Merge pull request #270 from mmcgrana/atomics-det
Deterministic example for atomics.
This commit is contained in:
Mark McGranaghan
GitHub
commit
fe9f3ab396
@ -7,9 +7,11 @@
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package main
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import "fmt"
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import "time"
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import "sync/atomic"
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import (
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"fmt"
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"sync"
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"sync/atomic"
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)
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func main() {
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@ -17,33 +19,34 @@ func main() {
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// (always-positive) counter.
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var ops uint64
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// 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.
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// A WaitGroup will help us wait for all goroutines
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// to finish their work.
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var wg sync.WaitGroup
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// We'll start 50 goroutines that each increment the
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// counter exactly 1000 times.
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for i := 0; i < 50; i++ {
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wg.Add(1)
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go func() {
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for {
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for c := 0; c < 1000; c++ {
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// To atomically increment the counter we
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// use `AddUint64`, giving it the memory
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// address of our `ops` counter with the
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// `&` syntax.
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atomic.AddUint64(&ops, 1)
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// Wait a bit between increments.
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time.Sleep(time.Millisecond)
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}
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wg.Done()
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}()
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}
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// Wait a second to allow some ops to accumulate.
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time.Sleep(time.Second)
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// Wait until all the goroutines are done.
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wg.Wait()
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// 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 `opsFinal` via
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// `LoadUint64`. As above we need to give this
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// function the memory address `&ops` from which to
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// fetch the value.
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opsFinal := atomic.LoadUint64(&ops)
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fmt.Println("ops:", opsFinal)
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// It's safe to access `ops` 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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// `atomic.LoadUint64`.
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fmt.Println("ops:", ops)
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}
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@ -1,2 +1,2 @@
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a4190094ea0405b5f2733101beb97939a1d43aee
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KDr9EMMPMgi
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8ebec0be3b167021c96b8b497d0e8c0a2ea99385
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F2pJfduyQiA
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@ -1,7 +1,11 @@
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# Running the program shows that we executed about
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# 40,000 operations.
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# We expect to get exactly 50,000 operations. Had we
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# used the non-atomic `ops++` 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 `-race` flag.
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$ go run atomic-counters.go
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ops: 41419
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ops: 50000
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# Next we'll look at mutexes, another tool for managing
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# state.
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71
public/atomic-counters
generated
71
public/atomic-counters
generated
@ -46,7 +46,7 @@ counters</em> accessed by multiple goroutines.</p>
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</td>
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<td class="code leading">
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<a href="http://play.golang.org/p/KDr9EMMPMgi"><img title="Run code" src="play.png" class="run" /></a><img title="Copy code" src="clipboard.png" class="copy" />
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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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@ -59,9 +59,11 @@ counters</em> accessed by multiple goroutines.</p>
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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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<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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@ -95,16 +97,28 @@ counters</em> accessed by multiple goroutines.</p>
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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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<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="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">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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@ -120,7 +134,8 @@ address of our <code>ops</code> counter with the
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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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<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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@ -128,13 +143,13 @@ address of our <code>ops</code> counter with the
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<tr>
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<td class="docs">
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<p>Wait a bit between increments.</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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<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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@ -144,12 +159,12 @@ address of our <code>ops</code> counter with the
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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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<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">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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<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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@ -157,18 +172,16 @@ address of our <code>ops</code> counter with the
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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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<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">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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<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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@ -181,14 +194,18 @@ fetch the value.</p>
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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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<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: 41419</span>
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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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@ -219,7 +236,7 @@ state.</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 \"fmt\"\u000Aimport \"time\"\u000Aimport \"sync/atomic\"\u000A');codeLines.push('func main() {\u000A');codeLines.push(' var ops uint64\u000A');codeLines.push(' for i := 0; i \x3C 50; i++ {\u000A go func() {\u000A for {\u000A');codeLines.push(' atomic.AddUint64(&ops, 1)\u000A');codeLines.push(' time.Sleep(time.Millisecond)\u000A }\u000A }()\u000A }\u000A');codeLines.push(' time.Sleep(time.Second)\u000A');codeLines.push(' opsFinal := atomic.LoadUint64(&ops)\u000A fmt.Println(\"ops:\", opsFinal)\u000A}\u000A');codeLines.push('');codeLines.push('');
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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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<script src="site.js" async></script>
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</body>
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