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Chapter 7. Interprocess communication 73

7.4.3 Point-to-point communication

Point-to-point communication is the most widely occuring form of data

transfer in parallel computing in shared-memory computers. There are

two basic routines for this purpose in the SHMEM library, shmem_get

and shmem_put. Since SHMEM routines are one-sided, only one of these

is needed to transfer data.

shmem_get

The C language syntax of a call to shmem_get is

void shmem_get(void *target, void *source, int len, int pe);

For Fortran 90, the syntax is

INTEGER :: len, pe

CALL SHMEM_GET(target, source, len, pe)

The routine shmem_get copies data of length len (in 8-byte words) from

the memory of PE pe. The copying is started at address source in this

PE, and the data is moved to the address target in the calling PE’s

memory. The calling PE is blocked during the transfer, i.e., it waits until

the transfer is ﬁnished before moving on to the next command in the

program.

shmem_put

The C syntax of a call to shmem_put is

void shmem_put(void *target, void *source, int len, int pe);

For Fortran 90 the syntax is

INTEGER :: len, pe

CALL SHMEM_PUT(target, source, len, pe)

The routine shmem_put copies data of length len (in 8-byte words) from

the memory of the calling PE starting at address source to address

target in the memory of PE pe. The calling PE is not blocked during

the transfer, but it continues with the program as soon as the transfer

is initiated. Therefore, the programmer should consider using some

synchronizing routine such as shmem_wait.

In terms of eﬃciency, there is not much diﬀerence between shmem_put

and shmem_get. For faster performance one should try to concentrate

data transfers into as few and large blocks of data as possible. Having

len a multiple of 8 is optimal for these routines.

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