https://blog.cndenis.com/tags/proplists.htmlRecent content in Proplists on cnDenis的笔记Hugo -- gohugo.iozh-cnFri, 30 May 2014 00:00:00 +0000https://blog.cndenis.com/Erlang/2014/05/faster_proplists_get_value.htmlFri, 30 May 2014 00:00:00 +0000https://blog.cndenis.com/Erlang/2014/05/faster_proplists_get_value.html<p>在cowboy的<a class="link" href="https://github.com/extend/cowboy/blob/master/src/cowboy_protocol.erl" target="_blank" rel="noopener" >cowboy_protocol.erl</a> 文件中看到如这样一下函数:</p> <div class="highlight"><pre tabindex="0" class="chroma"><code class="language-erlang" data-lang="erlang"><span class="line"><span class="cl"><span class="c">%% Faster alternative to proplists:get_value/3. </span></span></span><span class="line"><span class="cl"><span class="nf">get_value</span><span class="p">(</span><span class="nv">Key</span><span class="p">,</span> <span class="nv">Opts</span><span class="p">,</span> <span class="nv">Default</span><span class="p">)</span> <span class="o">-&gt;</span> </span></span><span class="line"><span class="cl"> <span class="k">case</span> <span class="nn">lists</span><span class="p">:</span><span class="nf">keyfind</span><span class="p">(</span><span class="nv">Key</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="nv">Opts</span><span class="p">)</span> <span class="k">of</span> </span></span><span class="line"><span class="cl"> <span class="p">{_,</span> <span class="nv">Value</span><span class="p">}</span> <span class="o">-&gt;</span> <span class="nv">Value</span><span class="p">;</span> </span></span><span class="line"><span class="cl"> <span class="p">_</span> <span class="o">-&gt;</span> <span class="nv">Default</span> </span></span><span class="line"><span class="cl"> <span class="k">end</span><span class="p">.</span> </span></span></code></pre></div><p>做了一下测试, 考虑到proplist一般都不长, 用一个10个元素的列表进行测试, 结果这个函数比 <code>proplists:get_value/3</code> 快5倍, 对于更长的列表差别更大.</p> <pre><code>bench:get_value/3 Single Process: 13035909 call per sec, 16777216 times in 1287 ms proplists:get_value/3 Single Process: 2273335 call per sec, 4194304 times in 1845 ms </code></pre> <p>速度差别原因主要是, <code>keyfind</code> 是BIF, 用C实现的, 而 <code>proplists:get_value/3</code> 是erlang实现的, 另外 <code>proplists:get_value/3</code> 把 atom 当作 {atom, true} 处理, 逻辑比 <code>keyfind</code> 复杂一些</p> <p>这里另外一个测试和讨论: <a class="link" href="http://www.ostinelli.net/erlang-listskeyfind-or-proplistsget_value/" target="_blank" rel="noopener" >http://www.ostinelli.net/erlang-listskeyfind-or-proplistsget_value/</a>,</p>