244 lines
8.4 KiB
Plaintext
Generated
244 lines
8.4 KiB
Plaintext
Generated
<!DOCTYPE html>
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<html>
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<head>
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<meta charset="utf-8">
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<title>Go by Example: Atomic Counters</title>
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<div class="example" id="atomic-counters">
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<h2><a href="./">Go by Example</a>: Atomic Counters</h2>
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<table>
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<tr>
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<td class="docs">
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<p>The primary mechanism for managing state in Go is
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communication over channels. We saw this for example
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with <a href="worker-pools">worker pools</a>. There are a few other
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options for managing state though. Here we’ll
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look at using the <code>sync/atomic</code> package for <em>atomic
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counters</em> accessed by multiple goroutines.</p>
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</td>
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<td class="code empty leading">
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</td>
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<tr>
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<td class="docs">
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</td>
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<td class="code leading">
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<a href="http://play.golang.org/p/F2pJfduyQiA"><img title="Run code" src="play.png" class="run" /></a><img title="Copy code" src="clipboard.png" class="copy" />
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<div class="highlight"><pre><span class="kn">package</span> <span class="nx">main</span>
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</pre></div>
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</td>
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</tr>
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<tr>
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<td class="docs">
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</td>
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<td class="code leading">
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<div class="highlight"><pre><span class="kn">import</span> <span class="p">(</span>
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<span class="s">"fmt"</span>
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<span class="s">"sync"</span>
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<span class="s">"sync/atomic"</span>
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<span class="p">)</span>
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</pre></div>
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</td>
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</tr>
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<tr>
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<td class="docs">
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</td>
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<td class="code leading">
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<div class="highlight"><pre><span class="kd">func</span> <span class="nx">main</span><span class="p">()</span> <span class="p">{</span>
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</pre></div>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>We’ll use an unsigned integer to represent our
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(always-positive) counter.</p>
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</td>
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<td class="code leading">
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<div class="highlight"><pre> <span class="kd">var</span> <span class="nx">ops</span> <span class="kt">uint64</span>
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</pre></div>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>A WaitGroup will help us wait for all goroutines
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to finish their work.</p>
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</td>
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<td class="code leading">
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<div class="highlight"><pre> <span class="kd">var</span> <span class="nx">wg</span> <span class="nx">sync</span><span class="p">.</span><span class="nx">WaitGroup</span>
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</pre></div>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>We’ll start 50 goroutines that each increment the
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counter exactly 1000 times.</p>
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</td>
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<td class="code leading">
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<div class="highlight"><pre> <span class="k">for</span> <span class="nx">i</span> <span class="o">:=</span> <span class="mi">0</span><span class="p">;</span> <span class="nx">i</span> <span class="p"><</span> <span class="mi">50</span><span class="p">;</span> <span class="nx">i</span><span class="o">++</span> <span class="p">{</span>
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<span class="nx">wg</span><span class="p">.</span><span class="nx">Add</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>
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</pre></div>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>To atomically increment the counter we
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use <code>AddUint64</code>, giving it the memory
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address of our <code>ops</code> counter with the
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<code>&</code> syntax.</p>
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</td>
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<td class="code leading">
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<div class="highlight"><pre> <span class="k">go</span> <span class="kd">func</span><span class="p">()</span> <span class="p">{</span>
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<span class="k">for</span> <span class="nx">c</span> <span class="o">:=</span> <span class="mi">0</span><span class="p">;</span> <span class="nx">c</span> <span class="p"><</span> <span class="mi">1000</span><span class="p">;</span> <span class="nx">c</span><span class="o">++</span> <span class="p">{</span>
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</pre></div>
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</td>
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</tr>
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<tr>
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<td class="docs">
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</td>
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<td class="code leading">
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<div class="highlight"><pre> <span class="nx">atomic</span><span class="p">.</span><span class="nx">AddUint64</span><span class="p">(</span><span class="o">&</span><span class="nx">ops</span><span class="p">,</span> <span class="mi">1</span><span class="p">)</span>
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<span class="p">}</span>
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<span class="nx">wg</span><span class="p">.</span><span class="nx">Done</span><span class="p">()</span>
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<span class="p">}()</span>
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<span class="p">}</span>
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</pre></div>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>Wait until all the goroutines are done.</p>
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</td>
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<td class="code leading">
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<div class="highlight"><pre> <span class="nx">wg</span><span class="p">.</span><span class="nx">Wait</span><span class="p">()</span>
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</pre></div>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>It’s safe to access <code>ops</code> now because we know
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no other goroutine is writing to it. Reading
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atomics safely while they are being updated is
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also possible, using functions like
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<code>atomic.LoadUint64</code>.</p>
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</td>
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<td class="code">
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<div class="highlight"><pre> <span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">"ops:"</span><span class="p">,</span> <span class="nx">ops</span><span class="p">)</span>
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<span class="p">}</span>
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</pre></div>
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</td>
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</tr>
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</table>
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<table>
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<tr>
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<td class="docs">
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<p>We expect to get exactly 50,000 operations. Had we
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used the non-atomic <code>ops++</code> to increment the counter,
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we’d likely get a different number, changing between
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runs, because the goroutines would interfere with
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each other. Moreover, we’d get data race failures
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when running with the <code>-race</code> flag.</p>
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</td>
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<td class="code leading">
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<div class="highlight"><pre><span class="gp">$</span> go run atomic-counters.go
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<span class="go">ops: 50000</span>
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</pre></div>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>Next we’ll look at mutexes, another tool for managing
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state.</p>
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</td>
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<td class="code empty">
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</td>
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</tr>
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</table>
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<p class="next">
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Next example: <a href="mutexes">Mutexes</a>.
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</p>
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<p class="footer">
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by <a href="https://markmcgranaghan.com">Mark McGranaghan</a> | <a href="https://github.com/mmcgrana/gobyexample/blob/master/examples/atomic-counters">source</a> | <a href="https://github.com/mmcgrana/gobyexample#license">license</a>
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</p>
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</div>
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<script>
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var codeLines = [];
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codeLines.push('');codeLines.push('package main\u000A');codeLines.push('import (\u000A \"fmt\"\u000A \"sync\"\u000A \"sync/atomic\"\u000A)\u000A');codeLines.push('func main() {\u000A');codeLines.push(' var ops uint64\u000A');codeLines.push(' var wg sync.WaitGroup\u000A');codeLines.push(' for i := 0; i \x3C 50; i++ {\u000A wg.Add(1)\u000A');codeLines.push(' go func() {\u000A for c := 0; c \x3C 1000; c++ {\u000A');codeLines.push(' atomic.AddUint64(&ops, 1)\u000A }\u000A wg.Done()\u000A }()\u000A }\u000A');codeLines.push(' wg.Wait()\u000A');codeLines.push(' fmt.Println(\"ops:\", ops)\u000A}\u000A');codeLines.push('');codeLines.push('');
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</script>
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