XBeach requires that the number of MPI partitions in the M-direction (usually aligned with the x or nx
dimension of your grid) does not exceed nx / 4. If you run your simulation on a machine with a large
number of vCPUs (for example, a machine with 96 vCPUs), XBeach may attempt to create more partitions
than the domain can accommodate, resulting in the error message:
Number of mpi domains in M-direction is greater than nx/4
XBeach cannot split into separate model domains.
For example, with a computational grid where:
nx = 360ny = 180The maximum number of MPI partitions in the M-direction is:
360 / 4 = 90If you use a machine with more than 90 vCPUs (e.g., c4-highmem-96 with 96 vCPUs), XBeach will try to split the domain into more partitions than allowed, leading to the error.
On a typical laptop, the number of available computational cores is much smaller than on high-performance machines, making it less likely to hit the partition limit. However, on larger machines or instances with more vCPUs, XBeach is more likely to run into this issue.
To avoid this error, use a VM instance with fewer vCPUs. Ensure that the number of vCPUs you allocate does not exceed nx / 4. For instance, with nx = 360, limit the number of vCPUs to 90 or fewer.
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