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<h1 id="Cache-Line"><a href="#Cache-Line" class="headerlink" title="Cache Line"></a>Cache Line</h1><p>数据经过主存、L3、L2、L1 Cache到CPU寄存器进行运算，访问的速度逐级提高，L3一般为多核共享，L2、L1一般为单核本地使用。</p>
<p>按照局部性原理，正在使用的数据的附近数据也大概率会被使用，所以CPU会按照<code>Cache Line</code>为一个单位加载一批连续的数据到缓存里，主流CPU一般为64B</p>
<p>例如你有一个数组，每个元素的类型为int64，大小为8B，当你用到第一个元素时，CPU会将另外7个元素也一起作为一个<code>Cache Line</code>加载到各级缓存中</p>
<h1 id="什么是False-sharing伪共享"><a href="#什么是False-sharing伪共享" class="headerlink" title="什么是False sharing伪共享"></a>什么是False sharing伪共享</h1><p><img src="/../images/false_sharing_01.png" alt="false sharing"></p>
<p>两个线程修改同一条<code>Cache Line</code>上的不同位置，发生了缓存不一致的情况，会导致L1、L2缓存上这条<code>Cache Line</code>被认为是无效数据</p>
<p>此时就需要<code>MESI缓存一致性协议</code>和<code>RFO请求</code>核间通信方式，来标记自己本地缓存上<code>Cache Line</code>的状态并通知另一个核是否需要更新缓存，最后通过相对比较慢的<code>L3 Cache</code>同步数据，甚至穿透到主存，总之这些都会带来开销</p>
<p>所以在这种情况下没有真正达到共享的目的，此为伪共享（其实应该翻译为错误共享）</p>
<p>那延伸一下，<code>Cache Line</code>如果大小为1B一个单位，只要不是修改到同一个字节的位置，那是不是就没有这个问题了？</p>
<p>但随之而来的是局部性的损失以及控制电路复杂度的提高，设计就是不断权衡利弊，这是另一个话题了</p>
<h1 id="如何验证伪共享问题呢，我要眼见为实"><a href="#如何验证伪共享问题呢，我要眼见为实" class="headerlink" title="如何验证伪共享问题呢，我要眼见为实"></a>如何验证伪共享问题呢，我要眼见为实</h1><h2 id="复现"><a href="#复现" class="headerlink" title="复现"></a>复现</h2><figure class="highlight golang"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">package</span> main</span><br><span class="line"></span><br><span class="line"><span class="keyword">import</span> (</span><br><span class="line"> <span class="string">"fmt"</span></span><br><span class="line"> <span class="string">"time"</span></span><br><span class="line">)</span><br><span class="line"></span><br><span class="line"><span class="comment">// a demo for false share</span></span><br><span class="line"><span class="keyword">type</span> MyInt64 <span class="keyword">struct</span> {</span><br><span class="line"> val <span class="type">int64</span> <span class="comment">// 8Byte</span></span><br><span class="line">}</span><br><span class="line"><span class="keyword">var</span> (</span><br><span class="line"> N = <span class="number">100000000</span></span><br><span class="line"> M = <span class="number">8</span></span><br><span class="line"> List = <span class="built_in">new</span>([<span class="number">8</span>]MyInt64)</span><br><span class="line">)</span><br><span class="line"><span class="function"><span class="keyword">func</span> <span class="title">run</span><span class="params">(idx <span class="type">int</span>)</span></span> {</span><br><span class="line"> <span class="keyword">for</span> i:=N; i><span class="number">0</span>; i-- {</span><br><span class="line"> List[idx].val = <span class="type">int64</span>(i)</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"><span class="function"><span class="keyword">func</span> <span class="title">main</span><span class="params">()</span></span> {</span><br><span class="line"> ch := <span class="built_in">make</span>(<span class="keyword">chan</span> <span class="type">int</span>, M) <span class="comment">// exit signal</span></span><br><span class="line"> start := time.Now()</span><br><span class="line"> <span class="keyword">for</span> i:=<span class="number">0</span>; i<M; i++ {</span><br><span class="line"> <span class="keyword">go</span> <span class="function"><span class="keyword">func</span><span class="params">(i <span class="type">int</span>)</span></span> {</span><br><span class="line"> run(i)</span><br><span class="line"> ch <- <span class="number">1</span></span><br><span class="line"> }(i)</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">for</span> i:=<span class="number">0</span>; i<M; i++ {</span><br><span class="line"> <-ch</span><br><span class="line"> }</span><br><span class="line"> fmt.Printf(<span class="string">"%v\n"</span>, time.Since(start))</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>我们构造一个长度为8的数组，每个元素都是一个大小为64bit的类型，共占64Byte</p>
<p>然后用8个协程分别更新各自索引位置的元素</p>
<p>执行 <code>go run main.go</code></p>
<p>此时耗时约在秒级：2.5329389s</p>
<h2 id="改进"><a href="#改进" class="headerlink" title="改进"></a>改进</h2><p>如果我们加上一些padding，此时数组中每个元素都是64Byte的类型，每个元素都占满一个<code>Cache</code>，使得每个核操作不同的<code>Cache Line</code></p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line">type padding int64</span><br><span class="line">type MyInt64 struct {</span><br><span class="line"> val int64 // 8Byte</span><br><span class="line"> pad1, pad2, pad3, pad4, pad5, pad6, pad7 padding // 56Byte</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>耗时约在百毫秒级别：113.9928ms</p>
<h1 id="总结"><a href="#总结" class="headerlink" title="总结"></a>总结</h1><p>所以可见，如果我们编写的程序恰巧遇到了<code>False Sharing</code>，会导致严重的性能问题。</p>
<p>目前<code>perf-c2c</code>等工具可以检测伪共享问题。开源的<code>Disruptor</code>框架也详细阐述了该问题</p>
<p>Golang在<code>sync.Pool</code>中也有对应的设计</p>
<figure class="highlight golang"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">type</span> poolLocal <span class="keyword">struct</span> {</span><br><span class="line"> poolLocalInternal</span><br><span class="line"></span><br><span class="line"> <span class="comment">// Prevents false sharing on widespread platforms with</span></span><br><span class="line"> <span class="comment">// 128 mod (cache line size) = 0 .</span></span><br><span class="line"> pad [<span class="number">128</span> - unsafe.Sizeof(poolLocalInternal{})%<span class="number">128</span>]<span class="type">byte</span></span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<h1 id="但是真的结束了吗？"><a href="#但是真的结束了吗？" class="headerlink" title="但是真的结束了吗？"></a>但是真的结束了吗？</h1><p>如果在上面的代码中不用数组，而是用slice会怎样？</p>
<p>slice底层是什么结构，会有什么影响？</p>
<figure class="highlight golang"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">type</span> slice <span class="keyword">struct</span> {</span><br><span class="line"> array unsafe.Pointer</span><br><span class="line"> <span class="built_in">len</span> <span class="type">int</span></span><br><span class="line"> <span class="built_in">cap</span> <span class="type">int</span></span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<hr>
<p>如果改成8个pad会怎样？哪种对齐的结构对编译器更加友好？</p>
<p>（实际8个pad速度更快，可以试试）</p>
<p>7个pad</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br></pre></td><td class="code"><pre><span class="line">MOVQ AX, SI</span><br><span class="line">SHLQ $6, AX</span><br><span class="line">MOVQ DX, (BX)(AX*1)</span><br><span class="line">DECQ DX</span><br><span class="line">MOVQ SI, AX</span><br><span class="line">TESTQ DX, DX</span><br><span class="line">JLE 71</span><br><span class="line">MOVQ main.List+8(SB), CX</span><br><span class="line">MOVQ main.List(SB), BX</span><br><span class="line">NOP</span><br><span class="line">CMPQ AX, CX</span><br></pre></td></tr></table></figure>
<p>8个pad</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line">LEAQ (AX)(AX*8), SI</span><br><span class="line">MOVQ DX, (BX)(SI*8)</span><br><span class="line">DECQ DX</span><br><span class="line">TESTQ DX, DX</span><br><span class="line">JLE 56</span><br><span class="line">MOVQ main.List(SB), BX</span><br><span class="line">TESTB AL, (BX)</span><br><span class="line">CMPQ AX, $8</span><br></pre></td></tr></table></figure>
<hr>
<p>如果关闭编译优化会怎样？或者说编译优化究竟在优化什么？</p>
<p>关闭编译优化：</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br></pre></td><td class="code"><pre><span class="line">go build -gcflags="-N -l -S" main.go</span><br><span class="line">0x000e 00014 (main.go:18) MOVQ AX, main.idx+40(SP)</span><br><span class="line">0x0013 00019 (main.go:19) MOVQ main.N(SB), DX</span><br><span class="line">0x001a 00026 (main.go:19) MOVQ DX, main.i+16(SP)</span><br><span class="line">0x001f 00031 (main.go:19) NOP</span><br><span class="line">0x0020 00032 (main.go:19) JMP 34</span><br><span class="line">0x0022 00034 (main.go:19) CMPQ main.i+16(SP), $0</span><br><span class="line">0x0028 00040 (main.go:19) JGT 44</span><br><span class="line">0x002a 00042 (main.go:19) JMP 91</span><br><span class="line">0x002c 00044 (main.go:20) MOVQ main.idx+40(SP), AX</span><br><span class="line">0x0031 00049 (main.go:20) MOVQ main.List+8(SB), CX</span><br><span class="line">0x0038 00056 (main.go:20) MOVQ main.List(SB), DX</span><br><span class="line">0x003f 00063 (main.go:20) MOVQ main.i+16(SP), BX</span><br><span class="line">0x0044 00068 (main.go:20) CMPQ CX, AX</span><br><span class="line">0x0047 00071 (main.go:20) JHI 75</span><br><span class="line">0x0049 00073 (main.go:20) JMP 101</span><br><span class="line">0x004b 00075 (main.go:20) LEAQ (DX)(AX*8), DX</span><br><span class="line">0x004f 00079 (main.go:20) MOVQ BX, (DX)</span><br><span class="line">0x0052 00082 (main.go:20) JMP 84</span><br><span class="line">0x0054 00084 (main.go:19) DECQ main.i+16(SP)</span><br><span class="line">0x0059 00089 (main.go:19) JMP 34</span><br><span class="line">0x005b 00091 (main.go:22) MOVQ 24(SP), BP</span><br><span class="line">0x0060 00096 (main.go:22) ADDQ $32, SP</span><br><span class="line">0x0064 00100 (main.go:22) RET</span><br><span class="line">0x0065 00101 (main.go:20) PCDATA $1, $0</span><br><span class="line">0x0065 00101 (main.go:20) CALL runtime.panicIndex(SB)</span><br><span class="line">0x006a 00106 (main.go:20) XCHGL AX, AX</span><br></pre></td></tr></table></figure>
<p>开启编译优化：</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br></pre></td><td class="code"><pre><span class="line">go build -gcflags=-S main.go</span><br><span class="line">0x000e 00014 (main.go:19) MOVQ main.N(SB), DX</span><br><span class="line">0x0015 00021 (main.go:19) JMP 32</span><br><span class="line">0x0017 00023 (main.go:20) MOVQ DX, (BX)(AX*8)</span><br><span class="line">0x001b 00027 (main.go:19) DECQ DX</span><br><span class="line">0x001e 00030 (main.go:19) NOP</span><br><span class="line">0x0020 00032 (main.go:19) TESTQ DX, DX</span><br><span class="line">0x0023 00035 (main.go:19) JLE 58</span><br><span class="line">0x0025 00037 (main.go:20) MOVQ main.List+8(SB), CX</span><br><span class="line">0x002c 00044 (main.go:20) MOVQ main.List(SB), BX</span><br><span class="line">0x0033 00051 (main.go:20) CMPQ AX, CX</span><br><span class="line">0x0036 00054 (main.go:20) JCS 23</span><br><span class="line">0x0038 00056 (main.go:20) JMP 68</span><br><span class="line">0x003a 00058 (main.go:22) MOVQ 16(SP), BP</span><br><span class="line">0x003f 00063 (main.go:22) ADDQ $24, SP</span><br><span class="line">0x0043 00067 (main.go:22) RET</span><br><span class="line">0x0044 00068 (main.go:20) PCDATA $1, $0</span><br><span class="line">0x0044 00068 (main.go:20) CALL runtime.panicIndex(SB)</span><br><span class="line">0x0049 00073 (main.go:20) XCHGL AX, AX</span><br></pre></td></tr></table></figure>
<p>可以看到把很多操作压在<code>DX</code>寄存器里操作了，不用内存取值了</p>
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<a href="/astronomy/spectrum-107-Piscium/" class="post-title-link" itemprop="url">107 Piscium 恒星光谱计划</a>
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<h1 id="前言"><a href="#前言" class="headerlink" title="前言"></a>前言</h1><p>记录一下实现一个想法的过程</p>
<h1 id="真实影像"><a href="#真实影像" class="headerlink" title="真实影像"></a>真实影像</h1><h2 id="远程天文台拍摄过程"><a href="#远程天文台拍摄过程" class="headerlink" title="远程天文台拍摄过程"></a>远程天文台拍摄过程</h2><p><img src="/images/107_psc_deepsky_shooting.jpg" alt="107_psc_deepsky_shooting"></p>
<h2 id="原图"><a href="#原图" class="headerlink" title="原图"></a>原图</h2><p>略</p>
<h2 id="处理过程"><a href="#处理过程" class="headerlink" title="处理过程"></a>处理过程</h2><p><img src="/images/107_psc_pixinsight.png" alt="107_psc_pixinsight"></p>
<h1 id="光谱图像制作"><a href="#光谱图像制作" class="headerlink" title="光谱图像制作"></a>光谱图像制作</h1><h2 id="获取原始数据"><a href="#获取原始数据" class="headerlink" title="获取原始数据"></a>获取原始数据</h2><ol>
<li>数据源：<a href="https://www.eso.org/sci/observing/phase3/data_streams.html">https://www.eso.org/sci/observing/phase3/data_streams.html</a></li>
<li>搜索HD编号：<code>HD 10476</code>，找到两份光谱数据，选择合适的下载，最好包含可见光波段</li>
</ol>
<p><img src="/images/107_psc_search_spectrum.png" alt="107_psc_search_spectrum"></p>
<h2 id="处理光谱数据"><a href="#处理光谱数据" class="headerlink" title="处理光谱数据"></a>处理光谱数据</h2><ol>
<li>读取数据，并转换合适的单位</li>
<li>降采样（可选）</li>
<li>Savitzky-Golay滤波</li>
<li>输出</li>
</ol>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">import</span> os</span><br><span class="line"><span class="keyword">from</span> astropy.io <span class="keyword">import</span> fits</span><br><span class="line"><span class="keyword">from</span> scipy.signal <span class="keyword">import</span> savgol_filter</span><br><span class="line"><span class="keyword">import</span> matplotlib.pyplot <span class="keyword">as</span> plt</span><br><span class="line"></span><br><span class="line"><span class="comment"># 导入文件</span></span><br><span class="line">filepath = os.path.join(<span class="string">r'./spectrum'</span>, <span class="string">'ADP.2016-09-27T07_02_47.049.fits'</span>)</span><br><span class="line">f = fits.<span class="built_in">open</span>(filepath)</span><br><span class="line">spec = f[<span class="number">1</span>]</span><br><span class="line">data = spec.data</span><br><span class="line"></span><br><span class="line"><span class="comment"># 获取数据，并转换单位</span></span><br><span class="line">wave_nm = [wvl/<span class="number">10</span> <span class="keyword">for</span> wvl <span class="keyword">in</span> <span class="built_in">list</span>(data.field(<span class="number">0</span>)[<span class="number">0</span>])]</span><br><span class="line">flux = <span class="built_in">list</span>(data.field(<span class="number">1</span>)[<span class="number">0</span>])</span><br><span class="line"></span><br><span class="line"><span class="comment"># 降采样</span></span><br><span class="line">wave_nm = wave_nm[<span class="number">0</span>::<span class="number">3</span>]</span><br><span class="line">flux = flux[<span class="number">0</span>::<span class="number">3</span>]</span><br><span class="line"></span><br><span class="line"><span class="comment"># Savitzky-Golay滤波</span></span><br><span class="line">flux = savgol_filter(flux, <span class="number">53</span>, <span class="number">1</span>, mode= <span class="string">'nearest'</span>)</span><br><span class="line"></span><br><span class="line"><span class="comment"># 设置输出图片尺寸、标题、纵轴、横轴</span></span><br><span class="line">plt.figure().set_size_inches(<span class="number">24</span>, <span class="number">4</span>)</span><br><span class="line">plt.title(<span class="string">'HD 10476 Spectrum Comparison'</span>)</span><br><span class="line">plt.xlabel(<span class="string">'Wavelength (nm)'</span>)</span><br><span class="line">plt.ylabel(<span class="string">'Flux (ADU s-1 mW-1)'</span>)</span><br><span class="line"></span><br><span class="line"><span class="comment"># 绘制</span></span><br><span class="line">plt.plot(wave_nm, flux)</span><br><span class="line">plt.show()</span><br></pre></td></tr></table></figure>
<p>滤波前的结果</p>
<p><img src="/images/107_psc_specturm_no_filter.png" alt="107_psc_specturm_no_filter"></p>
<p>滤波后的结果</p>
<p><img src="/images/107_psc_specturm.png" alt="107_psc_specturm"></p>
<h2 id="创建渐变光谱背景"><a href="#创建渐变光谱背景" class="headerlink" title="创建渐变光谱背景"></a>创建渐变光谱背景</h2><p>由于我的Adobe Photoshop 2022没有自带的光谱渐变，所以我只能手工创建</p>
<p>色阶并不是均分的，因为在人眼可见光中，红光占的谱线宽度比别的颜色要更宽，所以在编辑颜色的渐变时，给红色到黑色更长的渐变过程</p>
<p><img src="/images/107_psc_PS_gradient_editor.png" alt="107_psc_PS_gradient_editor"></p>
<p>合并图层</p>
<p><img src="/images/107_psc_PS_gradient_merge_layer.png" alt="107_psc_PS_gradient_merge_layer"></p>
<h2 id="用universe-sandbox软件做模拟效果图"><a href="#用universe-sandbox软件做模拟效果图" class="headerlink" title="用universe sandbox软件做模拟效果图"></a>用universe sandbox软件做模拟效果图</h2><p><img src="/images/107_psc_universe_sandbox.png" alt="107_psc_universe_sandbox"></p>
<h2 id="最后PS合成"><a href="#最后PS合成" class="headerlink" title="最后PS合成"></a>最后PS合成</h2><ol>
<li>添加背景</li>
<li>调整光谱背景的对比度</li>
<li>将python导出的光谱曲线导入</li>
<li>增加波长刻度</li>
<li>增加黑色的竖线，调整透明度</li>
<li>增加文字，外发光效果</li>
</ol>
<p><img src="/images/107_psc_PS_all_layers.png" alt="107_psc_PS_all_layers"></p>
<h1 id="后记"><a href="#后记" class="headerlink" title="后记"></a>后记</h1><p>我发现天文数据虽然很公开，但都是在自己平台发布自己天文设备采集到的数据，导致搜寻数据困难，而且数据格式不一，虽然有很多数据整合平台，但感觉反而更乱了，花了很大精力，开了无数个chrome标签页，爆了无数次内存，才终于找到可见光波段的光谱数据</p>
<p>滤波调参时，有一个重要参考，至少要将钠的D1、D2两条很近的吸收带分辨出来</p>
<p>虽然全球各地有很多远程天文台可以租用，总有夜晚晴朗的天文台，但仍然很容易观测失败，例如天空突然飞来一堆云、对焦失败、赤道仪故障星点拉线等</p>
<p><strong>用恒星光谱作为礼物可能是个还不错的选择，但这不是流水线式的工作，最重要的还是真诚。</strong></p>
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<h1 id="panic日志"><a href="#panic日志" class="headerlink" title="panic日志"></a>panic日志</h1><figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br></pre></td><td class="code"><pre><span class="line">[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x512f5d]</span><br><span class="line"></span><br><span class="line">goroutine 48438156 [running]:</span><br><span class="line">encoding/json.(*encodeState).marshal.func1(0xc009decb40)</span><br><span class="line"> /go/src/encoding/json/encode.go:326 +0x9a</span><br><span class="line">panic(0xd41f60, 0x187d170)</span><br><span class="line"> /go/src/runtime/panic.go:967 +0x15d</span><br><span class="line">encoding/json.(*encodeState).string(0xc004eba100, 0x0, 0x7, 0x1)</span><br><span class="line"> /go/src/encoding/json/encode.go:1003 +0x5d</span><br><span class="line">encoding/json.stringEncoder(0xc004eba100, 0xce54e0, 0xc008bfeea0, 0x198, 0x100)</span><br><span class="line"> /go/src/encoding/json/encode.go:644 +0x358</span><br><span class="line">encoding/json.structEncoder.encode(0xc00451c000, 0x13, 0x21, 0xc0058d99e0, 0xc004eba100, 0xe29520, 0xc008bfeea0, 0x199, 0xd30100)</span><br><span class="line"> /go/src/encoding/json/encode.go:758 +0x2bb</span><br><span class="line">encoding/json.ptrEncoder.encode(0xc0058d9a70, 0xc004eba100, 0xd26880, 0xc008bfeea0, 0x16, 0xd20100)</span><br><span class="line"> /go/src/encoding/json/encode.go:914 +0x116</span><br><span class="line">encoding/json.(*encodeState).reflectValue(0xc004eba100, 0xd26880, 0xc008bfeea0, 0x16, 0xc009de0100)</span><br><span class="line"> /go/src/encoding/json/encode.go:358 +0x82</span><br><span class="line">encoding/json.(*encodeState).marshal(0xc004eba100, 0xd26880, 0xc008bfeea0, 0x450100, 0x0, 0x0)</span><br><span class="line"> /go/src/encoding/json/encode.go:330 +0xf0</span><br><span class="line">encoding/json.Marshal(0xd26880, 0xc008bfeea0, 0x18cf180, 0x8, 0xc0029067a8, 0xc002038c40, 0x15)</span><br><span class="line"> /go/src/encoding/json/encode.go:161 +0x52</span><br><span class="line">...</span><br></pre></td></tr></table></figure>
<p>可以看到，是json encode string的时候发生的panic，这种情况比较少见。</p>
<p>重点是这一行日志：</p>
<p><code>encoding/json.(*encodeState).string(0xc004eba100, 0x0, 0x7, 0x1)</code></p>
<p><code>string()</code>函数的入参有0x0和0x7</p>
<p>实际上这里的0x0是指字符串变量指向内容的指针为0，表示长度的变量为0x7</p>
<h1 id="原理"><a href="#原理" class="headerlink" title="原理"></a>原理</h1><p><code>string</code>类型底层实现是一个指向内容的指针和一个表示长度的int</p>
<figure class="highlight go"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// StringHeader is the runtime representation of a string.</span></span><br><span class="line"><span class="comment">// It cannot be used safely or portably and its representation may</span></span><br><span class="line"><span class="comment">// change in a later release.</span></span><br><span class="line"><span class="comment">// Moreover, the Data field is not sufficient to guarantee the data</span></span><br><span class="line"><span class="comment">// it references will not be garbage collected, so programs must keep</span></span><br><span class="line"><span class="comment">// a separate, correctly typed pointer to the underlying data.</span></span><br><span class="line"><span class="keyword">type</span> StringHeader <span class="keyword">struct</span> {</span><br><span class="line"> Data <span class="type">uintptr</span></span><br><span class="line"> Len <span class="type">int</span></span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>对字符串的赋值，分为两部分，所以在go里对字符串进行并发读写的话，有可能会导致指向的内容和长度不匹配，导致一些奇怪的问题。</p>
<p>比如两个协程同时并发读写同一个字符串变量，一个赋值为”123”，另一个赋值为”123456”，该字符串有可能为”123”，但长度是6。</p>
<p>但是就算并发读写，Data指针指向的一直是正常地址，只是长度不一定匹配，又怎么会导致nil pointer的呢？</p>
<p>其实，在go里，有一种特殊优化，如果你把字符串赋值为””，即空字符串的话，Data指针是0，也即空指针。</p>
<p>我们来验证一下：</p>
<figure class="highlight go"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br><span class="line">51</span><br><span class="line">52</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">package</span> main</span><br><span class="line"></span><br><span class="line"><span class="keyword">import</span> (</span><br><span class="line"> <span class="string">"fmt"</span></span><br><span class="line"> <span class="string">"time"</span></span><br><span class="line">)</span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="keyword">type</span> AudioClips <span class="keyword">struct</span> {</span><br><span class="line"> Param *TableStruct</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="keyword">type</span> TableStruct <span class="keyword">struct</span> {</span><br><span class="line"> RequestId <span class="type">string</span></span><br><span class="line"> Type <span class="type">string</span></span><br><span class="line"> Organization <span class="type">string</span></span><br><span class="line"> BtId <span class="type">string</span></span><br><span class="line"> AppId <span class="type">string</span></span><br><span class="line"> TokenId <span class="type">string</span></span><br><span class="line"> Channel <span class="type">string</span></span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">func</span> <span class="title">f</span><span class="params">(org, serviceId, appId, channel <span class="type">string</span>)</span></span> {</span><br><span class="line"> curTime := time.Now().Format(layout)</span><br><span class="line"> fmt.Sprintf(<span class="string">"%s@%s@%s@%s@%s@%s"</span>, curTime, org, serviceId, appId, channel, <span class="string">"length"</span>)</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="keyword">var</span> layout <span class="type">string</span></span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">func</span> <span class="title">main</span><span class="params">()</span></span> {</span><br><span class="line"> a := AudioClips{</span><br><span class="line"> Param: &TableStruct{</span><br><span class="line"> AppId: <span class="string">""</span>,</span><br><span class="line"> Organization: <span class="string">"org"</span>,</span><br><span class="line"> Channel: <span class="string">"channel"</span>,</span><br><span class="line"> },</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="keyword">go</span> <span class="function"><span class="keyword">func</span><span class="params">()</span></span> {</span><br><span class="line"> <span class="keyword">for</span> {</span><br><span class="line"> f(a.Param.Organization, <span class="string">"serviceId"</span>, a.Param.AppId, a.Param.Channel)</span><br><span class="line"> }</span><br><span class="line"> }()</span><br><span class="line"></span><br><span class="line"> <span class="keyword">for</span> {</span><br><span class="line"> <span class="keyword">go</span> <span class="function"><span class="keyword">func</span><span class="params">()</span></span> {a.Param.AppId = <span class="string">""</span>}()</span><br><span class="line"> time.Sleep(<span class="number">10</span>*time.Nanosecond)</span><br><span class="line"> <span class="keyword">go</span> <span class="function"><span class="keyword">func</span><span class="params">()</span></span> {a.Param.AppId = <span class="string">"default"</span>}()</span><br><span class="line"> time.Sleep(<span class="number">10</span>*time.Nanosecond)</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>运行后发生panic</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br></pre></td><td class="code"><pre><span class="line">$ go run main.go</span><br><span class="line">panic: runtime error: invalid memory address or nil pointer dereference</span><br><span class="line">[signal 0xc0000005 code=0x0 addr=0x0 pc=0xe570a2]</span><br><span class="line"></span><br><span class="line">goroutine 34 [running]:</span><br><span class="line">fmt.(*buffer).writeString(...)</span><br><span class="line"> C:/Program Files/Go/src/fmt/print.go:82</span><br><span class="line">fmt.(*fmt).padString(0xc0000202b0, 0x0, 0x7)</span><br><span class="line"> C:/Program Files/Go/src/fmt/format.go:110 +0x98</span><br><span class="line">fmt.(*fmt).fmtS(0xc0000202b0, 0x0, 0x7)</span><br><span class="line"> C:/Program Files/Go/src/fmt/format.go:359 +0x68</span><br><span class="line">fmt.(*pp).fmtString(0xc000020270, 0x0, 0x7, 0x21700000073)</span><br><span class="line"> C:/Program Files/Go/src/fmt/print.go:446 +0x1d2</span><br><span class="line">fmt.(*pp).printArg(0xc000020270, 0xead6c0, 0xc0003de2b0, 0x73)</span><br><span class="line"> C:/Program Files/Go/src/fmt/print.go:694 +0x875</span><br><span class="line">fmt.(*pp).doPrintf(0xc000020270, 0xec9c4d, 0x11, 0xc000097f18, 0x6, 0x6)</span><br><span class="line"> C:/Program Files/Go/src/fmt/print.go:1026 +0x168</span><br><span class="line">fmt.Sprintf(0xec9c4d, 0x11, 0xc000097f18, 0x6, 0x6, 0x0, 0x0)</span><br><span class="line"> C:/Program Files/Go/src/fmt/print.go:219 +0x6d</span><br><span class="line">main.f(0xec799a, 0x3, 0xec8574, 0x9, 0x0, 0x7, 0xec7fd9, 0x7)</span><br><span class="line"> C:/Projects/test/11/main.go:43 +0x1de</span><br><span class="line">main.main.func1(0xc000194000)</span><br><span class="line"> C:/Projects/test/11/main.go:59 +0x8a</span><br><span class="line">created by main.main</span><br><span class="line"> C:/Projects/test/11/main.go:57 +0x91</span><br><span class="line">exit status 2</span><br></pre></td></tr></table></figure>
<p>看下汇编代码，略有精简</p>
<p><code>go tool compile -S -N -l main.go</code></p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br></pre></td><td class="code"><pre><span class="line">...</span><br><span class="line">"".main.func2 STEXT size=92 args=0x8 locals=0x8</span><br><span class="line"> 0x0000 00000 (main.go:47) TEXT "".main.func2(SB), ABIInternal, $8-8</span><br><span class="line"> 0x0000 00000 (main.go:47) MOVQ (TLS), CX</span><br><span class="line"> 0x0009 00009 (main.go:47) CMPQ SP, 16(CX)</span><br><span class="line"> 0x000d 00013 (main.go:47) PCDATA $0, $-2</span><br><span class="line"> 0x000d 00013 (main.go:47) JLS 85</span><br><span class="line"> 0x000f 00015 (main.go:47) PCDATA $0, $-1</span><br><span class="line"> 0x000f 00015 (main.go:47) SUBQ $8, SP</span><br><span class="line"> 0x0013 00019 (main.go:47) MOVQ BP, (SP)</span><br><span class="line"> 0x0017 00023 (main.go:47) LEAQ (SP), BP</span><br><span class="line"> 0x001b 00027 (main.go:47) MOVQ "".a+16(SP), CX</span><br><span class="line"> 0x0020 00032 (main.go:47) TESTB AL, (CX)</span><br><span class="line"> 0x0022 00034 (main.go:47) MOVQ $0, 72(CX)</span><br><span class="line"> 0x002a 00042 (main.go:47) LEAQ 64(CX), DI</span><br><span class="line"> 0x002e 00046 (main.go:47) CMPL runtime.writeBarrier(SB), $0</span><br><span class="line"> 0x0035 00053 (main.go:47) JEQ 57</span><br><span class="line"> 0x0037 00055 (main.go:47) JMP 76</span><br><span class="line"> 0x0039 00057 (main.go:47) MOVQ $0, 64(CX)</span><br><span class="line"> 0x0041 00065 (main.go:47) JMP 67</span><br><span class="line"> 0x0043 00067 (main.go:47) PCDATA $0, $-1</span><br><span class="line"> 0x0043 00067 (main.go:47) PCDATA $1, $-1</span><br><span class="line"> 0x0043 00067 (main.go:47) MOVQ (SP), BP</span><br><span class="line"> 0x0047 00071 (main.go:47) ADDQ $8, SP</span><br><span class="line"> 0x004b 00075 (main.go:47) RET</span><br><span class="line">...</span><br></pre></td></tr></table></figure>
<p><code>MOVQ $0, 72(CX)</code> 表示把立即数0存入CX地址+72，即把长度0存入Len</p>
<p><code>MOVQ $0, 64(CX)</code> 表示把立即数0存入CX地址+64，即指向字符串内容的指针值为0，也就是nil pointer</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br></pre></td><td class="code"><pre><span class="line">...</span><br><span class="line">"".main.func3 STEXT size=100 args=0x8 locals=0x8</span><br><span class="line"> 0x0000 00000 (main.go:49) TEXT "".main.func3(SB), ABIInternal, $8-8</span><br><span class="line"> 0x0000 00000 (main.go:49) MOVQ (TLS), CX</span><br><span class="line"> 0x0009 00009 (main.go:49) CMPQ SP, 16(CX)</span><br><span class="line"> 0x000d 00013 (main.go:49) PCDATA $0, $-2</span><br><span class="line"> 0x000d 00013 (main.go:49) JLS 93</span><br><span class="line"> 0x000f 00015 (main.go:49) PCDATA $0, $-1</span><br><span class="line"> 0x000f 00015 (main.go:49) SUBQ $8, SP</span><br><span class="line"> 0x0013 00019 (main.go:49) MOVQ BP, (SP)</span><br><span class="line"> 0x0017 00023 (main.go:49) LEAQ (SP), BP</span><br><span class="line"> 0x001b 00027 (main.go:49) MOVQ "".a+16(SP), CX</span><br><span class="line"> 0x0020 00032 (main.go:49) TESTB AL, (CX)</span><br><span class="line"> 0x0022 00034 (main.go:49) MOVQ $7, 72(CX)</span><br><span class="line"> 0x002a 00042 (main.go:49) LEAQ 64(CX), DI</span><br><span class="line"> 0x002e 00046 (main.go:49) PCDATA $0, $-2</span><br><span class="line"> 0x002e 00046 (main.go:49) PCDATA $1, $-2</span><br><span class="line"> 0x002e 00046 (main.go:49) CMPL runtime.writeBarrier(SB), $0</span><br><span class="line"> 0x0035 00053 (main.go:49) JEQ 57</span><br><span class="line"> 0x0037 00055 (main.go:49) JMP 79</span><br><span class="line"> 0x0039 00057 (main.go:49) LEAQ go.string."default"(SB), AX</span><br><span class="line"> 0x0040 00064 (main.go:49) MOVQ AX, 64(CX)</span><br><span class="line"> 0x0044 00068 (main.go:49) JMP 70</span><br><span class="line"> 0x0046 00070 (main.go:49) PCDATA $0, $-1</span><br><span class="line"> 0x0046 00070 (main.go:49) PCDATA $1, $-1</span><br><span class="line"> 0x0046 00070 (main.go:49) MOVQ (SP), BP</span><br><span class="line"> 0x004a 00074 (main.go:49) ADDQ $8, SP</span><br><span class="line"> 0x004e 00078 (main.go:49) RET</span><br><span class="line">...</span><br></pre></td></tr></table></figure>
<p><code>MOVQ $7, 72(CX)</code> 表示把立即数7存入CX地址+72，即把长度0存入Len</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">LEAQ go.string."default"(SB), AX</span><br><span class="line">MOVQ AX, 64(CX)</span><br></pre></td></tr></table></figure>
<p>表示把指向”default”的地址存入AX，再把AX存入64(CX)，即把指向字符串内容的指针赋值为”default”的地址</p>
<p>所以在json encode这个字符串的时候，可能会发生字符串有长度，但指向内容的指针是空指针，</p>
<p>可以通过len(str)>0这样的判断，但真正去取值的时候就发生了nil pointer panic。</p>
<h1 id="更进一步"><a href="#更进一步" class="headerlink" title="更进一步"></a>更进一步</h1><p>所以可见，如果某个字符串变量存储了很多的内容，可以通过赋值为空字符串进行释放。</p>
<p>那么问题又来了，如果按上面的分析，赋值为空字符串，指针就会为空，那为什么如下代码没有发生panic</p>
<figure class="highlight go"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">a := <span class="string">""</span></span><br><span class="line">p := &a</span><br></pre></td></tr></table></figure>
<p>其实编译器会判断到如果这个空字符串在后续是有取指针操作的话，加上如下汇编代码：</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line">0x003c 00060 (main.go:9) MOVQ "".&p+24(SP), DI</span><br><span class="line">0x0041 00065 (main.go:9) MOVQ "".&a+16(SP), AX</span><br><span class="line">0x0046 00070 (main.go:9) CMPL runtime.writeBarrier(SB), $0</span><br><span class="line">0x004d 00077 (main.go:9) JEQ 81</span><br><span class="line">0x004f 00079 (main.go:9) JMP 95</span><br><span class="line">0x0051 00081 (main.go:9) MOVQ AX, (DI)</span><br><span class="line">0x0054 00084 (main.go:9) JMP 86</span><br></pre></td></tr></table></figure>
<p>以保证可以正常取到指针</p>
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<a href="/golang/golang-panic-problem-1/" class="post-title-link" itemprop="url">Golang panic疑难杂症（一）</a>
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<p>项目里经常出现<code>panic: assignment to entry in nil map</code>的错误，问题其实很简单，但第一眼不一定能看出来。</p>
<p>复现demo：</p>
<figure class="highlight go"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">package</span> main</span><br><span class="line"></span><br><span class="line"><span class="keyword">import</span> (</span><br><span class="line"> <span class="string">"math/rand"</span></span><br><span class="line"> <span class="string">"runtime"</span></span><br><span class="line"> <span class="string">"sync"</span></span><br><span class="line"> <span class="string">"time"</span></span><br><span class="line">)</span><br><span class="line"></span><br><span class="line"><span class="keyword">type</span> Task <span class="keyword">struct</span> {</span><br><span class="line"> Result <span class="type">string</span></span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">func</span> <span class="params">(t *Task)</span></span> f(wg *sync.WaitGroup) <span class="keyword">map</span>[<span class="type">string</span>]<span class="keyword">interface</span>{} {</span><br><span class="line"> <span class="keyword">defer</span> wg.Done()</span><br><span class="line"> <span class="comment">// do something</span></span><br><span class="line"> time.Sleep(<span class="number">10</span>*time.Microsecond)</span><br><span class="line"> <span class="keyword">return</span> <span class="keyword">map</span>[<span class="type">string</span>]<span class="keyword">interface</span>{}{t.Result: rand.Int()}</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">func</span> <span class="title">run</span><span class="params">()</span></span> {</span><br><span class="line"> <span class="keyword">var</span> wg sync.WaitGroup</span><br><span class="line"> tasks := []Task{</span><br><span class="line"> {Result: <span class="string">"task1"</span>},</span><br><span class="line"> {Result: <span class="string">"task2"</span>},</span><br><span class="line"> }</span><br><span class="line"> result := <span class="built_in">make</span>([]<span class="keyword">map</span>[<span class="type">string</span>]<span class="keyword">interface</span>{}, <span class="number">2</span>)</span><br><span class="line"></span><br><span class="line"> <span class="keyword">for</span> idx, task := <span class="keyword">range</span> tasks {</span><br><span class="line"> wg.Add(<span class="number">1</span>)</span><br><span class="line"> <span class="keyword">go</span> <span class="function"><span class="keyword">func</span><span class="params">(i <span class="type">int</span>, t Task)</span></span> {</span><br><span class="line"> result[i] = t.f(&wg)</span><br><span class="line"> }(idx, task)</span><br><span class="line"> }</span><br><span class="line"> wg.Wait()</span><br><span class="line"></span><br><span class="line"> result[<span class="number">0</span>][<span class="string">"test"</span>] = <span class="number">0</span></span><br><span class="line"> result[<span class="number">1</span>][<span class="string">"test"</span>] = <span class="number">1</span></span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">func</span> <span class="title">main</span><span class="params">()</span></span> {</span><br><span class="line"> runtime.GOMAXPROCS(<span class="number">2</span>)</span><br><span class="line"> <span class="keyword">for</span> i := <span class="number">0</span>; i < <span class="number">10000</span>; i++ {</span><br><span class="line"> run()</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>代码逻辑是期望并发执行两个任务，然后使用得到的结果，但却发生了panic。</p>
<p>原因是在错误的地方使用了<code>defer wg.Done()</code>，内部函数f执行结束后，还没有将返回值赋值给<code>result[i]</code>，主线程就继续运行了，进而发生nil map panic。</p>
<p>前人挖的坑，后人的kpi。</p>
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<a href="/uncategorized/gitlab-ee-upgrade-13-to-14/" class="post-title-link" itemprop="url">Gitlab EE 从13.x版本升级至14.x注意点</a>
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<p>Gitlab 14.x相较于13.x删除了<code>unicorn</code>，使用<code>puma</code>。所以需要注释掉<code>gitlab.rb</code>中所有关于<code>unicorn</code>选项，打开<code>puma</code>的相关选项。</p>
<p>提供一份配置示例如下：</p>
<figure class="highlight ruby"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">################################################################################</span></span><br><span class="line"><span class="comment">## GitLab Unicorn</span></span><br><span class="line"><span class="comment">##! Tweak unicorn settings.</span></span><br><span class="line"><span class="comment">##! Docs: https://docs.gitlab.com/omnibus/settings/unicorn.html</span></span><br><span class="line"><span class="comment">################################################################################</span></span><br><span class="line"></span><br><span class="line"><span class="comment"># unicorn['enable'] = true</span></span><br><span class="line"><span class="comment"># unicorn['worker_timeout'] = 60</span></span><br><span class="line"><span class="comment">###! Minimum worker_processes is 2 at this moment</span></span><br><span class="line"><span class="comment">###! See https://gitlab.com/gitlab-org/gitlab-ce/issues/18771</span></span><br><span class="line"><span class="comment"># unicorn['worker_processes'] = 2</span></span><br><span class="line"><span class="comment"># unicorn['worker_memory_limit_min'] = "250 * 1 << 20"</span></span><br><span class="line"><span class="comment"># unicorn['worker_memory_limit_max'] = "400 * 1 << 20"</span></span><br><span class="line"></span><br><span class="line"><span class="comment">################################################################################</span></span><br><span class="line"><span class="comment">## GitLab Puma</span></span><br><span class="line"><span class="comment">##! Tweak puma settings. You should only use Unicorn or Puma, not both.</span></span><br><span class="line"><span class="comment">##! Docs: https://docs.gitlab.com/omnibus/settings/puma.html</span></span><br><span class="line"><span class="comment">################################################################################</span></span><br><span class="line"></span><br><span class="line">puma[<span class="string">'enable'</span>] = <span class="literal">true</span></span><br><span class="line"><span class="comment"># puma['ha'] = false</span></span><br><span class="line">puma[<span class="string">'worker_timeout'</span>] = <span class="number">300</span></span><br><span class="line">puma[<span class="string">'worker_processes'</span>] = <span class="number">4</span></span><br><span class="line">puma[<span class="string">'min_threads'</span>] = <span class="number">1</span></span><br><span class="line">puma[<span class="string">'max_threads'</span>] = <span class="number">16</span></span><br></pre></td></tr></table></figure>
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<a href="/golang/insights-into-empty-struct/" class="post-title-link" itemprop="url">空struct引发的一些探索</a>
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<h3 id="背景"><a href="#背景" class="headerlink" title="背景"></a>背景</h3><p>偶然发现项目代码中有如下的写法：</p>
<figure class="highlight go"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">type</span> Student <span class="keyword">struct</span> {</span><br><span class="line"> Name <span class="type">string</span></span><br><span class="line"> Age <span class="type">int</span></span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">func</span> <span class="title">NewStudent</span><span class="params">(s Student)</span></span> *Student {</span><br><span class="line"> <span class="keyword">return</span> &s</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>我们且不去探讨“这样写法有什么作用”、“为什么要这样写”，我们先就代码本身讨论。</p>
<p>众所周知，从严格意义上来说，golang函数调用中只有值传递的方式传递参数，所以这个函数会返回一个新的指针，并指向一个新的对象。</p>
<p>然后我就在思考，这种函数会被编译器特殊优化么？</p>
<h3 id="探索"><a href="#探索" class="headerlink" title="探索"></a>探索</h3><p>写一个简单demo测试一下：</p>
<figure class="highlight go"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">package</span> main</span><br><span class="line"></span><br><span class="line"><span class="keyword">import</span> <span class="string">"fmt"</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">type</span> A <span class="keyword">struct</span>{} <span class="comment">// 空struct</span></span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">func</span> <span class="title">NewStructA</span><span class="params">(a A)</span></span> *A {</span><br><span class="line"> <span class="keyword">return</span> &a</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">func</span> <span class="title">main</span><span class="params">()</span></span> {</span><br><span class="line"> a := A{}</span><br><span class="line"> newA := NewStructA(a)</span><br><span class="line"> fmt.Printf(<span class="string">"a: %p, newA: %p\n"</span>, &a, newA)</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>运行结果很amazing啊</p>
<figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">$ go run main.go</span><br><span class="line">a: 0x5a6d88, newA: 0x5a6d88</span><br></pre></td></tr></table></figure>
<p>两个指针居然完全相等，这不合常理啊，讲道理，值传递的话，return返回出来的应该是一个新对象的指针啊</p>
<p>不过仔细观察一下，这个例子中，A是一个空struct，函数里也不可能修改传进来的<code>A的实例a</code>，所以即使两个是相同的指针也没有任何影响。</p>
<p>那我们就去看看编译器是怎么做的吧。</p>
<p><code>-gcflags='-S'</code> 列出汇编代码</p>
<p><code>FUNCDATA</code>和<code>PCDATA</code>是编译器产生的，用于保存一些给垃圾收集的信息，在本例中可以过滤掉</p>
<figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line">$ go build -gcflags=<span class="string">'-S'</span> main.go 2>&1 | grep -v <span class="string">"FUNCDATA\|PCDATA"</span> | grep -A10 <span class="string">'"".NewStructA STEXT'</span></span><br><span class="line"></span><br><span class="line"><span class="string">""</span>.NewStructA STEXT nosplit size=13 args=0x8 locals=0x0</span><br><span class="line"> 0x0000 00000 (D:\projects\<span class="built_in">test</span>\go\struct\1\main.go:7) TEXT <span class="string">""</span>.NewStructA(SB), NOSPLIT|ABIInternal, <span class="variable">$0</span>-8</span><br><span class="line"> 0x0000 00000 (D:\projects\<span class="built_in">test</span>\go\struct\1\main.go:8) LEAQ runtime.zerobase(SB), AX</span><br><span class="line"> 0x0007 00007 (D:\projects\<span class="built_in">test</span>\go\struct\1\main.go:8) MOVQ AX, <span class="string">""</span>.~r1+8(SP)</span><br><span class="line"> 0x000c 00012 (D:\projects\<span class="built_in">test</span>\go\struct\1\main.go:8) RET</span><br><span class="line"> 0x0000 48 8d 05 00 00 00 00 48 89 44 24 08 c3 H......H.D$..</span><br><span class="line"> rel 3+4 t=16 runtime.zerobase+0</span><br><span class="line">...</span><br></pre></td></tr></table></figure>
<p><code>$0-8</code>中0代表的是分配的栈帧大小为0，8代表的是各个实参和返回值的总大小</p>
<p><img src="/images/caller.png" alt="caller"></p>
<p><code>LEAQ runtime.zerobase(SB), AX</code></p>
<p>把<code>runtime.zerobase(SB)</code>的地址传送到<code>AX</code>寄存器</p>
<p><code>MOVQ AX, "".~r1+8(SP)</code></p>
<p>把<code>AX</code>寄存器中的数据移动到<code>~r1+8(SP)</code>即返回值的位置</p>
<p><code>runtime.zerobase</code>是个什么东西？</p>
<p>翻看源码找到<code>runtime/malloc.go</code>826行</p>
<figure class="highlight go"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// base address for all 0-byte allocations</span></span><br><span class="line"><span class="keyword">var</span> zerobase <span class="type">uintptr</span></span><br></pre></td></tr></table></figure>
<p>是一个用于所有对内存零占用的结构的地址</p>
<p>所以空struct的地址就是zerobase，所以函数<code>NewStructA</code>才会返回相同的地址。</p>
<p>而且实际上前文所说的<code>$0-8中的8代表的是各个实参和返回值的总大小</code>，这个<code>8</code>也可以理解了，实参其实并没有占用栈空间，是返回值(空struct的指针)占用了栈空间的8个字节。</p>
<figure class="highlight go"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">package</span> main</span><br><span class="line"></span><br><span class="line"><span class="keyword">import</span> (</span><br><span class="line"> <span class="string">"fmt"</span></span><br><span class="line"> <span class="string">"unsafe"</span></span><br><span class="line">)</span><br><span class="line"></span><br><span class="line"><span class="keyword">type</span> A <span class="keyword">struct</span>{}</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">func</span> <span class="title">main</span><span class="params">()</span></span> {</span><br><span class="line"> a := A{}</span><br><span class="line"> fmt.Println(unsafe.Sizeof(a)) <span class="comment">// print 0</span></span><br><span class="line"> fmt.Println(unsafe.Sizeof(&a)) <span class="comment">// print 8</span></span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<h3 id="空struct特性"><a href="#空struct特性" class="headerlink" title="空struct特性"></a>空struct特性</h3><ol>
<li><p>以前一直不理解<code>chan struct{}</code>的意图，其实可以理解为<code>struct{}</code>是golang中最小的数据结构</p>
<figure class="highlight go"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line">done := <span class="built_in">make</span>(<span class="keyword">chan</span> <span class="keyword">struct</span>{})</span><br><span class="line"><span class="keyword">go</span> <span class="function"><span class="keyword">func</span><span class="params">()</span></span> {</span><br><span class="line"> <span class="comment">// ...</span></span><br><span class="line"> done <- <span class="keyword">struct</span>{}{}</span><br><span class="line">}()</span><br><span class="line"><- done</span><br></pre></td></tr></table></figure>
</li>
<li><p><code>map[string]struct{}</code>比<code>map[string]bool</code>更节约空间</p>
</li>
<li><pre><code class="go">var x [1000000000]struct{}
fmt.Println(unsafe.Sizeof(x)) // print 0
</code></pre>
</li>
</ol>
<h3 id="参考资料"><a href="#参考资料" class="headerlink" title="参考资料"></a>参考资料</h3><ul>
<li><p><a href="https://golang.org/doc/asm">A Quick Guide to Go’s Assembler</a></p>
</li>
<li><p><a href="https://blog.csdn.net/u010853261/article/details/101060546">golang汇编语言基础</a></p>
</li>
<li><p><a href="https://blog.csdn.net/Tencent_TEG/article/details/108413692">Golang 汇编入门知识总结</a></p>
</li>
<li><p><a href="https://blog.csdn.net/qq_34777600/article/details/87195673">空结构体struct{}解析</a></p>
</li>
</ul>
<h3 id="附录"><a href="#附录" class="headerlink" title="附录"></a>附录</h3><table>
<thead>
<tr>
<th>助记符</th>
<th>指令种类</th>
<th>用途</th>
<th>示例</th>
</tr>
</thead>
<tbody><tr>
<td>MOVQ</td>
<td>传送</td>
<td>数据传送</td>
<td>MOVQ 48, AX // 把 48 传送到 AX</td>
</tr>
<tr>
<td>LEAQ</td>
<td>传送</td>
<td>地址传送</td>
<td>LEAQ AX, BX // 把 AX 有效地址传送到 BX</td>
</tr>
<tr>
<td>PUSHQ</td>
<td>传送</td>
<td>栈压入</td>
<td>PUSHQ AX // 将 AX 内容送入栈顶位置</td>
</tr>
<tr>
<td>POPQ</td>
<td>传送</td>
<td>栈弹出</td>
<td>POPQ AX // 弹出栈顶数据后修改栈顶指针</td>
</tr>
<tr>
<td>ADDQ</td>
<td>运算</td>
<td>相加并赋值</td>
<td>ADDQ BX, AX // 等价于 AX+=BX</td>
</tr>
<tr>
<td>SUBQ</td>
<td>运算</td>
<td>相减并赋值</td>
<td>SUBQ BX, AX // 等价于 AX-=BX</td>
</tr>
<tr>
<td>CMPQ</td>
<td>运算</td>
<td>比较大小</td>
<td>CMPQ SI CX // 比较 SI 和 CX 的大小</td>
</tr>
<tr>
<td>CALL</td>
<td>转移</td>
<td>调用函数</td>
<td>CALL runtime.printnl(SB) // 发起调用</td>
</tr>
<tr>
<td>JMP</td>
<td>转移</td>
<td>无条件转移指令</td>
<td>JMP 0x0185 //无条件转至 0x0185 地址处</td>
</tr>
<tr>
<td>JLS</td>
<td>转移</td>
<td>条件转移指令</td>
<td>JLS 0x0185 //左边小于右边，则跳到 0x0185</td>
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<a href="/uncategorized/mac-idea-terminal-alt-key-solution/" class="post-title-link" itemprop="url">解决mac下IntelliJ IDEA的terminal无法使用快速按单词移动光标</a>
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<p>在mac下idea的terminal没办法使用option+左右方向键跳过单词，google后找到以下解决方案：</p>
<h3 id="非zsh用户"><a href="#非zsh用户" class="headerlink" title="非zsh用户"></a>非zsh用户</h3><p>Add this to ~/.zshrc:</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">"\e\eC": forward-word</span><br><span class="line">"\e\eD": backward-word</span><br></pre></td></tr></table></figure>
<h3 id="zsh用户"><a href="#zsh用户" class="headerlink" title="zsh用户"></a>zsh用户</h3><p>Add this to ~/.zshrc:</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">bindkey "\e\eOD" backward-word</span><br><span class="line">bindkey "\e\eOC" forward-word</span><br></pre></td></tr></table></figure>
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