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Chapter 4. Program development 33

The option -Xn or -X n is used to indicate how many processors you

want for your application. If you do not provide this option, the program

can be run on any number of processors using the mpprun command.

This kind of executable is called malleable.

Here is a typical example of generating and running a non-malleable

executable, which has to be run on a ﬁxed number of PEs:

t3e% f90 -X 16 -o prog.x prog.f90

t3e% ./prog.x

Here we are using 16 processors for our application. However, we can

also generate and run a malleable executable prog.x:

t3e% f90 -o prog.x prog.f90

t3e% mpprun -n 16 ./prog.x

t3e% mpprun -n 8 ./prog.x

Here the number of processors was given to the mpprun command and

the option -X was omitted. The program was ﬁrst run on 16 processors

and then on eight processors.

The same applies to the C/C++ compilers. Here is an example of com-

piling and running a C program:

t3e% cc -o prog.x prog.c

t3e% mpprun -n 8 ./prog.x

Here we used eight processors for running our malleable executable

prog.x.

The MPI, PVM, and SHMEM libraries are automatically linked to your

application, when needed. Therefore, you do not need to provide any

special ﬂags to be able to use, e.g., MPI calls. The Cray scientiﬁc library

(Libsci) is also automatically linked to your program.

The option -O indicates the optimization level used in the compilation.

If you are running production jobs, you should always turn code opti-

mization on! The default is moderate optimization, but you can request

more aggressive optimization.

The Fortran 90 compiler is discussed in more detail in Chapter 5,and

the C/C++ compiler in Chapter 6.

4.3 Libsci — Cray scientiﬁc library

Libsci is a collection of various mathematical subroutines. Most of the

routines solve some speciﬁc problem of linear algebra, but there are a

few routines for fast Fourier transforms as well.

Libsci is Cray’s own package of subroutines for computational tasks. Lib-

sci is divided into following sublibraries: BLAS 1,2,3, LAPACK, BLAS_S,

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