HP XC System 3.x Software manual Communication Between Nodes, Shared File View, Private File View

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+");

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" )

Communication Between Nodes

On the HP XC system, processes in an MPI application run on compute nodes and use the system interconnect for communication between the nodes. By default, intranode communication is done using shared memory between MPI processes. Refer to the HP-MPI documentation for information about selecting and overriding the default system interconnect.

Communication Between Nodes 97