如何利用SystemTap统计函数执行耗时详解
<p><span><strong>前言</strong></span></p>
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当我们需要对应用程序进行系能分析时，我们通常可以使用perf或者火焰图。 但是这些工具通常只能定性问题，发现那些函数占用cpu较多，需要优化。但是给不出定量的数据， 比如这个函数的耗时情况，它耗时1ms还是5ms。</p>
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因此在不在代码中加入统计耗时的代码的情况，我们可以使用SystemTap来统计应用程序的耗时情况。</p>
<p>
SystemTap可以跟踪内核函数和用户态进程，当我们跟踪用户态进程时，需要使用其process模块。</p>
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<span><strong>查找函数符号</strong></span></p>
<p>
很多情况下，代码在执行时，其函数符号并不一定是代码中写的名称，因此我们可以使用以下脚本打印 出应用程序中在调用的函数符号。</p>
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<code class="plain plain">probe process("/data0/app").function("*") {</code>
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<code class="plain spaces"> </code><code class="plain plain">println(probefunc())</code>
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<code class="plain plain">}</code>
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然后执行</p>
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<code class="bash plain">stap </code><code class="bash functions">echo</code><code class="bash plain">.stp</code>
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其会打印出/data0/app这个程序运行时调用到的各个函数名，此处最好填绝对路径。我们可以从中找到 我们需要统计的函数名称。</p>
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<span><strong>统计函数耗时</strong></span></p>
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我们可以使用SystemTap内置的直方图来展示耗时的分布。我们有两种直方图函数可以使用:</p>
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<code class="plain plain">@hist_linear(v, start, stop, interval) # 打印start-stop区间interval间隔的直方图</code>
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<code class="plain plain">@hist_log(v) # 打印以2为底指数分布的直方图</code>
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统计脚本：</p>
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<code class="cpp plain">global sends # 声明全局的统计存储容器</code>
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<code class="cpp plain">probe process(</code><code class="cpp string">"/data0/app"</code><code class="cpp plain">).function(</code><code class="cpp string">"git.intra.xx.send"</code><code class="cpp plain">).</code><code class="cpp keyword bold">return</code> <code class="cpp plain">{ # function中为函数名，同时支持通配符*等，在该函数</code><code class="cpp keyword bold">return</code><code class="cpp plain">时计算耗时</code>
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<code class="cpp spaces"> </code><code class="cpp plain">sends <<< gettimeofday_us() - @entry(gettimeofday_us()) # 以微秒精度来统计，entry方法将一个表达式放置于函数入口处</code>
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<code class="cpp plain">}</code>
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<code class="cpp plain">probe timer.s(10) { # 每10s打印一次直方图</code>
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<code class="cpp spaces"> </code><code class="cpp plain">print(@hist_log(sends))</code>
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<code class="cpp plain">}</code>
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然后执行<code>stap elaspe.stp</code>即可获得每10秒统计的结果，如果希望每10秒清空重新统计的话， 可以将打印函数修改为：</p>
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<code class="cpp plain">probe timer.s(10) { # 每10s打印一次直方图</code>
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<code class="cpp spaces"> </code><code class="cpp plain">print(@hist_log(sends))</code>
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<code class="cpp spaces"> </code><code class="cpp keyword bold">delete</code> <code class="cpp plain">sends # 清空数据</code>
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<code class="cpp plain">}</code>
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<span><strong>总结</strong></span></p>
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以上就是这篇文章的全部内容了，希望本文的内容对大家的学习或者工作能带来一定的帮助，如果有疑问大家可以留言交流，谢谢大家对的支持。</p>
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<strong>参考</strong></p>
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1.148. systemtap</p>
<p>
原文链接：https://lrita.github.io/2017/09/16/systemtap-get-function-elapse/</p>
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