baf22e84e7
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288 lines
13 KiB
Plaintext
288 lines
13 KiB
Plaintext
<!DOCTYPE html>
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<meta charset="utf-8">
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<title>Go by Example: Mutexes</title>
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<link rel=stylesheet href="site.css">
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<div class="example" id="mutexes">
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<h2><a href="./">Go by Example</a>: Mutexes</h2>
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<table>
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<td class="docs">
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<p>In the previous example we saw how to manage simple
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counter state using <a href="atomic-counters">atomic operations</a>.
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For more complex state we can use a <em><a href="http://en.wikipedia.org/wiki/Mutual_exclusion">mutex</a></em>
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to safely access data across multiple goroutines.</p>
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<td class="code empty leading">
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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/IRewFKz2OPN">
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<img title="Run code" class="run" src="data:image/png;base64,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" />
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</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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<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">"math/rand"</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="s">"time"</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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<td class="docs">
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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>For our example the <code>state</code> will be a map.</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">state</span> <span class="p">=</span> <span class="nb">make</span><span class="p">(</span><span class="kd">map</span><span class="p">[</span><span class="kt">int</span><span class="p">]</span><span class="kt">int</span><span class="p">)</span>
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</pre></div>
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</td>
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<td class="docs">
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<p>This <code>mutex</code> will synchronize access to <code>state</code>.</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">mutex</span> <span class="p">=</span> <span class="o">&</span><span class="nx">sync</span><span class="p">.</span><span class="nx">Mutex</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 keep track of how many read and write
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operations we do.</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">readOps</span> <span class="kt">uint64</span>
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<span class="kd">var</span> <span class="nx">writeOps</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>Here we start 100 goroutines to execute repeated
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reads against the state, once per millisecond in
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each goroutine.</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">r</span> <span class="o">:=</span> <span class="mi">0</span><span class="p">;</span> <span class="nx">r</span> <span class="p"><</span> <span class="mi">100</span><span class="p">;</span> <span class="nx">r</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="nx">total</span> <span class="o">:=</span> <span class="mi">0</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>For each read we pick a key to access,
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<code>Lock()</code> the <code>mutex</code> to ensure
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exclusive access to the <code>state</code>, read
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the value at the chosen key,
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<code>Unlock()</code> the mutex, and increment
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the <code>readOps</code> count.</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">key</span> <span class="o">:=</span> <span class="nx">rand</span><span class="p">.</span><span class="nx">Intn</span><span class="p">(</span><span class="mi">5</span><span class="p">)</span>
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<span class="nx">mutex</span><span class="p">.</span><span class="nx">Lock</span><span class="p">()</span>
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<span class="nx">total</span> <span class="o">+=</span> <span class="nx">state</span><span class="p">[</span><span class="nx">key</span><span class="p">]</span>
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<span class="nx">mutex</span><span class="p">.</span><span class="nx">Unlock</span><span class="p">()</span>
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<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">readOps</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>Wait a bit between reads.</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">Millisecond</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>We’ll also start 10 goroutines to simulate writes,
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using the same pattern we did for reads.</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">w</span> <span class="o">:=</span> <span class="mi">0</span><span class="p">;</span> <span class="nx">w</span> <span class="p"><</span> <span class="mi">10</span><span class="p">;</span> <span class="nx">w</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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<span class="nx">key</span> <span class="o">:=</span> <span class="nx">rand</span><span class="p">.</span><span class="nx">Intn</span><span class="p">(</span><span class="mi">5</span><span class="p">)</span>
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<span class="nx">val</span> <span class="o">:=</span> <span class="nx">rand</span><span class="p">.</span><span class="nx">Intn</span><span class="p">(</span><span class="mi">100</span><span class="p">)</span>
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<span class="nx">mutex</span><span class="p">.</span><span class="nx">Lock</span><span class="p">()</span>
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<span class="nx">state</span><span class="p">[</span><span class="nx">key</span><span class="p">]</span> <span class="p">=</span> <span class="nx">val</span>
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<span class="nx">mutex</span><span class="p">.</span><span class="nx">Unlock</span><span class="p">()</span>
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<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">writeOps</span><span class="p">,</span> <span class="mi">1</span><span class="p">)</span>
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<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">Millisecond</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>Let the 10 goroutines work on the <code>state</code> and
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<code>mutex</code> for a second.</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>Take and report final operation counts.</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">readOpsFinal</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">readOps</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">"readOps:"</span><span class="p">,</span> <span class="nx">readOpsFinal</span><span class="p">)</span>
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<span class="nx">writeOpsFinal</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">writeOps</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">"writeOps:"</span><span class="p">,</span> <span class="nx">writeOpsFinal</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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<td class="docs">
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<p>With a final lock of <code>state</code>, show how it ended up.</p>
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</td>
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<td class="code">
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<div class="highlight"><pre> <span class="nx">mutex</span><span class="p">.</span><span class="nx">Lock</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">"state:"</span><span class="p">,</span> <span class="nx">state</span><span class="p">)</span>
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<span class="nx">mutex</span><span class="p">.</span><span class="nx">Unlock</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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90,000 total operations against our <code>mutex</code>-synchronized
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<code>state</code>.</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 mutexes.go
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<span class="go">readOps: 83285</span>
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<span class="go">writeOps: 8320</span>
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<span class="go">state: map[1:97 4:53 0:33 2:15 3:2]</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 implementing this same state
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management task using only goroutines and channels.</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="stateful-goroutines">Stateful Goroutines</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/mutexes">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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