303 lines
14 KiB
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Generated
303 lines
14 KiB
Plaintext
Generated
<!DOCTYPE html>
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<meta charset="utf-8">
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<title>Go by Example: Mutexes</title>
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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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<tr>
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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 <a href="http://en.wikipedia.org/wiki/Mutual_exclusion"><em>mutex</em></a>
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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/CHCDredHCOz"><img title="Run code" src="play.png" class="run" /></a><img title="Copy code" src="clipboard.png" class="copy" />
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<pre class="chroma"><span class="kn">package</span> <span class="nx">main</span>
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</pre>
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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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<pre class="chroma"><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>
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</td>
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<td class="docs">
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<td class="code leading">
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<pre class="chroma"><span class="kd">func</span> <span class="nf">main</span><span class="p">()</span> <span class="p">{</span>
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</pre>
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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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<pre class="chroma">
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<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>
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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>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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<pre class="chroma">
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<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>
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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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<pre class="chroma">
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<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>
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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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<pre class="chroma">
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<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>
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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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<pre class="chroma">
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<span class="nx">key</span> <span class="o">:=</span> <span class="nx">rand</span><span class="p">.</span><span class="nf">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="nf">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="nf">Unlock</span><span class="p">()</span>
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<span class="nx">atomic</span><span class="p">.</span><span class="nf">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>
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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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<pre class="chroma">
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<span class="nx">time</span><span class="p">.</span><span class="nf">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>
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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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<pre class="chroma">
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<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="nf">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="nf">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="nf">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="nf">Unlock</span><span class="p">()</span>
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<span class="nx">atomic</span><span class="p">.</span><span class="nf">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="nf">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>
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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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<pre class="chroma">
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<span class="nx">time</span><span class="p">.</span><span class="nf">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>
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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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<pre class="chroma">
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<span class="nx">readOpsFinal</span> <span class="o">:=</span> <span class="nx">atomic</span><span class="p">.</span><span class="nf">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="nf">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="nf">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="nf">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>
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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>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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<pre class="chroma">
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<span class="nx">mutex</span><span class="p">.</span><span class="nf">Lock</span><span class="p">()</span>
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<span class="nx">fmt</span><span class="p">.</span><span class="nf">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="nf">Unlock</span><span class="p">()</span>
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<span class="p">}</span>
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</pre>
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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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<pre class="chroma">
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<span class="gp">$</span> go run mutexes.go
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<span class="go">readOps: 83285
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</span><span class="go">writeOps: 8320
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</span><span class="go">state: map[1:97 4:53 0:33 2:15 3:2]</span></pre>
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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> and <a href="https://eli.thegreenplace.net">Eli Bendersky</a> | <a href="https://github.com/mmcgrana/gobyexample">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 \"math/rand\"\u000A \"sync\"\u000A \"sync/atomic\"\u000A \"time\"\u000A)\u000A');codeLines.push('func main() {\u000A');codeLines.push(' var state \u003D make(map[int]int)\u000A');codeLines.push(' var mutex \u003D \u0026sync.Mutex{}\u000A');codeLines.push(' var readOps uint64\u000A var writeOps uint64\u000A');codeLines.push(' for r :\u003D 0; r \u003C 100; r++ {\u000A go func() {\u000A total :\u003D 0\u000A for {\u000A');codeLines.push(' key :\u003D rand.Intn(5)\u000A mutex.Lock()\u000A total +\u003D state[key]\u000A mutex.Unlock()\u000A atomic.AddUint64(\u0026readOps, 1)\u000A');codeLines.push(' time.Sleep(time.Millisecond)\u000A }\u000A }()\u000A }\u000A');codeLines.push(' for w :\u003D 0; w \u003C 10; w++ {\u000A go func() {\u000A for {\u000A key :\u003D rand.Intn(5)\u000A val :\u003D rand.Intn(100)\u000A mutex.Lock()\u000A state[key] \u003D val\u000A mutex.Unlock()\u000A atomic.AddUint64(\u0026writeOps, 1)\u000A time.Sleep(time.Millisecond)\u000A }\u000A }()\u000A }\u000A');codeLines.push(' time.Sleep(time.Second)\u000A');codeLines.push(' readOpsFinal :\u003D atomic.LoadUint64(\u0026readOps)\u000A fmt.Println(\"readOps:\", readOpsFinal)\u000A writeOpsFinal :\u003D atomic.LoadUint64(\u0026writeOps)\u000A fmt.Println(\"writeOps:\", writeOpsFinal)\u000A');codeLines.push(' mutex.Lock()\u000A fmt.Println(\"state:\", state)\u000A mutex.Unlock()\u000A}\u000A');codeLines.push('');codeLines.push('');
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</script>
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