Add a tutorial on configuring Dask for use with ESMValTool

_episodes/11-dask-configuration.md

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+---
+title: "Configuring Dask"
+teaching: 20 (+ optional 10)
+exercises: 40 (+ optional 20)
+compatibility: ESMValCore v2.10.0
+
+questions:
+  - "What is the Dask configuration file and how should I use it?"
+  - "What are Dask workers"
+  - "What is the Dask scheduler"
+
+objectives:
+  - "Understand the contents of the dask.yml file"
+  - "Prepare a personalized dask.yml file"
+
+keypoints:
+  - "The ``~/.esmvaltool/dask.yml`` file tells ESMValCore how to configure Dask."
+  - "``cluster`` can be used to start a new Dask cluster for each run."
+  - "``client`` can be used to connect to an already running Dask cluster."
+  - "The Dask default scheduler can be configured by editing the files in ``~/.config/dask``."
+  - "The Dask Dashboard can be used to see if the Dask workers have sufficient memory available."
+
+---
+
+## Introduction
+
+When processing larger amounts of data, and especially when the tool crashes
+when running a recipe because there is not enough memory available, it is
+usually beneficial to change the default [Dask configuration][dask-configuration].
+
+The preprocessor functions in ESMValCore use the
+[Iris](https://scitools-iris.readthedocs.io) library, which in turn uses Dask
+Arrays to be able to process datasets that are larger than the available memory.
+It is not necesary to understand how these work exactly to use the ESMValTool,
+but if you are interested there is a
+[Dask Array Tutorial](https://tutorial.dask.org/02_array.html) as a well as a
+[guide to "Lazy Data"](https://scitools-iris.readthedocs.io/
+en/stable/userguide/real_and_lazy_data.html)
+available. Lazy data is the term the Iris library uses for Dask Arrays.
+
+
+### Dask Workers
+The most important concept to understand when using Dask Arrays is the concept
+of a Dask *worker*. With Dask, computations are run in parallel by little
+Python programs that are called *workers*. These could be on running on the
+same machine that you are running ESMValTool on, or they could be on one or
+more other computers. Dask workers typically require 2 to 4 gigabytes (GiB) of
+memory (RAM) each. In order to avoid running out of memory, it is important
+to use only as many workers as your computer(s) have memory for. ESMValCore
+(or Dask) provide configuration files where you can configure the number of
+workers.
+
+Note that only array computations are run using Dask, so total runtime may not
+decrease as much as you might expect when you increase the number of Dask
+workers.
+
+### Dask Scheduler
+
+In order to distribute the computations over the workers, Dask makes use of a
+*scheduler*. There are two different schedulers available. The default
+scheduler can be a good choice for smaller computations that can run
+on a single computer, while the scheduler provided by the Dask Distributed
+package is more suitable for larger computations.
+
+> ## On using ``max_parallel_tasks``
+>
+> In the config-user.yml file, there is a setting called ``max_parallel_tasks``.
+> Any variable or diagnostic script in the recipe is considered a 'task' in this
+> context and this is set to a value larger than 1, these will be
+> processed in parallel on the computer running the ``esmvaltool`` command.
+>
+> With the Dask Distributed scheduler, all the tasks running in parallel
+> can use the same workers, but with the default scheduler each task will
+> start its own workers. If a recipe does not run with ``max_parallel_tasks``
+> set to a value larger than 1, try reducing the value or setting it to 1.
+> This is especially the case for recipes with high resolution data or many
+> datasets per variable.
+>
+{: .callout}
+
+## Starting a Dask distributed cluster
+
+The workers and the scheduler together are called a Dask "cluster".
+Let's start the the tutorial by configuring ESMValCore so it runs its
+computations on a cluster with just one worker.
+
+We use a text editor called ``nano`` to edit the configuration file:
+
+~~~bash
+  nano ~/.esmvaltool/dask.yml
+~~~
+
+Any other editor can be used, e.g. many systems have ``vi`` available.
+
+This file contains the settings for:
+
+- Starting a new cluster of Dask workers
+- Or alternatively: connecting to an existing cluster of Dask workers
+
+Add the following content to the file ``~/.esmvaltool/dask.yml``:
+
+```yaml
+cluster:
+  type: distributed.LocalCluster
+  n_workers: 1
+  threads_per_worker: 2
+  memory_limit: 4GiB
+```
+
+This tells ESMValCore to start a new cluster of one worker, that can use 2
+gigabytes (GiB) of memory and run computations using 2 threads. For a more
+extensive description of the available arguments and their values, see
+[``distributed.LocalCluster``][distributed-localcluster].
+
+To see this configuration in action, run we will run a version of
+[recipe_easy_ipcc.yml](https://docs.esmvaltool.org/
+en/latest/recipes/recipe_examples.html) with just two datasets.
+This recipe takes a few minutes to run, once you have the data available.
+Download the recipe [here](../files/recipe_easy_ipcc_short.yml) and run it
+with the command:
+
+~~~bash
+  esmvaltool run recipe_easy_ipcc_short.yml
+~~~
+
+After finding and downloading all the required input files, this will start
+the Dask scheduler and workers required for processing the data. A message that
+looks like this will appear on the screen:
+
+```
+2024-05-29 12:52:38,858 UTC [107445] INFO    Dask dashboard: http://127.0.0.1:8787/status
+```
+
+Open the Dashboard link in a browser to see the Dask Dashboard website.
+When the recipe has finished running, the Dashboard website will stop working.
+The top left panel shows the memory use of each of the workers, the panel on the
+right shows one row for each thread that is doing work, and the panel at the
+bottom shows the progress of all work that the scheduler currently has been
+asked to do.
+
+> ## Explore what happens if workers do not have enough memory
+>
+> Reduce the amount of memory that the workers are allowed to use to 2GiB and
+> run the recipe again. Watch what happens.
+>
+>> ## Solution
+>>
+>> We use `memory_limit` entry in the `~/.esmvaltool/dask.yml` file to set the
+>> amount of memory allowed to 2GiB:
+>>```yaml
+>> cluster:
+>>    type: distributed.LocalCluster
+>>    n_workers: 1
+>>    threads_per_worker: 2
+>>    memory_limit: 2GiB
+>>```
+>> Note that the bars representing the memory use turn orange as the worker
+>> reaches the maximum amount of memory it is allowed to use and it starts
+>> 'spilling' (writing data temporarily) to disk.
+>> The red blocks in the top right panel represent time spent reading/writing
+>> to disk. While 2 GiB per worker may be enough in other cases, it is
+>> apparently not enough for this recipe.
+>>
+>> Warning messages about high memory usage by workers and/or killed workers
+>> will also be written to the terminal, which may be convenient to diagnose
+>> issues that occurred while you were not watching the dashboard.
+>>
+> {: .solution}
+{: .challenge}
+
+
+> ## Tune the configuration to your own computer
+>
+> Look at how much memory you have available on your machine (e.g. by running
+> the command ``grep MemTotal /proc/meminfo`` on Linux), set the
+> ``memory_limit`` back to 4 GiB per worker and increase the number of Dask
+> workers so they use total amount available minus a few gigabytes for your
+> other work. Run the recipe again and notice that it completed faster.
+>
+> If are working on a computer that is shared with other users, please be
+> mindful of them and only use a modest amount of memory instead of all
+> available memory.
+>
+>> ## Solution
+>>
+>> For example, if your computer has 16 GiB of memory and you do not have too
+>> many other programs running, it can use 12 GiB of memory for Dask workers,
+>> so you can start 3 workers with 4 GiB of memory each.
+>>
+>> Use the `num_workers` entry in the `~/.esmvaltool/dask.yml` file to set the
+>> number of workers to 3:
+>>```yaml
+>> cluster:
+>>    type: distributed.LocalCluster
+>>    n_workers: 3
+>>    threads_per_worker: 2
+>>    memory_limit: 4GiB
+>>```
+>> and run the recipe again with the command
+>> ``esmvaltool run recipe_easy_ipcc_short.yml``.
+>> The time it took to run the recipe is printed to the screen.
+>>
+> {: .solution}
+{: .challenge}
+
+## Using an existing Dask Distributed cluster
+
+It can be useful to start the Dask Distributed cluster before
+running the ``esmvaltool`` command. For example, if you would like to keep the
+Dashboard available for further investigation after the recipe completes
+running, or if you are working from a Jupyter notebook environment, see
+[dask-labextension](https://github.com/dask/dask-labextension) and
+[dask_jobqueue interactive use][dask-jobqueue-interactive] for more information.
+
+To use a cluster that was started in some other way, the following configuration
+can be used in ``~/.esmvaltool/dask.yml``:
+
+```yaml
+client:
+  address: "tcp://127.0.0.1:33041"
+```
+where the address depends on the Dask cluster. Code to start a
+[``distributed.LocalCluster``][distributed-localcluster]
+that automatically scales between 0 and 2 workers depending on demand, could
+look like this:
+
+```python
+from time import sleep
+
+from distributed import LocalCluster
+
+if __name__ == '__main__':  # Remove this line when running from a Jupyter notebook
+    cluster = LocalCluster(
+        threads_per_worker=2,
+        memory_limit='4GiB',
+    )
+    cluster.adapt(minimum=0, maximum=2)
+    # Print connection information
+    print(f"Connect to the Dask Dashboard by opening {cluster.dashboard_link} in a browser.")
+    print("Add the following text to ~/.esmvaltool/dask.yml to connect to the cluster:" )
+    print("client:")
+    print(f'  address: "{cluster.scheduler_address}"')
+    # When running this as a Python script, the next two lines keep the cluster
+    # running for an hour.
+    hour = 3600 # seconds
+    sleep(1 * hour)
+    # Stop the cluster when you are done with it.
+    cluster.close()
+```
+
+> ## Start a cluster and use it
+>
+> Copy the Python code above into a file called ``start_dask_cluster.py`` (or
+into a Jupyter notebook if you prefer) and start the cluster using the command
+``python start_dask_cluster.py``. Edit the ``~/esmvaltool/dask.yml`` file so
+ESMValCore can connect to the cluster. Run the recipe again and notice that the
+Dashboard remains available after the recipe completes.
+>
+>> ## Solution
+>>
+>> If the script printed
+>> ```
+>> Connect to the Dask Dashboard by opening http://127.0.0.1:8787/status in a browser.
+>> Add the following text to ~/.esmvaltool/dask.yml to connect to the cluster:
+>> client:
+>>   address: "tcp://127.0.0.1:34827"
+>> ```
+>> to the screen, edit the file ``~/.esmvaltool/dask.yml`` so it contains the
+lines
+>> ```yaml
+>> client:
+>>   address: "tcp://127.0.0.1:34827"
+>> ```
+>> open the link "http://127.0.0.1:8787/status" in your browser and
+>> run the recipe again with the command ``esmvaltool run recipe_easy_ipcc_short.yml``.
+> {: .solution}
+{: .challenge}
+
+When running from a Jupyter notebook, don't forget to `close()` the cluster
+when you are running on an HPC facility (see below), to avoid wasting
+compute hours you are not using.
+
+## Using the Dask default scheduler
+
+It is recommended to use the Distributed scheduler explained above for
+processing larger amounts of data. However, in many cases the default scheduler
+is good enough. Note that it does not provide a Dashboard, so it is less
+instructive and that is why we did not use it earlier in this tutorial.
+
+To use the default scheduler, comment out all the contents of
+``~/.esmvaltool/dask.yml`` and create a file in ``~/.config/dask``, e.g. called
+``~/.config/dask/default.yml`` but the filename does not matter, with the
+contents:
+```yaml
+scheduler: threads
+num_workers: 4
+```
+to set the number of workers to 4. The ``scheduler`` can also be set to
+``synchronous``. In that case it will use a single thread, which may be useful
+for debugging.
+
+> ## Use the default scheduler
+>
+> Follow the instructions above to use the default scheduler and run the recipe
+> again. To keep track of the amount of memory used by the process, you can
+> start the ``top`` command in another terminal. The amount of memory is shown
+> in the ``RES`` column.
+>
+>> ## Solution
+>>
+>> The recipe runs a bit faster with this configuration and you may have seen
+>> a memory use of around 5 GB.
+>>
+> {: .solution}
+{: .challenge}
+
+## Optional: Using dask_jobqueue to run a Dask Cluster on an HPC system
+
+The [``dask_jobqueue``](https://jobqueue.dask.org) package provides functionality
+to start Dask Distributed clusters on High Performance Computing (HPC) or
+High Throughput Computing (HTC) systems. This section is optional and only
+useful if you have access to a such a system.
+
+An example configuration for the
+[Levante HPC system](https://docs.dkrz.de/doc/levante/index.html)
+could look like this:
+
+```yaml
+cluster:
+  type: dask_jobqueue.SLURMCluster  # Levante uses SLURM as a job scheduler
+  queue: compute                    # SLURM partition name
+  account: bk1088                   # SLURM account name
+  cores: 128                        # number of CPU cores per SLURM job
+  memory: 240GiB                    # amount of memory per SLURM job
+  processes: 64                     # number of Dask workers per SLURM job
+  interface: ib0                    # use the infiniband network interface for communication
+  local_directory: "/scratch/username/dask-tmp"  # directory for spilling to disk
+  n_workers: 64                     # total number of workers to start
+```
+
+In this example we use the popular SLURM scheduduler, but other schedulers are
+also supported, see [this list](https://jobqueue.dask.org/en/latest/api.html).
+
+In the above example, ESMValCore will start 64 Dask workers
+(with 128 / 64 = 2 threads each) and for that it will need to launch a single
+SLURM batch job on the ``compute`` partition. If you would set ``n_workers`` to
+e.g. 256, it would launch 4 SLURM batch jobs which would each start 64 workers
+for a total of 4 x 64 = 256 workers. In the above configuration, each worker is
+allowed to use 240 GiB per job / 64 workers per job = ~4 GiB per worker.
+
+It is important to read the documentation about your HPC system and answer
+questions such as:
+- Which batch scheduler does my HPC system use?
+- How many CPU cores are available per node (a computer in an HPC system)?
+- How much memory is available for use per node?
+- What is the fastest network interface (run `ip a` to find the available
+  interfaces, infiniband `ib*` is much faster than ethernet `eth*`)?
+- What path should I use for storing temporary files on the nodes (try to
+  avoid slower network storage if possible)?
+- Which computing queue has the best availability?
+- Can I use part of a node or do I need to use the full node?
+  - If you are always charged for using the full node, asking for only part of
+    a node is wasteful of computational resources.
+  - If you can ask for part of a node, make sure the amount of memory you
+    request matches the number of CPU cores if possible, or you will be charged
+    for a larger fraction of the node.
+
+in order to find the optimal configuration for your situation.
+
+> ## Tune the configuration to your own computer
+>
+> Answer the questions above and create an ``~/.esmvaltool/dask.yml`` file that
+> matches your situation. To benefit from using an HPC system, you will probably
+> need to run a larger recipe than the example we have used so far. You could
+> try the full version of that recipe (
+> ``esmvaltool run examples/recipe_easy_ipcc.yml``) or use your own recipe.
+> To understand how the different settings affect performance, you may want to
+> experiment with different configurations.
+>
+>> ## Solution
+>>
+>> The best configuration depends on the HPC system that you are using.
+>> Discuss your answer with the instructor and the class if possible.
+>> If you are taking this course by yourself, you can have a look at the
+>> [Dask configuration examples][dask-configuration] in the ESMValCore
+>> documentation.
+> {: .solution}
+{: .challenge}
+
+{% include links.md %}