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Input/output (IO) can also have a big impact on model performance. It is set by Several aspects of the model I/O performance can be controlled at runtime by setting the PIO* parameters in env_run.xml. The model developer can start by tuning the number of PIO io tasks (processes that perform I/O) and the stride between the I/O tasks by setting PIO_NUMTASKS and PIO_STRIDE in (I don't know which file or even if this is right)respectively in env_run.xml. All I know about PIO comes from this email by Mark Taylor:
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