169. Memory Allocation

In some systems, message-passing and remote-memory-access ( RMA) operations run faster when accessing specially allocated memory (e.g., memory that is shared by the other processes in the communicating group on an SMP). MPI provides a mechanism for allocating and freeing such special memory. The use of such memory for message-passing or RMA is not mandatory, and this memory can be used without restrictions as any other dynamically allocated memory. However, implementations may restrict the use of the MPI_WIN_LOCK and MPI_WIN_UNLOCK functions to windows allocated in such memory (see Section Lock .)

MPI_ALLOC_MEM(size, info, baseptr)
IN size	size of memory segment in bytes (nonnegative integer)
IN info	info argument (handle)
OUT baseptr	pointer to beginning of memory segment allocated

int MPI_Alloc_mem(MPI_Aint size, MPI_Info info, void *baseptr)

MPI_ALLOC_MEM(SIZE, INFO, BASEPTR, IERROR)
INTEGER INFO, IERROR
INTEGER(KIND=MPI_ADDRESS_KIND) SIZE, BASEPTR

void* MPI::Alloc_mem(MPI::Aint size, const MPI::Info& info)

The info argument can be used to provide directives that control the desired location of the allocated memory. Such a directive does not affect the semantics of the call. Valid info values are implementation-dependent; a null directive value of info = MPI_INFO_NULL is always valid.

The function MPI_ALLOC_MEM may return an error code of class MPI_ERR_NO_MEM to indicate it failed because memory is exhausted.

MPI_FREE_MEM(base)
IN base	initial address of memory segment allocated by MPI_ALLOC_MEM (choice)

int MPI_Free_mem(void *base)

MPI_FREE_MEM(BASE, IERROR)
<type> BASE(*)
INTEGER IERROR

void MPI::Free_mem(void *base)

The function MPI_FREE_MEM may return an error code of class MPI_ERR_BASE to indicate an invalid base argument.

Rationale.

The C and C++ bindings of MPI_ALLOC_MEM and MPI_FREE_MEM are similar to the bindings for the malloc and free C library calls: a call to MPI_Alloc_mem(..., &base) should be paired with a call to MPI_Free_mem(base) (one less level of indirection). Both arguments are declared to be of same type void* so as to facilitate type casting. The Fortran binding is consistent with the C and C++ bindings: the Fortran MPI_ALLOC_MEM call returns in baseptr the (integer valued) address of the allocated memory. The base argument of MPI_FREE_MEM is a choice argument, which passes (a reference to) the variable stored at that location. ( End of rationale.)

Advice to implementors.

If MPI_ALLOC_MEM allocates special memory, then a design similar to the design of C malloc and free functions has to be used, in order to find out the size of a memory segment, when the segment is freed. If no special memory is used, MPI_ALLOC_MEM simply invokes malloc, and MPI_FREE_MEM invokes free.

A call to MPI_ALLOC_MEM can be used in shared memory systems to allocate memory in a shared memory segment. ( End of advice to implementors.)

Example

Example of use of MPI_ALLOC_MEM, in Fortran with pointer support. We assume 4-byte REALs, and assume that pointers are address-sized.

REAL A 
POINTER (P, A(100,100))   ! no memory is allocated 
CALL MPI_ALLOC_MEM(4*100*100, MPI_INFO_NULL, P, IERR) 
! memory is allocated 
... 
A(3,5) = 2.71; 
... 
CALL MPI_FREE_MEM(A, IERR) ! memory is freed 

Example Same example, in C

float  (* f)[100][100] ; 
/* no memory is allocated */ 
MPI_Alloc_mem(sizeof(float)*100*100, MPI_INFO_NULL, &f); 
/* memory allocated */ 
... 
(*f)[5][3] = 2.71; 
... 
MPI_Free_mem(f);