.. _tune-faq: Ray Tune FAQ ------------ Here we try to answer questions that come up often. If you still have questions after reading this, let us know! .. contents:: :local: :depth: 1 Which search algorithm/scheduler should I choose? ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Ray Tune offers :ref:`many different search algorithms ` and :ref:`schedulers `. Deciding on which to use mostly depends on your problem: * Is it a small or large problem (how long does it take to train? How costly are the resources, like GPUs)? Can you run many trials in parallel? * How many hyperparameters would you like to tune? * What values are valid for hyperparameters? **If your model returns incremental results** (eg. results per epoch in deep learning, results per each added tree in GBDTs, etc.) using early stopping usually allows for sampling more configurations, as unpromising trials are pruned before they run their full course. Please note that not all search algorithms can use information from pruned trials. Early stopping cannot be used without incremental results - in case of the functional API, that means that ``tune.report()`` has to be called more than once - usually in a loop. **If your model is small**, you can usually try to run many different configurations. A **random search** can be used to generate configurations. You can also grid search over some values. You should probably still use :ref:`ASHA for early termination of bad trials ` (if your problem supports early stopping). **If your model is large**, you can try to either use **Bayesian Optimization-based search algorithms** like :ref:`BayesOpt ` or :ref:`Dragonfly ` to get good parameter configurations after few trials. :ref:`Ax ` is similar but more robust to noisy data. Please note that these algorithms only work well with **a small number of hyperparameters**. Alternatively, you can use :ref:`Population Based Training ` which works well with few trials, e.g. 8 or even 4. However, this will output a hyperparameter *schedule* rather than one fixed set of hyperparameters. **If you have a small number of hyperparameters**, Bayesian Optimization methods work well. Take a look at :ref:`BOHB ` or :ref:`Optuna ` with the :ref:`ASHA ` scheduler to combine the benefits of Bayesian Optimization with early stopping. **If you only have continuous values for hyperparameters** this will work well with most Bayesian Optimization methods. Discrete or categorical variables still work, but less good with an increasing number of categories. **If you have many categorical values for hyperparameters**, consider using random search, or a TPE-based Bayesian Optimization algorithm such as :ref:`Optuna ` or :ref:`HyperOpt `. **Our go-to solution** is usually to use **random search** with :ref:`ASHA for early stopping ` for smaller problems. Use :ref:`BOHB ` for **larger problems** with a **small number of hyperparameters** and :ref:`Population Based Training ` for **larger problems** with a **large number of hyperparameters** if a learning schedule is acceptable. How do I choose hyperparameter ranges? ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ A good start is to look at the papers that introduced the algorithms, and also to see what other people are using. Most algorithms also have sensible defaults for some of their parameters. For instance, `XGBoost's parameter overview `_ reports to use ``max_depth=6`` for the maximum decision tree depth. Here, anything between 2 and 10 might make sense (though that naturally depends on your problem). For **learning rates**, we suggest using a **loguniform distribution** between **1e-5** and **1e-1**: ``tune.loguniform(1e-5, 1e-1)``. For **batch sizes**, we suggest trying **powers of 2**, for instance, 2, 4, 8, 16, 32, 64, 128, 256, etc. The magnitude depends on your problem. For easy problems with lots of data, use higher batch sizes, for harder problems with not so much data, use lower batch sizes. For **layer sizes** we also suggest trying **powers of 2**. For small problems (e.g. Cartpole), use smaller layer sizes. For larger problems, try larger ones. For **discount factors** in reinforcement learning we suggest sampling uniformly between 0.9 and 1.0. Depending on the problem, a much stricter range above 0.97 or oeven above 0.99 can make sense (e.g. for Atari). How can I use nested/conditional search spaces? ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Sometimes you might need to define parameters whose value depend on the value of other parameters. Ray Tune offers some methods to define these. Nested spaces ''''''''''''' You can nest hyperparameter definition in sub dictionaries: .. code-block:: python config = { "a": { "x": tune.uniform(0, 10) }, "b": tune.choice([1, 2, 3]) } The trial config will be nested exactly like the input config. Conditional spaces '''''''''''''''''' :ref:`Custom and conditional search spaces are explained in detail here `. In short, you can pass custom functions to ``tune.sample_from()`` that can return values that depend on other values: .. code-block:: python config = { "a": tune.randint(5, 10) "b": tune.sample_from(lambda spec: np.random.randint(0, spec.config.a)) } Conditional grid search ''''''''''''''''''''''' If you would like to grid search over two parameters that depend on each other, this might not work out of the box. For instance say that *a* should be a value between 5 and 10 and *b* should be a value between 0 and a. In this case, we cannot use ``tune.sample_from`` because it doesn't support grid searching. The solution here is to create a list of valid *tuples* with the help of a helper function, like this: .. code-block:: python def _iter(): for a in range(5, 10): for b in range(a): yield a, b config = { "ab": tune.grid_search(list(_iter())), } Your trainable then can do something like ``a, b = config["ab"]`` to split the a and b variables and use them afterwards. How does early termination (e.g. Hyperband/ASHA) work? ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Early termination algorithms look at the intermediately reported values, e.g. what is reported to them via ``tune.report()`` after each training epoch. After a certain number of steps, they then remove the worst performing trials and keep only the best performing trials. Goodness of a trial is determined by ordering them by the objective metric, for instance accuracy or loss. In ASHA, you can decide how many trials are early terminated. ``reduction_factor=4`` means that only 25% of all trials are kept each time they are reduced. With ``grace_period=n`` you can force ASHA to train each trial at least for ``n`` epochs. Why are all my trials returning "1" iteration? ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ **This is most likely applicable for the Tune function API.** Ray Tune counts iterations internally every time ``tune.report()`` is called. If you only call ``tune.report()`` once at the end of the training, the counter has only been incremented once. If you're using the class API, the counter is increased after calling ``step()``. Note that it might make sense to report metrics more often than once. For instance, if you train your algorithm for 1000 timesteps, consider reporting intermediate performance values every 100 steps. That way, schedulers like Hyperband/ASHA can terminate bad performing trials early. What are all these extra outputs? ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ You'll notice that Ray Tune not only reports hyperparameters (from the ``config``) or metrics (passed to ``tune.report()``), but also some other outputs. .. code-block:: bash Result for easy_objective_c64c9112: date: 2020-10-07_13-29-18 done: false experiment_id: 6edc31257b564bf8985afeec1df618ee experiment_tag: 7_activation=tanh,height=-53.116,steps=100,width=13.885 hostname: ubuntu iterations: 0 iterations_since_restore: 1 mean_loss: 4.688385317424468 neg_mean_loss: -4.688385317424468 node_ip: 192.168.1.115 pid: 5973 time_since_restore: 7.605552673339844e-05 time_this_iter_s: 7.605552673339844e-05 time_total_s: 7.605552673339844e-05 timestamp: 1602102558 timesteps_since_restore: 0 training_iteration: 1 trial_id: c64c9112 See the :ref:`tune-autofilled-metrics` section for a glossary. How do I set resources? ~~~~~~~~~~~~~~~~~~~~~~~ If you want to allocate specific resources to a trial, you can use the ``resources_per_trial`` parameter of ``tune.run()``, to which you can pass a dict or a :class:`PlacementGroupFactory ` object: .. code-block:: python tune.run( train_fn, resources_per_trial={ "cpu": 2, "gpu": 0.5, "custom_resources": {"hdd": 80} } ) The example above showcases three things: 1. The `cpu` and `gpu` options set how many CPUs and GPUs are available for each trial, respectively. **Trials cannot request more resources** than these (exception: see 3). 2. It is possible to request **fractional GPUs**. A value of 0.5 means that half of the memory of the GPU is made available to the trial. You will have to make sure yourself that your model still fits on the fractional memory. 3. You can request custom resources you supplied to Ray when starting the cluster. Trials will only be scheduled on single nodes that can provide all resources you requested. One important thing to keep in mind is that each Ray worker (and thus each Ray Tune Trial) will only be scheduled on **one machine**. That means if you for instance request 2 GPUs for your trial, but your cluster consists of 4 machines with 1 GPU each, the trial will never be scheduled. In other words, you will have to make sure that your Ray cluster has machines that can actually fulfill your resource requests. In some cases your trainable might want to start other remote actors, for instance if you're leveraging distributed training via Ray Train. In these cases, you can use :ref:`placement groups ` to request additional resources: .. code-block:: python tune.run( train_fn, resources_per_trial=tune.PlacementGroupFactory([ {"CPU": 2, "GPU": 0.5, "hdd": 80}, {"CPU": 1}, {"CPU": 1}, ], strategy="PACK") Here, you're requesting 2 additional CPUs for remote tasks. These two additional actors do not necessarily have to live on the same node as your main trainable. In fact, you can control this via the ``strategy`` parameter. In this example, ``PACK`` will try to schedule the actors on the same node, but allows them to be scheduled on other nodes as well. Please refer to the :ref:`placement groups documentation ` to learn more about these placement strategies. Why is my training stuck and Ray reporting that pending actor or tasks cannot be scheduled? ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ This is usually caused by Ray actors or tasks being started by the trainable without the trainable resources accounting for them, leading to a deadlock. This can also be "stealthly" caused by using other libraries in the trainable that are based on Ray, such as Modin. In order to fix the issue, request additional resources for the trial using :ref:`placement groups `, as outlined in the section above. For example, if your trainable is using Modin dataframes, operations on those will spawn Ray tasks. By allocating an additional CPU bundle to the trial, those tasks will be able to run without being starved of resources. .. code-block:: python import modin.pandas as pd def train_fn(config, checkpoint_dir=None): # some Modin operations here tune.report(metric=metric) tune.run( train_fn, resources_per_trial=tune.PlacementGroupFactory([ {"CPU": 1}, # this bundle will be used by the trainable itself {"CPU": 1}, # this bundle will be used by Modin ], strategy="PACK") How can I pass further parameter values to my trainable? ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Ray Tune expects your trainable functions to accept only up to two parameters, ``config`` and ``checkpoint_dir``. But sometimes there are cases where you want to pass constant arguments, like the number of epochs to run, or a dataset to train on. Ray Tune offers a wrapper function to achieve just that, called :func:`tune.with_parameters() `: .. code-block:: python from ray import tune import numpy as np def train(config, checkpoint_dir=None, num_epochs=10, data=None): for i in range(num_epochs): for sample in data: # ... train on sample # Some huge dataset data = np.random.random(size=100000000) tune.run( tune.with_parameters(train, num_epochs=10, data=data)) This function works similarly to ``functools.partial``, but it stores the parameters directly in the Ray object store. This means that you can pass even huge objects like datasets, and Ray makes sure that these are efficiently stored and retrieved on your cluster machines. :func:`tune.with_parameters() ` also works with class trainables. Please see :ref:`here for further details ` and examples. How can I reproduce experiments? ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Reproducing experiments and experiment results means that you get the exact same results when running an experiment again and again. To achieve this, the conditions have to be exactly the same each time you run the exeriment. In terms of ML training and tuning, this mostly concerns the random number generators that are used for sampling in various places of the training and tuning lifecycle. Random number generators are used to create randomness, for instance to sample a hyperparameter value for a parameter you defined. There is no true randomness in computing, rather there are sophisticated algorithms that generate numbers that *seem* to be random and fulfill all properties of a random distribution. These algorithms can be *seeded* with an initial state, after which the generated random numbers are always the same. .. code-block:: python import random random.seed(1234) print([random.randint(0, 100) for _ in range(10)]) # The output of this will always be # [99, 56, 14, 0, 11, 74, 4, 85, 88, 10] The most commonly used random number generators from Python libraries are those in the native ``random`` submodule and the ``numpy.random`` module. .. code-block:: python # This should suffice to initialize the RNGs for most Python-based libraries import random import numpy as np random.seed(1234) np.random.seed(5678) In your tuning and training run, there are several places where randomness occurrs, and at all these places we will have to introduce seeds to make sure we get the same behavior. * **Search algorithm**: Search algorithms have to be seeded to generate the same hyperparameter configurations in each run. Some search algorithms can be explicitly instantiated with a random seed (look for a ``seed`` parameter in the constructor). For others, try to use the above code block. * **Schedulers**: Schedulers like Population Based Training rely on resampling some of the parameters, requiring randomness. Use the code block above to set the initial seeds. * **Training function**: In addition to initializing the configurations, the training functions themselves have to use seeds. This could concern e.g. the data splitting. You should make sure to set the seed at the start of your training function. PyTorch and TensorFlow use their own RNGs, which have to be initialized, too: .. code-block:: python import torch torch.manual_seed(0) import tensorflow as tf tf.random.set_seed(0) You should thus seed both Ray Tune's schedulers and search algorithms, and the training code. The schedulers and search algorithms should always be seeded with the same seed. This is also true for the training code, but often it is beneficial that the seeds differ *between different training runs*. Here's a blueprint on how to do all this in your training code: .. code-block:: python import random import numpy as np from ray import tune def trainable(config): # config["seed"] is set deterministically, but differs between training runs random.seed(config["seed"]) np.random.seed(config["seed"]) # torch.manual_seed(config["seed"]) # ... training code config = { "seed": tune.randint(0, 10000), # ... } if __name__ == "__main__": # Set seed for the search algorithms/schedulers random.seed(1234) np.random.seed(1234) # Don't forget to check if the search alg has a `seed` parameter tune.run( trainable, config=config ) **Please note** that it is not always possible to control all sources of non-determinism. For instance, if you use schedulers like ASHA or PBT, some trials might finish earlier than other trials, affecting the behavior of the schedulers. Which trials finish first can however depend on the current system load, network communication, or other factors in the envrionment that we cannot control with random seeds. This is also true for search algorithms such as Bayesian Optimization, which take previous results into account when sampling new configurations. This can be tackled by using the **synchronous modes** of PBT and Hyperband, where the schedulers wait for all trials to finish an epoch before deciding which trials to promote. We strongly advise to try reproduction on smaller toy problems first before relying on it for larger experiments. .. _tune-bottlenecks: How can I avoid bottlenecks? ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Sometimes you might run into a message like this: .. code-block:: The `experiment_checkpoint` operation took 2.43 seconds to complete, which may be a performance bottleneck Most commonly, the ``experiment_checkpoint`` operation is throwing this warning, but it might be something else, like ``process_trial_result``. These operations should usually take less than 500ms to complete. When it consistently takes longer, this might indicate a problem or inefficiencies. To get rid of this message, it is important to understand where it comes from. These are the main reasons this problem comes up: **The Trial config is very large** This is the case if you e.g. try to pass a dataset or other large object via the ``config`` parameter. If this is the case, the dataset is serialized and written to disk repeatedly during experiment checkpointing, which takes a long time. **Solution**: Use :func:`tune.with_parameters ` to pass large objects to function trainables via the objects store. For class trainables you can do this manually via ``ray.put()`` and ``ray.get()``. If you need to pass a class definition, consider passing an indicator (e.g. a string) instead and let the trainable select the class instead. Generally, your config dictionary should only contain primitive types, like numbers or strings. **The Trial result is very large** This is the case if you return objects, data, or other large objects via the return value of ``step()`` in your class trainable or to ``tune.report()`` in your function trainable. The effect is the same as above: The results are repeatedly serialized and written to disk, and this can take a long time. **Solution**: Usually you should be able to write data to the trial directory instead. You can then pass a filename back (or assume it is a known location). The trial dir is usually the current working directory. Class trainables have the ``Trainable.logdir`` property and function trainables the :func:`ray.tune.get_trial_dir` function to retrieve the logdir. If you really have to, you can also ``ray.put()`` an object to the Ray object store and retrieve it with ``ray.get()`` on the other side. Generally, your result dictionary should only contain primitive types, like numbers or strings. **You are training a large number of trials on a cluster, or you are saving huge checkpoints** Checkpoints and logs are synced between nodes - usually at least to the driver on the head node, but sometimes between worker nodes if needed (e.g. when using :ref:`Population Based Training `). If these checkpoints are very large (e.g. for NLP models), or if you are training a large number of trials, this syncing can take a long time. If nothing else is specified, syncing happens via SSH, which can lead to network overhead as connections are not kept open by Ray Tune. **Solution**: There are multiple solutions, depending on your needs: 1. You can disable syncing to the driver in the :class:`tune.SyncConfig `. In this case, logs and checkpoints will not be synced to the driver, so if you need to access them later, you will have to transfer them where you need them manually. 2. You can use :ref:`cloud checkpointing ` to save logs and checkpoints to a specified `upload_dir`. This is the preferred way to deal with this. All syncing will be taken care of automatically, as all nodes are able to access the cloud storage. Additionally, your results will be safe, so even when you're working on pre-emptible instances, you won't lose any of your data. **You are reporting results too often** Each result is processed by the search algorithm, trial scheduler, and callbacks (including loggers and the trial syncer). If you're reporting a large number of results per trial (e.g. multiple results per second), this can take a long time. **Solution**: The solution here is obvious: Just don't report results that often. In class trainables, ``step()`` should maybe process a larger chunk of data. In function trainables, you can report only every n-th iteration of the training loop. Try to balance the number of results you really need to make scheduling or searching decisions. If you need more fine grained metrics for logging or tracking, consider using a separate logging mechanism for this instead of the Ray Tune-provided progress logging of results.