User Guide & Configuring Tune

These pages will demonstrate the various features and configurations of Tune.


Before you continue, be sure to have read Key Concepts.

This document provides an overview of the core concepts as well as some of the configurations for running Tune.

Resources (Parallelism, GPUs, Distributed)


To run everything sequentially, use Ray Local Mode.

Parallelism is determined by resources_per_trial (defaulting to 1 CPU, 0 GPU per trial) and the resources available to Tune (ray.cluster_resources()).

By default, Tune automatically runs N concurrent trials, where N is the number of CPUs (cores) on your machine.

# If you have 4 CPUs on your machine, this will run 4 concurrent trials at a time., num_samples=10)

You can override this parallelism with resources_per_trial:

# If you have 4 CPUs on your machine, this will run 2 concurrent trials at a time., num_samples=10, resources_per_trial={"cpu": 2})

# If you have 4 CPUs on your machine, this will run 1 trial at a time., num_samples=10, resources_per_trial={"cpu": 4})

# Fractional values are also supported, (i.e., {"cpu": 0.5})., num_samples=10, resources_per_trial={"cpu": 0.5})

Tune will allocate the specified GPU and CPU from resources_per_trial to each individual trial. A trial will not be scheduled unless at least that amount of resources is available, preventing the cluster from being overloaded.

Using GPUs

To leverage GPUs, you must set gpu in{}). This will automatically set CUDA_VISIBLE_DEVICES for each trial.

# If you have 8 GPUs, this will run 8 trials at once., num_samples=10, resources_per_trial={"gpu": 1})

# If you have 4 CPUs on your machine and 1 GPU, this will run 1 trial at a time., num_samples=10, resources_per_trial={"cpu": 2, "gpu": 1})

You can find an example of this in the Keras MNIST example.


If ‘gpu’ is not set, CUDA_VISIBLE_DEVICES environment variable will be set as empty, disallowing GPU access.

Troubleshooting: Occasionally, you may run into GPU memory issues when running a new trial. This may be due to the previous trial not cleaning up its GPU state fast enough. To avoid this, you can use tune.utils.wait_for_gpu - see docstring.

Concurrent samples

If using a search algorithm, you may want to limit the number of trials that are being evaluated. For example, you may want to serialize the evaluation of trials to do sequential optimization.

In this case, ray.tune.suggest.ConcurrencyLimiter to limit the amount of concurrency:

algo = BayesOptSearch(utility_kwargs={
    "kind": "ucb",
    "kappa": 2.5,
    "xi": 0.0
algo = ConcurrencyLimiter(algo, max_concurrent=4)
scheduler = AsyncHyperBandScheduler()

See ConcurrencyLimiter (tune.suggest.ConcurrencyLimiter) for more details.

Distributed Tuning


This section covers how to run Tune across multiple machines. See Distributed Training for guidance in tuning distributed training jobs.

To attach to a Ray cluster, simply run ray.init before See Starting Ray via the CLI (ray start) for more information about ray.init:

# Connect to an existing distributed Ray cluster
ray.init(address=<ray_address>), num_samples=100, resources_per_trial={"cpu": 2, "gpu": 1})

Read more in the Tune distributed experiments guide.

Tune Distributed Training

To tune distributed training jobs, Tune provides a set of DistributedTrainableCreator for different training frameworks. Below is an example for tuning distributed TensorFlow jobs:

# Please refer to full example in
from ray.tune.integration.tensorflow import DistributedTrainableCreator
tf_trainable = DistributedTrainableCreator(

Read more about tuning distributed PyTorch, TensorFlow and Horovod jobs.

Search Space (Grid/Random)

You can specify a grid search or sampling distribution via the dict passed into

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
}, 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),
         "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 parameter. Read about this in the Search Space API page.

Auto-filled Metrics

You can log arbitrary values and metrics in both training APIs:

def trainable(config):
    for i in range(num_epochs):
        ..., metric_foo=random_metric_1, bar=metric_2)

class Trainable(tune.Trainable):
    def step(self):
        # don't call report here!
        return dict(acc=accuracy, metric_foo=random_metric_1, bar=metric_2)

During training, Tune will automatically log the below metrics in addition to the user-provided values. All of these can be used as stopping conditions or passed as a parameter to Trial Schedulers/Search Algorithms.

  • config: The hyperparameter configuration

  • date: String-formatted date and time when the result was processed

  • done: True if the trial has been finished, False otherwise

  • episodes_total: Total number of episodes (for RLLib trainables)

  • experiment_id: Unique experiment ID

  • experiment_tag: Unique experiment tag (includes parameter values)

  • hostname: Hostname of the worker

  • iterations_since_restore: The number of times has been called after restoring the worker from a checkpoint

  • node_ip: Host IP of the worker

  • pid: Process ID (PID) of the worker process

  • time_since_restore: Time in seconds since restoring from a checkpoint.

  • time_this_iter_s: Runtime of the current training iteration in seconds (i.e. one call to the trainable function or to _train() in the class API.

  • time_total_s: Total runtime in seconds.

  • timestamp: Timestamp when the result was processed

  • timesteps_since_restore: Number of timesteps since restoring from a checkpoint

  • timesteps_total: Total number of timesteps

  • training_iteration: The number of times has been called

  • trial_id: Unique trial ID

All of these metrics can be seen in the Trial.last_result dictionary.


When running a hyperparameter search, Tune can automatically and periodically save/checkpoint your model. This allows you to:

  • save intermediate models throughout training

  • use pre-emptible machines (by automatically restoring from last checkpoint)

  • Pausing trials when using Trial Schedulers such as HyperBand and PBT.

To use Tune’s checkpointing features, you must expose a checkpoint_dir argument in the function signature, and call tune.checkpoint_dir:

import os
import time
from ray import tune

def train_func(config, checkpoint_dir=None):
    start = 0
    if checkpoint_dir:
        with open(os.path.join(checkpoint_dir, "checkpoint")) as f:
            state = json.loads(
            start = state["step"] + 1

    for step in range(start, 100):

        # Obtain a checkpoint directory
        with tune.checkpoint_dir(step=step) as checkpoint_dir:
            path = os.path.join(checkpoint_dir, "checkpoint")
            with open(path, "w") as f:
                f.write(json.dumps({"step": step}))"world", ray="tune")

In this example, checkpoints will be saved by training iteration to local_dir/exp_name/trial_name/checkpoint_<step>.

You can restore a single trial checkpoint by using<checkpoint_dir>) By doing this, you can change whatever experiments’ configuration such as the experiment’s name:

# Restored previous trial from the given checkpoint
    name="RestoredExp", # The name can be different.
    stop={"training_iteration": 10}, # train 5 more iterations than previous
    config={"env": "CartPole-v0"},

Distributed Checkpointing

On a multinode cluster, Tune automatically creates a copy of all trial checkpoints on the head node. This requires the Ray cluster to be started with the cluster launcher and also requires rsync to be installed.

Note that you must use the tune.checkpoint_dir API to trigger syncing. Also, if running Tune on Kubernetes, be sure to use the KubernetesSyncer to transfer files between different pods.

If you do not use the cluster launcher, you should set up a NFS or global file system and disable cross-node syncing:

sync_config = tune.SyncConfig(sync_to_driver=False), sync_config=sync_config)

Handling Large Datasets

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.

from ray import tune

import numpy as np

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

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

Stopping Trials

You can control when trials are stopped early by passing the stop argument to This argument takes, a dictionary, a function, or a Stopper class as an argument.

If a dictionary is passed in, the keys may be any field in the return result of in the Function API or step() (including the results from step and auto-filled metrics).

In the example below, each trial will be stopped either when it completes 10 iterations OR when it reaches a mean accuracy of 0.98. These metrics are assumed to be increasing.

# training_iteration is an auto-filled metric by Tune.
    stop={"training_iteration": 10, "mean_accuracy": 0.98}

For more flexibility, you can pass in a function instead. If a function is passed in, it must take (trial_id, result) as arguments and return a boolean (True if trial should be stopped and False otherwise).

def stopper(trial_id, result):
    return result["mean_accuracy"] / result["training_iteration"] > 5, stop=stopper)

Finally, you can implement the Stopper abstract class for stopping entire experiments. For example, the following example stops all trials after the criteria is fulfilled by any individual trial, and prevents new ones from starting:

from ray.tune import Stopper

class CustomStopper(Stopper):
    def __init__(self):
        self.should_stop = False

    def __call__(self, trial_id, result):
        if not self.should_stop and result['foo'] > 10:
            self.should_stop = True
        return self.should_stop

    def stop_all(self):
        """Returns whether to stop trials and prevent new ones from starting."""
        return self.should_stop

stopper = CustomStopper(), stop=stopper)

Note that in the above example the currently running trials will not stop immediately but will do so once their current iterations are complete.

Ray Tune comes with a set of out-of-the-box stopper classes. See the Stopper documentation.


Tune by default will log results for Tensorboard, CSV, and JSON formats. If you need to log something lower level like model weights or gradients, see Trainable Logging.

Learn more about logging and customizations here: Loggers (tune.logger).

Tune will log the results of each trial to a subfolder under a specified local dir, which defaults to ~/ray_results.

# This logs to 2 different trial folders:
# ~/ray_results/trainable_name/trial_name_1 and ~/ray_results/trainable_name/trial_name_2
# trainable_name and trial_name are autogenerated., num_samples=2)

You can specify the local_dir and trainable_name:

# This logs to 2 different trial folders:
# ./results/test_experiment/trial_name_1 and ./results/test_experiment/trial_name_2
# Only trial_name is autogenerated., num_samples=2, local_dir="./results", name="test_experiment")

To specify custom trial folder names, you can pass use the trial_name_creator argument to This takes a function with the following signature:

def trial_name_string(trial):
        trial (Trial): A generated trial object.

        trial_name (str): String representation of Trial.
    return str(trial)

See the documentation on Trials: Trial.

Tensorboard (Logging)

Tune automatically outputs Tensorboard files during To visualize learning in tensorboard, install tensorboardX:

$ pip install tensorboardX

Then, after you run an experiment, you can visualize your experiment with TensorBoard by specifying the output directory of your results.

$ tensorboard --logdir=~/ray_results/my_experiment

If you are running Ray on a remote multi-user cluster where you do not have sudo access, you can run the following commands to make sure tensorboard is able to write to the tmp directory:

$ export TMPDIR=/tmp/$USER; mkdir -p $TMPDIR; tensorboard --logdir=~/ray_results

If using TF2, Tune also automatically generates TensorBoard HParams output, as shown below:
        "lr": tune.grid_search([1e-5, 1e-4]),
        "momentum": tune.grid_search([0, 0.9])

Console Output

User-provided fields will be outputted automatically on a best-effort basis. You can use a Reporter object to customize the console output.

== Status ==
Memory usage on this node: 11.4/16.0 GiB
Using FIFO scheduling algorithm.
Resources requested: 4/12 CPUs, 0/0 GPUs, 0.0/3.17 GiB heap, 0.0/1.07 GiB objects
Result logdir: /Users/foo/ray_results/myexp
Number of trials: 4 (4 RUNNING)
| Trial name           | status   | loc                 |    param1 | param2 |    acc | total time (s) |  iter |
| MyTrainable_a826033a | RUNNING  | | 0.303706  | 0.0761 | 0.1289 |        7.54952 |    15 |
| MyTrainable_a8263fc6 | RUNNING  | | 0.929276  | 0.158  | 0.4865 |        7.0501  |    14 |
| MyTrainable_a8267914 | RUNNING  | | 0.068426  | 0.0319 | 0.9585 |        7.0477  |    14 |
| MyTrainable_a826b7bc | RUNNING  | | 0.729127  | 0.0748 | 0.1797 |        7.05715 |    14 |

Uploading Results

If an upload directory is provided, Tune will automatically sync results from the local_dir to the given directory, natively supporting standard S3/gsutil/HDFS URIs.

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

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:

def custom_sync_func(source, target):
    # do arbitrary things inside
    sync_cmd = "s3 {source} {target}".format(
    sync_process = subprocess.Popen(sync_cmd, shell=True)

By default, syncing occurs every 300 seconds. To change the frequency of syncing, set the TUNE_CLOUD_SYNC_S environment variable in the driver 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 TUNE_GLOBAL_CHECKPOINT_S environment variable. So the true upload period is given by max(TUNE_CLOUD_SYNC_S, TUNE_GLOBAL_CHECKPOINT_S).

Using 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 sync_to_driver argument of tune.SyncConfig.

from ray.tune.integration.docker import DockerSyncer
sync_config = tune.SyncConfig(
    sync_to_driver=DockerSyncer), sync_config=sync_config)

Using Tune with Kubernetes

Tune automatically syncs files and checkpoints between different remote nodes as needed. To make this work in your Kubernetes cluster, you will need to pass a KubernetesSyncer to the sync_to_driver argument of tune.SyncConfig. You have to specify your Kubernetes namespace explicitly:

from ray.tune.integration.kubernetes import NamespacedKubernetesSyncer
sync_config = tune.SyncConfig(
), sync_config=sync_config)

Redirecting stdout and stderr to files

The stdout and stderr streams are usually printed to the console. For remote actors, Ray collects these logs and prints them to the head process.

However, if you would like to collect the stream outputs in files for later analysis or troubleshooting, Tune offers an utility parameter, log_to_file, for this.

By passing log_to_file=True to, stdout and stderr will be logged to trial_logdir/stdout and trial_logdir/stderr, respectively:

If you would like to specify the output files, you can either pass one filename, where the combined output will be stored, or two filenames, for stdout and stderr, respectively:
    log_to_file=("my_stdout.log", "my_stderr.log"))

The file names are relative to the trial’s logdir. You can pass absolute paths, too.

If log_to_file is set, Tune will automatically register a new logging handler for Ray’s base logger and log the output to the specified stderr output file.


Ray Tune supports callbacks that are called during various times of the training process. Callbacks can be passed as a parameter to, and the submethod will be invoked automatically.

This simple callback just prints a metric each time a result is received:

from ray import tune
from ray.tune import Callback

class MyCallback(Callback):
    def on_trial_result(self, iteration, trials, trial, result, **info):
        print(f"Got result: {result['metric']}")

def train(config):
    for i in range(10):

For more details and available hooks, please see the API docs for Ray Tune callbacks.


By default, Tune will run hyperparameter evaluations on multiple processes. However, if you need to debug your training process, it may be easier to do everything on a single process. You can force all Ray functions to occur on a single process with local_mode by calling the following before


Local mode with multiple configuration evaluations will interleave computation, so it is most naturally used when running a single configuration evaluation.

Note that local_mode has some known issues, so please read these tips for more info.

Stopping after the first failure

By default, will continue executing until all trials have terminated or errored. To stop the entire Tune run as soon as any trial errors:, fail_fast=True)

This is useful when you are trying to setup a large hyperparameter experiment.

Environment variables

Some of Ray Tune’s behavior can be configured using environment variables. These are the environment variables Ray Tune currently considers:

  • TUNE_CLUSTER_SSH_KEY: SSH key used by the Tune driver process to connect to remote cluster machines for checkpoint syncing. If this is not set, ~/ray_bootstrap_key.pem will be used.

  • TUNE_DISABLE_AUTO_CALLBACK_LOGGERS: Ray Tune automatically adds a CSV and JSON logger callback if they haven’t been passed. Setting this variable to 1 disables this automatic creation. Please note that this will most likely affect analyzing your results after the tuning run.

  • TUNE_DISABLE_AUTO_CALLBACK_SYNCER: Ray Tune automatically adds a Syncer callback to sync logs and checkpoints between different nodes if none has been passed. Setting this variable to 1 disables this automatic creation. Please note that this will most likely affect advanced scheduling algorithms like PopulationBasedTraining.

  • TUNE_DISABLE_AUTO_INIT: Disable automatically calling ray.init() if not attached to a Ray session.

  • TUNE_DISABLE_DATED_SUBDIR: Ray Tune automatically adds a date string to experiment directories when the name is not specified explicitly or the trainable isn’t passed as a string. Setting this environment variable to 1 disables adding these date strings.

  • TUNE_DISABLE_STRICT_METRIC_CHECKING: When you report metrics to Tune via and passed a metric parameter to, a scheduler, or a search algorithm, Tune will error if the metric was not reported in the result. Setting this environment variable to 1 will disable this check.

  • TUNE_FUNCTION_THREAD_TIMEOUT_S: Time in seconds the function API waits for threads to finish after instructing them to complete. Defaults to 2.

  • TUNE_GLOBAL_CHECKPOINT_S: Time in seconds that limits how often Tune’s experiment state is checkpointed. If not set this will default to 10.

  • TUNE_MAX_LEN_IDENTIFIER: Maximum length of trial subdirectory names (those with the parameter values in them)

  • TUNE_MAX_PENDING_TRIALS_PG: Maximum number of pending trials when placement groups are used. Defaults to 1000.

  • TUNE_RESULT_DIR: Directory where Ray Tune trial results are stored. If this is not set, ~/ray_results will be used.

  • TUNE_RESULT_BUFFER_LENGTH: Ray Tune can buffer results from trainables before they are passed to the driver. Enabling this might delay scheduling decisions, as trainables are speculatively continued. Setting this to 0 disables result buffering. Defaults to 1000 (results).

  • TUNE_RESULT_BUFFER_MAX_TIME_S: Similarly, Ray Tune buffers results up to number_of_trial/10 seconds, but never longer than this value. Defaults to 100 (seconds).

  • TUNE_RESULT_BUFFER_MIN_TIME_S: Additionally, you can specify a minimum time to buffer results. Defaults to 0.

  • TUNE_SYNCER_VERBOSITY: Amount of command output when using Tune with Docker Syncer. Defaults to 0.

  • TUNE_TRIAL_STARTUP_GRACE_PERIOD: Amount of time after starting a trial that Ray Tune checks for successful trial startups. After the grace period, Tune will block until a result from a running trial is received. Can be disabled by setting this to lower or equal to 0.

  • TUNE_WARN_THRESHOLD_S: Threshold for logging if an Tune event loop operation takes too long. Defaults to 0.5 (seconds).

  • TUNE_STATE_REFRESH_PERIOD: Frequency of updating the resource tracking from Ray. Defaults to 10 (seconds).

There are some environment variables that are mostly relevant for integrated libraries:

  • SIGOPT_KEY: SigOpt API access key.

  • WANDB_API_KEY: Weights and Biases API key. You can also use wandb login instead.

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.