asyncronous IO

[heikoklein]

SNAP uses a asynchronous task to read the next meteorology while working through the calculation loop.

Asynchronous handling needs ASYNC_IO.ON set in snap.input, though it only makes sense if more than 2 threads exist (OMP_NUM_THREADS).

async_io needs 30%-50% more memory for the additional meteorology buffers.
Performance benefits are up to a factor 2 if loop and io take the same time, but usually around 30%.

async_io with netcdf < 4.9.3 gives known warnings about can't locate attribute: '_QuantizeBitGroomNumberOfSignificantDigits'. This is a known bug in older versions of netcdf and is harmless. Check with nc_config --version.

ASYNC_IO.ON reduces the runtime of a standard 48h run with 3 isotopes on a server-CPU run with
time OMP_PLACES=cores OMP_NUM_THREADS=4 ~/Programme/Models/snap/src/naccident/bsnap_naccident snap.input
from real 2m28.153s to real 1m44.771s