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You can use an argument to the option to provide a list of device-indices.

The syntax is then to delimit ranks with colons [:], and threads by commas [,].

On a 4-GPU development machine with 16 visible cores, we often run classifications or refinements using: Which produces 4 working (slave) mpi-ranks, each with 4 threads.

This produces a single rank per card, but allows multiple CPU-cores to use each GPU, maximizing overall hardware utilization.

We often use this option if we don't have enough RAM to read in all the particles, but we have large enough fast SSD scratch disk(s) (e.g. Each node of our cluster has at least 64GB RAM, and an Intel(R) Xeon(R) CPU E5-2667 0 (@ 2.90GHz).Assuming you have a 4-GPU setup, the GPUs will be numbered 0, 1, 2 and 3.In general, you most likely will want to run a single MPI process on each GPU.If this flag is used without arguments, RELION will as default distribute its processes over all available (visible) GPUs.This default behaviour will likely be the preferred one if you do not need to share the computer with anyone else, and you only want to run a single job at a time.