Number of CPUs for planar domain LFRic setups #648
Denis Sergeev (dennissergeev)
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LFRic
Replies: 2 comments 10 replies
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Not sure that we have a guide. But you are right to consider the multigrid levels and so that it is the coarsest mesh that is partitioned. The next thing I would do is that if you have multiplied your total number of cells by 4, then I would also increase the cpus by 4 (so try 512 rather than 256). You may also need to increase the xios mpi/nodes. I'm not sure that we have a formula to work out the correct xios mpi? I would just go with the try multiplying that by 4 too. |
1 reply
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Thanks cjohnson-pi but increasing to 512 didn't help... By the xios mpi, do you mean |
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256x256 should/does work with 256CPUs - in fact there's a rose-stem job which does exactly this: https://github.com/MetOffice/lfric_apps/blob/main/rose-stem/site/common/lfric_atm/tasks_lfric_atm.cylc#L723 - what is the problem you are seeing? |
9 replies
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No, the list of diagnostics is the same - so I guess it's just the size of the arrays... How do I switch the server on? |
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So you'd need to set xios_server_mode=true and specify the number of processors for the server (32 probably okay). You'd also need to increase the number of nodes in the PBS select command by 1 to account for the inclusion of the server. I think there's something else that needs setting up to pass to the launch-exe script which determines how the processors are ordered between model and xios... |
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Hi iboutle, thanks for your help with this! Sorry to butt in but could I quickly ask a naive question... what exactly is the limit when running without the xios server? Is there a quantitative threshold in terms of the size of the simulation beyond which the use of the xios server is required? |
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iboutle Unfortunately it's still falling over - now a bit further than the diagnostic output: PET filesI might have done something wrong in the command directives though. Is there an example standalone suite with the XIOS server enabled? |
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Alex Corbett (@CorboPy) - I've no idea, I'm just guessing here! Denis Sergeev (@dennissergeev) not a simple one - you could look at u-dz462 or the rose-stem suite and try to piece it together from there, unless anyone else can help. |
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Is there a guide or a formula for how to choose the number of CPUs correctly for an LFRic setup on a planar mesh and the usual four multigrid levels?
For example, I don't understand why a 256x256 point domain doesn't run on 256 CPUs while a 128x128 point setup on 128 CPUs does. This is on Monsoon, so there are 128 cores per node.
Thank you.
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