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<h2 id="线程池的好处"><a href="#线程池的好处" class="headerlink" title="线程池的好处"></a>线程池的好处</h2><p>线程的开启也是一个相对缓慢的过程。</p>
<ul>
<li>如果不用线程池，在系统运行过程中实时开启线程，销毁线程。会极大增加系统的负载。导致系统的实时性减低，业务处理能力也会降低</li>
<li>服务器启动时，就事先创建好线程池，当业务来到时，直接从线程池取出一个线程来执行任务。任务完成后直接把线程归还给池，用于后续重复使用。</li>
</ul>
<h2 id="线程池的模式"><a href="#线程池的模式" class="headerlink" title="线程池的模式"></a>线程池的模式</h2><ul>
<li>固定大小</li>
<li>可变大小</li>
</ul>
<h2 id="线程同步"><a href="#线程同步" class="headerlink" title="线程同步"></a>线程同步</h2><p>并发编程的基本问题：</p>
<ul>
<li>互斥</li>
<li>通信</li>
</ul>
<p>我们希望有一段代码被<strong>原子式</strong>执行，由于单处理器的任务切换（中断）或者多处理器的并行，我们不能保证。</p>
<p>所以需要一种机制来保证原子性—–锁</p>
<p>操作系统给上层提供了一个API，使得应用程序可以使用锁来达到代码执行的原子性。</p>
<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="type">lock_t</span> mutex; <span class="comment">// some globally-allocated lock 'mutex'</span></span><br><span class="line">...</span><br><span class="line"><span class="built_in">lock</span>(&mutex);</span><br><span class="line">... <span class="comment">// 临界区</span></span><br><span class="line"><span class="built_in">unlock</span>(&mutex);</span><br></pre></td></tr></table></figure>
<p>调用者调用lock尝试获取锁，如果没有其他现场持有锁，线程就会获得锁，进入临界区。否则，他就会阻塞。直到锁可用。（具体实现比这复杂）</p>
<h3 id="锁的实现原理"><a href="#锁的实现原理" class="headerlink" title="锁的实现原理"></a>锁的实现原理</h3><p>锁定实现要求：</p>
<ul>
<li> 提供互斥（保证临界区执行的原子性）</li>
<li>公平性（不会有饿死线程）</li>
<li>性能</li>
</ul>
<h4 id="方式1：控制中断"><a href="#方式1：控制中断" class="headerlink" title="方式1：控制中断"></a>方式1：控制中断</h4><p>缺点：</p>
<ul>
<li>需要应用进程有特权操作，因为中断需要特权操作</li>
<li>只适合单处理器，多处理器无法保证互斥</li>
<li>会导致中断丢失</li>
<li>效率低。</li>
</ul>
<h4 id="方式-2：自旋锁"><a href="#方式-2：自旋锁" class="headerlink" title="方式 2：自旋锁"></a>方式 2：自旋锁</h4><p>需要硬件指令支持：<strong>test and set 指令 （原子交换）</strong></p>
<blockquote>
<p><strong>假设无硬件支持</strong>，我们很容易想到的一个锁（就是变量）实现是：(测试和设置不是原子操作)</p>
</blockquote>
<p>使用一个变量来标志锁是否被占有。这样进程在进入临界区时，会：</p>
<ul>
<li>检查标志，如果可以进入就进入临界区吗，设置锁标志。不能就阻塞等待。</li>
<li>在退出临界区时，清除标志。</li>
</ul>
<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">typedef</span> <span class="keyword">struct</span> <span class="title class_">lock_t</span> { <span class="type">int</span> flag; } <span class="type">lock_t</span>;</span><br><span class="line"><span class="function"><span class="type">void</span> <span class="title">init</span><span class="params">(<span class="type">lock_t</span> *mutex)</span> </span>{</span><br><span class="line"><span class="comment">// 0 -> lock is available, 1 -> held</span></span><br><span class="line">mutex->flag = <span class="number">0</span>;</span><br><span class="line">}</span><br><span class="line"><span class="function"><span class="type">void</span> <span class="title">lock</span><span class="params">(<span class="type">lock_t</span> *mutex)</span> </span>{</span><br><span class="line"><span class="keyword">while</span> (mutex->flag == <span class="number">1</span>) <span class="comment">// TEST the flag</span></span><br><span class="line">; <span class="comment">// spin-wait (do nothing)</span></span><br><span class="line">mutex->flag = <span class="number">1</span>;</span><br><span class="line"><span class="comment">// now SET it!</span></span><br><span class="line">}</span><br><span class="line"><span class="function"><span class="type">void</span> <span class="title">unlock</span><span class="params">(<span class="type">lock_t</span> *mutex)</span> </span>{</span><br><span class="line">mutex->flag = <span class="number">0</span>;</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>不能保证互斥：</p>
<ul>
<li><p>如果线程1在测试时，被中断到线程2，</p>
</li>
<li><p>线程2执行测试并成功设置了标志。此时又被中断</p>
</li>
<li><p>线程1，此时测试也会成功。两者同时进入了临界区</p>
</li>
</ul>
<p>性能问题：</p>
<p>自旋会导致白白浪费CPU资源</p>
<blockquote>
<p>test and set 硬件（原子交换）指令，我们可以实现一个满足要求的最简单的锁—-自旋锁</p>
</blockquote>
<p>test and set 的原理：</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line">// 这就是一个交换操作，只不过硬件保证了他的原子性。</span><br><span class="line">int TestAndSet(int *old_ptr, int new) {</span><br><span class="line"></span><br><span class="line">int old = *old_ptr; // 获取旧值</span><br><span class="line">*old_ptr = new;// 设置新值</span><br><span class="line">return old;// 返回旧值。</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>自旋锁的实现：</p>
<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">typedef</span> <span class="keyword">struct</span> <span class="title class_">lock_t</span></span><br><span class="line">{</span><br><span class="line"> <span class="type">int</span> flag;</span><br><span class="line">} <span class="type">lock_t</span>;</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="type">void</span> <span class="title">init</span><span class="params">(<span class="type">lock_t</span> *lock)</span> </span>{</span><br><span class="line"><span class="comment">// 0 indicates that lock is available, 1 that it is held</span></span><br><span class="line"> lock->flag = <span class="number">0</span>;</span><br><span class="line">}</span><br><span class="line"><span class="function"><span class="type">void</span> <span class="title">lock</span><span class="params">(<span class="type">lock_t</span> *lock)</span> </span>{</span><br><span class="line"> <span class="keyword">while</span> (<span class="built_in">TestAndSet</span>(&lock->flag, <span class="number">1</span>) == <span class="number">1</span>); <span class="comment">// spin-wait (do nothing)</span></span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="type">void</span> <span class="title">unlock</span><span class="params">(<span class="type">lock_t</span> *lock)</span> </span>{</span><br><span class="line"> lock->flag = <span class="number">0</span>;</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>工作场景模拟：</p>
<ul>
<li><p>当调用lock时，如果没有线程持有锁，flag=0，原子交换指令flag=1，返回旧flag=0，循环为假，跳出，获得锁。（这是原子操作，不会被中断）</p>
</li>
<li><p>如果在执行完原子指令之后，被中断（此时flag=1，）别的线程如果执行原子交换指令，while循环一致为真，导致自旋。保证了互斥</p>
</li>
<li><p>只有在第一个线程结束执行，调用unlock时，才会解除自旋。</p>
</li>
</ul>
<p>自旋锁：</p>
<p>保证了互斥，但没有保证公平。</p>
<p>有可能某个线程会一直自旋下去，导致饿死。</p>
<blockquote>
<p>compare-and-exchange 硬件支持的原子指令</p>
</blockquote>
<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// 如果满足期望的值，则会将锁交换。否则，什么也不做。返回锁原来的值。</span></span><br><span class="line"><span class="function"><span class="type">int</span> <span class="title">CompareAndSwap</span><span class="params">(<span class="type">int</span> *ptr, <span class="type">int</span> expected, <span class="type">int</span> <span class="keyword">new</span>)</span> </span></span><br><span class="line"><span class="function"></span>{</span><br><span class="line"> <span class="type">int</span> actual = *ptr;</span><br><span class="line"> <span class="keyword">if</span> (actual == expected)</span><br><span class="line"> *ptr = <span class="keyword">new</span>;</span><br><span class="line"> <span class="keyword">return</span> actual;</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>使用这条指令来实现自旋锁和上面的指令差不多：</p>
<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="type">void</span> <span class="title">lock</span><span class="params">(<span class="type">lock_t</span> *lock)</span> </span>{</span><br><span class="line"> <span class="keyword">while</span> (<span class="built_in">CompareAndSwap</span>(&lock->flag, <span class="number">0</span>, <span class="number">1</span>) == <span class="number">1</span>); <span class="comment">// spin</span></span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>在无等待同步（wait-free synchronization）时，这个指令比test and set强大、</p>
<blockquote>
<p>链接的加载（load-linked）和条件式存储（store-conditional）指令 原子指令</p>
</blockquote>
<p>连接加载：从内存中取出一个值到寄存器，（可以理解为解引用一个地址。）</p>
<p>条件式存储：只有上次从地址A取出的值，自从上次取出之后，没有被人家更改过（也就是说这个值还是那个值），才会去操作这个地址。）</p>
<h4 id="ticket-锁"><a href="#ticket-锁" class="headerlink" title="ticket 锁"></a>ticket 锁</h4><blockquote>
<p>获取并增加fetch-and-add 原子指令 </p>
</blockquote>
<p>它能原子地返回特定地址的旧值，并且让该值自增一。</p>
<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="type">int</span> <span class="title">FetchAndAdd</span><span class="params">(<span class="type">int</span> *ptr)</span> </span>{</span><br><span class="line"> <span class="type">int</span> old = *ptr;</span><br><span class="line"> *ptr = old + <span class="number">1</span>;</span><br><span class="line"> <span class="keyword">return</span> old;</span><br><span class="line">}</span><br><span class="line"><span class="keyword">typedef</span> <span class="keyword">struct</span> <span class="title class_">lock_t</span> {</span><br><span class="line"> <span class="type">int</span> ticket；<span class="comment">//车票</span></span><br><span class="line"> <span class="type">int</span> turn; <span class="comment">//（轮到自己）</span></span><br><span class="line">} <span class="type">lock_t</span>;</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="type">void</span> <span class="title">lock_init</span><span class="params">(<span class="type">lock_t</span> *lock)</span> </span>{</span><br><span class="line"> lock->ticket = <span class="number">0</span>;</span><br><span class="line"> lock->turn <span class="number">0</span>;</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="type">void</span> <span class="title">lock</span><span class="params">(<span class="type">lock_t</span> *lock)</span> </span>{</span><br><span class="line"> <span class="type">int</span> myturn = <span class="built_in">FetchAndAdd</span>(&lock->ticket);</span><br><span class="line"> <span class="keyword">while</span> (lock->turn != myturn); <span class="comment">// spin</span></span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="type">void</span> <span class="title">unlock</span><span class="params">(<span class="type">lock_t</span> *lock)</span> </span>{</span><br><span class="line"> <span class="built_in">FetchAndAdd</span>(&lock->turn);</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>相当于先买票，再排队。</p>
<p>使用两个变量，构建锁：</p>
<p>获取锁的方式：</p>
<ul>
<li>先买票（调用获取并增加指令），此时自己的lock->ticket 为1，表示自己有票了。</li>
<li>再去排队（while循环），如果轮到自己了，就可以获得锁。</li>
</ul>
<p><strong>既可以保证互斥也可以保证公平性。</strong></p>
<h4 id="自旋过长时间的应对方法-—-yield-系统调用"><a href="#自旋过长时间的应对方法-—-yield-系统调用" class="headerlink" title="自旋过长时间的应对方法 —- yield()系统调用"></a>自旋过长时间的应对方法 —- yield()系统调用</h4><p>在以上公平互斥锁的基础上，还有一个问题：</p>
<p>如果处于临界区中的线程被中断，其他线程就得长时间自旋等待。</p>
<p>解决办法：</p>
<ul>
<li>yield()系统调用：在线程发现锁被别人持有时，主动调用yield让出锁，是自己进入就绪态。</li>
</ul>
<p>在两个线程下工作的很好。</p>
<p>缺点：如果在许多线程下反复竞争一把锁时，一个线程长时间持有锁，会导致其他线程全部让出锁，知道持有锁的继续执行。</p>
<h4 id="解决让出过多问题-—-使用等待队列，睡眠代替自旋。"><a href="#解决让出过多问题-—-使用等待队列，睡眠代替自旋。" class="headerlink" title="解决让出过多问题 —- 使用等待队列，睡眠代替自旋。"></a>解决让出过多问题 —- 使用等待队列，睡眠代替自旋。</h4><p>前面一些方法的真正问题是存在太多的偶然性。调度程序决定如何调度。如果调度不合理，线程或者一直自旋（第一种方法），或者立刻让出 CPU（第二种方法）。无论哪种方法，都可能造成浪费，也能防止饿死。</p>
<p>因此，我们必须<strong>显式地施加某种控制，决定锁释放时，谁能抢到锁（排队）</strong>。为了做到这一点，我们需要操作系统的更多支持，并需要一个队列来保存等待锁的线程</p>
<p>睡眠与唤醒。</p>
<p>一个线程如果尝试获得锁，失败，自己会陷入睡眠。等到获得锁的线程结束执行时，他会唤醒正在睡眠的线程队列。队列中如果没有人有紧急情况，就会按排队顺序接着下一个线程执行。</p>
<p>例如，Linux 提供了 futex，具体来说，每个 futex 都关联一个特定的物理内存位置，也有一个事先建好的内核队列。调用者通过futex 调用（见下面的描述）来睡眠或者唤醒。<br>具体来说，有两个调用。调用 futex_wait(address, expected)时，如果 address 处的值等于expected，就会让调线程睡眠。否则，调用立刻返回。调用 futex_wake(address)唤醒等待队列中的一个线程。图 28.9 是 Linux 环境下的例子：</p>
<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="type">void</span> <span class="title">mutex_lock</span><span class="params">(<span class="type">int</span> *mutex)</span></span></span><br><span class="line"><span class="function"></span>{</span><br><span class="line"> <span class="type">int</span> v;</span><br><span class="line"> <span class="comment">/*首先测试mutixc值为是否0，如果是，说明没人等待，并且锁是空闲的。也就是说只有我自己想要锁。那我直接拿。并把最高位设置为1.*/</span></span><br><span class="line"> </span><br><span class="line"> <span class="keyword">if</span> (<span class="built_in">atomic_bit_test_set</span>(mutex, <span class="number">31</span>) == <span class="number">0</span>)</span><br><span class="line"> <span class="keyword">return</span>;</span><br><span class="line"> </span><br><span class="line"> </span><br><span class="line"> <span class="comment">// 否则，等待者加1.</span></span><br><span class="line"> <span class="built_in">atomic_increment</span>(mutex);</span><br><span class="line"> <span class="keyword">while</span> (<span class="number">1</span>)</span><br><span class="line"> {</span><br><span class="line"> <span class="comment">// 再次循环尝试获得锁。</span></span><br><span class="line"> <span class="keyword">if</span> (<span class="built_in">atomic_bit_test_set</span>(mutex, <span class="number">31</span>) == <span class="number">0</span>)</span><br><span class="line"> {</span><br><span class="line"> <span class="comment">// 如果，获得了，等待者减一。</span></span><br><span class="line"> <span class="built_in">atomic_decrement</span>(mutex);</span><br><span class="line"> <span class="keyword">return</span>;</span><br><span class="line"> }</span><br><span class="line"> <span class="comment">/* We have to wait now. First make sure the futex value</span></span><br><span class="line"><span class="comment"> we are monitoring is truly negative (i.e. locked).*/</span></span><br><span class="line"> v = *mutex;</span><br><span class="line"> <span class="keyword">if</span> (v >= <span class="number">0</span>)</span><br><span class="line"> <span class="keyword">continue</span>;</span><br><span class="line"> <span class="comment">// 如果执行到这，说明锁已经没了，我们需要等待。</span></span><br><span class="line"> <span class="built_in">futex_wait</span>(mutex, v);</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="type">void</span> <span class="title">mutex_unlock</span><span class="params">(<span class="type">int</span> *mutex)</span></span></span><br><span class="line"><span class="function"></span>{</span><br><span class="line"> <span class="comment">/* Adding 0x80000000 to the counter results in 0 if and only if</span></span><br><span class="line"><span class="comment"> there are not other interested threads */</span></span><br><span class="line"> <span class="keyword">if</span> (<span class="built_in">atomic_add_zero</span>(mutex, <span class="number">0x80000000</span>))</span><br><span class="line"> <span class="keyword">return</span>;</span><br><span class="line"> <span class="comment">/* There are other threads waiting for this mutex,</span></span><br><span class="line"><span class="comment"> wake one of them up.</span></span><br><span class="line"><span class="comment"> */</span></span><br><span class="line"> <span class="built_in">futex_wake</span>(mutex);</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>这段代码来自 nptl 库（gnu libc 库的一部分）[L09]中 lowlevellock.h，它很有趣。基本上，它利用一个整数，同时记录锁是否被持有（整数的最高位），以及等待者的个数（整数的其余所有位）。因此，如果锁是负的，它就被持有（因为最高位被设置，该位决定了整数的符号）。这段代码的有趣之处还在于，它展示了如何优化常见的情况，即没有竞争时：只有一个线程获取和释放锁，所做的工作很少（获取锁时测试和设置的原子位运算，释放锁时原子的加法）。</p>
<h4 id="两阶段锁"><a href="#两阶段锁" class="headerlink" title="两阶段锁"></a>两阶段锁</h4><p>两阶段锁的第一阶段会先自旋一段时间，希望它可以获取锁。<br>如果第一个自旋阶段没有获得锁，第二阶段调用者会睡眠，直到锁可用。</p>
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<h1 id="计算机系统中的并发"><a href="#计算机系统中的并发" class="headerlink" title="计算机系统中的并发"></a>计算机系统中的并发</h1><p>多处理器：有多个处理器插座。</p>
<p>多核：一个处理器插座，只能有一个处理器安装，一个处理器有多个核心（独立执行计算的单元），每个核心可以有多个硬件线程（hart）</p>
<h2 id="何为并发？"><a href="#何为并发？" class="headerlink" title="何为并发？"></a>何为并发？</h2><p>并发：单个系统在同时执行多个独立的任务。</p>
<ul>
<li>硬件并发：一台计算机系统提供多个核心、单个CPU提供多个硬件线程（hart）。</li>
<li>软件并发：单个核心执行<strong>任务切换</strong>（task switch）。</li>
</ul>
<p>如果只有纯硬件并发，那么计算有多少硬件线程，就只能执行多少任务。</p>
<p>而软件并发，理论上可以在迷惑应用程序和用户的前提下，“同时”执行多个任务。</p>
<p>因此，现代计算机系统的并发是二者的结合。</p>
<p>硬件并发和软件并发还是有着许多不同：</p>
<ul>
<li>内存模型的假设</li>
<li>任务切换的负担</li>
</ul>
<h2 id="并发的方式（手段）"><a href="#并发的方式（手段）" class="headerlink" title="并发的方式（手段）"></a>并发的方式（手段）</h2><ul>
<li>多进程（每个进程只有一个执行线程）</li>
<li>多线程（每个进程有多个执行线程）</li>
</ul>
<h3 id="多进程并发"><a href="#多进程并发" class="headerlink" title="多进程并发"></a>多进程并发</h3><p>每个进程独立获得资源，进程之间互不干扰。</p>
<p>通过进程间通信手段进行通信</p>
<ul>
<li>信号</li>
<li>共享内存</li>
<li>管道</li>
<li>信号量</li>
<li>消息队列</li>
<li>等</li>
</ul>
<p>缺点：</p>
<ul>
<li>进程切换开销大，导致速度慢</li>
<li>进程管理需要操作系统维持（通过维持某些数据结构）</li>
</ul>
<p>优点：</p>
<ul>
<li>各个任务之间独立，安全性高。</li>
</ul>
<h3 id="多线程并发"><a href="#多线程并发" class="headerlink" title="多线程并发"></a>多线程并发</h3><p>在单个进程中执行多个线程。</p>
<ul>
<li>线程之间共享同一进程的大部分资源</li>
<li>但可以有自己的执行序列（PC寄存器），因此，可以同时独立完成多个任务。</li>
</ul>
<p>缺点：</p>
<ul>
<li>同一个进程的共享数据的访问，要被正确同步。</li>
</ul>
<p>多个单线程/进程间的通信(包含启动)要比单一进程中的多线程间的通信(包括启动)的开销大，若不考虑共享内存可能会带来的问题，多线程将会成为主流语言(包括C++)更青睐的并发途径。此外，C++标准并未对进程间通信提供任何原生支持，所以使用多进程的方式实现，这会依赖与平台相关的API。因此，本书只关注使用多线程的并发，并且在此之后所提到“并发”，均假设为多线程来实现。</p>
<h3 id="并发与并行"><a href="#并发与并行" class="headerlink" title="并发与并行"></a>并发与并行</h3><p>不同的关注点</p>
<p><strong>并行</strong>关注点：更注重性能，如何利用当前硬件的所有资源，来提高任务处理速度。</p>
<p><strong>并发</strong>关注点：任务分离，任务响应。</p>
<h3 id="关注点分离"><a href="#关注点分离" class="headerlink" title="关注点分离"></a>关注点分离</h3><p>互相独立的功能模块，可以在同一应用（进程）中分离，并行执行完成的任务，再进行信息的交换（需要手动处理交互逻辑）。</p>
<p>这种思想使得，线程的数量不再依赖CPU中可用（hart）的数量，因为对线程的划分是基于概念上的设计，而不是一种增加吞吐量的尝试（根本目的并不是为了能够处理更多任务，但这样做确实大部分情况下都能增加吞吐量。）。</p>
<h2 id="并发的方式"><a href="#并发的方式" class="headerlink" title="并发的方式"></a>并发的方式</h2><ul>
<li>指令级并行</li>
<li>数据级并行</li>
<li>线程级并性（任务并行）</li>
<li>多机并行</li>
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<h2 id="博客搭建的步骤"><a href="#博客搭建的步骤" class="headerlink" title="博客搭建的步骤"></a>博客搭建的步骤</h2><p>要搭建个人博客，首先要了解具体要实施哪些步骤，需要哪些工具，在了解了这些之后，再根据自己的需要，就可以很清晰的搭建自己的理想博客了。</p>
<h2 id="前置知识"><a href="#前置知识" class="headerlink" title="前置知识"></a>前置知识</h2><ul>
<li>静态网站和动态网站</li>
</ul>
<p>静态网站：内容如果不被发布者修改源代码后，重新编译，重新生成。网站上的资源永远不会改变。（比如，一篇博客文章，不与用户有任何数据交互。）</p>
<p>动态网站：底层与数据库交互，有一些放在数据库中的动态资源是可以随着你的请求而产生变化。（比如: 一个登录页面。需要与底层数据库进行数据交换。）</p>
<ul>
<li>网页生成器</li>
</ul>
<p>我们在自己的电脑上写完了自己文章之后，我们想要把他放到网上供别人在线查看的话，基本上用户都会使用基于web的浏览器在线浏览你的博客。然而浏览器都是基于html的web页面。对于md格式的文本文件，浏览器无法直接展示在他的界面上。（也不是无法展示，而是他会以纯文本文件的形式展示。），如果你直接把这些文件放到网上（比如一个公开服务器上或者托管平台上）供其他人访问，他们看到的只能是界面丑陋，晦涩生硬的纯文本。</p>
<p>而html就是用来渲染这些文本，让他们更生动（比如有颜色，有图片，格式多样）更方便用户阅读。所以，我们写的博客要是能够转换成web页面，然后再放在网上让别人阅读，那就好了。</p>
<p>我能想到的最笨的写博客的方法就是，你直接把你的内容嵌套到html文件里。这样也就不用任何第三方工具了。（但这既费力又需要很强的知识储备。没人愿意这样干。）</p>
<p>所以静态网站生成器就是干这个工作的：他会把你的md文件，自动嵌入到web页面中。不用你自己手动做这些操作，你要做的就是使用这些工具，遵循他们的格式要求就可以生成一个静态网站了。</p>
<ul>
<li>部署</li>
</ul>
<p>上面说的网页生成器所做的事情是转换，资源仍然在你的本地电脑上，我们如何将资源放到网上呢？这个基本常识就是（将资源放到大家可以访问的Ip地址上，也就是自己的网站了。）</p>
<p>能做这件事的有很多：</p>
<ul>
<li>自己购买一个云服务器，然后，将生成的静态资源直接放服务器的磁盘上，别人就可以通过IP地址访问你的资源。</li>
<li>通过一些托管平台（比如GitPage）本质上和服务器是一样的，只不过是别人帮你托管。</li>
<li>其他方式</li>
</ul>
<p>完成上面所有步骤，网站部署大致流程就完成了。剩下的就是一些细节问题（比如，如何写博客，如何使用生成器生成自己想要的网站样式（模板））等等问题。</p>
<h2 id="正式搭建"><a href="#正式搭建" class="headerlink" title="正式搭建"></a>正式搭建</h2><p>工具：使用md写博客，使用hexo生成网页，使用GItPage托管（部署）网站。</p>
<h3 id="hexo-的使用"><a href="#hexo-的使用" class="headerlink" title="hexo 的使用"></a>hexo 的使用</h3><p>在有自己博客的前提下，开始下载hexo，生成静态网页。</p>
<ul>
<li>安装:hexo是使用js写的平台，需要安装nodejs，并使用nodejs 包管理工具，NPM安装hexo软件。</li>
</ul>
<figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">npm install -g hexo-cli</span><br></pre></td></tr></table></figure>
<ul>
<li>hexo是一个博客框架，首先需要生成一个初始博客模板（框架），命令为：</li>
</ul>
<figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">hexo init blog_dir_name && <span class="built_in">cd</span> my-blog</span><br><span class="line"><span class="comment"># blog_dir_name 是你想要放置你的网站的根目录</span></span><br></pre></td></tr></table></figure>
<ul>
<li>这个框架就是hexo为你生成的一个静态网页模板，你可以在本地直接测试他的生成效果。你自己的网页基本也是这样。如果测试效果满意，就可以推送到服务器上，网站就建成了。本地测试命令（需要进入到博客根目录中）。</li>
</ul>
<figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">hexo server </span><br><span class="line"><span class="comment"># 可以简写为hexo s</span></span><br><span class="line">INFO Hexo is running at http://0.0.0.0:4000/. Press Ctrl+C to stop.</span><br></pre></td></tr></table></figure>
<p>这会在本地启动服务器，你可以通过浏览器访问相应的端口进行访问。</p>
<ul>
<li>创建你的第一篇文章（帖子）</li>
</ul>
<p>开始新帖子时，最好使用“草稿”功能。默认情况下，草稿不会发布，因此您可以自由更改其他帖子，同时保持未完成的草稿不公开。以下命令</p>
<figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">hexo new draft <span class="string">"My First Blog Post"</span></span><br><span class="line"><span class="comment"># creates -> ./source/_drafts/My-First-Blog-Post.md</span></span><br></pre></td></tr></table></figure>
<p>草稿帖子都会放在./source/_drafts/目录下，默认名是你的标题名。</p>
<blockquote>
<p>使用 hexo new 命令时，可以使用除“draft”之外的其他模板。查看 ./scaffolds/ 文件夹并阅读 Hexo <a target="_blank" rel="noopener" href="https://hexo.io/docs/writing.html#Scaffolds">文档</a>。</p>
</blockquote>
<ul>
<li>要编辑草稿，请导航至 ./source/_drafts/My-First-Blog-Post.md 并使用您最喜欢的 Markdown 编辑器打开该文件。<br> 让我们在您的新帖子中添加副标题和一些段落内容……</li>
</ul>
<figure class="highlight markdown"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line">---</span><br><span class="line">title: My First Blog Post</span><br><span class="line"><span class="section">tags:</span></span><br><span class="line"><span class="section">---</span></span><br><span class="line"><span class="section">## Hello there</span></span><br><span class="line">This is some content.</span><br></pre></td></tr></table></figure>
<blockquote>
<p>位于 Markdown 文件顶部的破折号之间的内容称为“front-matter”。它是 Hexo 和活动主题使用的元数据。有关更多信息，请参阅 Front-Matter 上的 Hexo <a target="_blank" rel="noopener" href="https://hexo.io/docs/front-matter.html">文档</a>。</p>
</blockquote>
<p>保存对 Markdown 文件的更改将被正在运行的 hexo 服务器<strong>自动</strong>检测并重新生成为静态 HTML 文件，但您必须刷新浏览器才能查看更改。</p>
<p>如果您不喜欢每次都手动刷新浏览器，hexo-livereload 或 hexo-browsersync 插件可以自动执行此操作。</p>
<ul>
<li>安装 <code>hexo-browsersync</code> 插件（我个人最喜欢的）：</li>
</ul>
<figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">$ npm install hexo-browsersync --save</span><br><span class="line">$ hexo server --draft --open <span class="comment"># 重启服务器</span></span><br></pre></td></tr></table></figure>
<blockquote>
<p>其他 Hexo 插件可以使用 npm 以相同的方式轻松安装。许多插件都具有可以在项目的 <code>_config.yml </code>文件中进行调整的配置。您需要查阅每个插件的文档以了解其特定的配置属性。对于 hexo-browsersync，默认设置可以正常工作，不需要编辑 _config.yml 文件。</p>
</blockquote>
<h3 id="一些写作技巧"><a href="#一些写作技巧" class="headerlink" title="一些写作技巧"></a>一些写作技巧</h3><ul>
<li>显示更少的内容</li>
</ul>
<p>假设您有一篇很长的文章，并且不喜欢整篇文章显示在列表页面中……<br>您可以使用 <code><!-- more --> </code>在 Markdown 中标记一个位置，以将其从列表页面中隐藏。它将被替换为“阅读更多”链接，该链接将打开文章的其余内容。</p>
<figure class="highlight markdown"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line">---</span><br><span class="line">title: My First Blog Post</span><br><span class="line"><span class="section">tags:</span></span><br><span class="line"><span class="section">---</span></span><br><span class="line">This is a summary of the post.</span><br><span class="line"><!-- more --></span><br><span class="line"><span class="section">## Hello there</span></span><br><span class="line">This is some content.</span><br><span class="line"></span><br></pre></td></tr></table></figure>
<ul>
<li>插入图像</li>
</ul>
<p>图像和其他资源文件可以放置在 ./source/ 文件夹下的<strong>子目录</strong>中。使用这张来自维基百科的原始 A-Team 图片作为测试。下载并保存到此路径：<br>./source/images/Ateam.jpg</p>
<p>编辑您的原始帖子，插入引用 /images/Ateam.jpg 的 Markdown 图像链接：</p>
<figure class="highlight markdown"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line">---</span><br><span class="line">title: My First Blog Post</span><br><span class="line"><span class="section">tags:</span></span><br><span class="line"><span class="section">---</span></span><br><span class="line"><span class="section">## Hello there</span></span><br><span class="line">This is some content.</span><br><span class="line"></span><br><span class="line">![<span class="string">I love it when a plan comes together.</span>](<span class="link">/images/Ateam.jpg</span>)</span><br></pre></td></tr></table></figure>
<p>您应该在浏览器中看到类似这样的内容：</p>
<blockquote>
<p>Assets(就是你的图片什么的东西) can also be organized in <strong>folders for each post</strong>. It requires enabling the <code>post_asset_folder: true</code> setting in <code>_config.yml</code> and some knowledge of its relative path referencing behavior. See the <a target="_blank" rel="noopener" href="https://hexo.io/docs/asset-folders.html">Hexo documentation on Asset Folders</a> for more information.</p>
</blockquote>
<ul>
<li>发布草稿</li>
</ul>
<p>当需要将草稿移至“实时”帖子以供全世界查看时，请使用 hexo 发布命令：</p>
<figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">hexo publish My-First-Blog-Post</span><br></pre></td></tr></table></figure>
<p>运行此命令时会发生一些事情：</p>
<ol>
<li><p>markdown 文件 My-First-Blog-Post.md 从 ./source/_drafts/ 移动到 ./source/_posts。</p>
</li>
<li><p>文件的“front-matter”更改为包含发布日期：</p>
</li>
</ol>
<figure class="highlight markdown"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line">---</span><br><span class="line">title: My First Blog Post</span><br><span class="line">date: 2024-06-02 21:45:16 # <----</span><br><span class="line"><span class="section">tags:</span></span><br><span class="line"><span class="section">---</span></span><br><span class="line">...</span><br></pre></td></tr></table></figure>
<p>最后，准备整个站点以进行部署。运行<code>hexo generate</code>命令：</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"> hexo generate</span><br><span class="line"># generates -> ./public/</span><br></pre></td></tr></table></figure>
<p>运行网站所需的所有内容都将放置在<code> ./public</code> 文件夹中。您已准备好将此文件夹传输到您的公共网络<strong>服务器</strong>或 CDN。</p>
<p>现在您已经了解了一般工作流程，接下来探索一些增强网站的其他方法：</p>
<ul>
<li>Commit your changes with <a target="_blank" rel="noopener" href="http://rogerdudler.github.io/git-guide/">Git</a> so that you can always revert back to a good state if things go wrong. Read <a target="_blank" rel="noopener" href="http://jr0cket.co.uk/hexo/managing-hexo-website-content-with-git-and-github.html">jr0cket’s guide for Managing your Hexo website with Git and Github</a>.</li>
<li>Customize your blog with various <a target="_blank" rel="noopener" href="https://hexo.io/docs/configuration.html">configuration settings</a> inside the <code>_config.yml</code> file.</li>
<li><a target="_blank" rel="noopener" href="https://hexo.io/docs/deployment.html">Automate your deployment</a> to GitHub Pages, Heroku, Amazon S3, and other providers via the <code>hexo deploy</code> command.</li>
<li>Enhance the style and functionality of your articles with built-in <a target="_blank" rel="noopener" href="https://hexo.io/docs/tag-plugins.html">tag plugins</a>.</li>
<li>Browse the growing catalog of <a target="_blank" rel="noopener" href="https://hexo.io/plugins/">community Hexo plugins</a>.</li>
<li>Reconfigure the <a target="_blank" rel="noopener" href="http://jr0cket.co.uk/hexo/hexo-theme-simple-changes.html">default Hexo “landscape” theme</a>, try out some of the <a target="_blank" rel="noopener" href="https://hexo.io/themes/">other free themes</a> that are available, or <a target="_blank" rel="noopener" href="https://hexo.io/docs/themes.html">create your own</a>.</li>
<li><a target="_blank" rel="noopener" href="https://hexo.io/docs/migration.html">Migrate your old posts</a> from another blog system (Wordpress, Jekyll, Octopress, etc.) to Hexo.</li>
</ul>
<p>And to see the options for a particular command, use <code>hexo help {command}</code>:</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">$ hexo help new</span><br><span class="line">$ hexo help server</span><br></pre></td></tr></table></figure>
<hr>
<p>参考：</p>
<p><a target="_blank" rel="noopener" href="https://www.cgmartin.com/2016/01/03/getting-started-with-hexo-blog/">开始使用Hexo博客框架</a></p>
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