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<div class="slides">
<section>
<div class="center middle"><div style="width:100%"><div>
<h1 class=" text-2xl mt-2 font-sans">CSAPP 讨论会</h1>
<p class=" font-serif my-1">Week6 并发编程<br/>
2023/1/14</p>
</div>
</div>
</div>
</section>
<section>
<div>
<h2 class=" text-xl mt-2 pb-2 font-sans">并发编程要处理的问题</h2>
<ul class=" list-disc font-serif">
<li class=" ml-8">通信: 基于共享内存 或 基于消息传递</li>
<li class=" ml-8">同步: 各个逻辑之间的前后依赖关系<ul class=" list-disc font-serif">
<li class=" ml-8">自旋(忙等待)</li>
<li class=" ml-8">阻塞(sleep)</li>
</ul>
</li>
</ul>
<p class=" font-serif my-1">基于共享内存的同步问题：对资源的操作不是原子性</p>
<p class=" font-serif my-1">竞态条件成立的三个条件</p>
<ul class=" list-disc font-serif">
<li class=" ml-8">两个线程共享数据</li>
<li class=" ml-8">至少一个处理会对数据进行修改</li>
<li class=" ml-8">一个处理未完成之前，另一个介入进来(中间态被打断)</li>
</ul>
<p class=" font-serif my-1">从时间维度上来看, 对资源的操作要保证互斥: 数据的修改操作有且仅有一个程序在活动</p>
</div>
</section>
<section>
<section> <div>
<h2 class=" text-xl mt-2 pb-2 font-sans">共享内存同步</h2>
<p class=" font-serif my-1">通过 flag 来判断是否有线程对资源的操作，而对 flag 的操作也必须满足原子性，硬件指令来保障 ---> pthread_mutex (futex)</p>
<p class=" font-serif my-1">往下一步: 编程需要更方便的指令<br/>
condition variable, semaphore(信号量)</p>
<div class="codehilite"><pre class=" bg-gray-100 overflow-x-auto rounded p-2 mb-2 mt-2"><span/><code><span class="kt">void</span><span class="w"> </span><span class="nf">producer</span><span class="p">()</span><span class="w"> </span><span class="p">{</span><span class="w"/>
<span class="w"> </span><span class="n">P</span><span class="p">(</span><span class="o">&</span><span class="n">empty</span><span class="p">);</span><span class="w"> </span>
<span class="w"> </span><span class="c1">// do sth 假设线程安全</span>
<span class="w"> </span><span class="n">V</span><span class="p">(</span><span class="o">&</span><span class="n">fill</span><span class="p">);</span><span class="w"/>
<span class="p">}</span><span class="w"/>
<span class="kt">void</span><span class="w"> </span><span class="nf">consumer</span><span class="p">()</span><span class="w"> </span><span class="p">{</span><span class="w"/>
<span class="w"> </span><span class="n">P</span><span class="p">(</span><span class="o">&</span><span class="n">fill</span><span class="p">);</span><span class="w"/>
<span class="w"> </span><span class="c1">// do sth 假设线程安全</span>
<span class="w"> </span><span class="n">V</span><span class="p">(</span><span class="o">&</span><span class="n">empty</span><span class="p">);</span><span class="w"/>
<span class="p">}</span><span class="w"/>
</code></pre></div>
</div> </section>
<section> <div>
<h2 class=" text-xl mt-2 pb-2 font-sans">针对业务场景特定性的优化</h2>
<p class=" font-serif my-1">读读</p>
<ul class=" list-disc font-serif">
<li class=" ml-8">不需要上锁</li>
</ul>
<p class=" font-serif my-1">写写</p>
<ul class=" list-disc font-serif">
<li class=" ml-8">必须保证写的顺序性</li>
<li class=" ml-8"><red>Solution:</red>拆分数据, 线程各自计算(私有数据不需要上锁)，最后再合并(上锁同步)</li>
</ul>
<p class=" font-serif my-1">读写</p>
<ul class=" list-disc font-serif">
<li class=" ml-8">要么只能看到修改前的数据</li>
<li class=" ml-8">要么只能看到全部修改后的数据</li>
<li class=" ml-8">不能看到修改的中间态</li>
</ul>
</div> </section>
<section> <div>
<h2 class=" text-xl mt-2 pb-2 font-sans">针对业务场景特定性的优化</h2>
<p class=" font-serif my-1">锁的变体</p>
<ul class=" list-disc font-serif">
<li class=" ml-8">读写锁</li>
<li class=" ml-8">RCU 锁</li>
<li class=" ml-8">MCS 锁</li>
</ul>
</div> </section>
<section> <div>
<h2 class=" text-xl mt-2 pb-2 font-sans">事务内存</h2>
<div class="codehilite"><pre class=" bg-gray-100 overflow-x-auto rounded p-2 mb-2 mt-2"><span/><code><span class="n">atom</span><span class="w"> </span><span class="p">{</span><span class="w"/>
<span class="w"> </span><span class="c1">// operate(read or write) data code</span>
<span class="p">}</span><span class="w"/>
</code></pre></div>
<div class="codehilite"><pre class=" bg-gray-100 overflow-x-auto rounded p-2 mb-2 mt-2"><span/><code><span class="n">loop</span><span class="p">:</span>
<span class="n">data_before</span> <span class="o">:=</span> <span class="n">data</span>
<span class="k">try</span><span class="p">:</span>
<span class="o">//</span> <span class="n">operate</span> <span class="n">data</span> <span class="n">code</span>
<span class="k">if</span> <span class="ow">not</span> <span class="n">equal</span><span class="p">(</span><span class="n">data_before</span><span class="p">,</span> <span class="n">data_after</span><span class="p">):</span>
<span class="k">raise</span> <span class="n">error</span>
<span class="k">else</span><span class="p">:</span>
<span class="n">commit</span>
<span class="k">except</span><span class="p">:</span>
<span class="n">discard</span> <span class="n">data_after</span>
<span class="n">goto</span> <span class="n">loop</span>
</code></pre></div>
<p class=" font-serif my-1"><a href="https://www.bilibili.com/video/BV1tY411274s?t=672.7]">monitor(管程)</a></p>
<ul class=" list-disc font-serif">
<li class=" ml-8">通用的并发编程方法</li>
</ul>
</div> </section>
</section>
<section>
<div>
<h2 class=" text-xl mt-2 pb-2 font-sans">其他方面</h2>
<p class=" font-serif my-1">以上这些都是 data in share memory 所带来的问题<br/>
换种思路: inform data changes by sharing memory</p>
<p class=" font-serif my-1">函数式</p>
<ul class=" list-disc font-serif">
<li class=" ml-8">
<p class=" font-serif my-1">同步: future/promise</p>
</li>
<li class=" ml-8">
<p class=" font-serif my-1">消息传递</p>
</li>
<li class=" ml-8">
<p class=" font-serif my-1">asynchronous(异步) + 单线程执行</p>
</li>
</ul>
</div>
</section>
<section>
<div>
<h2 class=" text-xl mt-2 pb-2 font-sans">其他方面</h2>
<p class=" font-serif my-1">编译器隐藏依赖假设</p>
<ul class=" list-disc font-serif">
<li class=" ml-8">单线程模型编译</li>
</ul>
<p class=" font-serif my-1">我们的 CPU 也是一个编译器 </p>
<ul class=" list-disc font-serif">
<li class=" ml-8"><a href="https://research.swtch.com/hwmm">保证了单线程程序的正确性</a> </li>
<li class=" ml-8">同步原语: fence 指令</li>
</ul>
</div>
</section>
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