# Tag Info

11

Actor-Critic is not just a single algorithm, it should be viewed as a "family" of related techniques. They're all techniques based on the policy gradient theorem, which train some form of critic that computes some form of value estimate to plug into the update rule as a lower-variance replacement for the returns at the end of an episode. They all perform "...

6

The "trick" of subtracting a (state-dependent) baseline from the $Q(s, a)$ term in policy gradients to reduce variants (which is what is described in your "baseline reduction" link) is a different trick from the modifications to the rewards that you are asking about. The baseline subtraction trick for variance reduction does not appear to be present in the ...

4

For discrete action spaces, what is the purpose of the actor in Actor-Critic algorithms? In brief, it is the policy function $\pi(a|s)$. The critic (a state action function $v_{\pi}(s)$) is not used to derive a policy, and in "vanilla" Actor-Critic cannot be used in this way at all unless you have the full distribution model of the MDP. It just seems to ...

4

When using the loss function for the critic described in your question, the Actor-Critic is an on-policy approach (as are most Actor-Critic methods). Your intuition as to what it is learning seems to be quite close, but the notation/terminology is not quite on point. First it's important to realize that the $Q(s, a)$ critic is an estimator, we're training ...

3

The twist here is that the $a_{t+1}$ in (11) and the $\mu(s_{t+1})$ in (16) are the same and actually the $a_t$ in the on-policy case and the $a_t$ in the off-policy case are different. The key to the understanding is that in on-policy algorithms you have to use actions (and generally speaking trajectories) generated by the policy in the updating steps (to ...

3

I think what you mean to ask is how can differentiation occur when there's no obvious neural network function to differentiate? Don't worry - lots of people get confused about this, because it seems like an obvious hole in the puzzle. As mentioned by @AtillaOzgur, neural networks use partial differentiation through backpropagation. First, take the output ...

3

This is simply from definition of return in average reward setting (look at equation $10.9$). The "standard" TD error is defined as $$TD_{\text{error}} = R_{t+1} + V(S_{t+1}) - V(S_t)$$ In average reward setting, average reward $r(\pi)$ is subtracted from reward at $t$, $R_t$, so TD error in this case is TD_{\text{...

3

We subtract mean from values and divide it with standard deviation to get data with mean of zero and variance of one. The range of values per episode does not matter, it will always make it to have zero mean and variance of one in all cases. If the range is bigger ([100, 200]) then deviation will be bigger as well than for smaller range ([0, 1]) so we will ...

3

MDPs are strict generalisations of contextual bandits, adding time steps and state transitions, plus the concept of return as a measure of agent performance. Therefore, methods used in RL to solve MDPs will work to solve contextual bandits. You can either treat a contextual bandit as a series of 1-step episodes (with start state chosen randomly), or as a ...

2

You're right, the first time you run it the two policies ($\pi_{\theta old}$ and $\pi_\theta$) will be the same. This means your loss is simply the advantage (since you multiply the the ratio ($r(\theta)={\pi_\theta(a|s)\over\pi_{\theta old}(a|s)}$) by the advantage (so $loss=-r_t(\theta)A_t$). However, with PPO you run multiple epochs of training on the ...

2

According to Sutton and Barto, they are the same thing. Note 13.5-6 (page 338) of their Reinforcement Learning: An Introduction, 2nd Edition book: Actor-critic methods are sometimes referred to as advantage actor-critic methods in the literature

2

In the answer I'll be using notation similar to the one from the SAC paper. If we look at the standard objective function for policy gradient methods we have \begin{align} J_\pi &= V_\pi(s_t)\\ &= \mathbb E_{a_t \sim \pi(a|s_t)}[Q(s_t, a_t)]\\ &= \mathbb E_{a_t \sim \pi(a|s_t)}[ \mathbb E_{s_{t+1} \sim p(s|s_t, a_t)} [r(s_t, a_t) + V(s_{t+1})]]\\ ...

2

There are different actor-critic (AC) algorithms with different convergence guarantees. For example, AC algorithms where the critic is tabular have different convergence guarantees than AC algorithms where the critic is a neural network (function approximation). Most convergence proofs assume that the actor and the critic operate at different time scales, ...

2

Keeping this taxonomy intact for model-based Dynamic programming algorithms, I would argue that value iteration is a Actor only approach, and policy iteration is a Actor-Critic approach. However, not many people discuss the term Actor-Critic when referring to Policy Iteration. How come? Both policy iteration and value iteration are value-based approaches. ...

2

Let's say your old policy is $\pi_b$ and your current one is $\pi_a$. If you collected trajectory by using policy $\pi_b$ you would get return $G$ whose expected value is \begin{align} E_{\pi_b}[G_t|S_t = s] &= E_{\pi_b}[R_{t+1} + G_{t+1}]\\ &= \sum_a \pi_b(a|s) \sum_{s', r} p(s', r|s, a) [r + E_{\pi_b}[G_{t+1}|S_{t+1} = s']]\\ \end{align} You can ...

2

Yeah, it seems like it's a wrong implementation. vals_ref_v is a matrix of 1 row, and 128 columns. value_v.detach() is a matrix of 128 row

2

Here is the commit I fixed few minor errors, but the major one was when I saw what the line histories = [deque(maxlen=self.reward_steps)] * len(self.env.envs) was doing. It was just repeating the same queue. In [2]: histories = [deque(maxlen=5)] * 4 In [3]: histories ...

1

I changed the line adv_v = vals_ref_v - value_v.detach() to adv_v = vals_ref_v - value_v.squeeze(-1).detach(). It seems the convergence is much faster. According to the A2C algorithm, it is just logic to apply $Q(a, s) - V(s)$, where $Q(a, s)$ and $V(s)$ with the same shape. The call to detach() is important here as we don't want to propagate the PG into ...

1

I believe that the author is referring to how the networks are trained in Deep RL. Consider Deep Q-Learning where the $Q(s,a)$ is approximated using a neural network. Then the loss function used to train the network is $$\mathbb{E}[(r + \gamma \max_{a'} Q(s',a') - Q(s,a))^2]\;.$$ Here, $r + \gamma \max_{a'} Q(s',a')$ is your target, what you want your ...

1

What is the advantage of using more than one environment with a single agent? There are two main advantages to this approach: The dataset for training is closer to the independent, identically distributed (i.i.d.) ideal, important for theoretical and practical reasons when training a neural network. Samples taken from a single trajectory are not ...

1

In reinforcement learning, you can distinguish algorithms based on the functions they use to ultimately find the policy (which is the goal in RL anyway!). algorithms that attempt to find an optimal value function (an example is Q-learning, which attempts to find a state-action value function), then derive the policy from the value function algorithms that ...

1

Theoretically, nothing precludes the use of $\lambda$-returns in actor-critic methods. The $\lambda$-return is an unbiased estimator of the Monte Carlo (MC) return, which means they are essentially interchangeable. In fact, as discussed in High-Dimensional Continuous Control Using Generalized Advantage Estimation, using the $\lambda$-return instead of the MC ...

1

Unfortunately no, the way to go is track the total reward and see if it's increasing and converging eventually. Value loss isn't a useful metric as the loss can be 0 when the value network always predicts 0 and the agent doesn't collect any reward, meaning very poor behavior.

1

The paper Dota 2 with Large Scale Deep Reinforcement Learning goes into greater detail than the initial blog posts. They call their distributed training framework Rapid, which is also used in some of their robotics work, such as the paper Learning Dexterous In-Hand Manipulation, where they discuss a smaller scale deployment of Rapid (as compared to Dota2/...

1

OpenAI have a post on that: https://openai.com/blog/openai-five/ They use a myriad of rollout workers that collect data for 60 seconds and push that data to a GPU cluster where gradients are computed for batches of 4096 observations which are then averaged. PPO is actually designed to allow this kind of parallelisation as it uses trajectory segments with a ...

1

I believe if you run a single agent in multiple parallel environments many times you will get similar actions in similar states, the reason behind multiple agents is that you will have different agents with different parameters and you can also have different explicit exploration policies so your exploration will be better and you will learn more from ...

1

I found the error: division by zero when calculating the ratio, here. ratio = pi.prob(self.tfa) / oldpi.prob(self.tfa) I changed to: ratio = tf.divide(pi.prob(self.tfa), tf.maximum(oldpi.prob(self.tfa), 1e-5))

1

It might seem to give the same update direction but would it converge to desirable policy parameters? Actor-Critic is proposed alongside the policy gradient theorem in Sutton 1999. It is shown to maximize the state-value function. If you are able to show that the technique of yours is, in fact, maximizing some desirable objective function, you could propose ...

1

For the actor, I'd say the 'not mutually exclusive' option is more biologically plausible in the context of muscle systems, where the actions can be seen as simultaneous muscle activations. Maybe at a higher level, an agent thinks of the action as 'go north' or 'go south', but the final outputs which have to control muscles at a lower level have to represent ...

1

My understanding of the main idea behind A2C / A3C is that we run small segments of an episode to estimate the return using a trainable value function to compensate for the unseen final steps of the episode. This seems fairly accurate. The important thing to note is that the trainable value function is trained to predict values (specifically, advantage ...

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