During setup, there are two input files to be read. One is a parameters
file which is described in the previous section. Another is the target DC
file. They are described in some detail in the previous section.
For a large problem, the size of the DC file varies from several Mbytes
to several tens of Mbytes. One may use the sequential setup for each node,
where
each node opens the DC file and reads the data independently.
The argument for that is that the setup complexity is in the order of
where
is the maximum number of
unknowns of the system. Since UNIX sets a limit on the number of files
that can be open at any one time, for a coarse grain computer with a UNIX type
of operating system this may easily exceed the limited number of files
that can be opened at
the same time. Also only one node can read data at a time so the
rest of the nodes remain idle. This could be very time consuming.
A solution is to use parallel I/O to achieve high performance. On
the CM-5, CMMD provides a parallel I/O mode called global synchronous
broadcast, in which a global file is accessed by all nodes simultaneously,
with each node reading (or writing) the same data. This mode provides a
significant increase in flexibility and parallelism for I/O operations.
For Intel and IBM machines, there is a global broadcasting mode in which one node opens the file and reads the data sequentially then broadcasts the data to the rest of nodes.