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淺析 PHP 的「垃圾回收」機制
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給對象添加一個引用計數器，每當有一個地方引用它，計數器的值就加一。每當有一個引用失效，計數器的值就減一。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如果一個變量 value 的 refcount 減一之後等於 0，此 value 可以被","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"釋放","attrs":{}}],"attrs":{}},{"type":"text","text":"掉，","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"不屬於","attrs":{}}],"attrs":{}},{"type":"text","text":"垃圾。","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"垃圾回收器不會處理","attrs":{}},{"type":"text","text":" 。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如果一個變量 value 的 refcount 減一之後還是大於 0，此 value 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需要維護引用計數器，有一定的消耗。且較難處理循環引用的問題。後面也會講到如何解決這個問題。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"下面的例子說明引用計數的是如何變化的：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/6e/6ecd001be906c109e6d8a7c01a85d19f.png","alt":null,"title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"1.2 PHP 中的變量知識","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"每個 php 變量存在一個叫 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"zval","attrs":{}}],"attrs":{}},{"type":"text","text":" 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buf 數組中的位置保存到了 gc_info 中，當後續 value 的 refcount 變爲了 0。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"需要將其從 buf 中刪除時可以知道該 value 保存在哪個 gc_root_buffer 中。","attrs":{}}]}]}],"attrs":{}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"5. 釋放垃圾","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"由於回收方法 zend_gc_collect_cycles() 實在是太長，我把幾個關鍵步驟理出來了：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/00/008f107be893870bf1c73214eb72d7e2.png","alt":null,"title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"掃描根節點","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"收集根節點","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"調用回收器","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"清理變量","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"收集完成","attrs":{}}]}]}],"attrs":{}},{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"text","text":"四、總結","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"（1）PHP 的垃圾回收和 Java 的垃圾回收還是很有很大區別的，我們都以爲沒有高級語言會用到引用計數法來回收垃圾，但偏偏 PHP 用的是引用計數。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"（2）PHP 用了一套自己的算法來解決因循環引用而產生垃圾的問題，這套算法可以簡單理解爲先把可疑垃圾的引用計數減一來進行測試，如果引用計數確實等於 0 ，則標記顏色爲黑色，後續一起清理。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"（3）PHP 垃圾收集中總共用到了三種關鍵顏色： 白色- 垃圾， 黑色- 非垃圾， 紫色- 防止重複插入。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"歡迎關注我的公衆號：「","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"悟空聊架構","attrs":{}}],"attrs":{}},{"type":"text","text":"」","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"作者簡介：8 年互聯網職場老兵｜全棧工程師｜90 後超級奶爸｜開源踐行者｜公衆號萬粉原創號主。藍橋簽約作者，著有《JVM 性能調優實戰》專欄，手寫了一套 7 萬字 SpringCloud 實戰總結和 3 萬字分佈式算法總結。歡迎關注我的公衆號「悟空聊架構」，免費獲取資料學習。","attrs":{}}]}],"attrs":{}}]}
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