HP XC System 3.x Software - Manual (Page 109)

The modified Makefile is invoked as follows:
$ make PREFIX='srun -n1 -N1' MAKE_J='-j4'
11.4 Local Disks on Compute Nodes
The use of a local disk for private, temporary storage may be configured on the compute node s of your
HP XC system. Contact your system administrator to find out about the local disks configured on your
system.
A local disk is a temporary storage space and does not hold data across execution of applications. Therefore,
any information generated by the application during its execution is not saved on the local disk once the
application has completed.
11.5 I/O Performance Considerations
Before building and running your parallel application, I/O performance issues on the HP XC cluster must
be considered.
The I/O control system provides two basic types of standard file system views to the application:
• Shared
• Private
11.5.1 Shared File View
Although a file opened by multiple processes of an application is shared, each core maintains a private
file pointer and file position. This means that if a certain order of input or output from multiple cores is
desired, the application must synchronize its I/O requests or position its file pointer such that it acts on
the desired file location.
Output requests to standard output and standard error are line-buffered, which can be sufficient output
ordering in many cases. A similar effect for other files can be achieved by using append mode when
opening the file with the fopen system call:
fp = fopen ("myfile", "a+");
11.5.2 Private File View
Although the shared file approach improves ease of use for most applications, some applications, especially
those written for shared-nothing clusters, can require the use of file systems private to each node. To
accommodate these applications, the system must be configured with local disk.
For example, assume /tmp and/tmp1 have been configured on each compute node.
Now each process can open up a file named /tmp/myscratch or /tmp1/myotherscratch and each
would see a unique file pointer. If these file systems do not exist local to the node, an error results.
It is a good idea to use this option for temporary storage only, and make sure that the application deletes
the file at the end.
C example: fd = open ("/tmp/myscratch", flags)
Fortran example: open (unit=9, file="/tmp1/myotherscratch" )
11.6 Communication Between Nodes
On the HP XC system, processes in an MPI application run on compute node s and use the system
interconnect for communication between the nodes. By default, intranode communication is done using
shared memory between MPI processes. For information about selecting and overriding the default system
interconnect, see the HP-MPI documentation.
11.7 Using MPICH on the HP XC System
MPICH is a freely available portable implementation of MPI. For additional information on MPICH, see
the following URL:
http://www-unix.mcs.anl.gov/mpi/mpich1/ .
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