Deep Deterministic Policy Gradient (DDPG)

Overview

DDPG is a popular DRL algorithm for continuous control. It extends DQN to work with the continuous action space by introducing a deterministic actor that directly outputs continuous actions. DDPG also combines techniques from DQN, such as the replay buffer and target network.

Original paper:

Continuous control with deep reinforcement learning

Reference resources:

Implemented Variants

Variants Implemented	Description
`ddpg_continuous_action.py`, docs	For continuous action space

Below is our single-file implementation of DDPG:

`ddpg_continuous_action.py`

The ddpg_continuous_action.py has the following features:

For continuous action space
Works with the Box observation space of low-level features
Works with the Box (continuous) action space

Usage

poetry install
poetry install -E pybullet
python cleanrl/ddpg_continuous_action.py --help
python cleanrl/ddpg_continuous_action.py --env-id HopperBulletEnv-v0
poetry install -E mujoco # only works in Linux
python cleanrl/ddpg_continuous_action.py --env-id Hopper-v3

Explanation of the logged metrics

Running python cleanrl/ddpg_continuous_action.py will automatically record various metrics such as actor or value losses in Tensorboard. Below is the documentation for these metrics:

charts/episodic_return: episodic return of the game
charts/SPS: number of steps per second
losses/qf1_loss: the mean squared error (MSE) between the Q values at timestep $t$ and the Bellman update target estimated using the reward $r_t$ and the Q values at timestep $t+1$, thus minimizing the one-step temporal difference. Formally, it can be expressed by the equation below. $$ J(\theta^{Q}) = \mathbb{E}_{(s,a,r,s') \sim \mathcal{D}} \big[ (Q(s, a) - y)^2 \big], $$ with the Bellman update target $y = r + \gamma \, Q^{'}(s', a')$, where $a' \sim \mu^{'}(s')$, and the replay buffer $\mathcal{D}$.
losses/actor_loss: implemented as -qf1(data.observations, actor(data.observations)).mean(); it is the negative average Q values calculated based on the 1) observations and the 2) actions computed by the actor based on these observations. By minimizing actor_loss, the optimizer updates the actors parameter using the following gradient (Lillicrap et al., 2016, Algorithm 1)¹:

\[ \nabla_{\theta^{\mu}} J \approx \frac{1}{N}\sum_i\left.\left.\nabla_{a} Q\left(s, a \mid \theta^{Q}\right)\right|_{s=s_{i}, a=\mu\left(s_{i}\right)} \nabla_{\theta^{\mu}} \mu\left(s \mid \theta^{\mu}\right)\right|_{s_{i}} \]

losses/qf1_values: implemented as qf1(data.observations, data.actions).view(-1), it is the average Q values of the sampled data in the replay buffer; useful when gauging if under or over estimation happens.

Implementation details

Our ddpg_continuous_action.py is based on the OurDDPG.py from sfujim/TD3, which presents the the following implementation difference from (Lillicrap et al., 2016)¹:

ddpg_continuous_action.py uses a gaussian exploration noise $\mathcal{N}(0, 0.1)$, while (Lillicrap et al., 2016)¹ uses Ornstein-Uhlenbeck process with $\theta=0.15$ and $\sigma=0.2$.
ddpg_continuous_action.py runs the experiments using the openai/gym MuJoCo environments, while (Lillicrap et al., 2016)¹ uses their proprietary MuJoCo environments.

ddpg_continuous_action.py uses the following architecture:

class QNetwork(nn.Module):
    def __init__(self, env):
        super(QNetwork, self).__init__()
        self.fc1 = nn.Linear(np.array(env.single_observation_space.shape).prod() + np.prod(env.single_action_space.shape), 256)
        self.fc2 = nn.Linear(256, 256)
        self.fc3 = nn.Linear(256, 1)

    def forward(self, x, a):
        x = torch.cat([x, a], 1)
        x = F.relu(self.fc1(x))
        x = F.relu(self.fc2(x))
        x = self.fc3(x)
        return x


class Actor(nn.Module):
    def __init__(self, env):
        super(Actor, self).__init__()
        self.fc1 = nn.Linear(np.array(env.single_observation_space.shape).prod(), 256)
        self.fc2 = nn.Linear(256, 256)
        self.fc_mu = nn.Linear(256, np.prod(env.single_action_space.shape))

    def forward(self, x):
        x = F.relu(self.fc1(x))
        x = F.relu(self.fc2(x))
        return torch.tanh(self.fc_mu(x))

while (Lillicrap et al., 2016, see Appendix 7 EXPERIMENT DETAILS)¹ uses the following architecture (difference highlighted):

class QNetwork(nn.Module):
    def __init__(self, env):
        super(QNetwork, self).__init__()
        self.fc1 = nn.Linear(np.array(env.single_observation_space.shape).prod(), 400)
        self.fc2 = nn.Linear(400 + np.prod(env.single_action_space.shape), 300)
        self.fc3 = nn.Linear(300, 1)

    def forward(self, x, a):
        x = F.relu(self.fc1(x))
        x = torch.cat([x, a], 1)
        x = F.relu(self.fc2(x))
        x = self.fc3(x)
        return x


class Actor(nn.Module):
    def __init__(self, env):
        super(Actor, self).__init__()
        self.fc1 = nn.Linear(np.array(env.single_observation_space.shape).prod(), 400)
        self.fc2 = nn.Linear(400, 300)
        self.fc_mu = nn.Linear(300, np.prod(env.single_action_space.shape))

    def forward(self, x):
        x = F.relu(self.fc1(x))
        x = F.relu(self.fc2(x))
        return torch.tanh(self.fc_mu(x))

ddpg_continuous_action.py uses the following learning rates:

q_optimizer = optim.Adam(list(qf1.parameters()), lr=3e-4)
actor_optimizer = optim.Adam(list(actor.parameters()), lr=3e-4)

while (Lillicrap et al., 2016, see Appendix 7 EXPERIMENT DETAILS)¹ uses the following learning rates:

q_optimizer = optim.Adam(list(qf1.parameters()), lr=1e-4)
actor_optimizer = optim.Adam(list(actor.parameters()), lr=1e-3)

ddpg_continuous_action.py uses --batch-size=256 --tau=0.005, while (Lillicrap et al., 2016, see Appendix 7 EXPERIMENT DETAILS)¹ uses --batch-size=64 --tau=0.001

ddpg_continuous_action.py also adds support for handling continuous environments where the lower and higher bounds of the action space are not $[-1,1]$, or are asymmetric. The case where the bounds are not $[-1,1]$ is handled in DDPG.py (Fujimoto et al., 2018)² as follows:

class Actor(nn.Module):

    ...

    def forward(self, state):
        a = F.relu(self.l1(state))
        a = F.relu(self.l2(a))
        return self.max_action * torch.tanh(self.l3(a)) # Scale from [-1,1] to [-action_high, action_high]

On the other hand, in CleanRL's ddpg_continuous_action.py, the mean and the scale of the the action space are computed as action_bias and action_scale respectively. Those scalars are in turn used to scale the output of a tanh activation function in the actor to the original action space range:

class Actor(nn.Module):
    def __init__(self, env):
        ...
        # action rescaling
        self.register_buffer("action_scale", torch.FloatTensor((env.action_space.high - env.action_space.low) / 2.0))
        self.register_buffer("action_bias", torch.FloatTensor((env.action_space.high + env.action_space.low) / 2.0))

    def forward(self, x):
        x = F.relu(self.fc1(x))
        x = F.relu(self.fc2(x))
        x = torch.tanh(self.fc_mu(x))
        return x * self.action_scale + self.action_bias # Scale from [-1,1] to [-action_low, action_high]

Additionally, when drawing exploration noise that is added to the actions produced by the actor, CleanRL's ddpg_continuous_action.py centers the distribution the sampled from at action_bias, and the scale of the distribution is set to action_scale * exploration_noise.

Info

Note that Humanoid-v2, InvertedPendulum-v2, Pusher-v2 have action space bounds that are not the standard [-1, 1]. See below.

Ant-v2 Observation space: Box(-inf, inf, (111,), float64) Action space: Box(-1.0, 1.0, (8,), float32)
HalfCheetah-v2 Observation space: Box(-inf, inf, (17,), float64) Action space: Box(-1.0, 1.0, (6,), float32)
Hopper-v2 Observation space: Box(-inf, inf, (11,), float64) Action space: Box(-1.0, 1.0, (3,), float32)
Humanoid-v2 Observation space: Box(-inf, inf, (376,), float64) Action space: Box(-0.4, 0.4, (17,), float32)
InvertedDoublePendulum-v2 Observation space: Box(-inf, inf, (11,), float64) Action space: Box(-1.0, 1.0, (1,), float32)
InvertedPendulum-v2 Observation space: Box(-inf, inf, (4,), float64) Action space: Box(-3.0, 3.0, (1,), float32)
Pusher-v2 Observation space: Box(-inf, inf, (23,), float64) Action space: Box(-2.0, 2.0, (7,), float32)
Reacher-v2 Observation space: Box(-inf, inf, (11,), float64) Action space: Box(-1.0, 1.0, (2,), float32)
Swimmer-v2 Observation space: Box(-inf, inf, (8,), float64) Action space: Box(-1.0, 1.0, (2,), float32)
Walker2d-v2 Observation space: Box(-inf, inf, (17,), float64) Action space: Box(-1.0, 1.0, (6,), float32)

Experiment results

To run benchmark experiments, see benchmark/ddpg.sh. Specifically, execute the following command:

Below are the average episodic returns for ddpg_continuous_action.py (3 random seeds). To ensure the quality of the implementation, we compared the results against (Fujimoto et al., 2018)².

Environment	`ddpg_continuous_action.py`	`OurDDPG.py` (Fujimoto et al., 2018, Table 1)²	`DDPG.py` using settings from (Lillicrap et al., 2016)¹ in (Fujimoto et al., 2018, Table 1)²
HalfCheetah	9382.32 ± 1395.52	8577.29	3305.60
Walker2d	1598.35 ± 862.66	3098.11	1843.85
Hopper	1313.43 ± 684.46	1860.02	2020.46
Humanoid	897.74 ± 281.87	not available
Pusher	-34.45 ± 4.47	not available
InvertedPendulum	645.67 ± 270.31	1000.00 ± 0.00

Info

Note that ddpg_continuous_action.py uses gym MuJoCo v2 environments while OurDDPG.py (Fujimoto et al., 2018)² uses the gym MuJoCo v1 environments. According to the openai/gym#834, gym MuJoCo v2 environments should be equivalent to the gym MuJoCo v1 environments.

Also note the performance of our ddpg_continuous_action.py seems to be worse than the reference implementation on Walker2d and Hopper. This is likely due to openai/gym#938. We would have a hard time reproducing gym MuJoCo v1 environments because they have been long deprecated.

One other thing could cause the performance difference: the original code reported the average episodic return using determinisitc evaluation (i.e., without exploration noise), see sfujim/TD3/main.py#L15-L32, whereas we reported the episodic return during training and the policy gets updated between environments steps.

Learning curves:

Tracked experiments and game play videos:

Lillicrap, T.P., Hunt, J.J., Pritzel, A., Heess, N.M., Erez, T., Tassa, Y., Silver, D., & Wierstra, D. (2016). Continuous control with deep reinforcement learning. CoRR, abs/1509.02971. https://arxiv.org/abs/1509.02971 ↩↩↩↩↩↩↩↩
Fujimoto, S., Hoof, H.V., & Meger, D. (2018). Addressing Function Approximation Error in Actor-Critic Methods. ArXiv, abs/1802.09477. https://arxiv.org/abs/1802.09477 ↩↩↩↩↩