rllib-training.rst

Important

The ML team at Anyscale Inc., the company behind Ray, is looking for interns and full-time reinforcement learning engineers to help advance and maintain RLlib. If you have a background in ML/RL and are interested in making RLlib the industry-leading open-source RL library, apply here today. We'd be thrilled to welcome you on the team!

RLlib Training APIs

Getting Started

At a high level, RLlib provides an Trainer class which holds a policy for environment interaction. Through the trainer interface, the policy can be trained, checkpointed, or an action computed. In multi-agent training, the trainer manages the querying and optimization of multiple policies at once.

You can train a simple DQN trainer with the following command:

rllib train --run DQN --env CartPole-v0  # --config '{"framework": "tf2", "eager_tracing": true}' for eager execution

By default, the results will be logged to a subdirectory of ~/ray_results. This subdirectory will contain a file params.json which contains the hyperparameters, a file result.json which contains a training summary for each episode and a TensorBoard file that can be used to visualize training process with TensorBoard by running

tensorboard --logdir=~/ray_results

The rllib train command (same as the train.py script in the repo) has a number of options you can show by running:

rllib train --help
-or-
python ray/rllib/train.py --help

The most important options are for choosing the environment with --env (any OpenAI gym environment including ones registered by the user can be used) and for choosing the algorithm with --run (available options include SAC, PPO, PG, A2C, A3C, IMPALA, ES, DDPG, DQN, MARWIL, APEX, and APEX_DDPG).

Evaluating Trained Policies

In order to save checkpoints from which to evaluate policies, set --checkpoint-freq (number of training iterations between checkpoints) when running rllib train.

An example of evaluating a previously trained DQN policy is as follows:

rllib rollout \
    ~/ray_results/default/DQN_CartPole-v0_0upjmdgr0/checkpoint_1/checkpoint-1 \
    --run DQN --env CartPole-v0 --steps 10000

The rollout.py helper script reconstructs a DQN policy from the checkpoint located at ~/ray_results/default/DQN_CartPole-v0_0upjmdgr0/checkpoint_1/checkpoint-1 and renders its behavior in the environment specified by --env.

(Type rllib rollout --help to see the available evaluation options.)

For more advanced evaluation functionality, refer to Customized Evaluation During Training.

Configuration

Specifying Parameters

Each algorithm has specific hyperparameters that can be set with --config, in addition to a number of common hyperparameters. See the algorithms documentation for more information.

In an example below, we train A2C by specifying 8 workers through the config flag.

rllib train --env=PongDeterministic-v4 --run=A2C --config '{"num_workers": 8}'

Specifying Resources

You can control the degree of parallelism used by setting the num_workers hyperparameter for most algorithms. The Trainer will construct that many "remote worker" instances (see RolloutWorker class) that are constructed as ray.remote actors, plus exactly one "local worker", a RolloutWorker object that is not a ray actor, but lives directly inside the Trainer. For most algorithms, learning updates are performed on the local worker and sample collection from one or more environments is performed by the remote workers (in parallel). For example, setting num_workers=0 will only create the local worker, in which case both sample collection and training will be done by the local worker. On the other hand, setting num_workers=5 will create the local worker (responsible for training updates) and 5 remote workers (responsible for sample collection).

Since learning is most of the time done on the local worker, it may help to provide one or more GPUs to that worker via the num_gpus setting. Similarly, the resource allocation to remote workers can be controlled via num_cpus_per_worker, num_gpus_per_worker, and custom_resources_per_worker.

The number of GPUs can be fractional quantities (e.g. 0.5) to allocate only a fraction of a GPU. For example, with DQN you can pack five trainers onto one GPU by setting num_gpus: 0.2. Check out this fractional GPU example here as well that also demonstrates how environments (running on the remote workers) that require a GPU can benefit from the num_gpus_per_worker setting.

For synchronous algorithms like PPO and A2C, the driver and workers can make use of the same GPU. To do this for an amount of n GPUS:

gpu_count = n
num_gpus = 0.0001 # Driver GPU
num_gpus_per_worker = (gpu_count - num_gpus) / num_workers

If you specify num_gpus and your machine does not have the required number of GPUs available, a RuntimeError will be thrown by the respective worker. On the other hand, if you set num_gpus=0, your policies will be built solely on the CPU, even if GPUs are available on the machine.

Scaling Guide

Here are some rules of thumb for scaling training with RLlib.

1. If the environment is slow and cannot be replicated (e.g., since it requires interaction with physical systems), then you should use a sample-efficient off-policy algorithm such as :ref:`DQN <dqn>` or :ref:`SAC <sac>`. These algorithms default to num_workers: 0 for single-process operation. Make sure to set num_gpus: 1 if you want to use a GPU. Consider also batch RL training with the offline data API.
2. If the environment is fast and the model is small (most models for RL are), use time-efficient algorithms such as :ref:`PPO <ppo>`, :ref:`IMPALA <impala>`, or :ref:`APEX <apex>`. These can be scaled by increasing num_workers to add rollout workers. It may also make sense to enable vectorization for inference. Make sure to set num_gpus: 1 if you want to use a GPU. If the learner becomes a bottleneck, multiple GPUs can be used for learning by setting num_gpus > 1.
3. If the model is compute intensive (e.g., a large deep residual network) and inference is the bottleneck, consider allocating GPUs to workers by setting num_gpus_per_worker: 1. If you only have a single GPU, consider num_workers: 0 to use the learner GPU for inference. For efficient use of GPU time, use a small number of GPU workers and a large number of envs per worker.
4. Finally, if both model and environment are compute intensive, then enable remote worker envs with async batching by setting remote_worker_envs: True and optionally remote_env_batch_wait_ms. This batches inference on GPUs in the rollout workers while letting envs run asynchronously in separate actors, similar to the SEED architecture. The number of workers and number of envs per worker should be tuned to maximize GPU utilization. If your env requires GPUs to function, or if multi-node SGD is needed, then also consider :ref:`DD-PPO <ddppo>`.

Common Parameters

The following is a list of the common algorithm hyperparameters:

.. literalinclude:: ../../rllib/agents/trainer.py
   :language: python
   :start-after: __sphinx_doc_begin__
   :end-before: __sphinx_doc_end__

Tuned Examples

Some good hyperparameters and settings are available in the repository (some of them are tuned to run on GPUs). If you find better settings or tune an algorithm on a different domain, consider submitting a Pull Request!

You can run these with the rllib train command as follows:

rllib train -f /path/to/tuned/example.yaml

Basic Python API

The Python API provides the needed flexibility for applying RLlib to new problems. You will need to use this API if you wish to use custom environments, preprocessors, or models with RLlib.

Here is an example of the basic usage (for a more complete example, see custom_env.py):

import ray
import ray.rllib.agents.ppo as ppo
from ray.tune.logger import pretty_print

ray.init()
config = ppo.DEFAULT_CONFIG.copy()
config["num_gpus"] = 0
config["num_workers"] = 1
trainer = ppo.PPOTrainer(config=config, env="CartPole-v0")

# Can optionally call trainer.restore(path) to load a checkpoint.

for i in range(1000):
   # Perform one iteration of training the policy with PPO
   result = trainer.train()
   print(pretty_print(result))

   if i % 100 == 0:
       checkpoint = trainer.save()
       print("checkpoint saved at", checkpoint)

# Also, in case you have trained a model outside of ray/RLlib and have created
# an h5-file with weight values in it, e.g.
# my_keras_model_trained_outside_rllib.save_weights("model.h5")
# (see: https://keras.io/models/about-keras-models/)

# ... you can load the h5-weights into your Trainer's Policy's ModelV2
# (tf or torch) by doing:
trainer.import_model("my_weights.h5")
# NOTE: In order for this to work, your (custom) model needs to implement
# the `import_from_h5` method.
# See https://github.com/ray-project/ray/blob/master/rllib/tests/test_model_imports.py
# for detailed examples for tf- and torch trainers/models.

Note

It's recommended that you run RLlib trainers with :doc:`Tune <tune/index>`, for easy experiment management and visualization of results. Just set "run": ALG_NAME, "env": ENV_NAME in the experiment config.

All RLlib trainers are compatible with the :ref:`Tune API <tune-60-seconds>`. This enables them to be easily used in experiments with :doc:`Tune <tune/index>`. For example, the following code performs a simple hyperparam sweep of PPO:

import ray
from ray import tune

ray.init()
tune.run(
    "PPO",
    stop={"episode_reward_mean": 200},
    config={
        "env": "CartPole-v0",
        "num_gpus": 0,
        "num_workers": 1,
        "lr": tune.grid_search([0.01, 0.001, 0.0001]),
    },
)

Tune will schedule the trials to run in parallel on your Ray cluster:

== Status ==
Using FIFO scheduling algorithm.
Resources requested: 4/4 CPUs, 0/0 GPUs
Result logdir: ~/ray_results/my_experiment
PENDING trials:
 - PPO_CartPole-v0_2_lr=0.0001:     PENDING
RUNNING trials:
 - PPO_CartPole-v0_0_lr=0.01:       RUNNING [pid=21940], 16 s, 4013 ts, 22 rew
 - PPO_CartPole-v0_1_lr=0.001:      RUNNING [pid=21942], 27 s, 8111 ts, 54.7 rew

tune.run() returns an ExperimentAnalysis object that allows further analysis of the training results and retrieving the checkpoint(s) of the trained agent. It also simplifies saving the trained agent. For example:

# tune.run() allows setting a custom log directory (other than ``~/ray-results``)
# and automatically saving the trained agent
analysis = ray.tune.run(
    ppo.PPOTrainer,
    config=config,
    local_dir=log_dir,
    stop=stop_criteria,
    checkpoint_at_end=True)

# list of lists: one list per checkpoint; each checkpoint list contains
# 1st the path, 2nd the metric value
checkpoints = analysis.get_trial_checkpoints_paths(
    trial=analysis.get_best_trial("episode_reward_mean"),
    metric="episode_reward_mean")

# or simply get the last checkpoint (with highest "training_iteration")
last_checkpoint = analysis.get_last_checkpoint()
# if there are multiple trials, select a specific trial or automatically
# choose the best one according to a given metric
last_checkpoint = analysis.get_last_checkpoint(
    metric="episode_reward_mean", mode="max"
)

Loading and restoring a trained agent from a checkpoint is simple:

agent = ppo.PPOTrainer(config=config, env=env_class)
agent.restore(checkpoint_path)

Computing Actions

The simplest way to programmatically compute actions from a trained agent is to use trainer.compute_action(). This method preprocesses and filters the observation before passing it to the agent policy. Here is a simple example of testing a trained agent for one episode:

# instantiate env class
env = env_class(env_config)

# run until episode ends
episode_reward = 0
done = False
obs = env.reset()
while not done:
    action = agent.compute_action(obs)
    obs, reward, done, info = env.step(action)
    episode_reward += reward

For more advanced usage, you can access the workers and policies held by the trainer directly as compute_action() does:

class Trainer(Trainable):

  @PublicAPI
  def compute_action(self,
                     observation,
                     state=None,
                     prev_action=None,
                     prev_reward=None,
                     info=None,
                     policy_id=DEFAULT_POLICY_ID,
                     full_fetch=False):
      """Computes an action for the specified policy.

      Note that you can also access the policy object through
      self.get_policy(policy_id) and call compute_actions() on it directly.

      Arguments:
          observation (obj): observation from the environment.
          state (list): RNN hidden state, if any. If state is not None,
                        then all of compute_single_action(...) is returned
                        (computed action, rnn state, logits dictionary).
                        Otherwise compute_single_action(...)[0] is
                        returned (computed action).
          prev_action (obj): previous action value, if any
          prev_reward (int): previous reward, if any
          info (dict): info object, if any
          policy_id (str): policy to query (only applies to multi-agent).
          full_fetch (bool): whether to return extra action fetch results.
              This is always set to true if RNN state is specified.

      Returns:
          Just the computed action if full_fetch=False, or the full output
          of policy.compute_actions() otherwise.
      """

      if state is None:
          state = []
      preprocessed = self.workers.local_worker().preprocessors[
          policy_id].transform(observation)
      filtered_obs = self.workers.local_worker().filters[policy_id](
          preprocessed, update=False)
      if state:
          return self.get_policy(policy_id).compute_single_action(
              filtered_obs,
              state,
              prev_action,
              prev_reward,
              info,
              clip_actions=self.config["clip_actions"])
      res = self.get_policy(policy_id).compute_single_action(
          filtered_obs,
          state,
          prev_action,
          prev_reward,
          info,
          clip_actions=self.config["clip_actions"])
      if full_fetch:
          return res
      else:
          return res[0]  # backwards compatibility

Accessing Policy State

It is common to need to access a trainer's internal state, e.g., to set or get internal weights. In RLlib trainer state is replicated across multiple rollout workers (Ray actors) in the cluster. However, you can easily get and update this state between calls to train() via trainer.workers.foreach_worker() or trainer.workers.foreach_worker_with_index(). These functions take a lambda function that is applied with the worker as an arg. You can also return values from these functions and those will be returned as a list.

You can also access just the "master" copy of the trainer state through trainer.get_policy() or trainer.workers.local_worker(), but note that updates here may not be immediately reflected in remote replicas if you have configured num_workers > 0. For example, to access the weights of a local TF policy, you can run trainer.get_policy().get_weights(). This is also equivalent to trainer.workers.local_worker().policy_map["default_policy"].get_weights():

# Get weights of the default local policy
trainer.get_policy().get_weights()

# Same as above
trainer.workers.local_worker().policy_map["default_policy"].get_weights()

# Get list of weights of each worker, including remote replicas
trainer.workers.foreach_worker(lambda ev: ev.get_policy().get_weights())

# Same as above
trainer.workers.foreach_worker_with_index(lambda ev, i: ev.get_policy().get_weights())

Accessing Model State

Similar to accessing policy state, you may want to get a reference to the underlying neural network model being trained. For example, you may want to pre-train it separately, or otherwise update its weights outside of RLlib. This can be done by accessing the model of the policy:

Example: Preprocessing observations for feeding into a model

>>> import gym
>>> env = gym.make("Pong-v0")

# RLlib uses preprocessors to implement transforms such as one-hot encoding
# and flattening of tuple and dict observations.
>>> from ray.rllib.models.preprocessors import get_preprocessor
>>> prep = get_preprocessor(env.observation_space)(env.observation_space)
<ray.rllib.models.preprocessors.GenericPixelPreprocessor object at 0x7fc4d049de80>

# Observations should be preprocessed prior to feeding into a model
>>> env.reset().shape
(210, 160, 3)
>>> prep.transform(env.reset()).shape
(84, 84, 3)

Example: Querying a policy's action distribution

# Get a reference to the policy
>>> from ray.rllib.agents.ppo import PPOTrainer
>>> trainer = PPOTrainer(env="CartPole-v0", config={"framework": "tf2", "num_workers": 0})
>>> policy = trainer.get_policy()
<ray.rllib.policy.eager_tf_policy.PPOTFPolicy_eager object at 0x7fd020165470>

# Run a forward pass to get model output logits. Note that complex observations
# must be preprocessed as in the above code block.
>>> logits, _ = policy.model.from_batch({"obs": np.array([[0.1, 0.2, 0.3, 0.4]])})
(<tf.Tensor: id=1274, shape=(1, 2), dtype=float32, numpy=...>, [])

# Compute action distribution given logits
>>> policy.dist_class
<class_object 'ray.rllib.models.tf.tf_action_dist.Categorical'>
>>> dist = policy.dist_class(logits, policy.model)
<ray.rllib.models.tf.tf_action_dist.Categorical object at 0x7fd02301d710>

# Query the distribution for samples, sample logps
>>> dist.sample()
<tf.Tensor: id=661, shape=(1,), dtype=int64, numpy=..>
>>> dist.logp([1])
<tf.Tensor: id=1298, shape=(1,), dtype=float32, numpy=...>

# Get the estimated values for the most recent forward pass
>>> policy.model.value_function()
<tf.Tensor: id=670, shape=(1,), dtype=float32, numpy=...>

>>> policy.model.base_model.summary()
Model: "model"
_____________________________________________________________________
Layer (type)               Output Shape  Param #  Connected to
=====================================================================
observations (InputLayer)  [(None, 4)]   0
_____________________________________________________________________
fc_1 (Dense)               (None, 256)   1280     observations[0][0]
_____________________________________________________________________
fc_value_1 (Dense)         (None, 256)   1280     observations[0][0]
_____________________________________________________________________
fc_2 (Dense)               (None, 256)   65792    fc_1[0][0]
_____________________________________________________________________
fc_value_2 (Dense)         (None, 256)   65792    fc_value_1[0][0]
_____________________________________________________________________
fc_out (Dense)             (None, 2)     514      fc_2[0][0]
_____________________________________________________________________
value_out (Dense)          (None, 1)     257      fc_value_2[0][0]
=====================================================================
Total params: 134,915
Trainable params: 134,915
Non-trainable params: 0
_____________________________________________________________________

Example: Getting Q values from a DQN model

# Get a reference to the model through the policy
>>> from ray.rllib.agents.dqn import DQNTrainer
>>> trainer = DQNTrainer(env="CartPole-v0", config={"framework": "tf2"})
>>> model = trainer.get_policy().model
<ray.rllib.models.catalog.FullyConnectedNetwork_as_DistributionalQModel ...>

# List of all model variables
>>> model.variables()
[<tf.Variable 'default_policy/fc_1/kernel:0' shape=(4, 256) dtype=float32>, ...]

# Run a forward pass to get base model output. Note that complex observations
# must be preprocessed. An example of preprocessing is examples/saving_experiences.py
>>> model_out = model.from_batch({"obs": np.array([[0.1, 0.2, 0.3, 0.4]])})
(<tf.Tensor: id=832, shape=(1, 256), dtype=float32, numpy=...)

# Access the base Keras models (all default models have a base)
>>> model.base_model.summary()
Model: "model"
_______________________________________________________________________
Layer (type)                Output Shape    Param #  Connected to
=======================================================================
observations (InputLayer)   [(None, 4)]     0
_______________________________________________________________________
fc_1 (Dense)                (None, 256)     1280     observations[0][0]
_______________________________________________________________________
fc_out (Dense)              (None, 256)     65792    fc_1[0][0]
_______________________________________________________________________
value_out (Dense)           (None, 1)       257      fc_1[0][0]
=======================================================================
Total params: 67,329
Trainable params: 67,329
Non-trainable params: 0
______________________________________________________________________________

# Access the Q value model (specific to DQN)
>>> model.get_q_value_distributions(model_out)
[<tf.Tensor: id=891, shape=(1, 2)>, <tf.Tensor: id=896, shape=(1, 2, 1)>]

>>> model.q_value_head.summary()
Model: "model_1"
_________________________________________________________________
Layer (type)                 Output Shape              Param #
=================================================================
model_out (InputLayer)       [(None, 256)]             0
_________________________________________________________________
lambda (Lambda)              [(None, 2), (None, 2, 1), 66306
=================================================================
Total params: 66,306
Trainable params: 66,306
Non-trainable params: 0
_________________________________________________________________

# Access the state value model (specific to DQN)
>>> model.get_state_value(model_out)
<tf.Tensor: id=913, shape=(1, 1), dtype=float32>

>>> model.state_value_head.summary()
Model: "model_2"
_________________________________________________________________
Layer (type)                 Output Shape              Param #
=================================================================
model_out (InputLayer)       [(None, 256)]             0
_________________________________________________________________
lambda_1 (Lambda)            (None, 1)                 66049
=================================================================
Total params: 66,049
Trainable params: 66,049
Non-trainable params: 0
_________________________________________________________________

This is especially useful when used with custom model classes.

Advanced Python APIs

Custom Training Workflows

In the basic training example, Tune will call train() on your trainer once per training iteration and report the new training results. Sometimes, it is desirable to have full control over training, but still run inside Tune. Tune supports :ref:`custom trainable functions <trainable-docs>` that can be used to implement custom training workflows (example).

For even finer-grained control over training, you can use RLlib's lower-level building blocks directly to implement fully customized training workflows.

Global Coordination

Sometimes, it is necessary to coordinate between pieces of code that live in different processes managed by RLlib. For example, it can be useful to maintain a global average of a certain variable, or centrally control a hyperparameter used by policies. Ray provides a general way to achieve this through named actors (learn more about Ray actors here). These actors are assigned a global name and handles to them can be retrieved using these names. As an example, consider maintaining a shared global counter that is incremented by environments and read periodically from your driver program:

@ray.remote
class Counter:
   def __init__(self):
      self.count = 0
   def inc(self, n):
      self.count += n
   def get(self):
      return self.count

# on the driver
counter = Counter.options(name="global_counter").remote()
print(ray.get(counter.get.remote()))  # get the latest count

# in your envs
counter = ray.get_actor("global_counter")
counter.inc.remote(1)  # async call to increment the global count

Ray actors provide high levels of performance, so in more complex cases they can be used implement communication patterns such as parameter servers and allreduce.

Callbacks and Custom Metrics

You can provide callbacks to be called at points during policy evaluation. These callbacks have access to state for the current episode. Certain callbacks such as on_postprocess_trajectory, on_sample_end, and on_train_result are also places where custom postprocessing can be applied to intermediate data or results.

User-defined state can be stored for the episode in the episode.user_data dict, and custom scalar metrics reported by saving values to the episode.custom_metrics dict. These custom metrics will be aggregated and reported as part of training results. For a full example, see custom_metrics_and_callbacks.py.

.. autoclass:: ray.rllib.agents.callbacks.DefaultCallbacks
    :members:

Chaining Callbacks

Use the MultiCallbacks class to chaim multiple callbacks together.

.. autoclass:: ray.rllib.agents.callbacks.MultiCallbacks

Visualizing Custom Metrics

Custom metrics can be accessed and visualized like any other training result:

Customizing Exploration Behavior

RLlib offers a unified top-level API to configure and customize an agent’s exploration behavior, including the decisions (how and whether) to sample actions from distributions (stochastically or deterministically). The setup can be done via using built-in Exploration classes (see this package), which are specified (and further configured) inside Trainer.config["exploration_config"]. Besides using one of the available classes, one can sub-class any of these built-ins, add custom behavior to it, and use that new class in the config instead.

Every policy has-an Exploration object, which is created from the Trainer’s config[“exploration_config”] dict, which specifies the class to use via the special “type” key, as well as constructor arguments via all other keys, e.g.:

# in Trainer.config:
"exploration_config": {
    "type": "StochasticSampling",  # <- Special `type` key provides class information
    "[c'tor arg]" : "[value]",  # <- Add any needed constructor args here.
    # etc
}
# ...

The following table lists all built-in Exploration sub-classes and the agents that currently use these by default:

An Exploration class implements the get_exploration_action method, in which the exact exploratory behavior is defined. It takes the model’s output, the action distribution class, the model itself, a timestep (the global env-sampling steps already taken), and an explore switch and outputs a tuple of a) action and b) log-likelihood:

.. literalinclude:: ../../rllib/utils/exploration/exploration.py
   :language: python
   :start-after: __sphinx_doc_begin_get_exploration_action__
   :end-before: __sphinx_doc_end_get_exploration_action__

On the highest level, the Trainer.compute_action and Policy.compute_action(s) methods have a boolean explore switch, which is passed into Exploration.get_exploration_action. If explore=None, the value of Trainer.config[“explore”] is used, which thus serves as a main switch for exploratory behavior, allowing e.g. turning off any exploration easily for evaluation purposes (see :ref:`CustomEvaluation`).

The following are example excerpts from different Trainers' configs (see rllib/agents/trainer.py) to setup different exploration behaviors:

# All of the following configs go into Trainer.config.

# 1) Switching *off* exploration by default.
# Behavior: Calling `compute_action(s)` without explicitly setting its `explore`
# param will result in no exploration.
# However, explicitly calling `compute_action(s)` with `explore=True` will
# still(!) result in exploration (per-call overrides default).
"explore": False,

# 2) Switching *on* exploration by default.
# Behavior: Calling `compute_action(s)` without explicitly setting its
# explore param will result in exploration.
# However, explicitly calling `compute_action(s)` with `explore=False`
# will result in no(!) exploration (per-call overrides default).
"explore": True,

# 3) Example exploration_config usages:
# a) DQN: see rllib/agents/dqn/dqn.py
"explore": True,
"exploration_config": {
   # Exploration sub-class by name or full path to module+class
   # (e.g. “ray.rllib.utils.exploration.epsilon_greedy.EpsilonGreedy”)
   "type": "EpsilonGreedy",
   # Parameters for the Exploration class' constructor:
   "initial_epsilon": 1.0,
   "final_epsilon": 0.02,
   "epsilon_timesteps": 10000,  # Timesteps over which to anneal epsilon.
},

# b) DQN Soft-Q: In order to switch to Soft-Q exploration, do instead:
"explore": True,
"exploration_config": {
   "type": "SoftQ",
   # Parameters for the Exploration class' constructor:
   "temperature": 1.0,
},

# c) All policy-gradient algos and SAC: see rllib/agents/trainer.py
# Behavior: The algo samples stochastically from the
# model-parameterized distribution. This is the global Trainer default
# setting defined in trainer.py and used by all PG-type algos (plus SAC).
"explore": True,
"exploration_config": {
   "type": "StochasticSampling",
   "random_timesteps": 0,  # timesteps at beginning, over which to act uniformly randomly
},

Customized Evaluation During Training

RLlib will report online training rewards, however in some cases you may want to compute rewards with different settings (e.g., with exploration turned off, or on a specific set of environment configurations). You can evaluate policies during training by setting the evaluation_interval config, and optionally also evaluation_num_episodes, evaluation_config, evaluation_num_workers, and custom_eval_function (see trainer.py for further documentation).

By default, exploration is left as-is within evaluation_config. However, you can switch off any exploration behavior for the evaluation workers via:

# Switching off exploration behavior for evaluation workers
# (see rllib/agents/trainer.py)
"evaluation_config": {
   "explore": False
}

Note

Policy gradient algorithms are able to find the optimal policy, even if this is a stochastic one. Setting "explore=False" above will result in the evaluation workers not using this stochastic policy.

There is an end to end example of how to set up custom online evaluation in custom_eval.py. Note that if you only want to eval your policy at the end of training, you can set evaluation_interval: N, where N is the number of training iterations before stopping.

Below are some examples of how the custom evaluation metrics are reported nested under the evaluation key of normal training results:

------------------------------------------------------------------------
Sample output for `python custom_eval.py`
------------------------------------------------------------------------

INFO trainer.py:623 -- Evaluating current policy for 10 episodes.
INFO trainer.py:650 -- Running round 0 of parallel evaluation (2/10 episodes)
INFO trainer.py:650 -- Running round 1 of parallel evaluation (4/10 episodes)
INFO trainer.py:650 -- Running round 2 of parallel evaluation (6/10 episodes)
INFO trainer.py:650 -- Running round 3 of parallel evaluation (8/10 episodes)
INFO trainer.py:650 -- Running round 4 of parallel evaluation (10/10 episodes)

Result for PG_SimpleCorridor_2c6b27dc:
  ...
  evaluation:
    custom_metrics: {}
    episode_len_mean: 15.864661654135338
    episode_reward_max: 1.0
    episode_reward_mean: 0.49624060150375937
    episode_reward_min: 0.0
    episodes_this_iter: 133

------------------------------------------------------------------------
Sample output for `python custom_eval.py --custom-eval`
------------------------------------------------------------------------

INFO trainer.py:631 -- Running custom eval function <function ...>
Update corridor length to 4
Update corridor length to 7
Custom evaluation round 1
Custom evaluation round 2
Custom evaluation round 3
Custom evaluation round 4

Result for PG_SimpleCorridor_0de4e686:
  ...
  evaluation:
    custom_metrics: {}
    episode_len_mean: 9.15695067264574
    episode_reward_max: 1.0
    episode_reward_mean: 0.9596412556053812
    episode_reward_min: 0.0
    episodes_this_iter: 223
    foo: 1

Rewriting Trajectories

Note that in the on_postprocess_traj callback you have full access to the trajectory batch (post_batch) and other training state. This can be used to rewrite the trajectory, which has a number of uses including:

• Backdating rewards to previous time steps (e.g., based on values in info).
• Adding model-based curiosity bonuses to rewards (you can train the model with a custom model supervised loss).

To access the policy / model (policy.model) in the callbacks, note that info['pre_batch'] returns a tuple where the first element is a policy and the second one is the batch itself. You can also access all the rollout worker state using the following call:

from ray.rllib.evaluation.rollout_worker import get_global_worker

# You can use this from any callback to get a reference to the
# RolloutWorker running in the process, which in turn has references to
# all the policies, etc: see rollout_worker.py for more info.
rollout_worker = get_global_worker()

Policy losses are defined over the post_batch data, so you can mutate that in the callbacks to change what data the policy loss function sees.

Curriculum Learning

In Curriculum learning, the environment can be set to different difficulties (or "tasks") to allow for learning to progress through controlled phases (from easy to more difficult). RLlib comes with a basic curriculum learning API utilizing the TaskSettableEnv environment API. Your environment only needs to implement the set_task and get_task methods for this to work. You can then define an env_task_fn in your config, which receives the last training results and returns a new task for the env to be set to:

from ray.rllib.env.apis.task_settable_env import TaskSettableEnv

class MyEnv(TaskSettableEnv):
    def get_task(self):
        return self.current_difficulty

    def set_task(self, task):
        self.current_difficulty = task

def curriculum_fn(train_results, task_settable_env, env_ctx):
    # Very simple curriculum function.
    current_task = task_settable_env.get_task()
    new_task = current_task + 1
    return new_task

# Setup your Trainer's config like so:
config = {
    "env": MyEnv,
    "env_task_fn": curriculum_fn,
}
# Train using `tune.run` or `Trainer.train()` and the above config stub.
# ...

There are two more ways to use the RLlib's other APIs to implement curriculum learning.

Use the Trainer API and update the environment between calls to train(). This example shows the trainer being run inside a Tune function. This is basically the same as what the built-in env_task_fn API described above already does under the hood, but allows you to do even more customizations to your training loop.

import ray
from ray import tune
from ray.rllib.agents.ppo import PPOTrainer

def train(config, reporter):
    trainer = PPOTrainer(config=config, env=YourEnv)
    while True:
        result = trainer.train()
        reporter(**result)
        if result["episode_reward_mean"] > 200:
            task = 2
        elif result["episode_reward_mean"] > 100:
            task = 1
        else:
            task = 0
        trainer.workers.foreach_worker(
            lambda ev: ev.foreach_env(
                lambda env: env.set_task(task)))

num_gpus = 0
num_workers = 2

ray.init()
tune.run(
    train,
    config={
        "num_gpus": num_gpus,
        "num_workers": num_workers,
    },
    resources_per_trial=tune.PlacementGroupFactory(
        [{"CPU": 1}, {"GPU": num_gpus}] + [{"CPU": 1}] * num_workers
    ),
)

You could also use RLlib's callbacks API to update the environment on new training results:

import ray
from ray import tune

def on_train_result(info):
    result = info["result"]
    if result["episode_reward_mean"] > 200:
        task = 2
    elif result["episode_reward_mean"] > 100:
        task = 1
    else:
        task = 0
    trainer = info["trainer"]
    trainer.workers.foreach_worker(
        lambda ev: ev.foreach_env(
            lambda env: env.set_task(task)))

ray.init()
tune.run(
    "PPO",
    config={
        "env": YourEnv,
        "callbacks": {
            "on_train_result": on_train_result,
        },
    },
)

Debugging

Gym Monitor

The "monitor": true config can be used to save Gym episode videos to the result dir. For example:

rllib train --env=PongDeterministic-v4 \
    --run=A2C --config '{"num_workers": 2, "monitor": true}'

# videos will be saved in the ~/ray_results/<experiment> dir, for example
openaigym.video.0.31401.video000000.meta.json
openaigym.video.0.31401.video000000.mp4
openaigym.video.0.31403.video000000.meta.json
openaigym.video.0.31403.video000000.mp4

Eager Mode

Policies built with build_tf_policy (most of the reference algorithms are) can be run in eager mode by setting the "framework": "[tf2|tfe]" / "eager_tracing": true config options or using rllib train --config '{"framework": "tf2"}' [--trace]. This will tell RLlib to execute the model forward pass, action distribution, loss, and stats functions in eager mode.

Eager mode makes debugging much easier, since you can now use line-by-line debugging with breakpoints or Python print() to inspect intermediate tensor values. However, eager can be slower than graph mode unless tracing is enabled.

Using PyTorch

Trainers that have an implemented TorchPolicy, will allow you to run rllib train using the command line --torch flag. Algorithms that do not have a torch version yet will complain with an error in this case.

Episode Traces

You can use the data output API to save episode traces for debugging. For example, the following command will run PPO while saving episode traces to /tmp/debug.

rllib train --run=PPO --env=CartPole-v0 \
    --config='{"output": "/tmp/debug", "output_compress_columns": []}'

# episode traces will be saved in /tmp/debug, for example
output-2019-02-23_12-02-03_worker-2_0.json
output-2019-02-23_12-02-04_worker-1_0.json

Log Verbosity

You can control the trainer log level via the "log_level" flag. Valid values are "DEBUG", "INFO", "WARN" (default), and "ERROR". This can be used to increase or decrease the verbosity of internal logging. You can also use the -v and -vv flags. For example, the following two commands are about equivalent:

rllib train --env=PongDeterministic-v4 \
    --run=A2C --config '{"num_workers": 2, "log_level": "DEBUG"}'

rllib train --env=PongDeterministic-v4 \
    --run=A2C --config '{"num_workers": 2}' -vv

The default log level is WARN. We strongly recommend using at least INFO level logging for development.

Stack Traces

You can use the ray stack command to dump the stack traces of all the Python workers on a single node. This can be useful for debugging unexpected hangs or performance issues.

External Application API

In some cases (i.e., when interacting with an externally hosted simulator or production environment) it makes more sense to interact with RLlib as if it were an independently running service, rather than RLlib hosting the simulations itself. This is possible via RLlib's external applications interface (full documentation).

.. autoclass:: ray.rllib.env.policy_client.PolicyClient
    :members:

.. autoclass:: ray.rllib.env.policy_server_input.PolicyServerInput
    :members: