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Parallel I/O and Dense Matrices

Divide parallel I/O modes into two broad classes

1. There is one file pointer into the disk file. In this case there are two possibilities
   1. Only one process has access to the file pointer. Thus only that process can do I/O to the file, and has to scatter to, or gather from, the other processes when reading or writing the file.
   2. All processes in a group have access to the file pointer. Synchronization is required if the order in which data are written to, or read from, the file is important.
2. Each process in a group has its own file pointer. Again there are two main possibilities
   1. The file pointers can all access a global file space. In this case we refer to the file as a ``shared file.''
   2. Each file pointer can only access its own local file space. In this case we refer to the file as a ``distributed file.''