6a58750728
Consistently use `time.Sleep`, instead of `runtime.Gosched`, to ensure all goroutines can make progress. `Gosched` wasn't working in the playground (ref #149). Also stop trying to compare operation rates. This was tenuous given e.g. how short the programs ran for, and with the `Sleep`s we now expect the rates to be similar anyways.
313 lines
16 KiB
Plaintext
313 lines
16 KiB
Plaintext
<!DOCTYPE html>
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<title>Go by Example: Stateful Goroutines</title>
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<div class="example" id="stateful-goroutines">
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<h2><a href="./">Go by Example</a>: Stateful Goroutines</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 used explicit locking with
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<a href="mutexes">mutexes</a> to synchronize access to shared state
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across multiple goroutines. Another option is to use the
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built-in synchronization features of goroutines and
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channels to achieve the same result. This channel-based
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approach aligns with Go’s ideas of sharing memory by
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communicating and having each piece of data owned
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by exactly 1 goroutine.</p>
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</td>
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<td class="code empty leading">
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</td>
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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/P4SrrlosMp"><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="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/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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<tr>
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<td class="docs">
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<p>In this example our state will be owned by a single
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goroutine. This will guarantee that the data is never
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corrupted with concurrent access. In order to read or
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write that state, other goroutines will send messages
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to the owning goroutine and receive corresponding
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replies. These <code>readOp</code> and <code>writeOp</code> <code>struct</code>s
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encapsulate those requests and a way for the owning
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goroutine to respond.</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">type</span> <span class="nx">readOp</span> <span class="kd">struct</span> <span class="p">{</span>
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<span class="nx">key</span> <span class="kt">int</span>
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<span class="nx">resp</span> <span class="kd">chan</span> <span class="kt">int</span>
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<span class="p">}</span>
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<span class="kd">type</span> <span class="nx">writeOp</span> <span class="kd">struct</span> <span class="p">{</span>
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<span class="nx">key</span> <span class="kt">int</span>
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<span class="nx">val</span> <span class="kt">int</span>
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<span class="nx">resp</span> <span class="kd">chan</span> <span class="kt">bool</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>As before we’ll count how many operations we perform.</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> <span class="p">=</span> <span class="mi">0</span>
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<span class="kd">var</span> <span class="nx">writeOps</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>The <code>reads</code> and <code>writes</code> channels will be used by
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other goroutines to issue read and write requests,
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respectively.</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">reads</span> <span class="o">:=</span> <span class="nb">make</span><span class="p">(</span><span class="kd">chan</span> <span class="o">*</span><span class="nx">readOp</span><span class="p">)</span>
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<span class="nx">writes</span> <span class="o">:=</span> <span class="nb">make</span><span class="p">(</span><span class="kd">chan</span> <span class="o">*</span><span class="nx">writeOp</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>Here is the goroutine that owns the <code>state</code>, which
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is a map as in the previous example but now private
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to the stateful goroutine. This goroutine repeatedly
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selects on the <code>reads</code> and <code>writes</code> channels,
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responding to requests as they arrive. A response
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is executed by first performing the requested
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operation and then sending a value on the response
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channel <code>resp</code> to indicate success (and the desired
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value in the case of <code>reads</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="k">go</span> <span class="kd">func</span><span class="p">()</span> <span class="p">{</span>
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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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<span class="k">for</span> <span class="p">{</span>
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<span class="k">select</span> <span class="p">{</span>
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<span class="k">case</span> <span class="nx">read</span> <span class="o">:=</span> <span class="o"><-</span><span class="nx">reads</span><span class="p">:</span>
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<span class="nx">read</span><span class="p">.</span><span class="nx">resp</span> <span class="o"><-</span> <span class="nx">state</span><span class="p">[</span><span class="nx">read</span><span class="p">.</span><span class="nx">key</span><span class="p">]</span>
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<span class="k">case</span> <span class="nx">write</span> <span class="o">:=</span> <span class="o"><-</span><span class="nx">writes</span><span class="p">:</span>
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<span class="nx">state</span><span class="p">[</span><span class="nx">write</span><span class="p">.</span><span class="nx">key</span><span class="p">]</span> <span class="p">=</span> <span class="nx">write</span><span class="p">.</span><span class="nx">val</span>
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<span class="nx">write</span><span class="p">.</span><span class="nx">resp</span> <span class="o"><-</span> <span class="kc">true</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>This starts 100 goroutines to issue reads to the
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state-owning goroutine via the <code>reads</code> channel.
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Each read requires constructing a <code>readOp</code>, sending
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it over the <code>reads</code> channel, and the receiving the
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result over the provided <code>resp</code> channel.</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="k">for</span> <span class="p">{</span>
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<span class="nx">read</span> <span class="o">:=</span> <span class="o">&</span><span class="nx">readOp</span><span class="p">{</span>
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<span class="nx">key</span><span class="p">:</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">resp</span><span class="p">:</span> <span class="nb">make</span><span class="p">(</span><span class="kd">chan</span> <span class="kt">int</span><span class="p">)}</span>
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<span class="nx">reads</span> <span class="o"><-</span> <span class="nx">read</span>
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<span class="o"><-</span><span class="nx">read</span><span class="p">.</span><span class="nx">resp</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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<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 start 10 writes as well, using a similar
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approach.</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">write</span> <span class="o">:=</span> <span class="o">&</span><span class="nx">writeOp</span><span class="p">{</span>
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<span class="nx">key</span><span class="p">:</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="p">:</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">resp</span><span class="p">:</span> <span class="nb">make</span><span class="p">(</span><span class="kd">chan</span> <span class="kt">bool</span><span class="p">)}</span>
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<span class="nx">writes</span> <span class="o"><-</span> <span class="nx">write</span>
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<span class="o"><-</span><span class="nx">write</span><span class="p">.</span><span class="nx">resp</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 goroutines work 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>Finally, capture and report the op counts.</p>
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</td>
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<td class="code">
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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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<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 our program shows that the goroutine-based
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state management example completes about 80,000
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total 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 stateful-goroutines.go
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<span class="go">readOps: 71708</span>
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<span class="go">writeOps: 7177</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 this particular case the goroutine-based approach
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was a bit more involved than the mutex-based one. It
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might be useful in certain cases though, for example
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where you have other channels involved or when managing
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multiple such mutexes would be error-prone. You should
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use whichever approach feels most natural, especially
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with respect to understanding the correctness of your
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program.</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="sorting">Sorting</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/stateful-goroutines">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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