Tutorials & FAQ


We’d love to hear your feedback on using Tune - get in touch!

In this section, you can find material on how to use Tune and its various features. If any of the materials is out of date or broken, or if you’d like to add an example to this page, feel free to raise an issue on our Github repository.

Take a look at any of the below tutorials to get started with Tune.

Colab Exercises

Learn how to use Tune in your browser with the following Colab-based exercises.

Exercise Description Library Colab Link
Basics of using Tune. TF/Keras Tune Tutorial
Using Search algorithms and Trial Schedulers to optimize your model. Pytorch Tune Tutorial
Using Population-Based Training (PBT). Pytorch Tune Tutorial
Fine-tuning Huggingface Transformers with PBT. Huggingface Transformers/Pytorch Tune Tutorial

Tutorial source files can be found here.

What’s Next?

Check out:

Frequently asked questions

Here we try to answer questions that come up often. If you still have questions after reading this, let us know!

Which search algorithm/scheduler should I choose?

Ray Tune offers many different search algorithms and 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 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 BayesOpt or Dragonfly to get good parameter configurations after few trials. 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 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 BOHB or Optuna with the 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 Optuna or HyperOpt.

Our go-to solution is usually to use random search with ASHA for early stopping for smaller problems. Use BOHB for larger problems with a small number of hyperparameters and 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:

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

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:

config = {
    "a": tune.randint(5, 10)
    "b": tune.sample_from(lambda spec: np.random.randint(0, spec.config.a))

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.

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
  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 Auto-filled 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 PlacementGroupFactory object:

        "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 placement groups to request additional resources:

        {"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 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 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.

import modin.pandas as pd

def train_fn(config, checkpoint_dir=None):
    # some Modin operations here

        {"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 tune.with_parameters():

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.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.

tune.with_parameters() also works with class trainables. Please see 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.

import random
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.

# This should suffice to initialize the RNGs for most Python-based libraries
import random
import numpy as np

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:

import torch

import tensorflow as tf

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:

import random
import numpy as np
from ray import tune

def trainable(config):
    # config["seed"] is set deterministically, but differs between training runs
    # torch.manual_seed(config["seed"])
    # ... training code

config = {
    "seed": tune.randint(0, 10000),
    # ...

if __name__ == "__main__":
    # Set seed for the search algorithms/schedulers
    # Don't forget to check if the search alg has a `seed` parameter

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.

How can I avoid bottlenecks?

Sometimes you might run into a message like this:

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 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 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 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 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 the ray.tune.durable wrapper 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.

Further Questions or Issues?

You can post questions or issues or feedback through the following channels:

  1. Discussion Board: For questions about Ray usage or feature requests.

  2. GitHub Issues: For bug reports.

  3. Ray Slack: For getting in touch with Ray maintainers.

  4. StackOverflow: Use the [ray] tag questions about Ray.