INTRO_SHMEM(3) Open MPI INTRO_SHMEM(3)
NAME
intro_shmem - Introduction to the OpenSHMEM programming model
DESCRIPTION
The SHMEM programming model consists of library routines that provide
low-latency, high-bandwidth communication for use in highly paral-
lelized scalable programs. The routines in the OpenSHMEM application
programming interface (API) provide a programming model for exchanging
data between cooperating parallel processes. The resulting programs are
similar in style to Message Passing Interface (MPI) programs. The SHMEM
API can be used either alone or in combination with MPI routines in the
same parallel program.
An OpenSHMEM program is SPMD (single program, multiple data) in style.
The SHMEM processes, called processing elements or PEs, all start at
the same time and they all run the same program. Usually the PEs per-
form computation on their own subdomains of the larger problem and pe-
riodically communicate with other PEs to exchange information on which
the next computation phase depends.
The OpenSHMEM routines minimize the overhead associated with data
transfer requests, maximize bandwidth and minimize data latency. Data
latency is the period of time that starts when a PE initiates a trans-
fer of data and ends when a PE can use the data. OpenSHMEM routines
support remote data transfer through put operations, which transfer
data to a different PE, get operations, which transfer data from a dif-
ferent PE, and remote pointers, which allow direct references to data
objects owned by another PE. Other operations supported are collective
broadcast and reduction, barrier synchronization, and atomic memory op-
erations. An atomic memory operation is an atomic read-and-update oper-
ation, such as a fetch-and-increment, on a remote or local data object.
OPENSHMEM ROUTINES
This section lists the significant OpenSHMEM message-passing routines.
PE queries
* C/C++ only:
* _num_pes(3)
* _my_pe(3)
* Fortran only:
* NUM_PES(3)
* MY_PE(3)
Elemental data put routines
* C/C++ only:
* shmem_double_p(3)
* shmem_float_p(3)
* shmem_int_p(3)
* shmem_long_p(3)
* shmem_short_p.(3)
Block data put routines
* C/C++ and Fortran:
* shmem_put32(3)
* shmem_put64(3)
* shmem_put128(3)
* C/C++ only:
* shmem_double_put(3)
* shmem_float_put(3)
* shmem_int_put(3)
* shmem_long_put(3)
* shmem_short_put.(3)
* Fortran only:
* shmem_complex_put(3)
* shmem_integer_put(3)
* shmem_logical_put(3)
* shmem_real_put(3)
Elemental data get routines
* C/C++ only:
* shmem_double_g(3)
* shmem_float_g(3)
* shmem_int_g(3)
* shmem_long_g(3)
* shmem_short_g(3)
Block data get routines
* C/C++ and Fortran:
* shmem_get32(3)
* shmem_get64(3)
* shmem_get128(3)
* C/C++ only:
* shmem_double_get(3)
* shmem_float_get(3)
* shmem_int_get(3)
* shmem_long_get(3)
* shmem_short_get(3)
* Fortran only:
* shmem_complex_get(3)
* shmem_integer_get(3)
* shmem_logical_get(3)
* shmem_real_get(3)
Strided put routines
* C/C++ and Fortran:
* shmem_iput32(3)
* shmem_iput64(3)
* shmem_iput128(3)
* C/C++ only:
* shmem_double_iput(3)
* shmem_float_iput(3)
* shmem_int_iput(3)
* shmem_long_iput(3)
* shmem_short_iput(3)
* Fortran only:
* shmem_complex_iput(3)
* shmem_integer_iput(3)
* shmem_logical_iput(3)
* shmem_real_iput(3)
Strided get routines
* C/C++ and Fortran:
* shmem_iget32(3)
* shmem_iget64(3)
* shmem_iget128(3)
* C/C++ only:
* shmem_double_iget(3)
* shmem_float_iget(3)
* shmem_int_iget(3)
* shmem_long_iget(3)
* shmem_short_iget(3)
* Fortran only:
* shmem_complex_iget(3)
* shmem_integer_iget(3)
* shmem_logical_iget(3)
* shmem_real_iget(3)
Point-to-point synchronization routines
* C/C++ only:
* shmem_int_wait(3)
* shmem_int_wait_until(3)
* shmem_long_wait(3)
* shmem_long_wait_until(3)
* shmem_longlong_wait(3)
* shmem_longlong_wait_until(3)
* shmem_short_wait(3)
* shmem_short_wait_until(3)
* Fortran:
* shmem_int4_wait(3)
* shmem_int4_wait_until(3)
* shmem_int8_wait(3)
* shmem_int8_wait_until(3)
Barrier synchronization routines
* C/C++ and Fortran:
* shmem_barrier_all(3)
* shmem_barrier(3)
Atomic memory fetch-and-operate (fetch-op) routines
* C/C++ and Fortran:
* shmem_swap
Reduction routines
* C/C++ only:
* shmem_int_and_to_all(3)
* shmem_long_and_to_all(3)
* shmem_longlong_and_to_all(3)
* shmem_short_and_to_all(3)
* shmem_double_max_to_all(3)
* shmem_float_max_to_all(3)
* shmem_int_max_to_all(3)
* shmem_long_max_to_all(3)
* shmem_longlong_max_to_all(3)
* shmem_short_max_to_all(3)
* shmem_double_min_to_all(3)
* shmem_float_min_to_all(3)
* shmem_int_min_to_all(3)
* shmem_long_min_to_all(3)
* shmem_longlong_min_to_all(3)
* shmem_short_min_to_all(3)
* shmem_double_sum_to_all(3)
* shmem_float_sum_to_all(3)
* shmem_int_sum_to_all(3)
* shmem_long_sum_to_all(3)
* shmem_longlong_sum_to_all(3)
* shmem_short_sum_to_all(3)
* shmem_double_prod_to_all(3)
* shmem_float_prod_to_all(3)
* shmem_int_prod_to_all(3)
* shmem_long_prod_to_all(3)
* shmem_longlong_prod_to_all(3)
* shmem_short_prod_to_all(3)
* shmem_int_or_to_all(3)
* shmem_long_or_to_all(3)
* shmem_longlong_or_to_all(3)
* shmem_short_or_to_all(3)
* shmem_int_xor_to_all(3)
* shmem_long_xor_to_all(3)
* shmem_longlong_xor_to_all(3)
* shmem_short_xor_to_all(3)
* Fortran only:
* shmem_int4_and_to_all(3)
* shmem_int8_and_to_all(3)
* shmem_real4_max_to_all(3)
* shmem_real8_max_to_all(3)
* shmem_int4_max_to_all(3)
* shmem_int8_max_to_all(3)
* shmem_real4_min_to_all(3)
* shmem_real8_min_to_all(3)
* shmem_int4_min_to_all(3)
* shmem_int8_min_to_all(3)
* shmem_real4_sum_to_all(3)
* shmem_real8_sum_to_all(3)
* shmem_int4_sum_to_all(3)
* shmem_int8_sum_to_all(3)
* shmem_real4_prod_to_all(3)
* shmem_real8_prod_to_all(3)
* shmem_int4_prod_to_all(3)
* shmem_int8_prod_to_all(3)
* shmem_int4_or_to_all(3)
* shmem_int8_or_to_all(3)
* shmem_int4_xor_to_all(3)
* shmem_int8_xor_to_all(3)
Broadcast routines
* C/C++ and Fortran:
* shmem_broadcast32(3)
* shmem_broadcast64(3)
Cache management routines
* C/C++ and Fortran:
* shmem_udcflush(3)
* shmem_udcflush_line(3)
Byte-granularity block put routines
* C/C++ and Fortran
* shmem_putmem(3)
* shmem_getmem(3)
* Fortran only:
* shmem_character_put(3)
* shmem_character_get(3)
Collect routines
* C/C++ and Fortran:
* shmem_collect32(3)
* shmem_collect64(3)
* shmem_fcollect32(3)
* shmem_fcollect64(3)
Atomic memory fetch-and-operate (fetch-op) routines
* C/C++ only:
* shmem_double_swap(3)
* shmem_float_swap(3)
* shmem_int_cswap(3)
* shmem_int_fadd(3)
* shmem_int_finc(3)
* shmem_int_swap(3)
* shmem_long_cswap(3)
* shmem_long_fadd(3)
* shmem_long_finc(3)
* shmem_long_swap(3)
* shmem_longlong_cswap(3)
* shmem_longlong_fadd(3)
* shmem_longlong_finc(3)
* shmem_longlong_swap(3)
* Fortran only:
* shmem_int4_cswap(3)
* shmem_int4_fadd(3)
* shmem_int4_finc(3)
* shmem_int4_swap(3)
* shmem_int8_swap(3)
* shmem_real4_swap(3)
* shmem_real8_swap(3)
* shmem_int8_cswap(3)
Atomic memory operation routines
* Fortran only:
* shmem_int4_add(3)
* shmem_int4_inc(3)
Remote memory pointer function
* C/C++ and Fortran:
* shmem_ptr(3)
Reduction routines
* C/C++ only:
* shmem_longdouble_max_to_all(3)
* shmem_longdouble_min_to_all(3)
* shmem_longdouble_prod_to_all(3)
* shmem_longdouble_sum_to_all(3)
* Fortran only:
* shmem_real16_max_to_all(3)
* shmem_real16_min_to_all(3)
* shmem_real16_prod_to_all(3)
* shmem_real16_sum_to_all(3)
Accessibility query routines
* C/C++ and Fortran:
* shmem_pe_accessible(3)
* shmem_addr_accessible(3)
Symmetric Data Objects
Consistent with the SPMD nature of the OpenSHMEM programming model is
the concept of symmetric data objects. These are arrays or variables
that exist with the same size, type, and relative address on all PEs.
Another term for symmetric data objects is "remotely accessible data
objects". In the interface definitions for OpenSHMEM data transfer
routines, one or more of the parameters are typically required to be
symmetric or remotely accessible.
The following kinds of data objects are symmetric:
* Fortran data objects in common blocks or with the SAVE
attribute. These data objects must not be defined in a
dynamic shared object (DSO).
* Non-stack C and C++ variables. These data objects must
not be defined in a DSO.
* Fortran arrays allocated with shpalloc(3F)
* C and C++ data allocated by shmalloc(3C)
Collective Routines
Some SHMEM routines, for example, shmem_broadcast(3) and
shmem_float_sum_to_all(3), are classified as collective routines
because they distribute work across a set of PEs. They must be
called concurrently by all PEs in the active set defined by the
PE_start, logPE_stride, PE_size argument triplet. The following
man pages describe the OpenSHMEM collective routines:
* shmem_and(3)
* shmem_barrier(3)
* shmem_broadcast(3)
* shmem_collect(3)
* shmem_max(3)
* shmem_min(3)
* shmem_or(3)
* shmem_prod(3)
* shmem_sum(3)
* shmem_xor(3)
USING THE SYMMETRIC WORK ARRAY, PSYNC
Multiple pSync arrays are often needed if a particular PE calls as
OpenSHMEM collective routine twice without intervening barrier synchro-
nization. Problems would occur if some PEs in the active set for call 2
arrive at call 2 before processing of call 1 is complete by all PEs in
the call 1 active set. You can use shmem_barrier(3) or shmem_bar-
rier_all(3) to perform a barrier synchronization between consecutive
calls to OpenSHMEM collective routines.
There are two special cases:
* The shmem_barrier(3) routine allows the same pSync array to be
used on consecutive calls as long as the active PE set does not
change.
* If the same collective routine is called multiple times with the
same active set, the calls may alternate between two pSync ar-
rays. The SHMEM routines guarantee that a first call is com-
pletely finished by all PEs by the time processing of a third
call begins on any PE.
Because the SHMEM routines restore pSync to its original contents, mul-
tiple calls that use the same pSync array do not require that pSync be
reinitialized after the first call.
SHMEM ENVIRONMENT VARIABLES
This section lists the significant SHMEM environment variables.
* SMA_VERSION print the library version at start-up.
* SMA_INFO print helpful text about all these environment vari-
ables.
* SMA_SYMMETRIC_SIZE number of bytes to allocate for the symmetric
heap.
* SMA_DEBUG enable debugging messages.
The first call to SHMEM must be start_pes(3). This routines initialize
the SHMEM runtime.
Calling any other SHMEM routines beforehand has undefined behavior.
Multiple calls to this routine is not allowed.
COMPILING AND RUNNING OPENSHMEM PROGRAMS
The OpenSHMEM specification is silent regarding how OpenSHMEM programs
are compiled, linked and run. This section shows some examples of how
wrapper programs could be utilized to compile and launch applications.
The commands are styled after wrapper programs found in many MPI imple-
mentations.
The following sample command line demonstrates running an OpenSHMEM
Program using a wrapper script (oshrun in this case):
* C/C++:
oshcc c_program.c
* FORTRAN:
oshfort fortran_program.f
The following sample command line demonstrates running an OpenSHMEM
Program assuming that the library provides a wrapper script for such
purpose (named oshrun for this example):
oshrun -np 32 ./a.out
EXAMPLES
Example 1: The following Fortran OpenSHMEM program directs all PEs to
sum simultaneously the numbers in the VALUES variable across all PEs:
PROGRAM REDUCTION
REAL VALUES, SUM
COMMON /C/ VALUES
REAL WORK
CALL START_PES(0)
VALUES = MY_PE()
CALL SHMEM_BARRIER_ALL ! Synchronize all PEs
SUM = 0.0
DO I = 0, NUM_PES()-1
CALL SHMEM_REAL_GET(WORK, VALUES, 1, I) ! Get next value
SUM = SUM + WORK ! Sum it
ENDDO
PRINT *, 'PE ', MY_PE(), ' COMPUTED SUM=', SUM
CALL SHMEM_BARRIER_ALL
END
Example 2: The following C OpenSHMEM program transfers an array of 10
longs from PE 0 to PE 1:
#include <mpp/shmem.h>
main() {
long source[10] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
static long target[10];
shmem_init();
if (shmem_my_pe() == 0) {
/* put 10 elements into target on PE 1 */
shmem_long_put(target, source, 10, 1);
}
shmem_barrier_all(); /* sync sender and receiver */
if (shmem_my_pe() == 1)
printf("target[0] on PE %d is %d\n", shmem_my_pe(), target[0]);
}
SEE ALSO
The following man pages also contain information on OpenSHMEM routines.
See the specific man pages for implementation information.
shmem_add(3), shmem_and(3), shmem_barrier(3), shmem_barrier_all(3),
shmem_broadcast(3), shmem_cache(3), shmem_collect(3), shmem_cswap(3),
shmem_fadd(3), shmem_fence(3), shmem_finc(3), shmem_get(3),
shmem_iget(3), shmem_inc(3), shmem_iput(3), shmem_lock(3),
shmem_max(3), shmem_min(3), shmem_my_pe(3), shmem_or(3), shmem_prod(3),
shmem_put(3), shmem_quiet(3), shmem_short_g(3), shmem_short_p(3),
shmem_sum(3), shmem_swap(3), shmem_wait(3), shmem_xor(3), shmem_pe_ac-
cessible(3), shmem_addr_accessible(3), shmem_init(3), shmem_malloc(3C),
shmem_my_pe(3I), shmem_n_pes(3I)
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