https://morvanzhou.github.io/tutorials/machine-learning/reinforcement-learning/6-1-actor-critic/
http://www0.cs.ucl.ac.uk/staff/d.silver/web/Teaching.html
【强化学习】Actor-Critic详解
之前在强化学习分类中,我们提到了Policy-based与Value-based两种方式,然而有一种算法合并了Value-based (比如 Q learning) 和 Policy-based (比如 Policy Gradients) 两类强化学习算法,就是Actor-Critic方法
1、算法思想
Actor-Critic算法分为两部分,我们分开来看actor的前身是policy gradient他可以轻松地在连续动作空间内选择合适的动作,value-based的Qlearning做这件事就会因为空间过大而爆炸,但是又因为Actor是基于回合更新的所以学习效率比较慢,这时候我们发现可以使用一个value-based的算法作为Critic就可以实现单步更新。这样两种算法相互补充就形成了我们的Actor-Critic
Actor 基于概率选行为, Critic 基于 Actor 的行为评判行为的得分, Actor 根据 Critic 的评分修改选行为的概率。
Actor Critic优点:可以进行单步更新, 相较于传统的PG回合更新要快.
Actor Critic缺点:Actor的行为取决于 Critic 的Value,但是因为 Critic本身就很难收敛和actor一起更新的话就更难收敛了。(为了解决收敛问题, Deepmind 提出了 Actor Critic 升级版 Deep Deterministic Policy Gradient,后者融合了 DQN 的优势, 解决了收敛难的问题,之后我会详细解释这种算法)
2、公式推导
Actor(玩家):为了玩转这个游戏得到尽量高的reward,需要一个策略:输入state,输出action,即上面的第2步。(可以用神经网络来近似这个函数。剩下的任务就是如何训练神经网络,得更高的reward。这个网络就被称为actor)
Critic(评委):因为actor是基于策略policy的所以需要critic来计算出对应actor的value来反馈给actor,告诉他表现得好不好。所以就要使用到之前的Q值。(当然这个Q-function所以也可以用神经网络来近似。这个网络被称为critic。)
再提一下之前用过的符号
-策略 π ( s ) π(s) π(s)表示了agent的action,其输出是单个的action动作,而是选择动作的概率分布,所以一个状态下的所有动作概率加和应当为 1
π ( a ∣ s ) π(a|s) π(a∣s)表示策略
首先来看Critic的策略值函数即策略 π \pi π的 V π ( s ) V_\pi(s) Vπ(s)具体推导方式请参考之前Qlearning的推导方式
V π ( s ) = E π [ r + γ V π ( s ′ ) ] V_\pi(s)=E_\pi[r + \gamma V_\pi(s')] Vπ(s)=Eπ[r+γVπ(s′)]
策略的动作值函数如下
Q π ( s , a ) = R s a + γ V π ( s ′ ) Q_π(s,a)=R^a_s+γV_π(s′) Qπ(s,a)=Rsa+γVπ(s′)
此处提出了优势函数A,优势函数表示在状态 s 下,选择动作 a 有多好。如果 action a 比 average 要好,那么,advantage function 就是 positive 的,否则,就是 negative 的。
A π ( s , a ) = Q π ( s , a ) − V π ( s ) = r + γ V π ( s ′ ) − V π ( s ) A_\pi(s,a)=Q_π(s,a)-V_\pi(s)= r + γV_π(s′) -V_\pi(s) Aπ(s,a)=Qπ(s,a)−Vπ(s)=r+γVπ(s′)−Vπ(s)
前置条件下面两个公式是等价的因为使用了likelihood ratio似然比的方法,所以可能各种资料写的不一样,大家注意不要被搞蒙了
∇ θ π θ ( s , a ) = π θ ( s , a ) ∇ θ π θ ( s , a ) π θ ( s , a ) = π θ ( s , a ) ∇ θ l o g π θ ( s , a ) ∇_θπ_θ(s,a)=π_θ(s,a)\frac{∇_θπ_θ(s,a)}{π_θ(s,a)}=π_θ(s,a)∇_θlogπ_θ(s,a) ∇θπθ(s,a)=πθ(s,a)πθ(s,a)∇θπθ(s,a)=πθ(s,a)∇θlogπθ(s,a)
接下来我们看Actor,这部分假设采取policy Gradient这样的话使用策略梯度定理
∇ θ J ( θ ) = ∑ s ∈ S d ( s ) ∑ a ∈ A π θ ( s , a ) ∇ θ l o g π ( a ∣ s ; θ ) Q π ( s , a ) ∇_θJ(θ)=∑_{s∈S}d(s)∑_{a∈A}π_θ(s,a)∇_θlogπ(a|s;\theta)Q_{π}(s,a) ∇θJ(θ)=∑s∈Sd(s)∑a∈Aπθ(s,a)∇θlogπ(a∣s;θ)Qπ(s,a)
∇ θ J ( θ ) = E π θ [ ∇ θ l o g π θ ( s , a ) Q π θ ( s , a ) ] ∇_θJ(θ)=E_{π_θ}[∇_θlogπ_θ(s,a)Q_{π_θ}(s,a)] ∇θJ(θ)=Eπθ[∇θlogπθ(s,a)Qπθ(s,a)]
此处将 Q π ( s , a ) Q_{π}(s,a) Qπ(s,a)换成上文提到的 A π ( s , a ) A_\pi(s,a) Aπ(s,a)
∇ θ J ( θ ) = ∑ s ∈ S d ( s ) ∑ a ∈ A π θ ( s , a ) ∇ θ l o g π ( a ∣ s ; θ ) A π ( s , a ) ∇_θJ(θ)=∑_{s∈S}d(s)∑_{a∈A}π_θ(s,a)∇_θlogπ(a|s;\theta)A_{\pi}(s,a) ∇θJ(θ)=∑s∈Sd(s)∑a∈Aπθ(s,a)∇θlogπ(a∣s;θ)Aπ(s,a)
∇ θ J ( θ ) = E π θ [ ∇ θ l o g π θ ( s , a ) A π θ ( s , a ) ] ∇_θJ(θ)=E_{π_θ}[∇_θlogπ_θ(s,a)A_{\pi_θ}(s,a)] ∇θJ(θ)=Eπθ[∇θlogπθ(s,a)Aπθ(s,a)]
更新公式如下
下面几种形式都是一样的所以千万不要蒙
θ t + 1 ← θ t + α A π θ ( s , a ) ∇ θ l o g π θ ( s , a ) \theta_{t+1}←\theta_t+\alpha A_{\pi_θ}(s,a) ∇_θlogπ_θ(s,a) θt+1←θt+αAπθ(s,a)∇θlogπθ(s,a)
θ t + 1 ← θ t + α A π θ ( s , a ) ∇ θ π ( A t ∣ S t , θ ) π ( A t ∣ S t , θ ) \theta_{t+1}←\theta_t+\alpha A_{\pi_θ}(s,a)\frac{∇_θπ(A_t|S_t,\theta)}{π(A_t|S_t,\theta)} θt+1←θt+αAπθ(s,a)π(At∣St,θ)∇θπ(At∣St,θ)
θ t + 1 ← θ t + α ( r + γ V π ( s t + 1 ) − V π ( s t ) ) ∇ θ π ( A t ∣ S t , θ ) π ( A t ∣ S t , θ ) \theta_{t+1}←\theta_t+\alpha (r + γV_π(s_{t+1}) -V_\pi(s_t))\frac{∇_θπ(A_t|S_t,\theta)}{π(A_t|S_t,\theta)} θt+1←θt+α(r+γVπ(st+1)−Vπ(st))π(At∣St,θ)∇θπ(At∣St,θ)
损失函数如下
A可看做是常数所以可以求和平均打开期望,又因为损失函数要求最小所以加个"-"变换为解最小的问题
Actor: L π = − 1 n ∑ i = 1 n A π ( s , a ) l o g π ( s , a ) L_\pi =-\frac{1}{n}\sum_{i=1}^nA_{\pi}(s,a) logπ(s,a ) Lπ=−n1∑i=1nAπ(s,a)logπ(s,a)
值迭代可以直接使用均方误差MSE作为损失函数
Critic: L v = 1 n ∑ i = 1 n e i 2 L_v = \frac{1}{n}\sum_{i=1}^ne_i^2 Lv=n1∑i=1nei2
n-step的伪代码如下
3、代码实现
import numpy as npimport tensorflow as tfimport gymnp.random.seed(2)tf.set_random_seed(2) # reproducible# 超参数OUTPUT_GRAPH = FalseMAX_EPISODE = 3000DISPLAY_REWARD_THRESHOLD = 200 # 刷新阈值MAX_EP_STEPS = 1000 #最大迭代次数RENDER = False # 渲染开关GAMMA = 0.9# 衰变值LR_A = 0.001 # Actor学习率LR_C = 0.01# Critic学习率env = gym.make('CartPole-v0')env.seed(1) env = env.unwrappedN_F = env.observation_space.shape[0] # 状态空间N_A = env.action_space.n# 动作空间class Actor(object):def __init__(self, sess, n_features, n_actions, lr=0.001):self.sess = sessself.s = tf.placeholder(tf.float32, [1, n_features], "state")self.a = tf.placeholder(tf.int32, None, "act")self.td_error = tf.placeholder(tf.float32, None, "td_error") # TD_errorwith tf.variable_scope('Actor'):l1 = tf.layers.dense(inputs=self.s,units=20, # number of hidden unitsactivation=tf.nn.relu,kernel_initializer=tf.random_normal_initializer(0., .1), # weightsbias_initializer=tf.constant_initializer(0.1), # biasesname='l1')self.acts_prob = tf.layers.dense(inputs=l1,units=n_actions, # output unitsactivation=tf.nn.softmax, # get action probabilitieskernel_initializer=tf.random_normal_initializer(0., .1), # weightsbias_initializer=tf.constant_initializer(0.1), # biasesname='acts_prob')with tf.variable_scope('exp_v'):log_prob = tf.log(self.acts_prob[0, self.a])self.exp_v = tf.reduce_mean(log_prob * self.td_error) # advantage (TD_error) guided losswith tf.variable_scope('train'):self.train_op = tf.train.AdamOptimizer(lr).minimize(-self.exp_v) # minimize(-exp_v) = maximize(exp_v)def learn(self, s, a, td):s = s[np.newaxis, :]feed_dict = {self.s: s, self.a: a, self.td_error: td}_, exp_v = self.sess.run([self.train_op, self.exp_v], feed_dict)return exp_vdef choose_action(self, s):s = s[np.newaxis, :]probs = self.sess.run(self.acts_prob, {self.s: s}) # 获取所有操作的概率return np.random.choice(np.arange(probs.shape[1]), p=probs.ravel()) # return a intclass Critic(object):def __init__(self, sess, n_features, lr=0.01):self.sess = sessself.s = tf.placeholder(tf.float32, [1, n_features], "state")self.v_ = tf.placeholder(tf.float32, [1, 1], "v_next")self.r = tf.placeholder(tf.float32, None, 'r')with tf.variable_scope('Critic'):l1 = tf.layers.dense(inputs=self.s,units=20, # number of hidden unitsactivation=tf.nn.relu, # None# have to be linear to make sure the convergence of actor.# But linear approximator seems hardly learns the correct Q.kernel_initializer=tf.random_normal_initializer(0., .1), # weightsbias_initializer=tf.constant_initializer(0.1), # biasesname='l1')self.v = tf.layers.dense(inputs=l1,units=1, # output unitsactivation=None,kernel_initializer=tf.random_normal_initializer(0., .1), # weightsbias_initializer=tf.constant_initializer(0.1), # biasesname='V')with tf.variable_scope('squared_TD_error'):self.td_error = self.r + GAMMA * self.v_ - self.vself.loss = tf.square(self.td_error) # TD_error = (r+gamma*V_next) - V_evalwith tf.variable_scope('train'):self.train_op = tf.train.AdamOptimizer(lr).minimize(self.loss)def learn(self, s, r, s_):s, s_ = s[np.newaxis, :], s_[np.newaxis, :]v_ = self.sess.run(self.v, {self.s: s_})td_error, _ = self.sess.run([self.td_error, self.train_op],{self.s: s, self.v_: v_, self.r: r})return td_errorsess = tf.Session()actor = Actor(sess, n_features=N_F, n_actions=N_A, lr=LR_A) # 初始化Actorcritic = Critic(sess, n_features=N_F, lr=LR_C)# 初始化Criticsess.run(tf.global_variables_initializer()) # 初始化参数if OUTPUT_GRAPH:tf.summary.FileWriter("logs/", sess.graph) # 输出日志# 开始迭代过程 对应伪代码部分for i_episode in range(MAX_EPISODE):s = env.reset() # 环境初始化t = 0track_r = [] # 每回合的所有奖励while True:if RENDER: env.render()a = actor.choose_action(s) # Actor选取动作s_, r, done, info = env.step(a) # 环境反馈if done: r = -20 # 回合结束的惩罚track_r.append(r) # 记录回报值rtd_error = critic.learn(s, r, s_) # Critic 学习actor.learn(s, a, td_error) # Actor 学习s = s_t += 1if done or t >= MAX_EP_STEPS:# 回合结束, 打印回合累积奖励ep_rs_sum = sum(track_r)if 'running_reward' not in globals():running_reward = ep_rs_sumelse:running_reward = running_reward * 0.95 + ep_rs_sum * 0.05if running_reward > DISPLAY_REWARD_THRESHOLD: RENDER = True # renderingprint("episode:", i_episode, " reward:", int(running_reward))break