A2C算法原理及代码实现

本文主要参考王树森老师的强化学习课程

1.A2C算法原理

A2C算法是策略学习中比较经典的一个算法，是在 Barto 等人1983年提出的。我们知道策略梯度方法用策略梯度更新策略网络参数 θ，从而增大目标函数，即下面的随机梯度：

Actor-Critic 方法中用一个神经网络近似动作价值函数 Q π (s,a)，这个神经网络叫做“价值网络”，记为 q(s,a;w)，其中的 w 表示神经网络中可训练的参数。价值网络的输入是状态 s，输出是每个动作的价值。动作空间 A 中有多少种动作，那么价值网络的输出就是多少维的向量，向量每个元素对应一个动作。举个例子，动作空间是 A = {左,右,上}，价值网络的输出是 ：

神经网络可以采用以下结构：

虽然价值网络 q(s,a;w) 与DQN有相同的结构，但是两者的意义不同，训练算法也不同。、

• 价值网络是对动作价值函数 Q π (s,a) 的近似。而 DQN 则是对最优动作价值函数Q ⋆ (s,a) 的近似。
• 对价值网络的训练使用的是SARSA算法，它属于同策略，不能用经验回放。对DQN的训练使用的是 Q 学习算法，它属于异策略，可以用经验回放。

Actor-Critic 翻译成“演员—评论家”方法。策略网络 π(a|s;θ) 相当于演员，它基于状态 s 做出动作 a。价值网络 q(s,a;w) 相当于评论家，它给演员的表现打分，量化在状态 s的情况下做出动作 a 的好坏程度。策略网络（演员）和价值网络（评委）的关系如下图所示。

2. A2C算法训练流程

设当前策略网络参数是θnow ，价值网络参数是Wnow 。执行下面的步骤，将参数更新成 θnew 和 Wnew ：

3.A2C代码实现

基于pytorch在gym基础环境中选择经典环境cartpole-v0倒立摆进行验证。

3.1 算法代码：

import torch.optim as optim
import torch.nn as nn
import torch.nn.functional as F
from torch.distributions import Categorical
class ActorCritic(nn.Module):
    ''' A2C网络模型，包含一个Actor和Critic
    '''
    def __init__(self, input_dim, output_dim, hidden_dim):
        super(ActorCritic, self).__init__()
        self.critic = nn.Sequential(
            nn.Linear(input_dim, hidden_dim),
            nn.ReLU(),
            nn.Linear(hidden_dim, 1)
        )
        
        self.actor = nn.Sequential(
            nn.Linear(input_dim, hidden_dim),
            nn.ReLU(),
            nn.Linear(hidden_dim, output_dim),
            nn.Softmax(dim=1),
        )
        
    def forward(self, x):
        value = self.critic(x)
        probs = self.actor(x)
        dist  = Categorical(probs)
        return dist, value
class A2C:
    ''' A2C算法
    '''
    def __init__(self,state_dim,action_dim,cfg) -> None:
        self.gamma = cfg.gamma
        self.device = cfg.device
        self.model = ActorCritic(state_dim, action_dim, cfg.hidden_size).to(self.device)
        self.optimizer = optim.Adam(self.model.parameters())
    def compute_returns(self,next_value, rewards, masks):
        R = next_value
        returns = []
        for step in reversed(range(len(rewards))):
            R = rewards[step] + self.gamma * R * masks[step]
            returns.insert(0, R)
        return returns

3.2 实验代码：

import sys
import os
curr_path = os.path.dirname(os.path.abspath(__file__))  # 当前文件所在绝对路径
parent_path = os.path.dirname(curr_path)  # 父路径
sys.path.append(parent_path)  # 添加路径到系统路径
import gym
import numpy as np
import torch
import torch.optim as optim
import datetime
from common.multiprocessing_env import SubprocVecEnv
from a2c import ActorCritic
from common.utils import save_results, make_dir
from common.utils import plot_rewards
curr_time = datetime.datetime.now().strftime("%Y%m%d-%H%M%S") # 获取当前时间
algo_name = 'A2C'  # 算法名称
env_name = 'CartPole-v0'  # 环境名称
class A2CConfig:
    def __init__(self) -> None:
        self.algo_name = algo_name# 算法名称
        self.env_name = env_name # 环境名称
        self.n_envs = 8 # 异步的环境数目
        self.gamma = 0.99 # 强化学习中的折扣因子
        self.hidden_dim = 256
        self.lr = 1e-3 # learning rate
        self.max_frames = 30000
        self.n_steps = 5
        self.device = torch.device("cuda" if torch.cuda.is_available() else "cpu")
class PlotConfig:
    def __init__(self) -> None:
        self.algo_name = algo_name # 算法名称
        self.env_name = env_name # 环境名称
        self.device = torch.device("cuda" if torch.cuda.is_available() else "cpu")  # 检测GPU
        self.result_path = curr_path+"/outputs/" + self.env_name + \
            '/'+curr_time+'/results/'  # 保存结果的路径
        self.model_path = curr_path+"/outputs/" + self.env_name + \
            '/'+curr_time+'/models/'  # 保存模型的路径
        self.save = True # 是否保存图片
def make_envs(env_name):
    def _thunk():
        env = gym.make(env_name)
        env.seed(2)
        return env
    return _thunk
def ceshi_env(env,model,vis=False):
    state = env.reset()
    if vis: env.render()
    done = False
    total_reward = 0
    while not done:
        state = torch.FloatTensor(state).unsqueeze(0).to(cfg.device)
        dist, _ = model(state)
        next_state, reward, done, _ = env.step(dist.sample().cpu().numpy()[0])
        state = next_state
        if vis: env.render()
        total_reward += reward
    return total_reward
def compute_returns(next_value, rewards, masks, gamma=0.99):
    R = next_value
    returns = []
    for step in reversed(range(len(rewards))):
        R = rewards[step] + gamma * R * masks[step]
        returns.insert(0, R)
    return returns
def train(cfg,envs):
    print('开始训练!')
    print(f'环境：{cfg.env_name}, 算法：{cfg.algo_name}, 设备：{cfg.device}')
    env = gym.make(cfg.env_name) # a single env
    env.seed(10)
    state_dim = envs.observation_space.shape[0]
    action_dim = envs.action_space.n
    model = ActorCritic(state_dim, action_dim, cfg.hidden_dim).to(cfg.device)
    optimizer = optim.Adam(model.parameters())
    frame_idx = 0
    test_rewards = []
    test_ma_rewards = []
    state = envs.reset()
    while frame_idx < cfg.max_frames:
        log_probs = []
        values    = []
        rewards   = []
        masks     = []
        entropy = 0
        # rollout trajectory
        for _ in range(cfg.n_steps):
            state = torch.FloatTensor(state).to(cfg.device)
            dist, value = model(state)
            action = dist.sample()
            next_state, reward, done, _ = envs.step(action.cpu().numpy())
            log_prob = dist.log_prob(action)
            entropy += dist.entropy().mean()
            log_probs.append(log_prob)
            values.append(value)
            rewards.append(torch.FloatTensor(reward).unsqueeze(1).to(cfg.device))
            masks.append(torch.FloatTensor(1 - done).unsqueeze(1).to(cfg.device))
            state = next_state
            frame_idx += 1
            if frame_idx % 100 == 0:
                test_reward = np.mean([ceshi_env(env,model) for _ in range(10)])
                print(f"frame_idx:{frame_idx}, test_reward:{test_reward}")
                test_rewards.append(test_reward)
                if test_ma_rewards:
                    test_ma_rewards.append(0.9*test_ma_rewards[-1]+0.1*test_reward)
                else:
                    test_ma_rewards.append(test_reward) 
                # plot(frame_idx, test_rewards)   
        next_state = torch.FloatTensor(next_state).to(cfg.device)
        _, next_value = model(next_state)
        returns = compute_returns(next_value, rewards, masks)
        log_probs = torch.cat(log_probs)
        returns   = torch.cat(returns).detach()
        values    = torch.cat(values)
        advantage = returns - values
        actor_loss  = -(log_probs * advantage.detach()).mean()
        critic_loss = advantage.pow(2).mean()
        loss = actor_loss + 0.5 * critic_loss - 0.001 * entropy
        optimizer.zero_grad()
        loss.backward()
        optimizer.step()
    print('完成训练！')
    return test_rewards, test_ma_rewards
if __name__ == "__main__":
    cfg = A2CConfig()
    plot_cfg = PlotConfig()
    envs = [make_envs(cfg.env_name) for i in range(cfg.n_envs)]
    envs = SubprocVecEnv(envs) 
    # 训练
    rewards,ma_rewards = train(cfg,envs)
    make_dir(plot_cfg.result_path,plot_cfg.model_path)
    save_results(rewards, ma_rewards, tag='train', path=plot_cfg.result_path) # 保存结果
    plot_rewards(rewards, ma_rewards, plot_cfg, tag="train") # 画出结果

3.2 一些依赖的文件（common文件夹）

3.2.1 multiprocessing_env.py(来自 openai baseline，用于多线程环境)

# 该代码来自 openai baseline，用于多线程环境
# https://github.com/openai/baselines/tree/master/baselines/common/vec_env
import numpy as np
from multiprocessing import Process, Pipe
def worker(remote, parent_remote, env_fn_wrapper):
    parent_remote.close()
    env = env_fn_wrapper.x()
    while True:
        cmd, data = remote.recv()
        if cmd == 'step':
            ob, reward, done, info = env.step(data)
            if done:
                ob = env.reset()
            remote.send((ob, reward, done, info))
        elif cmd == 'reset':
            ob = env.reset()
            remote.send(ob)
        elif cmd == 'reset_task':
            ob = env.reset_task()
            remote.send(ob)
        elif cmd == 'close':
            remote.close()
            break
        elif cmd == 'get_spaces':
            remote.send((env.observation_space, env.action_space))
        else:
            raise NotImplementedError
class VecEnv(object):
    """
    An abstract asynchronous, vectorized environment.
    """
    def __init__(self, num_envs, observation_space, action_space):
        self.num_envs = num_envs
        self.observation_space = observation_space
        self.action_space = action_space
    def reset(self):
        """
        Reset all the environments and return an array of
        observations, or a tuple of observation arrays.
        If step_async is still doing work, that work will
        be cancelled and step_wait() should not be called
        until step_async() is invoked again.
        """
        pass
    def step_async(self, actions):
        """
        Tell all the environments to start taking a step
        with the given actions.
        Call step_wait() to get the results of the step.
        You should not call this if a step_async run is
        already pending.
        """
        pass
    def step_wait(self):
        """
        Wait for the step taken with step_async().
        Returns (obs, rews, dones, infos):
         - obs: an array of observations, or a tuple of
                arrays of observations.
         - rews: an array of rewards
         - dones: an array of "episode done" booleans
         - infos: a sequence of info objects
        """
        pass
    def close(self):
        """
        Clean up the environments' resources.
        """
        pass
    def step(self, actions):
        self.step_async(actions)
        return self.step_wait()
    
class CloudpickleWrapper(object):
    """
    Uses cloudpickle to serialize contents (otherwise multiprocessing tries to use pickle)
    """
    def __init__(self, x):
        self.x = x
    def __getstate__(self):
        import cloudpickle
        return cloudpickle.dumps(self.x)
    def __setstate__(self, ob):
        import pickle
        self.x = pickle.loads(ob)
        
class SubprocVecEnv(VecEnv):
    def __init__(self, env_fns, spaces=None):
        """
        envs: list of gym environments to run in subprocesses
        """
        self.waiting = False
        self.closed = False
        nenvs = len(env_fns)
        self.nenvs = nenvs
        self.remotes, self.work_remotes = zip(*[Pipe() for _ in range(nenvs)])
        self.ps = [Process(target=worker, args=(work_remote, remote, CloudpickleWrapper(env_fn)))
            for (work_remote, remote, env_fn) in zip(self.work_remotes, self.remotes, env_fns)]
        for p in self.ps:
            p.daemon = True # if the main process crashes, we should not cause things to hang
            p.start()
        for remote in self.work_remotes:
            remote.close()
        self.remotes[0].send(('get_spaces', None))
        observation_space, action_space = self.remotes[0].recv()
        VecEnv.__init__(self, len(env_fns), observation_space, action_space)
    def step_async(self, actions):
        for remote, action in zip(self.remotes, actions):
            remote.send(('step', action))
        self.waiting = True
    def step_wait(self):
        results = [remote.recv() for remote in self.remotes]
        self.waiting = False
        obs, rews, dones, infos = zip(*results)
        return np.stack(obs), np.stack(rews), np.stack(dones), infos
    def reset(self):
        for remote in self.remotes:
            remote.send(('reset', None))
        return np.stack([remote.recv() for remote in self.remotes])
    def reset_task(self):
        for remote in self.remotes:
            remote.send(('reset_task', None))
        return np.stack([remote.recv() for remote in self.remotes])
    def close(self):
        if self.closed:
            return
        if self.waiting:
            for remote in self.remotes:            
                remote.recv()
        for remote in self.remotes:
            remote.send(('close', None))
        for p in self.ps:
            p.join()
            self.closed = True
            
    def __len__(self):
        return self.nenvs

3.2.2 utils.py(主要是文件创建与绘图函数)

import os
import numpy as np
from pathlib import Path
import matplotlib.pyplot as plt
# import seaborn as sns
from matplotlib.font_manager import FontProperties  # 导入字体模块
def chinese_font():
    ''' 设置中文字体，注意需要根据自己电脑情况更改字体路径，否则还是默认的字体
    '''
    try:
        font = FontProperties(
        fname='/System/Library/Fonts/STHeiti Light.ttc', size=15) # fname系统字体路径，此处是mac的
    except:
        font = None
    return font
def plot_rewards_cn(rewards, ma_rewards, plot_cfg, tag='train'):
    ''' 中文画图
    '''
    # sns.set()
    plt.figure()
    plt.title(u"{}环境下{}算法的学习曲线".format(plot_cfg.env_name,
              plot_cfg.algo_name), fontproperties=chinese_font())
    plt.xlabel(u'回合数', fontproperties=chinese_font())
    plt.plot(rewards)
    plt.plot(ma_rewards)
    plt.legend((u'奖励', u'滑动平均奖励',), loc="best", prop=chinese_font())
    if plot_cfg.save:
        plt.savefig(plot_cfg.result_path+f"{tag}_rewards_curve_cn")
    # plt.show()
def plot_rewards(rewards, ma_rewards, plot_cfg, tag='train'):
    # sns.set()
    plt.figure()  # 创建一个图形实例，方便同时多画几个图
    plt.title("learning curve on {} of {} for {}".format(
        plot_cfg.device, plot_cfg.algo_name, plot_cfg.env_name))
    plt.xlabel('epsiodes')
    plt.plot(rewards, label='rewards')
    plt.plot(ma_rewards, label='ma rewards')
    plt.legend()
    if plot_cfg.save:
        plt.savefig(plot_cfg.result_path+"{}_rewards_curve".format(tag))
    plt.show()
def plot_losses(losses, algo="DQN", save=True, path='./'):
    # sns.set()
    plt.figure()
    plt.title("loss curve of {}".format(algo))
    plt.xlabel('epsiodes')
    plt.plot(losses, label='rewards')
    plt.legend()
    if save:
        plt.savefig(path+"losses_curve")
    plt.show()
def save_results(rewards, ma_rewards, tag='train', path='./results'):
    ''' 保存奖励
    '''
    np.save(path+'{}_rewards.npy'.format(tag), rewards)
    np.save(path+'{}_ma_rewards.npy'.format(tag), ma_rewards)
    print('结果保存完毕！')
def make_dir(*paths):
    ''' 创建文件夹
    '''
    for path in paths:
        Path(path).mkdir(parents=True, exist_ok=True)
def del_empty_dir(*paths):
    ''' 删除目录下所有空文件夹
    '''
    for path in paths:
        dirs = os.listdir(path)
        for dir in dirs:
            if not os.listdir(os.path.join(path, dir)):
                os.removedirs(os.path.join(path, dir))

4 实验结果