File Consistency and Semantics

The routines ending in _atomicity allow you to set or query whether a file is in atomic or nonatomic mode. In atomic mode, all operations within the communicator group that opens a file are completed as if sequentialized into some serial order. In nonatomic mode, no such guarantee is made. In nonatomic mode, MPI_File_sync can be used to assure weak consistency.

The default mode varies with the number of nodes you are using. If you are running a job on a single node, a file is in nonatomic mode by default when it is opened. If you are running a job on more than one node, a file is in atomic mode by default.

MPI_File_set_atomicity is a collective call that sets the consistency semantics for data-access operations. All the processes in the group must pass identical values for both the file handle and the Boolean flag that indicates whether atomic mode is set.

MPI_File_get_atomicity returns the current consistency semantics for data-access operations. Again, a Boolean flag indicates whether the atomic mode is set.

In some cases, setting atomicity to false may provide better performance. The default atomicity value on a cluster is true. The lack of synchronization among the distributed caches on a cluster will often prevent your data from completing in the desired state. In these circumstances, you may suffer performance disadvantages with atomicity set to true, especially when the data accesses overlap.