Load mesh limit open calls per rank

[mo-marqh]

PR Summary

Sci/Tech Reviewer: Andrew Coughtrie (@andrewcoughtrie)
Code Reviewer: Harry Shepherd (@harry-shepherd)

This changes delivers a targeted and limited scope refactor of the interaction between the components/driver/source/driver_mesh_mod.f90 mesh initialisation and the infrastructure/source/io/ugrid_2d_mod netCDF interface.

currently each read operation opens the file, reads a snippet of data, then closes the file.
this happens many times on each rank, e.g. 10 times for an lfric_atm C896 rank.
this puts excessive load on parallel file systems and their metadata server provision.
combined with loading from a single mesh file across thousands of ranks, this load becomes unmanageable.

This change alters the internal interface to open the file once per rank, read all required information and close the file.
this reduces the number of file operations by a factor (e.g. factor 10 at C896)

this is a stand alone change, but it is intended to work with pre-partitioned meshes to manage load for large runs.

Code Quality Checklist

• I have performed a self-review of my own code
• My code follows the project's style guidelines
• Comments have been included that aid understanding and enhance the readability of the code
• My changes generate no new warnings
• All automated checks in the CI pipeline have completed successfully

Testing

• I have tested this change locally, using the LFRic Core rose-stem suite
• If required (e.g. API changes) I have also run the LFRic Apps test suite using this branch
• If any tests fail (rose-stem or CI) the reason is understood and acceptable (e.g. kgo changes)
• I have added tests to cover new functionality as appropriate (e.g. system tests, unit tests, etc.)
• Any new tests have been assigned an appropriate amount of compute resource and have been allocated to an appropriate testing group (i.e. the developer tests are for jobs which use a small amount of compute resource and complete in a matter of minutes)

trac.log

Test Suite Results - lfric_core - load_mesh_limit_open_calls_per_rank/run3

Suite Information

Item	Value
Suite Name	load_mesh_limit_open_calls_per_rank/run3
Suite User	mark.hedley
Workflow Start	2026-08-10T10:51:47
Groups Run	all

Dependency	Reference	Main Like
lfric_core	mo-marqh/lfric_core@load_mesh_limit_open_calls_per_rank	False
SimSys_Scripts	MetOffice/SimSys_Scripts@cab3315	True

Task Information

✅ succeeded tasks - 433

Security Considerations

• I have reviewed my changes for potential security issues
• Sensitive data is properly handled (if applicable)
• Authentication and authorisation are properly implemented (if applicable)

Performance Impact

• Performance of the code has been considered and, if applicable, suitable performance measurements have been conducted

AI Assistance and Attribution

• Some of the content of this change has been produced with the assistance of Generative AI tool name (e.g., Met Office Github Copilot Enterprise, Github Copilot Personal, ChatGPT GPT-4, etc) and I have followed the Simulation Systems AI policy (including attribution labels)

Documentation

• Where appropriate I have updated documentation related to this change and confirmed that it builds correctly

PSyclone Approval

• If you have edited any PSyclone-related code (e.g. PSyKAl-lite, Kernel interface, optimisation scripts, LFRic data structure code) then please contact the TCD Team

Sci/Tech Review

• I understand this area of code and the changes being added
• The proposed changes correspond to the pull request description
• Documentation is sufficient (do documentation papers need updating)
• Sufficient testing has been completed

(Please alert the code reviewer via a tag when you have approved the SR)

Code Review

• All dependencies have been resolved
• Related Issues have been properly linked and addressed
• CLA compliance has been confirmed
• Code quality standards have been met
• Tests are adequate and have passed
• Documentation is complete and accurate
• Security considerations have been addressed
• Performance impact is acceptable