Cypress CSC-1200T Guía de usuario descargar pdf (Pagina 35)

Chapter 4. Program development 35

4.3.3 BLACS

Both BLAS and LAPACK are developed for single processor computa-

tions. In order to solve problems of linear algebra on parallel machines

where matrices can be distributed over several processors, we need to

communicate data between the processors. For this purpose there is a

special library called BLACS (Basic Linear Algebra Communication Sub-

routines).

The routines in BLACS can be divided into three classes. First, there

are communication routines for sending and receiving parts of matrices

between two or more processors. Second, there are global reduction rou-

tines in which all processors take part. An example of these is ﬁnding

the element of the largest absolute value in a distributed matrix. Third,

there are a few general support routines for setting up the communica-

tion network.

4.3.4 PBLAS and ScaLAPACK

PBLAS (Parallel BLAS) and ScaLAPACK (Scalable LAPACK) are parallelized

versions of BLAS and LAPACK, respectively. The names of the multipro-

cessor routines in these libraries are almost the same as the ones used

for the corresponding single processor routines, except for an initial P

for “parallel”.

For some obscure reason, Cray has not documented the PBLAS library

at all, except for a short notice on the ScaLAPACK manual pages about

PBLAS being supported. For ScaLAPACK, the situation is somewhat bet-

ter, since for all available routines there is a manual page. On the other

hand, the current implementation of ScaLAPACK on the T3E does not

support all routines available in the public domain version. See Table 4.1

for the existing ScaLAPACK routines.

4.3.5 Details

All of the above mentioned libraries follow a naming convention. This

dictates that any subroutine operating with single precision ﬂoating-

point numbers should be given a name beginning with S. Correspond-

ingly, those routines accepting double precision ﬂoating point numbers

as arguments have a name beginning with D (not counting the letter P

for parallel versions, which precedes the actual name).

However, since the single precision ﬂoating point numbers on the T3E

have 8 bytes, which on most other computers corresponds to the double

precision, you should make sure that you change not only the type deﬁ-

nitions of the variables but also all calls to BLAS etc. accordingly. Note

that on the T3E there are no BLAS routines starting with the letter D.

1 2 ... 30 31 32 33 34 35 36 37 38 39 40 ... 123 124

Sin comentarios

Cypress CSC-1200T Guía de usuario Pagina 35