CDDS: Overriding --gres=tmp Needs Two Broadcasts?

Oct 28, 2025 by Admin 50 views

CDDS: Overriding `--gres=tmp` Needs Two Broadcast Commands?

Hey guys! Let's dive into a quirky issue in CDDS (Climate Data Delivery Service) where overriding the --gres=tmp directive requires not one, but two broadcast commands. This might sound a bit puzzling, but don't worry, we'll break it down. This article will explore the reasons behind this behavior, focusing on how the MIP_CONVERT_TMP_SPACE variable and the check_disk_usage function interact within the CDDS workflow. Understanding this interplay is crucial for effectively managing temporary space allocation and preventing potential errors during data conversion processes. So, let’s unravel this together!

The `--gres=tmp` Override Conundrum

So, the core of the issue lies in how CDDS handles temporary space allocation for its processes, specifically the mip_convert_wrapper. The --gres directive, which stands for "generic resource," is used to request resources, in this case, temporary disk space (tmp). Now, the value for this directive is initially set using a Jinja2 template variable called MIP_CONVERT_TMP_SPACE. You can find this in the rose-suite.conf file within the CDDS repository.

Diving Deep into the Code

To be precise, you can check it out here:

https://github.com/MetOffice/CDDS/blob/ac9ae76006c11f050072a94a920352e248b47334/cdds/cdds/workflows/conversion/rose-suite.conf#L32

This line essentially defines the default amount of temporary space allocated for the conversion processes. Now, here's where the fun begins! If you try to increase this limit using a cylc broadcast command (which is used to send commands to running workflows), you might still encounter errors. Why? Because the check_disk_usage function, responsible for verifying if enough temporary space is available, might be using the old value of MIP_CONVERT_TMP_SPACE.

The `check_disk_usage` Function and Its Role

The check_disk_usage function lives within the mip_convert_wrapper and its job is quite simple: make sure the usage of $TMPDIR (the temporary directory) doesn't exceed the allocated limit. This check is crucial to prevent processes from running out of space and crashing. The function receives the allowed temporary space limit (max_temp_space_in_mb) as an argument. The problem arises because this argument might not reflect the newly broadcasted value. Here's the relevant code snippet:

https://github.com/MetOffice/CDDS/blob/ac9ae76006c11f050072a94a920352e248b47334/cdds/cdds/convert/mip_convert_wrapper/wrapper.py#L48

As you can see, the value used by check_disk_usage is determined within the wrapper script, and if this script hasn't been updated with the broadcasted value, you're in for a surprise!

The Error Manifestation

This discrepancy can lead to a MipConvertWrapperDiskUsageError, which looks something like this:

Usage of $TMPDIR measured at 16739MB, which exceeds allocation of 8192MB
Usage of $TMPDIR measured at 16739MB, which exceeds allocation of 8192MB
Traceback (most recent call last):
  File "{redacted}/conda_environments/cdds-3.3.0/lib/python3.10/site-packages/cdds/convert/command_line.py", line 109, in main_run_mip_convert
    exit_code = run_mip_convert_wrapper()
  File "{redacted}/conda_environments/cdds-3.3.0/lib/python3.10/site-packages/cdds/convert/mip_convert_wrapper/wrapper.py", line 129, in run_mip_convert_wrapper
    check_disk_usage(staging_dir, max_temp_space_in_mb)
  File "{redacted}/conda_environments/cdds-3.3.0/lib/python3.10/site-packages/cdds/convert/mip_convert_wrapper/actions.py", line 210, in check_disk_usage
    raise MipConvertWrapperDiskUsageError(msg1)
cdds.convert.exceptions.MipConvertWrapperDiskUsageError: Usage of $TMPDIR measured at 16739MB, which exceeds allocation of 8192MB

Even though you've broadcasted a new, higher limit, the wrapper is still using the old value, resulting in this error. Frustrating, right?

Why Two Broadcast Commands Are Needed

So, why the need for two broadcast commands? It boils down to how the configuration and the wrapper script are updated within the CDDS workflow. Here's the likely scenario:

First Broadcast: The first cylc broadcast command updates the MIP_CONVERT_TMP_SPACE variable in the suite configuration. This tells the system, "Hey, we want to increase the temporary space!"
The Catch: However, this update doesn't automatically propagate to the running mip_convert_wrapper script. The script might have already been initialized with the old configuration.
Second Broadcast (Likely Implicit): A second broadcast or a mechanism within the CDDS workflow is then required to trigger an update or re-initialization of the mip_convert_wrapper script itself. This ensures that the script picks up the new MIP_CONVERT_TMP_SPACE value.

In essence, the first broadcast updates the configuration, and the second broadcast (or a similar action) updates the running script that uses that configuration. This two-step process is crucial to ensure that the check_disk_usage function receives the correct, updated value for the maximum temporary space.

A Closer Look at the Workflow

Imagine it like this: you change the speed limit sign on a road (first broadcast), but the cars already on the road are still following their old maps (wrapper script with old configuration). You need to somehow update their maps (second broadcast/script re-initialization) so they know the new speed limit.

Potential Solutions and Best Practices

Now that we understand the problem, what are some potential solutions or best practices to avoid this double-broadcast dance?

Centralized Configuration Management: A more robust configuration management system could ensure that updates to variables like MIP_CONVERT_TMP_SPACE are automatically and consistently propagated to all relevant components, including running scripts.
Dynamic Configuration Loading: The mip_convert_wrapper script could be designed to dynamically load the configuration each time it runs, rather than relying on a pre-initialized value. This would guarantee that it always uses the latest settings.
Clear Documentation: Obviously! Clear documentation outlining this behavior is essential. Users need to be aware of the two-broadcast requirement to avoid unexpected errors.
Wrapper Script Updates: The workflow should ensure that the wrapper script is updated or re-initialized after configuration changes. This might involve a specific broadcast command or a mechanism within the workflow to trigger a script refresh.

The Importance of Understanding the System

Ultimately, understanding the inner workings of CDDS, particularly how configuration changes are propagated, is key to avoiding these kinds of issues. Knowing that a simple cylc broadcast might not be enough to fully update the system can save you a lot of headaches down the road.

In Conclusion: Mastering the `--gres=tmp` Override

So, there you have it! Overriding --gres=tmp in CDDS requires a bit of finesse, often necessitating two broadcast commands due to the interplay between the MIP_CONVERT_TMP_SPACE variable and the check_disk_usage function. By understanding this mechanism, you can avoid those pesky MipConvertWrapperDiskUsageError messages and ensure your data conversion processes run smoothly. Remember, the first broadcast updates the configuration, and the second (or a similar process) updates the running script. Keep this in mind, and you'll be a CDDS --gres=tmp master in no time! Keep exploring, keep learning, and happy data converting! Remember to always double-check your configurations and ensure that all components are aligned with your intended settings. This proactive approach will not only prevent errors but also enhance the overall efficiency and reliability of your CDDS workflows. Until next time, happy computing! 🚀