Often a communication with the same argument list is repeatedly executed within the inner loop of a parallel computation. In such a situation, it may be possible to optimize the communication by binding the list of communication arguments to a persistent communication request once and, then, repeatedly using the request to initiate and complete messages. The persistent request thus created can be thought of as a communication port or a ``half-channel.'' It does not provide the full functionality of a conventional channel, since there is no binding of the send port to the receive port. This construct allows reduction of the overhead for communication between the process and communication controller, but not of the overhead for communication between one communication controller and another. It is not necessary that messages sent with a persistent request be received by a receive operation using a persistent request, or vice versa.
A persistent communication request is created using one of the five following calls. These calls involve no communication.
| MPI_SEND_INIT(buf, count, datatype, dest, tag, comm, request) | |
| IN buf | initial address of send buffer (choice) |
| IN count | number of elements sent (non-negative integer) |
| IN datatype | type of each element (handle) |
| IN dest | rank of destination (integer) |
| IN tag | message tag (integer) |
| IN comm | communicator (handle) |
| OUT request | communication request (handle) |
int MPI_Send_init(void* buf, int count, MPI_Datatype datatype, int dest, int tag, MPI_Comm comm, MPI_Request *request)
MPI_SEND_INIT(BUF, COUNT, DATATYPE, DEST, TAG, COMM, REQUEST, IERROR)
int MPI_Bsend_init(void* buf, int count, MPI_Datatype datatype, int dest, int tag, MPI_Comm comm, MPI_Request *request)
MPI_BSEND_INIT(BUF, COUNT, DATATYPE, DEST, TAG, COMM, REQUEST, IERROR)
int MPI_Ssend_init(void* buf, int count, MPI_Datatype datatype, int dest, int tag, MPI_Comm comm, MPI_Request *request)
MPI_SSEND_INIT(BUF, COUNT, DATATYPE, DEST, TAG, COMM, REQUEST, IERROR)
int MPI_Rsend_init(void* buf, int count, MPI_Datatype datatype, int dest, int tag, MPI_Comm comm, MPI_Request *request)
MPI_RSEND_INIT(BUF, COUNT, DATATYPE, DEST, TAG, COMM, REQUEST, IERROR)
int MPI_Recv_init(void* buf, int count, MPI_Datatype datatype, int source, int tag, MPI_Comm comm, MPI_Request *request)
MPI_RECV_INIT(BUF, COUNT, DATATYPE, SOURCE, TAG, COMM, REQUEST, IERROR)
A persistent communication request is inactive after it was created
--- no active communication is attached to the request.
A communication (send or receive) that uses a persistent request
is initiated by the function MPI_START.
int MPI_Start(MPI_Request *request)
MPI_START(REQUEST, IERROR)
The argument, request, is a handle returned
by one of the previous five calls. The associated request should be
inactive. The request becomes active once the call is made.
If the request is for a send with ready mode, then
a matching receive should be posted before the call is made. The
communication buffer should not be
modified
after the call, and
until the operation completes.
The call is local, with similar semantics to the nonblocking
communication operations described in
Section Nonblocking Communication
. That is,
a call to MPI_START with a
request created by MPI_SEND_INIT
starts a
communication in the same manner as a call to MPI_ISEND;
a call to MPI_START with a
request created by MPI_BSEND_INIT
starts a
communication in the same manner as a call to
MPI_IBSEND; and so on.
int MPI_Startall(int count, MPI_Request *array_of_requests)
MPI_STARTALL(COUNT, ARRAY_OF_REQUESTS, IERROR)
Start all communications associated with requests in
array_of_requests. A call to
A communication started with a call to MPI_START or
MPI_STARTALL is
completed by a call to MPI_WAIT, MPI_TEST, or
one of the derived functions described in
Section Multiple Completions
. The request becomes inactive after
successful completion of such call. The request is not deallocated
and it can be activated anew by an MPI_START or
MPI_STARTALL call.
A persistent request is deallocated by a call to
MPI_REQUEST_FREE
(Section Communication Completion
).
The call to MPI_REQUEST_FREE can occur at any point in the program
after the persistent request was created. However, the request will be
deallocated only after it becomes inactive.
Active receive requests should not be freed. Otherwise, it will not be
possible to check that the receive has completed.
It is preferable, in general, to free requests when they are inactive. If this
rule is followed, then the functions
described in this section will be invoked
in a sequence of the form,
A send operation initiated with MPI_START can be matched with
any receive operation and, likewise, a receive operation initiated
with MPI_START can receive messages generated by any send
operation.
To prevent problems with the argument copying and register optimization done
by Fortran compilers, please note the hints in subsections ``Problems Due to
Data Copying and Sequence Association,'' and ``A Problem with
Register Optimization''
in Section Problems With Fortran Bindings for MPI
on
pages Problems Due to Data Copying and Sequence Association
and A Problem with Register Optimization
.
( End of advice to users.)
<type> BUF(*)
INTEGER REQUEST, COUNT, DATATYPE, DEST, TAG, COMM, REQUEST, IERROR
{ MPI::Prequest MPI::Comm::Send_init(const void* buf, int count, const MPI::Datatype& datatype, int dest, int tag) const (binding deprecated, see Section Deprecated since MPI-2.2
) }
Creates a persistent communication request
for a standard mode send operation, and binds to it all the
arguments of a send operation.
MPI_BSEND_INIT(buf, count, datatype, dest, tag,
comm, request) IN buf initial address of send buffer (choice) IN count number of elements sent (non-negative
integer) IN datatype type of each element (handle) IN dest rank of destination (integer) IN tag message tag (integer) IN comm communicator (handle) OUT request communication request (handle)
<type> BUF(*)
INTEGER REQUEST, COUNT, DATATYPE, DEST, TAG, COMM, REQUEST, IERROR
{ MPI::Prequest MPI::Comm::Bsend_init(const void* buf, int count, const MPI::Datatype& datatype, int dest, int tag) const (binding deprecated, see Section Deprecated since MPI-2.2
) }
Creates a persistent communication request for a buffered mode send.
MPI_SSEND_INIT(buf, count, datatype, dest, tag,
comm, request) IN buf initial address of send buffer (choice) IN count number of elements sent (non-negative
integer) IN datatype type of each element (handle) IN dest rank of destination (integer) IN tag message tag (integer) IN comm communicator (handle) OUT request communication request (handle)
<type> BUF(*)
INTEGER COUNT, DATATYPE, DEST, TAG, COMM, REQUEST, IERROR
{ MPI::Prequest MPI::Comm::Ssend_init(const void* buf, int count, const MPI::Datatype& datatype, int dest, int tag) const (binding deprecated, see Section Deprecated since MPI-2.2
) }
Creates a persistent communication object
for a synchronous mode send operation.
MPI_RSEND_INIT(buf, count, datatype, dest, tag,
comm, request) IN buf initial address of send buffer (choice) IN count number of elements sent (non-negative
integer) IN datatype type of each element (handle) IN dest rank of destination (integer) IN tag message tag (integer) IN comm communicator (handle) OUT request communication request (handle)
<type> BUF(*)
INTEGER COUNT, DATATYPE, DEST, TAG, COMM, REQUEST, IERROR
{ MPI::Prequest MPI::Comm::Rsend_init(const void* buf, int count, const MPI::Datatype& datatype, int dest, int tag) const (binding deprecated, see Section Deprecated since MPI-2.2
) }
Creates a persistent communication object
for a ready mode send operation.
MPI_RECV_INIT(buf, count, datatype, source, tag,
comm, request) OUT buf initial address of receive buffer (choice) IN count number of elements received (non-negative
integer) IN datatype type of each element (handle) IN source rank of source or MPI_ANY_SOURCE (integer) IN tag message tag or MPI_ANY_TAG (integer) IN comm communicator (handle) OUT request communication request (handle)
<type> BUF(*)
INTEGER COUNT, DATATYPE, SOURCE, TAG, COMM, REQUEST, IERROR
{ MPI::Prequest MPI::Comm::Recv_init(void* buf, int count, const MPI::Datatype& datatype, int source, int tag) const (binding deprecated, see Section Deprecated since MPI-2.2
) }
Creates a persistent communication request
for a receive operation. The argument buf is marked as OUT
because the user gives permission to write on the receive buffer by passing the
argument to MPI_RECV_INIT.
MPI_START(request) INOUT request communication request (handle)
INTEGER REQUEST, IERROR
{ void MPI::Prequest::Start() (binding deprecated, see Section Deprecated since MPI-2.2
) }
MPI_STARTALL(count, array_of_requests) IN count list length (non-negative
integer) INOUT array_of_requests array of requests (array of handle)
INTEGER COUNT, ARRAY_OF_REQUESTS(*), IERROR
{ static void MPI::Prequest::Startall(int count, MPI::Prequest array_of_requests[]) (binding deprecated, see Section Deprecated since MPI-2.2
) }
MPI_STARTALL(count, array_of_requests) has the
same effect as calls to
MPI_START (&array_of_requests[i]),
executed for i=0 ,..., count-1, in some arbitrary order.
where
* indicates zero or more repetitions.
If the same communication object is used in several concurrent
threads, it is the user's responsibility to coordinate calls so that the
correct sequence is obeyed.
Advice to users.
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