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3.4 Understanding parallel I/O

In parallel execution, each processor has its own slice of data (Kohn-Sham orbitals, charge density, etc), that have to be written to temporary files during the calculation, or to data files at the end of the calculation. This can be done in two different ways: There is a third format, no longer used for final data but used for scratch and restart files:

The directory for data is specified in input variables outdir and prefix (the former can be specified as well in environment variable ESPRESSO_TMPDIR): outdir/prefix.save. A copy of pseudopotential files is also written there. If some processor cannot access the data directory, the pseudopotential files are read instead from the pseudopotential directory specified in input data. Unpredictable results may follow if those files are not the same as those in the data directory!

IMPORTANT: Avoid I/O to network-mounted disks (via NFS) as much as you can! Ideally the scratch directory outdir should be a modern Parallel File System. If you do not have any, you can use local scratch disks (i.e. each node is physically connected to a disk and writes to it) but you may run into trouble anyway if you need to access your files that are scattered in an unpredictable way across disks residing on different nodes.

You can use input variable disk_io to vary the amount of I/O done by pw.x. Since v.5.1, the default value is disk_io='low', so the code will store wavefunctions into RAM and not on disk during the calculation. Specify disk_io='medium' only if you have too many k-points and you run into trouble with memory; choose disk_io='none' if you do not need to keep final data files.


next up previous contents
Next: 3.5 Tricks and problems Up: 3 Parallelism Previous: 3.3 Parallelization levels   Contents