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type Map struct { mu Mutex // 排他锁，用于对dirty map操作时候加锁处理 read atomic.Value // read map // dirty map。新增key时候，只写入dirty map中，需要使用mu dirty map[interface{}]*entry // 用来记录从read map中读取key时miss的次数 misses int } sync.Map结构体中read字段是atomic.Value类型，底层是readOnly结构体：
type readOnly struct { m map[interface{}]*entry amended bool // 当amended为true时候，表示sync.Map中的key也存在dirty map中 } read map和dirty map的value类型是*entry， entry结构体定义如下：
// expunged用来标记从dirty map删除掉了 var expunged = unsafe.Pointer(new(interface{})) type entry struct { // 如果p == nil 说明对应的entry已经被删除掉了， 且m.dirty == nil // 如果 p == expunged 说明对应的entry已经被删除了，但m.">
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<meta property="og:description" content="并发Map - sync.Map # 源码分析 # sync.Map的结构：
type Map struct { mu Mutex // 排他锁，用于对dirty map操作时候加锁处理 read atomic.Value // read map // dirty map。新增key时候，只写入dirty map中，需要使用mu dirty map[interface{}]*entry // 用来记录从read map中读取key时miss的次数 misses int } sync.Map结构体中read字段是atomic.Value类型，底层是readOnly结构体：
type readOnly struct { m map[interface{}]*entry amended bool // 当amended为true时候，表示sync.Map中的key也存在dirty map中 } read map和dirty map的value类型是*entry， entry结构体定义如下：
// expunged用来标记从dirty map删除掉了 var expunged = unsafe.Pointer(new(interface{})) type entry struct { // 如果p == nil 说明对应的entry已经被删除掉了， 且m.dirty == nil // 如果 p == expunged 说明对应的entry已经被删除了，但m." />
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<strong>并发Map - sync.Map</strong>
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<article class="markdown"><h2 id="并发map---syncmap">
并发Map - sync.Map
<a class="anchor" href="#%e5%b9%b6%e5%8f%91map---syncmap">#</a>
</h2>
<h3 id="源码分析">
源码分析
<a class="anchor" href="#%e6%ba%90%e7%a0%81%e5%88%86%e6%9e%90">#</a>
</h3>
<p><strong>sync.Map的结构：</strong></p>
<div class="highlight"><pre tabindex="0" style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-go" data-lang="go"><span style="display:flex;"><span><span style="color:#66d9ef">type</span> <span style="color:#a6e22e">Map</span> <span style="color:#66d9ef">struct</span> {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">mu</span> <span style="color:#a6e22e">Mutex</span> <span style="color:#75715e">// 排他锁，用于对dirty map操作时候加锁处理
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span>
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">read</span> <span style="color:#a6e22e">atomic</span>.<span style="color:#a6e22e">Value</span> <span style="color:#75715e">// read map
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span>
</span></span><span style="display:flex;"><span> <span style="color:#75715e">// dirty map。新增key时候，只写入dirty map中，需要使用mu
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#a6e22e">dirty</span> <span style="color:#66d9ef">map</span>[<span style="color:#66d9ef">interface</span>{}]<span style="color:#f92672">*</span><span style="color:#a6e22e">entry</span>
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span> <span style="color:#75715e">// 用来记录从read map中读取key时miss的次数
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#a6e22e">misses</span> <span style="color:#66d9ef">int</span>
</span></span><span style="display:flex;"><span>}
</span></span></code></pre></div><p>sync.Map结构体中read字段是<code>atomic.Value</code>类型，底层是<strong>readOnly结构体</strong>：</p>
<div class="highlight"><pre tabindex="0" style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-go" data-lang="go"><span style="display:flex;"><span><span style="color:#66d9ef">type</span> <span style="color:#a6e22e">readOnly</span> <span style="color:#66d9ef">struct</span> {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">m</span> <span style="color:#66d9ef">map</span>[<span style="color:#66d9ef">interface</span>{}]<span style="color:#f92672">*</span><span style="color:#a6e22e">entry</span>
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">amended</span> <span style="color:#66d9ef">bool</span> <span style="color:#75715e">// 当amended为true时候，表示sync.Map中的key也存在dirty map中
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span>}
</span></span></code></pre></div><p>read map和dirty map的value类型是*entry， entry结构体定义如下：</p>
<div class="highlight"><pre tabindex="0" style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-go" data-lang="go"><span style="display:flex;"><span><span style="color:#75715e">// expunged用来标记从dirty map删除掉了
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span><span style="color:#66d9ef">var</span> <span style="color:#a6e22e">expunged</span> = <span style="color:#a6e22e">unsafe</span>.<span style="color:#a6e22e">Pointer</span>(new(<span style="color:#66d9ef">interface</span>{}))
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span><span style="color:#66d9ef">type</span> <span style="color:#a6e22e">entry</span> <span style="color:#66d9ef">struct</span> {
</span></span><span style="display:flex;"><span> <span style="color:#75715e">// 如果p == nil 说明对应的entry已经被删除掉了， 且m.dirty == nil
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span>
</span></span><span style="display:flex;"><span> <span style="color:#75715e">// 如果 p == expunged 说明对应的entry已经被删除了，但m.dirty != nil，且该entry不存在m.dirty中
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span>
</span></span><span style="display:flex;"><span> <span style="color:#75715e">// 上述两种情况外，entry则是合法的值并且在m.read.m[key]中存在
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#75715e">// 如果m.dirty != nil，entry也会在m.dirty[key]中
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span>
</span></span><span style="display:flex;"><span> <span style="color:#75715e">// p指针指向sync.Map中key对应的Value
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#a6e22e">p</span> <span style="color:#a6e22e">unsafe</span>.<span style="color:#a6e22e">Pointer</span> <span style="color:#75715e">// *interface{}
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span>}
</span></span></code></pre></div><p>对Map的操作可以分为四类：</p>
<ol>
<li>Add key-value 新增key-value</li>
<li>Update key-value 更新key对应的value值</li>
<li>Get Key-value 获取Key对应的Value值</li>
<li>Delete Key 删除key</li>
</ol>
<p>我们来看看新增和更新操作：</p>
<div class="highlight"><pre tabindex="0" style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-go" data-lang="go"><span style="display:flex;"><span><span style="color:#75715e">// Store用来新增和更新操作
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span><span style="color:#66d9ef">func</span> (<span style="color:#a6e22e">m</span> <span style="color:#f92672">*</span><span style="color:#a6e22e">Map</span>) <span style="color:#a6e22e">Store</span>(<span style="color:#a6e22e">key</span>, <span style="color:#a6e22e">value</span> <span style="color:#66d9ef">interface</span>{}) {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">read</span>, <span style="color:#a6e22e">_</span> <span style="color:#f92672">:=</span> <span style="color:#a6e22e">m</span>.<span style="color:#a6e22e">read</span>.<span style="color:#a6e22e">Load</span>().(<span style="color:#a6e22e">readOnly</span>)
</span></span><span style="display:flex;"><span> <span style="color:#75715e">// 如果read map存在该key，且该key对应的value不是expunged时(准确的说key对应的value, value是*entry类型，entry的p字段指向不是expunged时)，
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#75715e">// 则使用cas更新value，此操作是原子性的
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#66d9ef">if</span> <span style="color:#a6e22e">e</span>, <span style="color:#a6e22e">ok</span> <span style="color:#f92672">:=</span> <span style="color:#a6e22e">read</span>.<span style="color:#a6e22e">m</span>[<span style="color:#a6e22e">key</span>]; <span style="color:#a6e22e">ok</span> <span style="color:#f92672">&amp;&amp;</span> <span style="color:#a6e22e">e</span>.<span style="color:#a6e22e">tryStore</span>(<span style="color:#f92672">&amp;</span><span style="color:#a6e22e">value</span>) {
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">return</span>
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">m</span>.<span style="color:#a6e22e">mu</span>.<span style="color:#a6e22e">Lock</span>() <span style="color:#75715e">// 先加锁，然后重新读取一次read map，目的是防止dirty map升级到read map（并发Load操作时候），read map更改了。
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#a6e22e">read</span>, <span style="color:#a6e22e">_</span> = <span style="color:#a6e22e">m</span>.<span style="color:#a6e22e">read</span>.<span style="color:#a6e22e">Load</span>().(<span style="color:#a6e22e">readOnly</span>)
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">if</span> <span style="color:#a6e22e">e</span>, <span style="color:#a6e22e">ok</span> <span style="color:#f92672">:=</span> <span style="color:#a6e22e">read</span>.<span style="color:#a6e22e">m</span>[<span style="color:#a6e22e">key</span>]; <span style="color:#a6e22e">ok</span> { <span style="color:#75715e">// 若read map存在此key，此时就是map的更新操作
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#66d9ef">if</span> <span style="color:#a6e22e">e</span>.<span style="color:#a6e22e">unexpungeLocked</span>() { <span style="color:#75715e">// 将value由expunged更改成nil，
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#75715e">// 若成功则表明dirty map中不存在此key，把key-value添加到dirty map中
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#a6e22e">m</span>.<span style="color:#a6e22e">dirty</span>[<span style="color:#a6e22e">key</span>] = <span style="color:#a6e22e">e</span>
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">e</span>.<span style="color:#a6e22e">storeLocked</span>(<span style="color:#f92672">&amp;</span><span style="color:#a6e22e">value</span>) <span style="color:#75715e">// 更改value。value是指针类型(*entry)，read map和dirty map的value都指向该值。
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> } <span style="color:#66d9ef">else</span> <span style="color:#66d9ef">if</span> <span style="color:#a6e22e">e</span>, <span style="color:#a6e22e">ok</span> <span style="color:#f92672">:=</span> <span style="color:#a6e22e">m</span>.<span style="color:#a6e22e">dirty</span>[<span style="color:#a6e22e">key</span>]; <span style="color:#a6e22e">ok</span> {<span style="color:#75715e">// 若dirty map存在该key，则直接更改value
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#a6e22e">e</span>.<span style="color:#a6e22e">storeLocked</span>(<span style="color:#f92672">&amp;</span><span style="color:#a6e22e">value</span>)
</span></span><span style="display:flex;"><span> } <span style="color:#66d9ef">else</span> { <span style="color:#75715e">// 若read map和dirty map中都不存在该key，其实就是map的新增key-value操作
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#66d9ef">if</span> !<span style="color:#a6e22e">read</span>.<span style="color:#a6e22e">amended</span> {<span style="color:#75715e">// amended为true时表示sync.Map部分key存在dirty map中
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#75715e">// dirtyLocked()做两件事情：
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#75715e">// 1. 若dirty map等于nil，则初始化dirty map。
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#75715e">// 2. 遍历read map，将read map中的key-value复制到dirty map中，从read map中复制的key-value时，value是nil或expunged的(因为nil和expunged是key删除了的）不进行复制。
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#75715e">// 同时若value值为nil，则顺便更改成expunged（用来标记dirty map不包含此key)
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span>
</span></span><span style="display:flex;"><span> <span style="color:#75715e">// 思考🤔：为啥dirtyLocked()要干事情2，即将read map的key-value复制到dirty map中？
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#a6e22e">m</span>.<span style="color:#a6e22e">dirtyLocked</span>()
</span></span><span style="display:flex;"><span> <span style="color:#75715e">// 该新增key-value将添加dirty map中，所以将read map的amended设置为true。当amended为true时候，从sync.Map读取key时候，优先从read map中读取，若read map读取时候不到时候，会从dirty map中读取
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#a6e22e">m</span>.<span style="color:#a6e22e">read</span>.<span style="color:#a6e22e">Store</span>(<span style="color:#a6e22e">readOnly</span>{<span style="color:#a6e22e">m</span>: <span style="color:#a6e22e">read</span>.<span style="color:#a6e22e">m</span>, <span style="color:#a6e22e">amended</span>: <span style="color:#66d9ef">true</span>})
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span> <span style="color:#75715e">// 添加key-value到dirty map中
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#a6e22e">m</span>.<span style="color:#a6e22e">dirty</span>[<span style="color:#a6e22e">key</span>] = <span style="color:#a6e22e">newEntry</span>(<span style="color:#a6e22e">value</span>)
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span> <span style="color:#75715e">// 释放锁
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#a6e22e">m</span>.<span style="color:#a6e22e">mu</span>.<span style="color:#a6e22e">Unlock</span>()
</span></span><span style="display:flex;"><span>}
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span><span style="color:#66d9ef">func</span> (<span style="color:#a6e22e">e</span> <span style="color:#f92672">*</span><span style="color:#a6e22e">entry</span>) <span style="color:#a6e22e">tryStore</span>(<span style="color:#a6e22e">i</span> <span style="color:#f92672">*</span><span style="color:#66d9ef">interface</span>{}) <span style="color:#66d9ef">bool</span> {
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">for</span> {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">p</span> <span style="color:#f92672">:=</span> <span style="color:#a6e22e">atomic</span>.<span style="color:#a6e22e">LoadPointer</span>(<span style="color:#f92672">&amp;</span><span style="color:#a6e22e">e</span>.<span style="color:#a6e22e">p</span>)
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">if</span> <span style="color:#a6e22e">p</span> <span style="color:#f92672">==</span> <span style="color:#a6e22e">expunged</span> {
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">return</span> <span style="color:#66d9ef">false</span>
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">if</span> <span style="color:#a6e22e">atomic</span>.<span style="color:#a6e22e">CompareAndSwapPointer</span>(<span style="color:#f92672">&amp;</span><span style="color:#a6e22e">e</span>.<span style="color:#a6e22e">p</span>, <span style="color:#a6e22e">p</span>, <span style="color:#a6e22e">unsafe</span>.<span style="color:#a6e22e">Pointer</span>(<span style="color:#a6e22e">i</span>)) {
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">return</span> <span style="color:#66d9ef">true</span>
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span>}
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span><span style="color:#66d9ef">func</span> (<span style="color:#a6e22e">e</span> <span style="color:#f92672">*</span><span style="color:#a6e22e">entry</span>) <span style="color:#a6e22e">unexpungeLocked</span>() (<span style="color:#a6e22e">wasExpunged</span> <span style="color:#66d9ef">bool</span>) {
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">return</span> <span style="color:#a6e22e">atomic</span>.<span style="color:#a6e22e">CompareAndSwapPointer</span>(<span style="color:#f92672">&amp;</span><span style="color:#a6e22e">e</span>.<span style="color:#a6e22e">p</span>, <span style="color:#a6e22e">expunged</span>, <span style="color:#66d9ef">nil</span>)
</span></span><span style="display:flex;"><span>}
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span><span style="color:#66d9ef">func</span> (<span style="color:#a6e22e">e</span> <span style="color:#f92672">*</span><span style="color:#a6e22e">entry</span>) <span style="color:#a6e22e">storeLocked</span>(<span style="color:#a6e22e">i</span> <span style="color:#f92672">*</span><span style="color:#66d9ef">interface</span>{}) {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">atomic</span>.<span style="color:#a6e22e">StorePointer</span>(<span style="color:#f92672">&amp;</span><span style="color:#a6e22e">e</span>.<span style="color:#a6e22e">p</span>, <span style="color:#a6e22e">unsafe</span>.<span style="color:#a6e22e">Pointer</span>(<span style="color:#a6e22e">i</span>))
</span></span><span style="display:flex;"><span>}
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span><span style="color:#66d9ef">func</span> (<span style="color:#a6e22e">m</span> <span style="color:#f92672">*</span><span style="color:#a6e22e">Map</span>) <span style="color:#a6e22e">dirtyLocked</span>() {
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">if</span> <span style="color:#a6e22e">m</span>.<span style="color:#a6e22e">dirty</span> <span style="color:#f92672">!=</span> <span style="color:#66d9ef">nil</span> {
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">return</span>
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">read</span>, <span style="color:#a6e22e">_</span> <span style="color:#f92672">:=</span> <span style="color:#a6e22e">m</span>.<span style="color:#a6e22e">read</span>.<span style="color:#a6e22e">Load</span>().(<span style="color:#a6e22e">readOnly</span>)
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">m</span>.<span style="color:#a6e22e">dirty</span> = make(<span style="color:#66d9ef">map</span>[<span style="color:#66d9ef">interface</span>{}]<span style="color:#f92672">*</span><span style="color:#a6e22e">entry</span>, len(<span style="color:#a6e22e">read</span>.<span style="color:#a6e22e">m</span>))
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">for</span> <span style="color:#a6e22e">k</span>, <span style="color:#a6e22e">e</span> <span style="color:#f92672">:=</span> <span style="color:#66d9ef">range</span> <span style="color:#a6e22e">read</span>.<span style="color:#a6e22e">m</span> {
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">if</span> !<span style="color:#a6e22e">e</span>.<span style="color:#a6e22e">tryExpungeLocked</span>() {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">m</span>.<span style="color:#a6e22e">dirty</span>[<span style="color:#a6e22e">k</span>] = <span style="color:#a6e22e">e</span>
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span>}
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span><span style="color:#66d9ef">func</span> (<span style="color:#a6e22e">e</span> <span style="color:#f92672">*</span><span style="color:#a6e22e">entry</span>) <span style="color:#a6e22e">tryExpungeLocked</span>() (<span style="color:#a6e22e">isExpunged</span> <span style="color:#66d9ef">bool</span>) {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">p</span> <span style="color:#f92672">:=</span> <span style="color:#a6e22e">atomic</span>.<span style="color:#a6e22e">LoadPointer</span>(<span style="color:#f92672">&amp;</span><span style="color:#a6e22e">e</span>.<span style="color:#a6e22e">p</span>)
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">for</span> <span style="color:#a6e22e">p</span> <span style="color:#f92672">==</span> <span style="color:#66d9ef">nil</span> {
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">if</span> <span style="color:#a6e22e">atomic</span>.<span style="color:#a6e22e">CompareAndSwapPointer</span>(<span style="color:#f92672">&amp;</span><span style="color:#a6e22e">e</span>.<span style="color:#a6e22e">p</span>, <span style="color:#66d9ef">nil</span>, <span style="color:#a6e22e">expunged</span>) {
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">return</span> <span style="color:#66d9ef">true</span>
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">p</span> = <span style="color:#a6e22e">atomic</span>.<span style="color:#a6e22e">LoadPointer</span>(<span style="color:#f92672">&amp;</span><span style="color:#a6e22e">e</span>.<span style="color:#a6e22e">p</span>)
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">return</span> <span style="color:#a6e22e">p</span> <span style="color:#f92672">==</span> <span style="color:#a6e22e">expunged</span>
</span></span><span style="display:flex;"><span>}
</span></span></code></pre></div><p>接下来看看Map的Get操作：</p>
<div class="highlight"><pre tabindex="0" style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-go" data-lang="go"><span style="display:flex;"><span><span style="color:#75715e">// Load方法用来获取key对应的value值，返回的ok表名key是否存在sync.Map中
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span><span style="color:#66d9ef">func</span> (<span style="color:#a6e22e">m</span> <span style="color:#f92672">*</span><span style="color:#a6e22e">Map</span>) <span style="color:#a6e22e">Load</span>(<span style="color:#a6e22e">key</span> <span style="color:#66d9ef">interface</span>{}) (<span style="color:#a6e22e">value</span> <span style="color:#66d9ef">interface</span>{}, <span style="color:#a6e22e">ok</span> <span style="color:#66d9ef">bool</span>) {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">read</span>, <span style="color:#a6e22e">_</span> <span style="color:#f92672">:=</span> <span style="color:#a6e22e">m</span>.<span style="color:#a6e22e">read</span>.<span style="color:#a6e22e">Load</span>().(<span style="color:#a6e22e">readOnly</span>)
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">e</span>, <span style="color:#a6e22e">ok</span> <span style="color:#f92672">:=</span> <span style="color:#a6e22e">read</span>.<span style="color:#a6e22e">m</span>[<span style="color:#a6e22e">key</span>]
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">if</span> !<span style="color:#a6e22e">ok</span> <span style="color:#f92672">&amp;&amp;</span> <span style="color:#a6e22e">read</span>.<span style="color:#a6e22e">amended</span> { <span style="color:#75715e">// 若key不存在read map中，且dirty map包含sync.Map中key情况下
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#a6e22e">m</span>.<span style="color:#a6e22e">mu</span>.<span style="color:#a6e22e">Lock</span>() <span style="color:#75715e">// 加锁
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#a6e22e">read</span>, <span style="color:#a6e22e">_</span> = <span style="color:#a6e22e">m</span>.<span style="color:#a6e22e">read</span>.<span style="color:#a6e22e">Load</span>().(<span style="color:#a6e22e">readOnly</span>) <span style="color:#75715e">// 再次从read map读取key
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#a6e22e">e</span>, <span style="color:#a6e22e">ok</span> = <span style="color:#a6e22e">read</span>.<span style="color:#a6e22e">m</span>[<span style="color:#a6e22e">key</span>]
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">if</span> !<span style="color:#a6e22e">ok</span> <span style="color:#f92672">&amp;&amp;</span> <span style="color:#a6e22e">read</span>.<span style="color:#a6e22e">amended</span> {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">e</span>, <span style="color:#a6e22e">ok</span> = <span style="color:#a6e22e">m</span>.<span style="color:#a6e22e">dirty</span>[<span style="color:#a6e22e">key</span>] <span style="color:#75715e">// 从dirty map中读取key
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#75715e">// missLocked() 首先将misses计数加1，misses用来表明read map读取key没有命中的次数。
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#75715e">// 若misses次数多于dirty map中元素个数时候，则将dirty map升级为read map，dirty map设置为nil, amended置为false
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#a6e22e">m</span>.<span style="color:#a6e22e">missLocked</span>()
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">m</span>.<span style="color:#a6e22e">mu</span>.<span style="color:#a6e22e">Unlock</span>()
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">if</span> !<span style="color:#a6e22e">ok</span> { <span style="color:#75715e">// read map 和 dirty map中都不存在该key
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#66d9ef">return</span> <span style="color:#66d9ef">nil</span>, <span style="color:#66d9ef">false</span>
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span> <span style="color:#75715e">// 加载value值
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#66d9ef">return</span> <span style="color:#a6e22e">e</span>.<span style="color:#a6e22e">load</span>()
</span></span><span style="display:flex;"><span>}
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span><span style="color:#66d9ef">func</span> (<span style="color:#a6e22e">e</span> <span style="color:#f92672">*</span><span style="color:#a6e22e">entry</span>) <span style="color:#a6e22e">load</span>() (<span style="color:#a6e22e">value</span> <span style="color:#66d9ef">interface</span>{}, <span style="color:#a6e22e">ok</span> <span style="color:#66d9ef">bool</span>) {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">p</span> <span style="color:#f92672">:=</span> <span style="color:#a6e22e">atomic</span>.<span style="color:#a6e22e">LoadPointer</span>(<span style="color:#f92672">&amp;</span><span style="color:#a6e22e">e</span>.<span style="color:#a6e22e">p</span>)
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">if</span> <span style="color:#a6e22e">p</span> <span style="color:#f92672">==</span> <span style="color:#66d9ef">nil</span> <span style="color:#f92672">||</span> <span style="color:#a6e22e">p</span> <span style="color:#f92672">==</span> <span style="color:#a6e22e">expunged</span> { <span style="color:#75715e">// 若value值是nil或expunged，返回nil, false，表示key不存在
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#66d9ef">return</span> <span style="color:#66d9ef">nil</span>, <span style="color:#66d9ef">false</span>
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">return</span> <span style="color:#f92672">*</span>(<span style="color:#f92672">*</span><span style="color:#66d9ef">interface</span>{})(<span style="color:#a6e22e">p</span>), <span style="color:#66d9ef">true</span>
</span></span><span style="display:flex;"><span>}
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span><span style="color:#66d9ef">func</span> (<span style="color:#a6e22e">m</span> <span style="color:#f92672">*</span><span style="color:#a6e22e">Map</span>) <span style="color:#a6e22e">missLocked</span>() {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">m</span>.<span style="color:#a6e22e">misses</span><span style="color:#f92672">++</span>
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">if</span> <span style="color:#a6e22e">m</span>.<span style="color:#a6e22e">misses</span> &lt; len(<span style="color:#a6e22e">m</span>.<span style="color:#a6e22e">dirty</span>) {
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">return</span>
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span> <span style="color:#75715e">// 新创建一个readOnly对象，其中amended为false, 并将m.dirty直接赋值给该对象的m字段，
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#75715e">// 这也是上面思考中的dirtyLocked为什么要干事情2的原因，因为通过2操作之后，m.dirty已包含read map中的所有key，可以直接拿来创建readOnly。
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#a6e22e">m</span>.<span style="color:#a6e22e">read</span>.<span style="color:#a6e22e">Store</span>(<span style="color:#a6e22e">readOnly</span>{<span style="color:#a6e22e">m</span>: <span style="color:#a6e22e">m</span>.<span style="color:#a6e22e">dirty</span>})
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">m</span>.<span style="color:#a6e22e">dirty</span> = <span style="color:#66d9ef">nil</span>
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">m</span>.<span style="color:#a6e22e">misses</span> = <span style="color:#ae81ff">0</span>
</span></span><span style="display:flex;"><span>}
</span></span></code></pre></div><p>在接着看看Map的删除操作：</p>
<div class="highlight"><pre tabindex="0" style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-go" data-lang="go"><span style="display:flex;"><span><span style="color:#75715e">// Delete用于删除key
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span><span style="color:#66d9ef">func</span> (<span style="color:#a6e22e">m</span> <span style="color:#f92672">*</span><span style="color:#a6e22e">Map</span>) <span style="color:#a6e22e">Delete</span>(<span style="color:#a6e22e">key</span> <span style="color:#66d9ef">interface</span>{}) {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">read</span>, <span style="color:#a6e22e">_</span> <span style="color:#f92672">:=</span> <span style="color:#a6e22e">m</span>.<span style="color:#a6e22e">read</span>.<span style="color:#a6e22e">Load</span>().(<span style="color:#a6e22e">readOnly</span>)
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">e</span>, <span style="color:#a6e22e">ok</span> <span style="color:#f92672">:=</span> <span style="color:#a6e22e">read</span>.<span style="color:#a6e22e">m</span>[<span style="color:#a6e22e">key</span>]
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">if</span> !<span style="color:#a6e22e">ok</span> <span style="color:#f92672">&amp;&amp;</span> <span style="color:#a6e22e">read</span>.<span style="color:#a6e22e">amended</span> {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">m</span>.<span style="color:#a6e22e">mu</span>.<span style="color:#a6e22e">Lock</span>()
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">read</span>, <span style="color:#a6e22e">_</span> = <span style="color:#a6e22e">m</span>.<span style="color:#a6e22e">read</span>.<span style="color:#a6e22e">Load</span>().(<span style="color:#a6e22e">readOnly</span>)
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">e</span>, <span style="color:#a6e22e">ok</span> = <span style="color:#a6e22e">read</span>.<span style="color:#a6e22e">m</span>[<span style="color:#a6e22e">key</span>]
</span></span><span style="display:flex;"><span> <span style="color:#75715e">// 若read map不存在该key，但dirty map中存在该key。则直接调用delete，删除dirty map中该key
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#66d9ef">if</span> !<span style="color:#a6e22e">ok</span> <span style="color:#f92672">&amp;&amp;</span> <span style="color:#a6e22e">read</span>.<span style="color:#a6e22e">amended</span> {
</span></span><span style="display:flex;"><span> delete(<span style="color:#a6e22e">m</span>.<span style="color:#a6e22e">dirty</span>, <span style="color:#a6e22e">key</span>)
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">m</span>.<span style="color:#a6e22e">mu</span>.<span style="color:#a6e22e">Unlock</span>()
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">if</span> <span style="color:#a6e22e">ok</span> {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">e</span>.delete()
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span>}
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span><span style="color:#66d9ef">func</span> (<span style="color:#a6e22e">e</span> <span style="color:#f92672">*</span><span style="color:#a6e22e">entry</span>) delete() (<span style="color:#a6e22e">hadValue</span> <span style="color:#66d9ef">bool</span>) {
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">for</span> {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">p</span> <span style="color:#f92672">:=</span> <span style="color:#a6e22e">atomic</span>.<span style="color:#a6e22e">LoadPointer</span>(<span style="color:#f92672">&amp;</span><span style="color:#a6e22e">e</span>.<span style="color:#a6e22e">p</span>)
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">if</span> <span style="color:#a6e22e">p</span> <span style="color:#f92672">==</span> <span style="color:#66d9ef">nil</span> <span style="color:#f92672">||</span> <span style="color:#a6e22e">p</span> <span style="color:#f92672">==</span> <span style="color:#a6e22e">expunged</span> { <span style="color:#75715e">// 若entry中p已经是nil或者expunged则直接返回
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#66d9ef">return</span> <span style="color:#66d9ef">false</span>
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">if</span> <span style="color:#a6e22e">atomic</span>.<span style="color:#a6e22e">CompareAndSwapPointer</span>(<span style="color:#f92672">&amp;</span><span style="color:#a6e22e">e</span>.<span style="color:#a6e22e">p</span>, <span style="color:#a6e22e">p</span>, <span style="color:#66d9ef">nil</span>) { <span style="color:#75715e">// 将entry中的p设置为nil
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#66d9ef">return</span> <span style="color:#66d9ef">true</span>
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span>}
</span></span></code></pre></div><p>sync.Map还提供遍历key-value功能：</p>
<div class="highlight"><pre tabindex="0" style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-go" data-lang="go"><span style="display:flex;"><span><span style="color:#75715e">// Range方法接受一个迭代回调函数，用来处理遍历的key和value
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span><span style="color:#66d9ef">func</span> (<span style="color:#a6e22e">m</span> <span style="color:#f92672">*</span><span style="color:#a6e22e">Map</span>) <span style="color:#a6e22e">Range</span>(<span style="color:#a6e22e">f</span> <span style="color:#66d9ef">func</span>(<span style="color:#a6e22e">key</span>, <span style="color:#a6e22e">value</span> <span style="color:#66d9ef">interface</span>{}) <span style="color:#66d9ef">bool</span>) {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">read</span>, <span style="color:#a6e22e">_</span> <span style="color:#f92672">:=</span> <span style="color:#a6e22e">m</span>.<span style="color:#a6e22e">read</span>.<span style="color:#a6e22e">Load</span>().(<span style="color:#a6e22e">readOnly</span>)
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">if</span> <span style="color:#a6e22e">read</span>.<span style="color:#a6e22e">amended</span> { <span style="color:#75715e">// 若dirty map中包含sync.Map中key时候
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#a6e22e">m</span>.<span style="color:#a6e22e">mu</span>.<span style="color:#a6e22e">Lock</span>()
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">read</span>, <span style="color:#a6e22e">_</span> = <span style="color:#a6e22e">m</span>.<span style="color:#a6e22e">read</span>.<span style="color:#a6e22e">Load</span>().(<span style="color:#a6e22e">readOnly</span>)
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">if</span> <span style="color:#a6e22e">read</span>.<span style="color:#a6e22e">amended</span> {<span style="color:#75715e">// 加锁之后，再次判断，是为了防止并发调用Load方法时候，dirty map升级为read map时候，amended为false情况
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#75715e">// read.amended为true的时候，m.dirty包含sync.Map中所有的key
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#a6e22e">read</span> = <span style="color:#a6e22e">readOnly</span>{<span style="color:#a6e22e">m</span>: <span style="color:#a6e22e">m</span>.<span style="color:#a6e22e">dirty</span>}
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">m</span>.<span style="color:#a6e22e">read</span>.<span style="color:#a6e22e">Store</span>(<span style="color:#a6e22e">read</span>)
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">m</span>.<span style="color:#a6e22e">dirty</span> = <span style="color:#66d9ef">nil</span>
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">m</span>.<span style="color:#a6e22e">misses</span> = <span style="color:#ae81ff">0</span>
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">m</span>.<span style="color:#a6e22e">mu</span>.<span style="color:#a6e22e">Unlock</span>()
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">for</span> <span style="color:#a6e22e">k</span>, <span style="color:#a6e22e">e</span> <span style="color:#f92672">:=</span> <span style="color:#66d9ef">range</span> <span style="color:#a6e22e">read</span>.<span style="color:#a6e22e">m</span> {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">v</span>, <span style="color:#a6e22e">ok</span> <span style="color:#f92672">:=</span> <span style="color:#a6e22e">e</span>.<span style="color:#a6e22e">load</span>()
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">if</span> !<span style="color:#a6e22e">ok</span> {
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">continue</span>
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">if</span> !<span style="color:#a6e22e">f</span>(<span style="color:#a6e22e">k</span>, <span style="color:#a6e22e">v</span>) { <span style="color:#75715e">//执行迭代回调函数，当返回false时候，停止迭代
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> <span style="color:#66d9ef">break</span>
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span>}
</span></span></code></pre></div><h3 id="为什么不使用syncmutexmap实现并发的map呢">
为什么不使用sync.Mutex+map实现并发的map呢？
<a class="anchor" href="#%e4%b8%ba%e4%bb%80%e4%b9%88%e4%b8%8d%e4%bd%bf%e7%94%a8syncmutexmap%e5%ae%9e%e7%8e%b0%e5%b9%b6%e5%8f%91%e7%9a%84map%e5%91%a2">#</a>
</h3>
<p>这个问题可以换个问法就是sync.Map相比sync.Mutex+map实现并发map有哪些优势？</p>
<p>sync.Map优势在于当key存在read map时候，如果进行Store操作，可以使用原子性操作更新，而sync.Mutex+map形式每次写操作都要加锁，这个成本更高。</p>
<p>另外并发读写两个不同的key时候，写操作需要加锁，而读操作是不需要加锁的。</p>
<h3 id="读少写多情况下并发map应该怎么设计">
读少写多情况下并发map，应该怎么设计？
<a class="anchor" href="#%e8%af%bb%e5%b0%91%e5%86%99%e5%a4%9a%e6%83%85%e5%86%b5%e4%b8%8b%e5%b9%b6%e5%8f%91map%e5%ba%94%e8%af%a5%e6%80%8e%e4%b9%88%e8%ae%be%e8%ae%a1">#</a>
</h3>
<p>这种情况下，可以使用分片锁，跟据key进行hash处理后，找到其对应读写锁，然后进行锁定处理。通过分片锁机制，可以降低锁的粒度来实现读少写多情况下高并发。可以参见
<a href="https://github.com/orcaman/concurrent-map">orcaman/concurrent-map</a>实现。</p>
<h3 id="总结">
总结
<a class="anchor" href="#%e6%80%bb%e7%bb%93">#</a>
</h3>
<ul>
<li>sync.Map是不能值传递（狭义的）的</li>
<li>sync.Map采用空间换时间策略。其底层结构存在两个map，分别是read map和dirty map。当读取操作时候，优先从read map中读取，是不需要加锁的，若key不存在read map中时候，再从dirty map中读取，这个过程是加锁的。当新增key操作时候，只会将新增key添加到dirty map中，此操作是加锁的，但不会影响read map的读操作。当更新key操作时候，如果key已存在read map中时候，只需无锁更新更新read map就行，负责加锁处理在dirty map中情况了。总之sync.Map会优先从read map中读取、更新、删除，因为对read map的读取不需要锁</li>
<li>当sync.Map读取key操作时候，若从read map中一直未读到，若dirty map中存在read map中不存在的keys时，则会把dirty map升级为read map，这个过程是加锁的。这样下次读取时候只需要考虑从read map读取，且读取过程是无锁的</li>
<li>延迟删除机制，删除一个键值时只是打上删除标记，只有在提升dirty map为read map的时候才清理删除的数据</li>
<li>sync.Map中的dirty map要么是nil，要么包含read map中所有未删除的key-value。</li>
<li>sync.Map适用于读多写少场景。根据
<a href="https://golang.org/pkg/sync/#Map">包官方文档介绍</a>，它特别适合这两个场景：1. 一个key只写入一次但读取多次时，比如在只会增长的缓存中；2. 当多个goroutine读取、写入和更新不相交的键值对时。</li>
</ul>
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