## 6.2.1. Window Creation

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The initialization operation allows each process in an intracommunicator group to specify, in a collective operation, a ``window'' in its memory that is made accessible to accesses by remote processes. The call returns an opaque object that represents the group of processes that own and access the set of windows, and the attributes of each window, as specified by the initialization call.

MPI_WIN_CREATE(base, size, disp_unit, info, comm, win)
IN base | initial address of window (choice)
IN size | size of window in bytes (nonnegative integer)
IN disp_unit | local unit size for displacements, in bytes (positive integer)
IN info | info argument (handle)
IN comm | communicator (handle)
OUT win | window object returned by the call (handle)

`int MPI_Win_create(void *base, MPI_Aint size, int disp_unit, MPI_Info info, MPI_Comm comm, MPI_Win *win)
`

`MPI_WIN_CREATE(BASE, SIZE, DISP_UNIT, INFO, COMM, WIN, IERROR)
<type> BASE(*)
INTEGER(KIND=MPI_ADDRESS_KIND) SIZE
INTEGER DISP_UNIT, INFO, COMM, WIN, IERROR
`

`static MPI::Win MPI::Win::Create(const void* base, MPI::Aint size, int disp_unit, const MPI::Info& info, const MPI::Intracomm& comm)
`

This is a collective call executed by all processes in the group of comm. It returns a window object that can be used by these processes to perform RMA operations. Each process specifies a window of existing memory that it exposes to RMA accesses by the processes in the group of comm. The window consists of size bytes, starting at address base. A process may elect to expose no memory by specifying size = 0.

The displacement unit argument is provided to facilitate address arithmetic in RMA operations: the target displacement argument of an RMA operation is scaled by the factor disp_unit specified by the target process, at window creation.

Rationale.

The window size is specified using an address sized integer, so as to allow windows that span more than 4 GB of address space. (Even if the physical memory size is less than 4 GB, the address range may be larger than 4 GB, if addresses are not contiguous.) ( End of rationale.)
Advice to users.

Common choices for disp_unit are 1 (no scaling), and (in C syntax) `sizeof(type)`, for a window that consists of an array of elements of type `type`. The later choice will allow one to use array indices in RMA calls, and have those scaled correctly to byte displacements, even in a heterogeneous environment. ( End of advice to users.)
The info argument provides optimization hints to the runtime about the expected usage pattern of the window. The following info key is predefined:

  no_locks --- if set to true, then the implementation may assume that the local window is never locked (by a call to MPI_WIN_LOCK). This implies that this window is not used for 3-party communication, and RMA can be implemented with no (less) asynchronous agent activity at this process.

The various processes in the group of comm may specify completely different target windows, in location, size, displacement units and info arguments. As long as all the get, put and accumulate accesses to a particular process fit their specific target window this should pose no problem. The same area in memory may appear in multiple windows, each associated with a different window object. However, concurrent communications to distinct, overlapping windows may lead to erroneous results.

Advice to users.

A window can be created in any part of the process memory. However, on some systems, the performance of windows in memory allocated by MPI_ALLOC_MEM (Section Memory Allocation ) will be better. Also, on some systems, performance is improved when window boundaries are aligned at ``natural'' boundaries (word, double-word, cache line, page frame, etc.). ( End of advice to users.)
Advice to implementors.

In cases where RMA operations use different mechanisms in different memory areas (e.g., load/store in a shared memory segment, and an asynchronous handler in private memory), the MPI_WIN_CREATE call needs to figure out which type of memory is used for the window. To do so, MPI maintains, internally, the list of memory segments allocated by MPI_ALLOC_MEM, or by other, implementation specific, mechanisms, together with information on the type of memory segment allocated. When a call to MPI_WIN_CREATE occurs, then MPI checks which segment contains each window, and decides, accordingly, which mechanism to use for RMA operations.

Vendors may provide additional, implementation-specific mechanisms to allow ``good'' memory to be used for static variables.

Implementors should document any performance impact of window alignment. ( End of advice to implementors.)

MPI_WIN_FREE(win)
INOUT win | window object (handle)

`int MPI_Win_free(MPI_Win *win)
`

`MPI_WIN_FREE(WIN, IERROR)
INTEGER WIN, IERROR
`

`void MPI::Win::Free()
`

Frees the window object win and returns a null handle (equal to MPI_WIN_NULL). This is a collective call executed by all processes in the group associated with win. MPI_WIN_FREE(win) can be invoked by a process only after it has completed its involvement in RMA communications on window win: i.e., the process has called MPI_WIN_FENCE, or called MPI_WIN_WAIT to match a previous call to MPI_WIN_POST or called MPI_WIN_COMPLETE to match a previous call to MPI_WIN_START or called MPI_WIN_UNLOCK to match a previous call to MPI_WIN_LOCK. When the call returns, the window memory can be freed.

Advice to implementors.

MPI_WIN_FREE requires a barrier synchronization: no process can return from free until all processes in the group of win called free. This, to ensure that no process will attempt to access a remote window (e.g., with lock/unlock) after it was freed. ( End of advice to implementors.)

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MPI-2.0 of July 18, 1997
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