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redis-is-not-only-single-threaded.html
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<h1 class="post-title">Redis is not only single threaded</h1>
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<time>Mar 21, 2019</time>
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<p>常聽到或是看到人說 Redis 是 single threaded，但始終沒有看到有人貼出程式碼證明這件事情。</p>
<p>事實上這是錯的，我們到 Github 直接搜尋 <code>pthread_create</code>，就可以發現其實在 <code>redis/src/bio.c</code> 裡面是有開 thread 來做 Background I/O 的，而且 redis 使用的 malloc lib 也就是 jemalloc，裡面也是會開 multithread 的。</p>
<p><a href="https://github.com/antirez/redis/blob/3d07ed983e8bc55e7d066558de0244bc77dbeba3/src/bio.c#L118-L128">請見原始碼</a></p>
<figure class="highlight"><pre><code class="language-c" data-lang="c"><span></span><span class="cm">/* bio.c#L118-L128 */</span>
<span class="cm">/* Ready to spawn our threads. We use the single argument the thread</span>
<span class="cm">* function accepts in order to pass the job ID the thread is</span>
<span class="cm">* responsible of. */</span>
<span class="k">for</span> <span class="p">(</span><span class="n">j</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span> <span class="n">j</span> <span class="o"><</span> <span class="n">BIO_NUM_OPS</span><span class="p">;</span> <span class="n">j</span><span class="o">++</span><span class="p">)</span> <span class="p">{</span>
<span class="kt">void</span> <span class="o">*</span><span class="n">arg</span> <span class="o">=</span> <span class="p">(</span><span class="kt">void</span><span class="o">*</span><span class="p">)(</span><span class="kt">unsigned</span> <span class="kt">long</span><span class="p">)</span> <span class="n">j</span><span class="p">;</span>
<span class="k">if</span> <span class="p">(</span><span class="n">pthread_create</span><span class="p">(</span><span class="o">&</span><span class="kr">thread</span><span class="p">,</span><span class="o">&</span><span class="n">attr</span><span class="p">,</span><span class="n">bioProcessBackgroundJobs</span><span class="p">,</span><span class="n">arg</span><span class="p">)</span> <span class="o">!=</span> <span class="mi">0</span><span class="p">)</span> <span class="p">{</span>
<span class="n">serverLog</span><span class="p">(</span><span class="n">LL_WARNING</span><span class="p">,</span><span class="s">"Fatal: Can't initialize Background Jobs."</span><span class="p">);</span>
<span class="n">exit</span><span class="p">(</span><span class="mi">1</span><span class="p">);</span>
<span class="p">}</span>
<span class="n">bio_threads</span><span class="p">[</span><span class="n">j</span><span class="p">]</span> <span class="o">=</span> <span class="kr">thread</span><span class="p">;</span>
<span class="p">}</span></code></pre></figure>
<p>會說 redis 是 single threaded，純粹只是因為 main worker thread 只有一條，這和說 Node.js 是 single threaded 是相同的誤會。</p>
<p>之後來補一下為何要用 single main thread 實作，和他的優缺點。</p>
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