man shmem_barrier for more information.

7.4.6 Example of using SHMEM

The following simple code illustrates the use of some of the routines

discussed above. Each PE has two four-component vectors of integers,

called source and target, and PE number 0 copies its source vector

into the location of target vector of PE number 1. Finally, target vector

is used to compute the values of vector c.

Note that in the Fortran 90 version the attribute SAVE is assigned to

both source and target vectors in order to make them symmetric data

objects. The call to barrier routine is necessary, because without it,

PE number 1 might use the original target vector while computing the

value of c, not the one passed by PE number 0.

Here is the example program in Fortran 90:

PROGRAM shmemex

IMPLICIT NONE

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