【rl-agents代码学习】02——DQN算法

Highway-env Intersection

本文将继续探索rl-agents中相关DQN算法的实现。下面的介绍将会以intersection这个环境为例，首先介绍一下Highway-env中的intersection-v1。Highway-env中相关文档——http://highway-env.farama.org/environments/intersection/。

highway-env中的环境可以通过配置文件进行修改， observations, actions, dynamics 以及rewards等信息都是以字典的形式存储在配置文件中。

PS：DQN、DuelingDQN算法原理可参考【强化学习】10 —— DQN算法【强化学习】11 —— Double DQN算法与Dueling DQN算法

import gymnasium as gym
import pprint
from matplotlib import pyplot as pltenv = gym.make("intersection-v1", render_mode='rgb_array')
pprint.pprint(env.unwrapped.config)

输出config，可以看到如下信息：

{'action': {'dynamical': True,'lateral': True,'longitudinal': True,'steering_range': [-1.0471975511965976, 1.0471975511965976],'type': 'ContinuousAction'},'arrived_reward': 1,'centering_position': [0.5, 0.6],'collision_reward': -5,'controlled_vehicles': 1,'destination': 'o1','duration': 13,'high_speed_reward': 1,'initial_vehicle_count': 10,'manual_control': False,'normalize_reward': False,'observation': {'features': ['presence','x','y','vx','vy','long_off','lat_off','ang_off'],'type': 'Kinematics','vehicles_count': 5},'offroad_terminal': False,'offscreen_rendering': False,'other_vehicles_type': 'highway_env.vehicle.behavior.IDMVehicle','policy_frequency': 1,'real_time_rendering': False,'render_agent': True,'reward_speed_range': [7.0, 9.0],'scaling': 7.15,'screen_height': 600,'screen_width': 600,'show_trajectories': False,'simulation_frequency': 15,'spawn_probability': 0.6}

之后可以通过以下代码输出图像：

plt.imshow(env.render())
plt.show()

输出observation，可以看到是一个5*8的array，：

[[ 1.0000000e+00  9.9999998e-03  1.0000000e+00  0.0000000e+00-1.2500000e-01  6.3297665e+01  0.0000000e+00  0.0000000e+00][ 1.0000000e+00  1.3849856e-01 -1.0000000e+00 -9.9416278e-021.2500000e-01  8.1300293e+01  1.0361128e-15  0.0000000e+00][ 1.0000000e+00 -2.0000000e-02 -1.0000000e+00  0.0000000e+002.2993930e-01  6.5756187e+01  2.8473811e-15  0.0000000e+00][ 0.0000000e+00  0.0000000e+00  0.0000000e+00  0.0000000e+000.0000000e+00  0.0000000e+00  0.0000000e+00  0.0000000e+00][ 0.0000000e+00  0.0000000e+00  0.0000000e+00  0.0000000e+000.0000000e+00  0.0000000e+00  0.0000000e+00  0.0000000e+00]]

observation的解释如下，

通过以下代码，可以将action的类型变为离散的空间。

env.unwrapped.configure({"action": {'longitudinal': True,"type": "DiscreteMetaAction"}
})

rl-agents之DQN

A neural-network model is used to estimate the state-action value function and produce a greedy optimal policy.

Implemented variants:

• Double DQN
• Dueling architecture
• N-step targets

References:

Playing Atari with Deep Reinforcement Learning, Mnih V. et al (2013).
Deep Reinforcement Learning with Double Q-learning, van Hasselt H. et al. (2015).
Dueling Network Architectures for Deep Reinforcement Learning, Wang Z. et al. (2015).

Query agent for actions sequence

由上一节所知，通过调用run_episodes函数，进行具体的agent训练。其中会调用step函数，并执行self.agent.plan(self.observation)。对于DQNAgent的实现，首先由AbstractAgent类实现plan,之后plan函数会调用act函数:

    def step(self):"""Plan a sequence of actions according to the agent policy, and step the environment accordingly."""# Query agent for actions sequenceactions = self.agent.plan(self.observation)

// rl_agents/agents/common/abstract.py
class AbstractAgent(Configurable, ABC):def __init__(self, config=None):super(AbstractAgent, self).__init__(config)self.writer = None  # Tensorboard writerself.directory = None  # Run directory@abstractmethoddef act(self, state):"""Pick an action:param state: s, the current state of the agent:return: a, the action to perform"""raise NotImplementedError()def plan(self, state):"""Plan an optimal trajectory from an initial state.:param state: s, the initial state of the agent:return: [a0, a1, a2...], a sequence of actions to perform"""return [self.act(state)]

DQN抽象类AbstractDQNAgent继承自AbstractStochasticAgent，AbstractStochasticAgent继承自AbstractAgent，在DQN抽象类AbstractDQNAgent中实现对act函数的重写：

    def act(self, state, step_exploration_time=True):"""Act according to the state-action value model and an exploration policy:param state: current state:param step_exploration_time: step the exploration schedule:return: an action"""self.previous_state = stateif step_exploration_time:self.exploration_policy.step_time()# Handle multi-agent observations# TODO: it would be more efficient to forward a batch of statesif isinstance(state, tuple):return tuple(self.act(agent_state, step_exploration_time=False) for agent_state in state)# Single-agent settingvalues = self.get_state_action_values(state)self.exploration_policy.update(values)return self.exploration_policy.sample()

探索策略

首先来看一下exploration_policy 的实现：

        self.exploration_policy = exploration_factory(self.config["exploration"], self.env.action_space)

探索策略加载的配置文件部分：

"exploration": {"method": "EpsilonGreedy","tau": 15000,"temperature": 1.0,"final_temperature": 0.05
}

跳转到exploration_factory，可以看到主要实现了三类探索策略，具体的内容会在后面部分进行介绍：

• Greedy
• $ϵ$-Greedy
• Boltzmann

def exploration_factory(exploration_config, action_space):"""Handles creation of exploration policies:param exploration_config: configuration dictionary of the policy, must contain a "method" key:param action_space: the environment action space:return: a new exploration policy"""from rl_agents.agents.common.exploration.boltzmann import Boltzmannfrom rl_agents.agents.common.exploration.epsilon_greedy import EpsilonGreedyfrom rl_agents.agents.common.exploration.greedy import Greedyif exploration_config['method'] == 'Greedy':return Greedy(action_space, exploration_config)elif exploration_config['method'] == 'EpsilonGreedy':return EpsilonGreedy(action_space, exploration_config)elif exploration_config['method'] == 'Boltzmann':return Boltzmann(action_space, exploration_config)else:raise ValueError("Unknown exploration method")

神经网络实现

接着获取$Q(s,a)$值

    def get_state_action_values(self, state):""":param state: s, an environment state:return: [Q(a1,s), ..., Q(an,s)] the array of its action-values for each actions"""return self.get_batch_state_action_values([state])[0]

调用了抽象方法get_batch_state_action_values

    @abstractmethoddef get_batch_state_action_values(self, states):"""Get the state-action values of several states:param states: [s1; ...; sN] an array of states:return: values:[[Q11, ..., Q1n]; ...] the array of all action values for each state"""raise NotImplementedError

接着来看DQNAgent中的具体实现：

class DQNAgent(AbstractDQNAgent):def __init__(self, env, config=None):super(DQNAgent, self).__init__(env, config)size_model_config(self.env, self.config["model"])self.value_net = model_factory(self.config["model"])self.target_net = model_factory(self.config["model"])self.target_net.load_state_dict(self.value_net.state_dict())self.target_net.eval()logger.debug("Number of trainable parameters: {}".format(trainable_parameters(self.value_net)))self.device = choose_device(self.config["device"])self.value_net.to(self.device)self.target_net.to(self.device)self.loss_function = loss_function_factory(self.config["loss_function"])self.optimizer = optimizer_factory(self.config["optimizer"]["type"],self.value_net.parameters(),**self.config["optimizer"])self.steps = 0def get_batch_state_action_values(self, states):return self.value_net(torch.tensor(states, dtype=torch.float).to(self.device)).data.cpu().numpy()

value_net的实现依赖于model_factory，其中的配置文件部分如下：

    "model": {"type": "MultiLayerPerceptron","layers": [128, 128]},

再进入model_factory，主要实现了四类网络：

• MultiLayerPerceptron
• DuelingNetwork
• ConvolutionalNetwork
• EgoAttentionNetwork

这里我们暂且先分析多层感知机MultiLayerPerceptron（即普通DQN）。

// rl_agents/agents/common/models.py
def model_factory(config: dict) -> nn.Module:if config["type"] == "MultiLayerPerceptron":return MultiLayerPerceptron(config)elif config["type"] == "DuelingNetwork":return DuelingNetwork(config)elif config["type"] == "ConvolutionalNetwork":return ConvolutionalNetwork(config)elif config["type"] == "EgoAttentionNetwork":return EgoAttentionNetwork(config)else:raise ValueError("Unknown model type")

MultiLayerPerceptron类继承自BaseModule，BaseModule继承自torch.nn.Module。根据配置文件baseline.json，可以看到MultiLayerPerceptron类的sizes为[128, 128]，激活函数为RELU。我们可以注意到，网络实现中有reshape操作，因为state的输入是5*8的矩阵，通过reshape，可以将其转换为一维的向量。最终网络结构类似于下图。

class MultiLayerPerceptron(BaseModule, Configurable):def __init__(self, config):super().__init__()Configurable.__init__(self, config)sizes = [self.config["in"]] + self.config["layers"] self.activation = activation_factory(self.config["activation"])layers_list = [nn.Linear(sizes[i], sizes[i + 1]) for i in range(len(sizes) - 1)]self.layers = nn.ModuleList(layers_list)if self.config.get("out", None):self.predict = nn.Linear(sizes[-1], self.config["out"])@classmethoddef default_config(cls):return {"in": None,"layers": [64, 64],"activation": "RELU","reshape": "True","out": None}def forward(self, x):if self.config["reshape"]:x = x.reshape(x.shape[0], -1)  # We expect a batch of vectorsfor layer in self.layers:x = self.activation(layer(x))if self.config.get("out", None):x = self.predict(x)return x

获取$Q$之后，探索策略进行更新，并sample一个action。以$ϵ$-Greedy为例，因为$ϵ$-Greedy继承DiscreteDistribution，所以主要关注DiscreteDistribution中的相关实现。

    def act(self, state, step_exploration_time=True):...self.exploration_policy.update(values)return self.exploration_policy.sample()

rl_agents/agents/common/exploration/epsilon_greedy.pydef update(self, values):"""Update the action distribution parameters:param values: the state-action values:param step_time: whether to update epsilon schedule"""self.optimal_action = np.argmax(values)self.epsilon = self.config['final_temperature'] + \(self.config['temperature'] - self.config['final_temperature']) * \np.exp(- self.time / self.config['tau'])if self.writer:self.writer.add_scalar('exploration/epsilon', self.epsilon, self.time)

class DiscreteDistribution(Configurable, ABC):def __init__(self, config=None, **kwargs):super(DiscreteDistribution, self).__init__(config)self.np_random = None@abstractmethoddef get_distribution(self):""":return: a distribution over actions {action:probability}"""raise NotImplementedError()def sample(self):""":return: an action sampled from the distribution"""distribution = self.get_distribution()return self.np_random.choice(list(distribution.keys()), 1, p=np.array(list(distribution.values())))[0]

可以看到首先需要获得action的一个分布，这部分在$ϵ$-Greedy中的实现为：

    def get_distribution(self):distribution = {action: self.epsilon / self.action_space.n for action in range(self.action_space.n)}distribution[self.optimal_action] += 1 - self.epsilonreturn distribution

get_distribution 函数返回一个动作的概率分布字典。字典的键是动作，字典的值是动作被选择的概率。概率分布的计算方式为：每个动作都有一个基础概率 self.epsilon / self.action_space.n，其中 self.action_space.n 是动作的总数，即每个动作被选择的概率相等，这是基于探索的角度。同时，最优动作 self.optimal_action 会额外获得一个概率增量 1 - self.epsilon，这是基于利用的角度，即利用已知的最优动作。

sample 函数根据 get_distribution 函数得到的动作概率分布进行采样，返回一个动作。具体地，使用 np_random.choice 函数，其参数包括动作列表和对应的动作概率分布列表，返回的是一个根据给定概率分布随机采样的动作。

小结1

到此，act函数返回一个待执行的action，此部分的框图如下所示：

之后这几步在上一讲已经讨论过http://t.csdnimg.cn/ddpVJ。

        # Forward the actions to the environment viewertry:self.env.unwrapped.viewer.set_agent_action_sequence(actions)except AttributeError:pass# Step the environmentprevious_observation, action = self.observation, actions[0]transition = self.wrapped_env.step(action)self.observation, reward, done, truncated, info = transitionterminal = done or truncated# Call callbackif self.step_callback_fn is not None:self.step_callback_fn(self.episode, self.wrapped_env, self.agent, transition, self.writer)

Record the experience

现在step函数中只剩下这一步，我们再来看这一步的实现。

        # Record the experience.try:self.agent.record(previous_observation, action, reward, self.observation, done, info)except NotImplementedError:pass

直接跳转到AbstractDQNAgent类中查看相关实现

    def record(self, state, action, reward, next_state, done, info):"""Record a transition by performing a Deep Q-Network iteration- push the transition into memory- sample a minibatch- compute the bellman residual loss over the minibatch- perform one gradient descent step- slowly track the policy network with the target network:param state: a state:param action: an action:param reward: a reward:param next_state: a next state:param done: whether state is terminal"""if not self.training:returnif isinstance(state, tuple) and isinstance(action, tuple):  # Multi-agent setting[self.memory.push(agent_state, agent_action, reward, agent_next_state, done, info)for agent_state, agent_action, agent_next_state in zip(state, action, next_state)]else:  # Single-agent settingself.memory.push(state, action, reward, next_state, done, info)batch = self.sample_minibatch()if batch:loss, _, _ = self.compute_bellman_residual(batch)self.step_optimizer(loss)self.update_target_network()

Replaybuffer

self.memory是Replaybuffer的一个实现

  self.memory = ReplayMemory(self.config)

• push函数的实现可以提升运算速率。
• 在强化学习中，经常需要从经验回放缓存（这里就是self.memory）中抽样出一批数据来更新模型。而这里的n-step是一个常用的技巧，它表明在预测下一个状态时，不仅仅使用当前的状态和动作，还使用接下来的n-1个状态和动作。当n为1时，这就是常见的单步过渡；当n大于1时，这就是n步采样。

rl_agents/agents/common/memory.py
class ReplayMemory(Configurable):"""Container that stores and samples transitions."""def __init__(self, config=None, transition_type=Transition):super(ReplayMemory, self).__init__(config)self.capacity = int(self.config['memory_capacity'])self.transition_type = transition_typeself.memory = []self.position = 0@classmethoddef default_config(cls):return dict(memory_capacity=10000,n_steps=1,gamma=0.99)def push(self, *args):"""Saves a transition."""if len(self.memory) < self.capacity:self.memory.append(None)self.position = len(self.memory) - 1elif len(self.memory) > self.capacity:self.memory = self.memory[:self.capacity]# Faster than append and popself.memory[self.position] = self.transition_type(*args)self.position = (self.position + 1) % self.capacitydef sample(self, batch_size, collapsed=True):"""Sample a batch of transitions.If n_steps is greater than one, the batch will be composed of lists of successive transitions.:param batch_size: size of the batch:param collapsed: whether successive transitions must be collapsed into one n-step transition.:return: the sampled batch"""# TODO: use agent's np_random for seedingif self.config["n_steps"] == 1:# Directly sample transitionsreturn random.sample(self.memory, batch_size)else:# Sample initial transition indexesindexes = random.sample(range(len(self.memory)), batch_size)# Get the batch of n-consecutive-transitions starting from sampled indexesall_transitions = [self.memory[i:i+self.config["n_steps"]] for i in indexes]# Collapse transitionsreturn map(self.collapse_n_steps, all_transitions) if collapsed else all_transitionsdef collapse_n_steps(self, transitions):"""Collapse n transitions <s,a,r,s',t> of a trajectory into one transition <s0, a0, Sum(r_i), sp, tp>.We start from the initial state, perform the first action, and then the return estimate is formed byaccumulating the discounted rewards along the trajectory until a terminal state or the end of thetrajectory is reached.:param transitions: A list of n successive transitions:return: The corresponding n-step transition"""state, action, cumulated_reward, next_state, done, info = transitions[0]discount = 1for transition in transitions[1:]:if done:breakelse:_, _, reward, next_state, done, info = transitiondiscount *= self.config['gamma']cumulated_reward += discount*rewardreturn state, action, cumulated_reward, next_state, done, infodef __len__(self):return len(self.memory)def is_full(self):return len(self.memory) == self.capacitydef is_empty(self):return len(self.memory) == 0

回到record代码中，首先将采样到的数据放入Replaybuffer，当采样数据量大于batch_size时，从Replaybuffer中采样。

    def sample_minibatch(self):if len(self.memory) < self.config["batch_size"]:return Nonetransitions = self.memory.sample(self.config["batch_size"])return Transition(*zip(*transitions))

compute_bellman_residual

之后便利用bellman方程进行更新：

loss, _, _ = self.compute_bellman_residual(batch)

    def compute_bellman_residual(self, batch, target_state_action_value=None):# Compute concatenate the batch elementsif not isinstance(batch.state, torch.Tensor):# logger.info("Casting the batch to torch.tensor")state = torch.cat(tuple(torch.tensor([batch.state], dtype=torch.float))).to(self.device)action = torch.tensor(batch.action, dtype=torch.long).to(self.device)reward = torch.tensor(batch.reward, dtype=torch.float).to(self.device)next_state = torch.cat(tuple(torch.tensor([batch.next_state], dtype=torch.float))).to(self.device)terminal = torch.tensor(batch.terminal, dtype=torch.bool).to(self.device)batch = Transition(state, action, reward, next_state, terminal, batch.info)# Compute Q(s_t, a) - the model computes Q(s_t), then we select the# columns of actions takenstate_action_values = self.value_net(batch.state)state_action_values = state_action_values.gather(1, batch.action.unsqueeze(1)).squeeze(1)if target_state_action_value is None:with torch.no_grad():# Compute V(s_{t+1}) for all next states.next_state_values = torch.zeros(batch.reward.shape).to(self.device)if self.config["double"]:# Double Q-learning: pick best actions from policy network_, best_actions = self.value_net(batch.next_state).max(1)# Double Q-learning: estimate action values from target networkbest_values = self.target_net(batch.next_state).gather(1, best_actions.unsqueeze(1)).squeeze(1)else:best_values, _ = self.target_net(batch.next_state).max(1)next_state_values[~batch.terminal] = best_values[~batch.terminal]# Compute the expected Q valuestarget_state_action_value = batch.reward + self.config["gamma"] * next_state_values# Compute lossloss = self.loss_function(state_action_values, target_state_action_value)return loss, target_state_action_value, batch

• with torch.no_grad():用于禁止在其作用域内进行梯度计算
• 实现了DoubleDQN
• self.loss_function = loss_function_factory(self.config["loss_function"])loss函数包括以下几种：

def loss_function_factory(loss_function):if loss_function == "l2":return F.mse_losselif loss_function == "l1":return F.l1_losselif loss_function == "smooth_l1":return F.smooth_l1_losselif loss_function == "bce":return F.binary_cross_entropyelse:raise ValueError("Unknown loss function : {}".format(loss_function))

step_optimizer

对梯度进行了截断

    def step_optimizer(self, loss):# Optimize the modelself.optimizer.zero_grad()loss.backward()for param in self.value_net.parameters():param.grad.data.clamp_(-1, 1)self.optimizer.step()

update_target_network

更新目标网络

    def update_target_network(self):self.steps += 1if self.steps % self.config["target_update"] == 0:self.target_net.load_state_dict(self.value_net.state_dict())

小结2

到此，整个DQN算法实现完毕，record部分的框图如下：

exploration_policy

这部分主要实现了三种策略：

• Greedy
• $ϵ$-Greedy
• Boltzmann

此部分可以参考：【强化学习】02—— 探索与利用

Greedy

Greedy贪婪策略即选择最优的策略$a_t={\mathrm{argmax}}_{a\in \mathcal{A}}Q(s,a)$

class Greedy(DiscreteDistribution):"""Always use the optimal action"""def __init__(self, action_space, config=None):super(Greedy, self).__init__(config)self.action_space = action_spaceif isinstance(self.action_space, spaces.Tuple):self.action_space = self.action_space.spaces[0]if not isinstance(self.action_space, spaces.Discrete):raise TypeError("The action space should be discrete")self.values = Noneself.seed()def get_distribution(self):optimal_action = np.argmax(self.values)return {action: 1 if action == optimal_action else 0 for action in range(self.action_space.n)}def update(self, values):self.values = values

$ϵ$-Greedy

$ϵ$-Greedy公式如下：
$$a_t=\{\begin{array}{llc}\arg {\max }_{a\in \mathcal{A}}\hat{Q}(a),& \text{采样概率:1-}ϵ& \\ \text{从 }\mathcal{A}\text{ 中随机选择},& \text{采样概率: }ϵ& \end{array}$$
这里实现的其实是衰减贪心策略，衰减曲线如下图所示。
$$\begin{array}{rl}ϵ& =\text{final-temperature}+(\text{temperature}-\text{final-temperature})\ast e^{\frac{-t}{\tau }}\end{array}$$

class EpsilonGreedy(DiscreteDistribution):"""Uniform distribution with probability epsilon, and optimal action with probability 1-epsilon"""def __init__(self, action_space, config=None):super(EpsilonGreedy, self).__init__(config)self.action_space = action_spaceif isinstance(self.action_space, spaces.Tuple):self.action_space = self.action_space.spaces[0]if not isinstance(self.action_space, spaces.Discrete):raise TypeError("The action space should be discrete")self.config['final_temperature'] = min(self.config['temperature'], self.config['final_temperature'])self.optimal_action = Noneself.epsilon = 0self.time = 0self.writer = Noneself.seed()@classmethoddef default_config(cls):return dict(temperature=1.0,final_temperature=0.1,tau=5000)def get_distribution(self):distribution = {action: self.epsilon / self.action_space.n for action in range(self.action_space.n)}distribution[self.optimal_action] += 1 - self.epsilonreturn distributiondef update(self, values):"""Update the action distribution parameters:param values: the state-action values:param step_time: whether to update epsilon schedule"""self.optimal_action = np.argmax(values)self.epsilon = self.config['final_temperature'] + \(self.config['temperature'] - self.config['final_temperature']) * \np.exp(- self.time / self.config['tau'])if self.writer:self.writer.add_scalar('exploration/epsilon', self.epsilon, self.time)def step_time(self):self.time += 1def set_time(self, time):self.time = timedef set_writer(self, writer):self.writer = writer

Boltzmann

玻尔兹曼分布（Boltzmann Distribution）是描述分子在热力学平衡时分布的概率分布函数。它表明在给定的能量状态下，不同的微观状态出现的概率是不同的，且符合一个指数函数形式。

在热力学中，任何物质在一定温度下都会具有一定的热运动，这些热运动状态可以用分子内能或动能来描述。而玻尔兹曼分布表明了在相同温度下，分子在所有可能状态之间的分布概率。其表达式为：

$$P(E_i)=\frac{e^{-E_i/kT}}{{\sum }_j{e}^{-E_j/kT}}$$

其中，$P(E_i)$为分子处于能量状态$E_i$的概率，$k$为玻尔兹曼常数，$T$为温度，$E_j$为所有可以达到的能量状态。

可以看到，玻尔兹曼分布中每个能量状态的出现概率与其能量成负指数关系，因此能量较小的状态出现的概率更大。这符合熵增加的趋势，即越有序的状态出现的概率越小。

class Boltzmann(DiscreteDistribution):"""Uniform distribution with probability epsilon, and optimal action with probability 1-epsilon"""def __init__(self, action_space, config=None):super(Boltzmann, self).__init__(config)self.action_space = action_spaceif not isinstance(self.action_space, spaces.Discrete):raise TypeError("The action space should be discrete")self.values = Noneself.seed()@classmethoddef default_config(cls):return dict(temperature=0.5)def get_distribution(self):actions = range(self.action_space.n)if self.config['temperature'] > 0:weights = np.exp(self.values / self.config['temperature'])else:weights = np.zeros((len(actions),))weights[np.argmax(self.values)] = 1return {action: weights[action] / np.sum(weights) for action in actions}def update(self, values):self.values = values

运行结果

运行命令与方法在上一讲已经介绍【rl-agents代码学习】01——总体框架。

超参数设置采用默认设置，使用DQN算法分别运行4000steps和20000steps。使用Tensorboard查看结果：

 tensorboard --logdir C:\Users\16413\Desktop\rl-agents-master\scripts\out\IntersectionEnv\DQNAgent\baseline_20231113-123234_7944\

4000steps


可以看到最后的episode reward大致在3左右。

20000steps

可以看到最后的episode reward大致在3左右。