Procedure 2-1.

1. Convert all global and static data to TASKCOMMON data.

   In a multitasking environment, all members of the multitasking group can access all global or static data because they share one user address space. An MPI process is described as a separate address space. To emulate an MPI process in a multitasking environment, all global or static data that can be modified during the course of execution of a program must be treated as data local to each task. This is done by placing the data in TASKCOMMON blocks. TASKCOMMON storage is a mechanism that is used in multitasked programs to provide a separate copy of data for each member of the multitasking group. TASKCOMMON data is still globally accessible across functions within a multitasked program, but it is private to each specific task. Fortran examples of global or static data that must be placed in TASKCOMMON include data that resides in COMMON blocks and data that appears in DATA or SAVE statements. In C, you must place all data that is declared static (either locally or globally) or data declared at a global level (outside of any function) in TASKCOMMON.

   Because changing your program so that all global and static data is private is tedious and makes a program less portable, Cray provides support in the form of compile-time command line options to do the conversions. You can convert most global and static data to TASKCOMMON data automatically by using the following command line options:

   • For C programs:

     cc -h taskprivate

     
     

   • For Fortran programs:

     f90 -a taskcommon

     
     

   When you are placing data in TASKCOMMON, there may be cases in which the compiler cannot do the conversion because of insufficient information. The compiler notes these cases by issuing a warning during compilation. For such cases, you must convert the data by hand. Most of the time, these cases are related to initialization that involves Fortran DATA or SAVE statements or C initialized static variables, and you might need to change only how or when the data is initialized for it to be successfully placed in TASKCOMMON.

   The following is an example of a case that the compiler cannot handle:

   int a; int b = &a

   If variable a resides in TASKCOMMON, its address will not be known until run time; therefore, the compiling system cannot initialize it. In this case, the initialization must be handled within the user program.

2. Use the cc(1) or f90(1) commands to build your shared memory MPI program, as in the following examples:

   C programs:

   cc -htaskprivate -D_MULTIP_ -L$MPTDIR/lib/multi file.c

   
   

   For C programs, the -D and -L options are needed to access the reentrant version of libc. This version is required to provide safe access to libc routines in a multitasking environment. When the mpt module is loaded, the module software sets $MPTDIR automatically and points to the default MPT software library. (For information on using modules, see Installing Programming Environment Products.) To make compiling in C easier, the environment variable $LIBCM is also set automatically when the mpt module is loaded. You can use $LIBCM with the cc(1) command to request the reentrant version of libc. $LIBCM is set to the following value:

   -D_MULTIP_ -L$MPTDIR/lib/multi

   
   

   The following example uses $LIBCM:

   cc -htaskprivate $LIBCM file.c

   
   

   Fortran programs:

   f90 -ataskcommon file.f

   
   

3. Select private I/O if private Fortran file unit numbers are desired.

   In a multitasking environment, Fortran unit numbers are, by default, shared by all members of the multitasking group. This behavior forces all files to be shared among MPI processes. The user can request that files be private to each MPI process by specifying the private I/O option on the assign(1) command. The examples in Table 2-1, request private I/O.

   Table 2-1. assign examples

   Example	Description
   assign -P private u:10	Specifies that unit 10 should be private to any MPI process that opens it.
   assign -P private p:%	Specifies that all named Fortran units should be private to any MPI process that opens them. This includes all units connected to regular files and excludes units such as 5 and 6, which are connected to stdin, stdout, or stderr by default.
   assign -P global u:0 assign -P global u:5 assign -P global u:6 assign -P global u:100 assign -P global u:101 assign -P global u:102	This set of assign commands can be used in conjunction with assign -P private g:all to retain units connected by default to stdin, stdout, and stderr as global units. A unit connected to these standard files cannot be a private unit.

   For more information on private I/O functionality on Cray PVP systems, see the assign(1) man page.

4. Run the application. To start or run an application that uses the shared memory version of MPI, you must use the -nt option on the mpirun(1) command (for example, mpirun -nt 4 compute).