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 File View |
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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 |
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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" )
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