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Deterministic example for atomics.

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Fixes #265
This commit is contained in:
Eli Bendersky
2019-09-05 10:50:09 -07:00
parent 13b0da17de
commit ef8f0e3831
4 changed files with 66 additions and 64 deletions
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39
examples/atomic-counters/atomic-counters.go
View File

@@ -7,9 +7,11 @@
package main
import "fmt"
import "time"
import "sync/atomic"
import (
"fmt"
"sync"
"sync/atomic"
)
func main() {
@@ -17,33 +19,28 @@ func main() {
// (always-positive) counter.
var ops uint64
// To simulate concurrent updates, we'll start 50
// goroutines that each increment the counter about
// once a millisecond.
// A WaitGroup will help us wait for all goroutines
// to finish their work.
var wg sync.WaitGroup
// We'll start 50 goroutines that each increment the
// counter exactly 1000 times.
for i := 0; i < 50; i++ {
wg.Add(1)
go func() {
for {
for c := 0; c < 1000; c++ {
// To atomically increment the counter we
// use `AddUint64`, giving it the memory
// address of our `ops` counter with the
// `&` syntax.
atomic.AddUint64(&ops, 1)
// Wait a bit between increments.
time.Sleep(time.Millisecond)
}
wg.Done()
}()
}
// Wait a second to allow some ops to accumulate.
time.Sleep(time.Second)
// In order to safely use the counter while it's still
// being updated by other goroutines, we extract a
// copy of the current value into `opsFinal` via
// `LoadUint64`. As above we need to give this
// function the memory address `&ops` from which to
// fetch the value.
opsFinal := atomic.LoadUint64(&ops)
fmt.Println("ops:", opsFinal)
// Wait until all the goroutines are done.
wg.Wait()
fmt.Println("ops:", ops)
}

4
examples/atomic-counters/atomic-counters.hash
View File

@@ -1,2 +1,2 @@
a4190094ea0405b5f2733101beb97939a1d43aee
KDr9EMMPMgi
103c9b7d036e3a5c14dc481755b78b10dc9f894e
GRkVf6J1--B

10
examples/atomic-counters/atomic-counters.sh
View File

@@ -1,7 +1,11 @@
# Running the program shows that we executed about
# 40,000 operations.
# We expect to get exactly 50,000 operations. Had we
# used the non-atomic `ops++` to increment the counter,
# we'd likely get a different number, changing between
# runs, because the goroutines would interfere with
# each other. Moreover, we'd get data race failures
# when running with the `-race` flag.
$ go run atomic-counters.go
ops: 41419
ops: 50000
# Next we'll look at mutexes, another tool for managing
# state.

77
public/atomic-counters generated
View File

@@ -46,7 +46,7 @@ counters</em> accessed by multiple goroutines.</p>
</td>
<td class="code leading">
<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" />
<a href="http://play.golang.org/p/GRkVf6J1--B"><img title="Run code" src="play.png" class="run" /></a><img title="Copy code" src="clipboard.png" class="copy" />
<div class="highlight"><pre><span class="kn">package</span> <span class="nx">main</span>
</pre></div>
@@ -59,9 +59,11 @@ counters</em> accessed by multiple goroutines.</p>
</td>
<td class="code leading">
<div class="highlight"><pre><span class="kn">import</span> <span class="s">&quot;fmt&quot;</span>
<span class="kn">import</span> <span class="s">&quot;time&quot;</span>
<span class="kn">import</span> <span class="s">&quot;sync/atomic&quot;</span>
<div class="highlight"><pre><span class="kn">import</span> <span class="p">(</span>
<span class="s">&quot;fmt&quot;</span>
<span class="s">&quot;sync&quot;</span>
<span class="s">&quot;sync/atomic&quot;</span>
<span class="p">)</span>
</pre></div>
</td>
@@ -95,16 +97,28 @@ counters</em> accessed by multiple goroutines.</p>
<tr>
<td class="docs">
<p>To simulate concurrent updates, we&rsquo;ll start 50
goroutines that each increment the counter about
once a millisecond.</p>
<p>A WaitGroup will help us wait for all goroutines
to finish their work.</p>
</td>
<td class="code leading">
<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>
</pre></div>
</td>
</tr>
<tr>
<td class="docs">
<p>We&rsquo;ll start 50 goroutines that each increment the
counter exactly 1000 times.</p>
</td>
<td class="code leading">
<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">&lt;</span> <span class="mi">50</span><span class="p">;</span> <span class="nx">i</span><span class="o">++</span> <span class="p">{</span>
<span class="k">go</span> <span class="kd">func</span><span class="p">()</span> <span class="p">{</span>
<span class="k">for</span> <span class="p">{</span>
<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>
</pre></div>
</td>
@@ -120,7 +134,8 @@ address of our <code>ops</code> counter with the
</td>
<td class="code leading">
<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">&amp;</span><span class="nx">ops</span><span class="p">,</span> <span class="mi">1</span><span class="p">)</span>
<div class="highlight"><pre> <span class="k">go</span> <span class="kd">func</span><span class="p">()</span> <span class="p">{</span>
<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">&lt;</span> <span class="mi">1000</span><span class="p">;</span> <span class="nx">c</span><span class="o">++</span> <span class="p">{</span>
</pre></div>
</td>
@@ -128,13 +143,13 @@ address of our <code>ops</code> counter with the
<tr>
<td class="docs">
<p>Wait a bit between increments.</p>
</td>
<td class="code leading">
<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>
<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">&amp;</span><span class="nx">ops</span><span class="p">,</span> <span class="mi">1</span><span class="p">)</span>
<span class="p">}</span>
<span class="nx">wg</span><span class="p">.</span><span class="nx">Done</span><span class="p">()</span>
<span class="p">}()</span>
<span class="p">}</span>
</pre></div>
@@ -144,31 +159,13 @@ address of our <code>ops</code> counter with the
<tr>
<td class="docs">
<p>Wait a second to allow some ops to accumulate.</p>
</td>
<td class="code leading">
<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>
</pre></div>
</td>
</tr>
<tr>
<td class="docs">
<p>In order to safely use the counter while it&rsquo;s still
being updated by other goroutines, we extract a
copy of the current value into <code>opsFinal</code> via
<code>LoadUint64</code>. As above we need to give this
function the memory address <code>&amp;ops</code> from which to
fetch the value.</p>
<p>Wait until all the goroutines are done.</p>
</td>
<td class="code">
<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">&amp;</span><span class="nx">ops</span><span class="p">)</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">&quot;ops:&quot;</span><span class="p">,</span> <span class="nx">opsFinal</span><span class="p">)</span>
<div class="highlight"><pre> <span class="nx">wg</span><span class="p">.</span><span class="nx">Wait</span><span class="p">()</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">&quot;ops:&quot;</span><span class="p">,</span> <span class="nx">ops</span><span class="p">)</span>
<span class="p">}</span>
</pre></div>
@@ -181,14 +178,18 @@ fetch the value.</p>
<tr>
<td class="docs">
<p>Running the program shows that we executed about
40,000 operations.</p>
<p>We expect to get exactly 50,000 operations. Had we
used the non-atomic <code>ops++</code> to increment the counter,
we&rsquo;d likely get a different number, changing between
runs, because the goroutines would interfere with
each other. Moreover, we&rsquo;d get data race failures
when running with the <code>-race</code> flag.</p>
</td>
<td class="code leading">
<div class="highlight"><pre><span class="gp">$</span> go run atomic-counters.go
<span class="go">ops: 41419</span>
<span class="go">ops: 50000</span>
</pre></div>
</td>
@@ -219,7 +220,7 @@ state.</p>
</div>
<script>
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