Ray Tune FAQ

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

What are Hyperparameters?

What are hyperparameters? And how are they different from model parameters?

In supervised learning, we train a model with labeled data so the model can properly identify new data values. Everything about the model is defined by a set of parameters, such as the weights in a linear regression. These are model parameters; they are learned during training.


In contrast, the hyperparameters define structural details about the kind of model itself, like whether or not we are using a linear regression or classification, what architecture is best for a neural network, how many layers, what kind of filters, etc. They are defined before training, not learned.


Other quantities considered hyperparameters include learning rates, discount rates, etc. If we want our training process and resulting model to work well, we first need to determine the optimal or near-optimal set of hyperparameters.

How do we determine the optimal hyperparameters? The most direct approach is to perform a loop where we pick a candidate set of values from some reasonably inclusive list of possible values, train a model, compare the results achieved with previous loop iterations, and pick the set that performed best. This process is called Hyperparameter Tuning or Optimization (HPO). And hyperparameters are specified over a configured and confined search space, collectively defined for each hyperparameter in a config dictionary.

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 How to use log metrics in Tune? 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:

    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 placement groups to request additional resources:

            {"CPU": 2, "GPU": 0.5, "hdd": 80},
            {"CPU": 1},
            {"CPU": 1},

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.

def train_fn(config, checkpoint_dir=None):
    # some Modin operations here
    # import modin.pandas as pd

            {"CPU": 1},  # this bundle will be used by the trainable itself
            {"CPU": 1},  # this bundle will be used by Modin

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=5, 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=5, 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

output = [random.randint(0, 100) for _ in range(10)]

# The output will always be the same.
assert output == [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 occurs, 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
    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.

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 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.

How can I develop and test Tune locally?

First, follow the instructions in Building Ray (Python Only) to develop Tune without compiling Ray. After Ray is set up, run pip install -r ray/python/ray/tune/requirements-dev.txt to install all packages required for Tune development. Now, to run all Tune tests simply run:

pytest ray/python/ray/tune/tests/

If you plan to submit a pull request, we recommend you to run unit tests locally beforehand to speed up the review process. Even though we have hooks to run unit tests automatically for each pull request, it’s usually quicker to run them on your machine first to avoid any obvious mistakes.

How can I get started contributing to Tune?

We use Github to track issues, feature requests, and bugs. Take a look at the ones labeled “good first issue” and “help wanted” for a place to start. Look for issues with “[tune]” in the title.


If raising a new issue or PR related to Tune, be sure to include “[tune]” in the title and add a tune label.

For project organization, Tune maintains a relatively up-to-date organization of issues on the Tune Github Project Board. Here, you can track and identify how issues are organized.

How can I make my Tune experiments reproducible?

Exact reproducibility of machine learning runs is hard to achieve. This is even more true in a distributed setting, as more non-determinism is introduced. For instance, if two trials finish at the same time, the convergence of the search algorithm might be influenced by which trial result is processed first. This depends on the searcher - for random search, this shouldn’t make a difference, but for most other searchers it will.

If you try to achieve some amount of reproducibility, there are two places where you’ll have to set random seeds:

  1. On the driver program, e.g. for the search algorithm. This will ensure that at least the initial configurations suggested by the search algorithms are the same.

  2. In the trainable (if required). Neural networks are usually initialized with random numbers, and many classical ML algorithms, like GBDTs, make use of randomness. Thus you’ll want to make sure to set a seed here so that the initialization is always the same.

Here is an example that will always produce the same result (except for trial runtimes).

import numpy as np
from ray import tune

def train(config):
    # Set seed for trainable random result.
    # If you remove this line, you will get different results
    # each time you run the trial, even if the configuration
    # is the same.
    random_result = np.random.uniform(0, 100, size=1).item()

# Set seed for Ray Tune's random search.
# If you remove this line, you will get different configurations
# each time you run the script.
    config={"seed": tune.randint(0, 1000)},

Some searchers use their own random states to sample new configurations. These searchers usually accept a seed parameter that can be passed on initialization. Other searchers use Numpy’s np.random interface - these seeds can be then set with np.random.seed(). We don’t offer an interface to do this in the searcher classes as setting a random seed globally could have side effects. For instance, it could influence the way your dataset is split. Thus, we leave it up to the user to make these global configuration changes.

How can I use large datasets in Tune?

You often will want to compute a large object (e.g., training data, model weights) on the driver and use that object within each trial.

Tune provides a wrapper function tune.with_parameters() that allows you to broadcast large objects to your trainable. Objects passed with this wrapper will be stored on the Ray object store and will be automatically fetched and passed to your trainable as a parameter.


If the objects are small in size or already exist in the Ray Object Store, there’s no need to use tune.with_parameters(). You can use partials or pass in directly to config instead.

from ray import tune
import numpy as np

def f(config, data=None):
    # use data

data = np.random.random(size=100000000)

tune.run(tune.with_parameters(f, data=data))

How can I upload my Tune results to cloud storage?

If an upload directory is provided, Tune will automatically sync results from the local_dir to the given directory, natively supporting standard URIs for systems like S3, gsutil or HDFS. Here is an example of uploading to S3, using a bucket called my-log-dir:


You can customize this to specify arbitrary storages with the syncer argument in tune.SyncConfig. This argument supports either strings with the same replacement fields OR arbitrary functions.

        upload_dir="s3://my-log-dir", syncer=custom_sync_str_or_func

If a string is provided, then it must include replacement fields {source} and {target}, like s3 sync {source} {target}. Alternatively, a function can be provided with the following signature:

import subprocess

def custom_sync_func(source, target):
    # run other workload here
    sync_cmd = "s3 {source} {target}".format(source=source, target=target)
    sync_process = subprocess.Popen(sync_cmd, shell=True)

By default, syncing occurs every 300 seconds. To change the frequency of syncing, set the sync_period attribute of the sync config to the desired syncing period.

Note that uploading only happens when global experiment state is collected, and the frequency of this is determined by the sync period. So the true upload period is given by max(sync period, TUNE_GLOBAL_CHECKPOINT_S).

Make sure that worker nodes have the write access to the cloud storage. Failing to do so would cause error messages like Error message (1): fatal error: Unable to locate credentials. For AWS set up, this involves adding an IamInstanceProfile configuration for worker nodes. Please see here for more tips.

How can I use Tune with Docker?

Tune automatically syncs files and checkpoints between different remote containers as needed.

To make this work in your Docker cluster, e.g. when you are using the Ray autoscaler with docker containers, you will need to pass a DockerSyncer to the syncer argument of tune.SyncConfig.

from ray import tune
from ray.tune.integration.docker import DockerSyncer

sync_config = tune.SyncConfig(syncer=DockerSyncer)

tune.run(train, sync_config=sync_config)

How can I use Tune with Kubernetes?

Ray Tune automatically synchronizes files and checkpoints between different remote nodes as needed. This usually happens via SSH, but this can be a performance bottleneck, especially when running many trials in parallel.

Instead you should use shared storage for checkpoints so that no additional synchronization across nodes is necessary. There are two main options.

First, you can use the SyncConfig to store your logs and checkpoints on cloud storage, such as AWS S3 or Google Cloud Storage:

from ray import tune

    # ...,

Second, you can set up a shared file system like NFS. If you do this, disable automatic trial syncing:

from ray import tune

    # ...,
        # Do not sync because we are on shared storage

Lastly, if you still want to use SSH for trial synchronization, but are not running on the Ray cluster launcher, you might need to pass a KubernetesSyncer to the syncer argument of tune.SyncConfig. You have to specify your Kubernetes namespace explicitly:

from ray.tune.integration.kubernetes import NamespacedKubernetesSyncer

sync_config = tune.SyncConfig(syncer=NamespacedKubernetesSyncer("ray"))

tune.run(train, sync_config=sync_config)

Please note that we strongly encourage you to use one of the other two options instead, as they will result in less overhead and don’t require pods to SSH into each other.

How do I configure search spaces?

You can specify a grid search or sampling distribution via the dict passed into tune.run(config=...).

parameters = {
    "qux": tune.sample_from(lambda spec: 2 + 2),
    "bar": tune.grid_search([True, False]),
    "foo": tune.grid_search([1, 2, 3]),
    "baz": "asd",  # a constant value

tune.run(train_fn, config=parameters)

By default, each random variable and grid search point is sampled once. To take multiple random samples, add num_samples: N to the experiment config. If grid_search is provided as an argument, the grid will be repeated num_samples of times.

# num_samples=10 repeats the 3x3 grid search 10 times, for a total of 90 trials
        "alpha": tune.uniform(100, 200),
        "beta": tune.sample_from(lambda spec: spec.config.alpha * np.random.normal()),
        "nn_layers": [
            tune.grid_search([16, 64, 256]),
            tune.grid_search([16, 64, 256]),

Note that search spaces may not be interoperable across different search algorithms. For example, for many search algorithms, you will not be able to use a grid_search or sample_from parameters. Read about this in the Search Space API page.