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<h1 id="_1">并发与并行<a class="headerlink" href="#_1" title="Permanent link">&para;</a></h1>
<p><em>并发与并行是程序同时处理多件事情的方式。本文涵盖并发与并行的区别、同步原语、经典并发问题、死锁、无锁数据结构、并行编程模型、异步编程和扩展定律——这些概念支撑着多线程服务器、分布式训练和每一个现代应用程序。</em></p>
<ul>
<li>单个CPU核心一次执行一条指令。但现代系统有8个、64个甚至数千个核心(GPU)。即使在单核上,我们也希望处理多个任务:一边下载文件一边渲染界面一边处理用户输入。<strong>并发</strong><strong>并行</strong>是管理多个活动的两种策略。</li>
</ul>
<h2 id="vs">并发 vs 并行<a class="headerlink" href="#vs" title="Permanent link">&para;</a></h2>
<p><img alt="并发在单核上交错执行任务;并行在多核上同时执行任务" src="../../images/concurrency_vs_parallelism.svg" /></p>
<ul>
<li>
<p><strong>并发</strong>是关于<em>管理</em>多个任务。任务通过交错进行:任务A运行一会儿,然后任务B,然后回到A。在单核上,并发创造了同时执行的假象。这些任务并非真正同时执行;它们轮流进行。</p>
</li>
<li>
<p><strong>并行</strong>是关于<em>执行</em>多个任务同时进行。有<span class="arithmatex">\(n\)</span>个核心,<span class="arithmatex">\(n\)</span>个任务可以真正同时运行。并行需要多个硬件执行单元。</p>
</li>
<li>
<p>类比:并发是一个厨师交替切菜和搅拌锅。并行是两个厨师各自同时做一个任务。一个系统可以是并发但不并行的(单核,任务交错),并行但不并发的(多核运行独立程序,没有交互),或者两者兼有(多核运行互相交错交互的任务)。</p>
</li>
<li>
<p>在ML中,并发出现在数据加载中(数据预处理与GPU计算重叠),而并行出现在分布式训练中(多个GPU同时计算梯度,第6章)。</p>
</li>
</ul>
<h2 id="_2">同步原语<a class="headerlink" href="#_2" title="Permanent link">&para;</a></h2>
<ul>
<li>
<p>当多个线程共享数据时,<strong>同步</strong>防止竞态条件。竞态条件发生在结果依赖于线程执行的不可预测顺序时。</p>
</li>
<li>
<p>考虑两个线程同时增加一个共享计数器:<code>counter += 1</code>。这实际上是三个操作:(1)读取计数器,(2)加1,(3)写入计数器。如果两个线程读取相同的值(比如5),都加1,都写入6,计数器最终为6而不是正确的7。一次增加丢失了。</p>
</li>
<li>
<p><strong>互斥锁</strong>(互斥排斥锁)确保一次只有一个线程访问临界区。一个线程在进入临界区前<strong>获取</strong>锁,之后<strong>释放</strong>锁。任何其他试图获取已被持有锁的线程将阻塞直到锁被释放。</p>
</li>
</ul>
<div class="highlight"><pre><span></span><code><a id="__codelineno-0-1" name="__codelineno-0-1" href="#__codelineno-0-1"></a>lock.acquire()
<a id="__codelineno-0-2" name="__codelineno-0-2" href="#__codelineno-0-2"></a>counter += 1 # 一次只有一个线程在此
<a id="__codelineno-0-3" name="__codelineno-0-3" href="#__codelineno-0-3"></a>lock.release()
</code></pre></div>
<ul>
<li>
<p>互斥锁是正确的,但会引入<strong>争用</strong>:如果许多线程竞争同一个锁,它们花费时间等待而不是计算。这限制了可扩展性。极端情况下,所有线程都想要同一个锁,会使整个程序串行化。</p>
</li>
<li>
<p><strong>信号量</strong>泛化了互斥锁。计数信号量维护一个计数器:<code>wait()</code> 递减计数器(如果会变负则阻塞),<code>signal()</code> 递增计数器。初始化为1的信号量行为类似互斥锁。初始化为<span class="arithmatex">\(n\)</span>的信号量允许最多<span class="arithmatex">\(n\)</span>个线程同时进入临界区(适用于资源池如数据库连接)。</p>
</li>
<li>
<p><strong>条件变量</strong>让一个线程等待直到某个特定条件满足。该线程释放一个锁,在条件变量上等待,当另一个线程发出该条件的信号时被唤醒。这避免了忙等待(在一个循环中反复检查条件,浪费CPU)。</p>
</li>
<li>
<p><strong>监视器</strong>将互斥锁与条件变量和共享数据捆绑为一个单一抽象。Java的 <code>synchronized</code> 关键字和Python的 <code>threading.Condition</code> 实现了类似监视器的语义。</p>
</li>
<li>
<p><strong>读写锁</strong>区分读线程(可以共享访问,因为读取不会修改数据)和写线程(需要独占访问)。多个读线程可以同时持有锁,但一个写线程会阻塞所有读线程和其他写线程。当读操作远多于写操作时(例如,提供预测的缓存模型),这是最优的。</p>
</li>
</ul>
<h2 id="_3">经典并发问题<a class="headerlink" href="#_3" title="Permanent link">&para;</a></h2>
<ul>
<li>
<p><strong>生产者-消费者</strong>(有界缓冲区):生产者生成项目并将其放入固定大小的缓冲区;消费者移除项目。挑战:缓冲区满时生产者必须等待,缓冲区空时消费者必须等待,且两者必须防止损坏缓冲区。</p>
</li>
<li>
<p>解决方案使用两个信号量(一个计数空位,一个计数满位)加上一个用于缓冲区本身的互斥锁。这是大多数消息队列、日志系统和数据管道背后的模式。</p>
</li>
<li>
<p><strong>读者-写者</strong>:多个读者可以同时读取,但写者需要独占访问。挑战是公平性:如果读者源源不断地到来,写者可能饥饿(永远得不到访问)。解决方案要么优先考虑读者,要么优先考虑写者,要么公平地交替。</p>
</li>
<li>
<p><strong>哲学家就餐问题</strong>:五位哲学家围坐在一张有五个叉子的桌子旁。每人需要两把叉子才能吃饭。如果所有五位同时拿起左边的叉子,没人能拿起右边的叉子,所有人都饿死(死锁)。解决方案包括:同时拿起两把叉子(原子操作),引入不对称性(一位哲学家先拿右边的叉子),或者使用服务员(限制用餐人数为4的信号量)。</p>
</li>
</ul>
<h2 id="_4">死锁<a class="headerlink" href="#_4" title="Permanent link">&para;</a></h2>
<ul>
<li><strong>死锁</strong>发生在一组线程各自等待集合中另一个线程持有的资源,形成一个依赖循环。没有人能继续。</li>
</ul>
<p><img alt="死锁:线程A持有锁1想要锁2,线程B持有锁2想要锁1——循环等待" src="../../images/deadlock_cycle.svg" /></p>
<ul>
<li>
<p>死锁的四个<strong>必要条件</strong>(必须同时满足):</p>
<ol>
<li><strong>互斥</strong>:资源一次只能被一个线程持有。</li>
<li><strong>持有并等待</strong>:一个线程持有一个资源的同时等待另一个资源。</li>
<li><strong>不可剥夺</strong>:资源不能被强制从线程中拿走。</li>
<li><strong>循环等待</strong>:等待图中存在一个循环。</li>
</ol>
</li>
<li>
<p><strong>死锁预防</strong>打破四个条件之一:</p>
<ul>
<li>消除循环等待:对资源施加全序。所有线程以相同的顺序获取资源。如果每个线程总是在获取锁A之后才获取锁B,则不可能有循环。</li>
<li>消除持有并等待:要求线程一次性(原子地)请求所有资源。</li>
</ul>
</li>
<li>
<p><strong>死锁避免</strong>动态决定是否批准一个资源请求可能导致死锁。<strong>银行家算法</strong>维护每个线程的最大可能需求,仅批准使系统保持"安全状态"(所有线程最终都能完成的状态)的请求。该算法每个请求 <span class="arithmatex">\(O(n^2 m)\)</span><span class="arithmatex">\(n\)</span>个线程,<span class="arithmatex">\(m\)</span>种资源类型),对大多数实际系统来说过于昂贵。</p>
</li>
<li>
<p><strong>死锁检测</strong>让死锁发生,然后检测它们(通过在等待图中找到循环)并恢复(通过杀死一个线程或回滚一个事务)。</p>
</li>
<li>
<p>在实践中,大多数系统对常见情况使用预防(资源排序),对罕见情况使用检测。数据库系统是经典例子:它们检测事务之间的死锁并中止一个来打破循环。</p>
</li>
</ul>
<h2 id="_5">无锁和免等待数据结构<a class="headerlink" href="#_5" title="Permanent link">&para;</a></h2>
<ul>
<li>
<p>锁引入了争用、优先级反转和死锁风险。<strong>无锁</strong>数据结构完全避免使用锁,使用硬件提供的<strong>原子操作</strong></p>
</li>
<li>
<p>关键的原子操作是<strong>比较并交换(CAS</strong>:原子地检查一个内存位置是否具有期望的值,如果是,则将其替换为新值。伪代码:</p>
</li>
</ul>
<div class="highlight"><pre><span></span><code><a id="__codelineno-1-1" name="__codelineno-1-1" href="#__codelineno-1-1"></a>CAS(address, expected, new_value):
<a id="__codelineno-1-2" name="__codelineno-1-2" href="#__codelineno-1-2"></a> if *address == expected:
<a id="__codelineno-1-3" name="__codelineno-1-3" href="#__codelineno-1-3"></a> *address = new_value
<a id="__codelineno-1-4" name="__codelineno-1-4" href="#__codelineno-1-4"></a> return true
<a id="__codelineno-1-5" name="__codelineno-1-5" href="#__codelineno-1-5"></a> else:
<a id="__codelineno-1-6" name="__codelineno-1-6" href="#__codelineno-1-6"></a> return false
</code></pre></div>
<ul>
<li>
<p>CAS实现为单个硬件指令,因此即使没有锁也是原子的。无锁算法使用重试循环中的CAS:读取当前值,计算新值,尝试CAS。如果另一个线程在此期间修改了该值,CAS失败,线程重试。</p>
</li>
<li>
<p><strong>无锁</strong>:至少一个线程在有限步骤内取得进展(不可能死锁,但个别线程在争用下可能无限重试)。</p>
</li>
<li>
<p><strong>免等待</strong>:每个线程在有限步骤内取得进展(最强保证,但最难实现)。</p>
</li>
<li>
<p>无锁的堆栈、队列和哈希映射广泛用于高性能系统。Java的 <code>ConcurrentHashMap</code> 和Go的原子操作都建立在CAS之上。</p>
</li>
</ul>
<h2 id="_6">并行编程模型<a class="headerlink" href="#_6" title="Permanent link">&para;</a></h2>
<ul>
<li><strong>共享内存</strong>并行:所有线程访问同一内存空间。同步是程序员的责任。<strong>OpenMP</strong>提供编译器指令来并行化循环:</li>
</ul>
<div class="highlight"><pre><span></span><code><a id="__codelineno-2-1" name="__codelineno-2-1" href="#__codelineno-2-1"></a><span class="cp">#pragma omp parallel for</span>
<a id="__codelineno-2-2" name="__codelineno-2-2" href="#__codelineno-2-2"></a><span class="k">for</span><span class="w"> </span><span class="p">(</span><span class="kt">int</span><span class="w"> </span><span class="n">i</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span><span class="w"> </span><span class="n">i</span><span class="w"> </span><span class="o">&lt;</span><span class="w"> </span><span class="n">n</span><span class="p">;</span><span class="w"> </span><span class="n">i</span><span class="o">++</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<a id="__codelineno-2-3" name="__codelineno-2-3" href="#__codelineno-2-3"></a><span class="w"> </span><span class="n">result</span><span class="p">[</span><span class="n">i</span><span class="p">]</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">compute</span><span class="p">(</span><span class="n">data</span><span class="p">[</span><span class="n">i</span><span class="p">]);</span>
<a id="__codelineno-2-4" name="__codelineno-2-4" href="#__codelineno-2-4"></a><span class="p">}</span>
</code></pre></div>
<ul>
<li>
<p>编译器将循环迭代拆分到可用的核心上。OpenMP对数据并行工作负载(对许多数据点执行相同操作)很有效,广泛用于科学计算。</p>
</li>
<li>
<p><strong>消息传递</strong>并行:每个进程有自己的内存。通信通过发送和接收消息实现。<strong>MPI</strong>(消息传递接口)是跨节点分布式计算的标准:</p>
</li>
</ul>
<div class="highlight"><pre><span></span><code><a id="__codelineno-3-1" name="__codelineno-3-1" href="#__codelineno-3-1"></a><span class="n">MPI_Send</span><span class="p">(</span><span class="n">data</span><span class="p">,</span><span class="w"> </span><span class="n">count</span><span class="p">,</span><span class="w"> </span><span class="n">MPI_FLOAT</span><span class="p">,</span><span class="w"> </span><span class="n">dest</span><span class="p">,</span><span class="w"> </span><span class="n">tag</span><span class="p">,</span><span class="w"> </span><span class="n">MPI_COMM_WORLD</span><span class="p">);</span>
<a id="__codelineno-3-2" name="__codelineno-3-2" href="#__codelineno-3-2"></a><span class="n">MPI_Recv</span><span class="p">(</span><span class="n">data</span><span class="p">,</span><span class="w"> </span><span class="n">count</span><span class="p">,</span><span class="w"> </span><span class="n">MPI_FLOAT</span><span class="p">,</span><span class="w"> </span><span class="n">src</span><span class="p">,</span><span class="w"> </span><span class="n">tag</span><span class="p">,</span><span class="w"> </span><span class="n">MPI_COMM_WORLD</span><span class="p">,</span><span class="w"> </span><span class="o">&amp;</span><span class="n">status</span><span class="p">);</span>
</code></pre></div>
<ul>
<li>
<p>MPI可扩展到数千个节点,因为没有需要同步的共享状态。分布式深度学习(第6章)使用集合操作如 <code>MPI_AllReduce</code>(环状 all-reduce)来跨GPU同步梯度。</p>
</li>
<li>
<p><strong>GPU并行</strong>遵循<strong>SIMT</strong>(单指令多线程)模型:数千个线程在不同数据上执行相同的指令。这非常适合矩阵运算(第2章),其中相同的乘加操作应用于每个元素。我们将在后续章节中详细介绍GPU编程。</p>
</li>
</ul>
<h2 id="_7">异步与事件驱动编程<a class="headerlink" href="#_7" title="Permanent link">&para;</a></h2>
<ul>
<li>
<p>并非所有并发都需要线程。<strong>异步</strong>编程使用<strong>事件循环</strong>在单个线程中处理许多I/O密集型任务。</p>
</li>
<li>
<p>事件循环维护一个任务队列。当一个任务需要等待I/O(网络响应、文件读取)时,它注册一个回调并交出控制权。事件循环选取下一个就绪的任务。当I/O完成时,回调被排队并最终执行。等待期间没有线程被阻塞。</p>
</li>
<li>
<p><strong>协程</strong>是可以暂停和恢复的函数。<code>async/await</code> 语法(Python、JavaScript、Rust)使协程看起来像常规的顺序代码:</p>
</li>
</ul>
<div class="highlight"><pre><span></span><code><a id="__codelineno-4-1" name="__codelineno-4-1" href="#__codelineno-4-1"></a><span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">fetch_data</span><span class="p">(</span><span class="n">url</span><span class="p">):</span>
<a id="__codelineno-4-2" name="__codelineno-4-2" href="#__codelineno-4-2"></a> <span class="n">response</span> <span class="o">=</span> <span class="k">await</span> <span class="n">http_get</span><span class="p">(</span><span class="n">url</span><span class="p">)</span> <span class="c1"># 在此暂停,事件循环运行其他任务</span>
<a id="__codelineno-4-3" name="__codelineno-4-3" href="#__codelineno-4-3"></a> <span class="k">return</span> <span class="n">process</span><span class="p">(</span><span class="n">response</span><span class="p">)</span> <span class="c1"># 响应到达时恢复</span>
</code></pre></div>
<ul>
<li>
<p><code>await</code> 关键字暂停协程并将控制权返回给事件循环。当等待的操作完成时,协程从中断处恢复。这是协作式多任务:协程自愿放弃控制,不同于抢占式多任务中OS强制切换线程。</p>
</li>
<li>
<p>异步适用于具有许多并发连接的<strong>I/O密集型</strong>工作负载(处理数千个客户的Web服务器)。它不适用于<strong>CPU密集型</strong>工作(单线程事件循环无法利用多核)。对于CPU密集型工作,请使用线程或进程。</p>
</li>
<li>
<p>Python的<strong>全局解释器锁(GIL</strong>阻止线程真正的并行:一次只有一个线程可以执行Python字节码。这就是为什么Python对CPU并行使用多处理(独立的进程,每个有自己的解释器),对I/O并发使用异步。GIL正在Python 3.13+中被移除(自由线程Python),这将启用真正的多线程并行。</p>
</li>
</ul>
<h2 id="_8">扩展定律<a class="headerlink" href="#_8" title="Permanent link">&para;</a></h2>
<ul>
<li><strong>阿姆达尔定律</strong>描述了并行化程序的理论加速。如果程序的<span class="arithmatex">\(p\)</span>部分是可并行的,其余 <span class="arithmatex">\(1-p\)</span> 部分是串行的:</li>
</ul>
<div class="arithmatex">\[\text{加速比}(n) = \frac{1}{(1-p) + \frac{p}{n}}\]</div>
<p><img alt="阿姆达尔定律:串行部分限制了最大加速比——10%串行意味着最大10x,无论多少核心" src="../../images/amdahl_serial_bottleneck.svg" /></p>
<ul>
<li>
<p>其中<span class="arithmatex">\(n\)</span>是处理器数量。当 <span class="arithmatex">\(n \to \infty\)</span> 时,最大加速比趋近于 <span class="arithmatex">\(\frac{1}{1-p}\)</span>。如果95%的程序是并行的,最大加速比为 <span class="arithmatex">\(\frac{1}{0.05} = 20\times\)</span>,无论你添加多少核心。串行部分就是瓶颈。</p>
</li>
<li>
<p>这对ML有深远影响:如果数据加载花费训练时间的10%并且是串行的,增加更多GPU最多只能将训练加速10倍。10%的串行瓶颈限制了所有东西(这就是为什么高效的数据管道和I/O与计算重叠很重要,第6章)。</p>
</li>
<li>
<p><strong>古斯塔夫森定律</strong>提供了更乐观的视角。它不是在固定问题规模并添加处理器,而是固定总时间并问可以做多少额外工作。如果并行部分随问题规模扩展:</p>
</li>
</ul>
<div class="arithmatex">\[\text{加速比}(n) = 1 - p + p \cdot n\]</div>
<ul>
<li>这是关于<span class="arithmatex">\(n\)</span>线性的。论证是:用更多处理器,我们解决更大的问题,而不是更快地解决同一问题。在ML中,这对应于用更多GPU增加批量大小(弱扩展),而不是保持批量大小固定(强扩展)。</li>
</ul>
<h2 id="colab">编程任务(使用CoLab或笔记本)<a class="headerlink" href="#colab" title="Permanent link">&para;</a></h2>
<ol>
<li>
<p>演示竞态条件。两个线程在没有同步的情况下增加一个共享计数器,观察丢失的更新。
<div class="highlight"><pre><span></span><code><a id="__codelineno-5-1" name="__codelineno-5-1" href="#__codelineno-5-1"></a><span class="kn">import</span><span class="w"> </span><span class="nn">threading</span>
<a id="__codelineno-5-2" name="__codelineno-5-2" href="#__codelineno-5-2"></a>
<a id="__codelineno-5-3" name="__codelineno-5-3" href="#__codelineno-5-3"></a><span class="n">counter</span> <span class="o">=</span> <span class="mi">0</span>
<a id="__codelineno-5-4" name="__codelineno-5-4" href="#__codelineno-5-4"></a>
<a id="__codelineno-5-5" name="__codelineno-5-5" href="#__codelineno-5-5"></a><span class="k">def</span><span class="w"> </span><span class="nf">increment</span><span class="p">(</span><span class="n">n</span><span class="p">):</span>
<a id="__codelineno-5-6" name="__codelineno-5-6" href="#__codelineno-5-6"></a> <span class="k">global</span> <span class="n">counter</span>
<a id="__codelineno-5-7" name="__codelineno-5-7" href="#__codelineno-5-7"></a> <span class="k">for</span> <span class="n">_</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="n">n</span><span class="p">):</span>
<a id="__codelineno-5-8" name="__codelineno-5-8" href="#__codelineno-5-8"></a> <span class="n">counter</span> <span class="o">+=</span> <span class="mi">1</span> <span class="c1"># 不是原子的:读、加、写</span>
<a id="__codelineno-5-9" name="__codelineno-5-9" href="#__codelineno-5-9"></a>
<a id="__codelineno-5-10" name="__codelineno-5-10" href="#__codelineno-5-10"></a><span class="n">threads</span> <span class="o">=</span> <span class="p">[</span><span class="n">threading</span><span class="o">.</span><span class="n">Thread</span><span class="p">(</span><span class="n">target</span><span class="o">=</span><span class="n">increment</span><span class="p">,</span> <span class="n">args</span><span class="o">=</span><span class="p">(</span><span class="mi">100000</span><span class="p">,))</span> <span class="k">for</span> <span class="n">_</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">4</span><span class="p">)]</span>
<a id="__codelineno-5-11" name="__codelineno-5-11" href="#__codelineno-5-11"></a><span class="k">for</span> <span class="n">t</span> <span class="ow">in</span> <span class="n">threads</span><span class="p">:</span> <span class="n">t</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
<a id="__codelineno-5-12" name="__codelineno-5-12" href="#__codelineno-5-12"></a><span class="k">for</span> <span class="n">t</span> <span class="ow">in</span> <span class="n">threads</span><span class="p">:</span> <span class="n">t</span><span class="o">.</span><span class="n">join</span><span class="p">()</span>
<a id="__codelineno-5-13" name="__codelineno-5-13" href="#__codelineno-5-13"></a>
<a id="__codelineno-5-14" name="__codelineno-5-14" href="#__codelineno-5-14"></a><span class="nb">print</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;Expected: </span><span class="si">{</span><span class="mi">4</span><span class="w"> </span><span class="o">*</span><span class="w"> </span><span class="mi">100000</span><span class="si">}</span><span class="s2">&quot;</span><span class="p">)</span>
<a id="__codelineno-5-15" name="__codelineno-5-15" href="#__codelineno-5-15"></a><span class="nb">print</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;Actual: </span><span class="si">{</span><span class="n">counter</span><span class="si">}</span><span class="s2">&quot;</span><span class="p">)</span>
<a id="__codelineno-5-16" name="__codelineno-5-16" href="#__codelineno-5-16"></a><span class="nb">print</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;Lost updates: </span><span class="si">{</span><span class="mi">4</span><span class="w"> </span><span class="o">*</span><span class="w"> </span><span class="mi">100000</span><span class="w"> </span><span class="o">-</span><span class="w"> </span><span class="n">counter</span><span class="si">}</span><span class="s2">&quot;</span><span class="p">)</span>
</code></pre></div></p>
</li>
<li>
<p>用锁修复竞态条件并测量开销。
<div class="highlight"><pre><span></span><code><a id="__codelineno-6-1" name="__codelineno-6-1" href="#__codelineno-6-1"></a><span class="kn">import</span><span class="w"> </span><span class="nn">threading</span>
<a id="__codelineno-6-2" name="__codelineno-6-2" href="#__codelineno-6-2"></a><span class="kn">import</span><span class="w"> </span><span class="nn">time</span>
<a id="__codelineno-6-3" name="__codelineno-6-3" href="#__codelineno-6-3"></a>
<a id="__codelineno-6-4" name="__codelineno-6-4" href="#__codelineno-6-4"></a><span class="n">lock</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">Lock</span><span class="p">()</span>
<a id="__codelineno-6-5" name="__codelineno-6-5" href="#__codelineno-6-5"></a><span class="n">counter</span> <span class="o">=</span> <span class="mi">0</span>
<a id="__codelineno-6-6" name="__codelineno-6-6" href="#__codelineno-6-6"></a>
<a id="__codelineno-6-7" name="__codelineno-6-7" href="#__codelineno-6-7"></a><span class="k">def</span><span class="w"> </span><span class="nf">increment_locked</span><span class="p">(</span><span class="n">n</span><span class="p">):</span>
<a id="__codelineno-6-8" name="__codelineno-6-8" href="#__codelineno-6-8"></a> <span class="k">global</span> <span class="n">counter</span>
<a id="__codelineno-6-9" name="__codelineno-6-9" href="#__codelineno-6-9"></a> <span class="k">for</span> <span class="n">_</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="n">n</span><span class="p">):</span>
<a id="__codelineno-6-10" name="__codelineno-6-10" href="#__codelineno-6-10"></a> <span class="k">with</span> <span class="n">lock</span><span class="p">:</span>
<a id="__codelineno-6-11" name="__codelineno-6-11" href="#__codelineno-6-11"></a> <span class="n">counter</span> <span class="o">+=</span> <span class="mi">1</span>
<a id="__codelineno-6-12" name="__codelineno-6-12" href="#__codelineno-6-12"></a>
<a id="__codelineno-6-13" name="__codelineno-6-13" href="#__codelineno-6-13"></a><span class="n">start</span> <span class="o">=</span> <span class="n">time</span><span class="o">.</span><span class="n">time</span><span class="p">()</span>
<a id="__codelineno-6-14" name="__codelineno-6-14" href="#__codelineno-6-14"></a><span class="n">threads</span> <span class="o">=</span> <span class="p">[</span><span class="n">threading</span><span class="o">.</span><span class="n">Thread</span><span class="p">(</span><span class="n">target</span><span class="o">=</span><span class="n">increment_locked</span><span class="p">,</span> <span class="n">args</span><span class="o">=</span><span class="p">(</span><span class="mi">100000</span><span class="p">,))</span> <span class="k">for</span> <span class="n">_</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">4</span><span class="p">)]</span>
<a id="__codelineno-6-15" name="__codelineno-6-15" href="#__codelineno-6-15"></a><span class="k">for</span> <span class="n">t</span> <span class="ow">in</span> <span class="n">threads</span><span class="p">:</span> <span class="n">t</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
<a id="__codelineno-6-16" name="__codelineno-6-16" href="#__codelineno-6-16"></a><span class="k">for</span> <span class="n">t</span> <span class="ow">in</span> <span class="n">threads</span><span class="p">:</span> <span class="n">t</span><span class="o">.</span><span class="n">join</span><span class="p">()</span>
<a id="__codelineno-6-17" name="__codelineno-6-17" href="#__codelineno-6-17"></a><span class="n">elapsed</span> <span class="o">=</span> <span class="n">time</span><span class="o">.</span><span class="n">time</span><span class="p">()</span> <span class="o">-</span> <span class="n">start</span>
<a id="__codelineno-6-18" name="__codelineno-6-18" href="#__codelineno-6-18"></a>
<a id="__codelineno-6-19" name="__codelineno-6-19" href="#__codelineno-6-19"></a><span class="nb">print</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;Counter: </span><span class="si">{</span><span class="n">counter</span><span class="si">}</span><span class="s2"> (correct: </span><span class="si">{</span><span class="mi">4</span><span class="w"> </span><span class="o">*</span><span class="w"> </span><span class="mi">100000</span><span class="si">}</span><span class="s2">)&quot;</span><span class="p">)</span>
<a id="__codelineno-6-20" name="__codelineno-6-20" href="#__codelineno-6-20"></a><span class="nb">print</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;Time with lock: </span><span class="si">{</span><span class="n">elapsed</span><span class="si">:</span><span class="s2">.3f</span><span class="si">}</span><span class="s2">s&quot;</span><span class="p">)</span>
</code></pre></div></p>
</li>
<li>
<p>可视化阿姆达尔定律。绘制不同并行比例下加速比与处理器数量的关系图。
<div class="highlight"><pre><span></span><code><a id="__codelineno-7-1" name="__codelineno-7-1" href="#__codelineno-7-1"></a><span class="kn">import</span><span class="w"> </span><span class="nn">jax.numpy</span><span class="w"> </span><span class="k">as</span><span class="w"> </span><span class="nn">jnp</span>
<a id="__codelineno-7-2" name="__codelineno-7-2" href="#__codelineno-7-2"></a><span class="kn">import</span><span class="w"> </span><span class="nn">matplotlib.pyplot</span><span class="w"> </span><span class="k">as</span><span class="w"> </span><span class="nn">plt</span>
<a id="__codelineno-7-3" name="__codelineno-7-3" href="#__codelineno-7-3"></a>
<a id="__codelineno-7-4" name="__codelineno-7-4" href="#__codelineno-7-4"></a><span class="n">n_procs</span> <span class="o">=</span> <span class="n">jnp</span><span class="o">.</span><span class="n">arange</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">65</span><span class="p">)</span>
<a id="__codelineno-7-5" name="__codelineno-7-5" href="#__codelineno-7-5"></a>
<a id="__codelineno-7-6" name="__codelineno-7-6" href="#__codelineno-7-6"></a><span class="k">for</span> <span class="n">p</span><span class="p">,</span> <span class="n">color</span> <span class="ow">in</span> <span class="p">[(</span><span class="mf">0.5</span><span class="p">,</span> <span class="s2">&quot;#e74c3c&quot;</span><span class="p">),</span> <span class="p">(</span><span class="mf">0.9</span><span class="p">,</span> <span class="s2">&quot;#f39c12&quot;</span><span class="p">),</span> <span class="p">(</span><span class="mf">0.95</span><span class="p">,</span> <span class="s2">&quot;#27ae60&quot;</span><span class="p">),</span> <span class="p">(</span><span class="mf">0.99</span><span class="p">,</span> <span class="s2">&quot;#3498db&quot;</span><span class="p">)]:</span>
<a id="__codelineno-7-7" name="__codelineno-7-7" href="#__codelineno-7-7"></a> <span class="n">speedup</span> <span class="o">=</span> <span class="mi">1</span> <span class="o">/</span> <span class="p">((</span><span class="mi">1</span> <span class="o">-</span> <span class="n">p</span><span class="p">)</span> <span class="o">+</span> <span class="n">p</span> <span class="o">/</span> <span class="n">n_procs</span><span class="p">)</span>
<a id="__codelineno-7-8" name="__codelineno-7-8" href="#__codelineno-7-8"></a> <span class="n">plt</span><span class="o">.</span><span class="n">plot</span><span class="p">(</span><span class="n">n_procs</span><span class="p">,</span> <span class="n">speedup</span><span class="p">,</span> <span class="n">color</span><span class="o">=</span><span class="n">color</span><span class="p">,</span> <span class="n">linewidth</span><span class="o">=</span><span class="mi">2</span><span class="p">,</span> <span class="n">label</span><span class="o">=</span><span class="sa">f</span><span class="s2">&quot;p=</span><span class="si">{</span><span class="n">p</span><span class="si">}</span><span class="s2">&quot;</span><span class="p">)</span>
<a id="__codelineno-7-9" name="__codelineno-7-9" href="#__codelineno-7-9"></a> <span class="c1"># 最大加速比线</span>
<a id="__codelineno-7-10" name="__codelineno-7-10" href="#__codelineno-7-10"></a> <span class="n">plt</span><span class="o">.</span><span class="n">axhline</span><span class="p">(</span><span class="mi">1</span> <span class="o">/</span> <span class="p">(</span><span class="mi">1</span> <span class="o">-</span> <span class="n">p</span><span class="p">),</span> <span class="n">color</span><span class="o">=</span><span class="n">color</span><span class="p">,</span> <span class="n">linestyle</span><span class="o">=</span><span class="s2">&quot;--&quot;</span><span class="p">,</span> <span class="n">alpha</span><span class="o">=</span><span class="mf">0.3</span><span class="p">)</span>
<a id="__codelineno-7-11" name="__codelineno-7-11" href="#__codelineno-7-11"></a>
<a id="__codelineno-7-12" name="__codelineno-7-12" href="#__codelineno-7-12"></a><span class="n">plt</span><span class="o">.</span><span class="n">xlabel</span><span class="p">(</span><span class="s2">&quot;处理器数量&quot;</span><span class="p">)</span>
<a id="__codelineno-7-13" name="__codelineno-7-13" href="#__codelineno-7-13"></a><span class="n">plt</span><span class="o">.</span><span class="n">ylabel</span><span class="p">(</span><span class="s2">&quot;加速比&quot;</span><span class="p">)</span>
<a id="__codelineno-7-14" name="__codelineno-7-14" href="#__codelineno-7-14"></a><span class="n">plt</span><span class="o">.</span><span class="n">title</span><span class="p">(</span><span class="s2">&quot;阿姆达尔定律:串行比例限制加速比&quot;</span><span class="p">)</span>
<a id="__codelineno-7-15" name="__codelineno-7-15" href="#__codelineno-7-15"></a><span class="n">plt</span><span class="o">.</span><span class="n">legend</span><span class="p">()</span>
<a id="__codelineno-7-16" name="__codelineno-7-16" href="#__codelineno-7-16"></a><span class="n">plt</span><span class="o">.</span><span class="n">grid</span><span class="p">(</span><span class="kc">True</span><span class="p">)</span>
<a id="__codelineno-7-17" name="__codelineno-7-17" href="#__codelineno-7-17"></a><span class="n">plt</span><span class="o">.</span><span class="n">show</span><span class="p">()</span>
</code></pre></div></p>
</li>
</ol>
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