221 lines
7.5 KiB
Plaintext
221 lines
7.5 KiB
Plaintext
<!DOCTYPE html>
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<html>
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<meta http-eqiv="content-type" content="text/html;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/2h8nvrnaHP"><img title="Run code" src="play.png" class="run" /></a>
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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="s">"fmt"</span>
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<span class="kn">import</span> <span class="s">"time"</span>
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<span class="kn">import</span> <span class="s">"sync/atomic"</span>
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<span class="kn">import</span> <span class="s">"runtime"</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> <span class="p">=</span> <span class="mi">0</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 simulate concurrent updates, we’ll start 50
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goroutines that each increment the counter about
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once a millisecond.</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="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="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="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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</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>Allow other goroutines to proceed.</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">runtime</span><span class="p">.</span><span class="nx">Gosched</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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<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 a second to allow some ops to accumulate.</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">time</span><span class="p">.</span><span class="nx">Sleep</span><span class="p">(</span><span class="nx">time</span><span class="p">.</span><span class="nx">Second</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>In order to safely use the counter while it’s still
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being updated by other goroutines, we extract a
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copy of the current value into <code>opsFinal</code> via
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<code>LoadUint64</code>. As above we need to give this
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function the memory address <code>&ops</code> from which to
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fetch the value.</p>
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</td>
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<td class="code">
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<div class="highlight"><pre> <span class="nx">opsFinal</span> <span class="o">:=</span> <span class="nx">atomic</span><span class="p">.</span><span class="nx">LoadUint64</span><span class="p">(</span><span class="o">&</span><span class="nx">ops</span><span class="p">)</span>
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<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">opsFinal</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>Running the program shows that we executed about
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40,000 operations.</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: 40200</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://twitter.com/mmcgrana">@mmcgrana</a> | <a href="mailto:mmcgrana@gmail.com">feedback</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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</body>
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