What's JVM-垃圾收集器與內存分配策略

原創

2021-07-16 10:59

收集方式	解釋
Minor GC/Young GC	僅對新生代進行垃圾回收
Major GC/Old GC	只對老年代進行垃圾收集/對整堆進行垃圾收集
Mixed GC	對整個新生代和部分老年代的垃圾收集
Full GC	對整個Java堆和方法區的垃圾收集

"}}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"3.2.2. 標記-清除算法","attrs":{}}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"🥭標記就是判斷對象是否屬於垃圾的過程，這個可由可達性分析算法進行標記，在此不再描述。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"🍍清除就是把標記過的區域清空。","attrs":{}}]}]}],"attrs":{}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/bf/bf96236ccfb50c0d0d8276278e8d3bc7.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":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"這個算法足夠簡單，但是它有兩個缺點：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"numberedlist","attrs":{"start":1,"normalizeStart":1},"content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"執行效率不穩定","attrs":{}},{"type":"text","text":"，它的執行效率隨着堆中需要回收的對象數量增加而下降。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","text":"第二個是","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"碎片化問題","attrs":{}},{"type":"text","text":"，過多的碎片會導致沒有足夠大的空間分配對象進而觸發進一步垃圾收集。","attrs":{}}]}]}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"3.2.3. 標記-複製算法","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"爲了解決標記清除算法的執行效率不穩定問題，引入了標記-複製算法。","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":"🥥標記還是判斷對象是否需要回收。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"🥝複製則是把需要保留的對象複製到另一半區域。","attrs":{}}]}]}],"attrs":{}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/77/77f4a582ad574f885f11ab7b08cb2b1b.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":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"此算法會把堆分爲兩個區域，其中一個保留，另一個存放對象；每次垃圾回收就把需要存活的對象複製到另一半內存，然後清空整個內存半區，這樣這一半就完全空閒，而且所有的存活對象都會整齊地排列在另一半中。下次同樣這樣操作。","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":"這種算法的缺點顯而易見：每次只有一半內存得以使用，未免有點太浪費空間了。","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":"爲了解決這個問題，可以把內存劃分比例換一下，因爲統計發現，絕大多數新生代對象撐不過第一輪垃圾回收。","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":"目前有一種更好的解決方案：把內存劃分成兩個較小的Survivor(以下簡稱S)和一個較大的Eden(以下簡稱E)。","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"每次只使用一個S和一個E來分配內存","attrs":{}},{"type":"text","text":"。垃圾回收時，把這個E和S上的存活對象移動到另一個S上，然後清空E和剛剛那個S。","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":"凡事總有個例外，如果S不夠容納一次GC之後的存活對象，就需要老年代的部分區域來存放。這部分實現的安全性由JVM擔保。","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":"由此可見這個算法主要用於新生代的GC。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"3.2.4. 標記-整理算法","attrs":{}}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"🍅標記依舊是判斷對象是否需要回收。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"🍆而整理則是把存活對象移動到一端，然後直接清除邊界以外的內存區域即可。","attrs":{}}]}]}],"attrs":{}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/b5/b5c83cdcb915583e9ace193a7d65da16.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":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"這個算法如果用在老年代上的話，估計不是很理想，因爲老年代存活對象太多了。而且這個算法在移動對象時，必須暫停用戶程序，就像JOJO裏Dio喊出：The World！全世界凍結一樣。然後搬運它需要搬運的對象。","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":"是否移動對象有弊有利。移動的好處在於內存空間整齊，方便新空間的申請，且加大系統吞吐量；弊端就是","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"會因爲移動對象而產生過多的停頓","attrs":{}},{"type":"text","text":"。","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":"還有一種中和式，就是在內存碎片多到無法容忍時進行整理，CMS便是這樣的原理。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"3.3. HotSpot算法細節","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"3.3.1. 根節點枚舉","attrs":{}}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"🥑在JVM裏，固定可作爲根節點的有全局性引用和執行上下文(棧幀中的本地變量表)。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"🥦迄今爲止，","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"所有的根節點枚舉都是需要暫停用戶程序的","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":"🥬通過一種稱爲","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"OOPMap的數據結構，JVM可以快速得知所有引用的位置","attrs":{}},{"type":"text","text":"。","attrs":{}}]}]}],"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":"因爲在","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"類加載時會確定對象偏移量多少的位置上，數據是什麼類型","attrs":{}},{"type":"text","text":"，加上即時編譯時，也會記錄棧和寄存器裏哪些保存的是引用類型，所以最終可以","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"直接得到所有引用的位置","attrs":{}},{"type":"text","text":"。然後把它們加入到OOPMap，進而選出根節點。","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":"JIT記錄OOPMap具體過程：一個線程意味着一個棧，一個棧由多個棧幀組成，一個棧幀對應着一個方法，一個方法裏面可能有多個安全點(見下面)。 GC發生時，程序首先運行到最近的一個安全點停下來，然後更新自己的OOPMap ，記下棧上哪些位置代表着引用。枚舉根節點時，遞歸遍歷每個棧幀的OOPMap，通過棧中記錄的被引用對象的內存地址，即可找到所有的GC Roots。","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":"OOPMap還可用作準確式GC(以下內容爲","attrs":{}},{"type":"link","attrs":{"href":"https://my.oschina.net/u/1757225/blog/1583822","title":"","type":null},"content":[{"type":"text","text":"轉載","attrs":{}}]},{"type":"text","text":")。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"numberedlist","attrs":{"start":1,"normalizeStart":1},"content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","text":"保守式GC在進行GC的時候，會從一些已知的位置(GC Roots)開始掃描內存，掃描到一個數字就判斷他是不是可能是指向GC堆中的一個指針(這裏會涉及上下邊界檢查(GC堆的上下界是已知的)、對齊檢查(通常分配空間的時候會有對齊要求，假如說是4字節對齊，那麼不能被4整除的數字就肯定不是指針)，之類的)。然後一直遞歸的掃描下去，最後完成可達性分析。這種模糊的判斷方法因爲無法準確判斷一個位置上是否是真的指向GC堆中的指針，所以被命名爲保守式GC。這種可達性分析的方式因爲不需要準確的判斷出一個指針，所以效率快，但是也正因爲這種特點，它存在下面兩個明顯的缺點：","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":"因爲是模糊的檢查，所以對於一些已經死掉的對象，很可能會被誤認爲仍有地方引用他們，GC也就自然不會回收他們，從而引起了無用的內存佔用，就是典型的佔着茅坑不拉屎，造成資源浪費。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"由於不知道疑似指針是否真的是指針，所以它們的值都不能改寫；移動對象就意味着要修正指針。換言之，對象就不可移動了。有一種辦法可以在使用保守式GC的同時支持對象的移動，那就是增加一個間接層，不直接通過指針來實現引用，而是添加一層“句柄”(handle)在中間，所有引用先指到一個句柄表裏，再從句柄表找到實際對象。這樣，要移動對象的話，只要修改句柄表裏的內容即可。但是這樣的話引用的訪問速度就降低了。Sun 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VM用過這種全handle的設計，但效果實在算不上好。","attrs":{}}]}]}],"attrs":{}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"numberedlist","attrs":{"start":2,"normalizeStart":2},"content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","text":"準確式GC","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":"與保守式GC相對的就是準確式GC，何爲準確式GC？就是我們準確的知道，某個位置上面是否是指針，對於Java來說，就是知道對於某個位置上的數據是什麼類型的，這樣就可以判斷出所有的位置上的數據是不是指向GC堆的引用，包括棧和寄存器裏的數據。","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":"網上看了下說是實現這種要求的方法有好幾種，但是在java中實現的方式是：從我外部記錄下類型信息，存成映射表，在HotSpot中把這種映射表稱之爲OOPMap，不同的虛擬機名稱可能不一樣。","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":"實現這種功能，需要虛擬機的解釋器和JIT編譯器支持，由他們來生成OOPMap。生成這樣的映射表一般有兩種方式：","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":"每次都遍歷原始的映射表，循環的一個個偏移量掃描過去；這種用法也叫“解釋式”；","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"爲每個映射表生成一塊定製的掃描代碼（想像掃描映射表的循環被展開的樣子），以後每次要用映射表就直接執行生成的掃描代碼；這種用法也叫“編譯式”。總而言之，GC開始的時候，就通過OOPMap這樣的一個映射表知道，在對象內的什麼偏移量上是什麼類型的數據，而且特定的位置記錄下棧和寄存器中哪些位置是引用。","attrs":{}}]}]}],"attrs":{}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"3.3.2. 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支持的最大虛擬地址空間爲47位，也就是128TB，和最大物理地址空間46位，也就是64TB。而在64位機中，Linux保留了高18位，如果我們可以縮減堆地址的話，那就會有部分高位怎麼也用不到。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/7d/7d2079a3e4c6cde7d77817d711c47899.jpeg","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":"image","attrs":{"src":"https://static001.geekbang.org/infoq/89/89ea1561de319cef53ff6f1ba27aaa41.jpeg","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":"image","attrs":{"src":"https://static001.geekbang.org/infoq/ab/ab0389856a1c188b79e7316d3de334e8.jpeg","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":"image","attrs":{"src":"https://static001.geekbang.org/infoq/9d/9d1761ae18b3546fde6aca42db5f9f7b.jpeg","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":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"所以ZGC最高支持4TB堆和64位機，這樣第43-46位就可爲我們所用(實際上目前ZGC支持到了16TB，這個是後話)。","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":"把4位分成：保留位-Remapped位-Mark1位-Mark0位這四種便可以實現把引用狀態信息保存在指針中。","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":"染色指針有三大優勢：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"numberedlist","attrs":{"start":1,"normalizeStart":1},"content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","text":"某個Region一旦被移走就立刻可用，因爲移走之後指向這個位置的指針被標記爲Remapped，接下來通過“自愈”技術修復即可。所以下一次訪問這裏的指針會自動重定向到新的位置。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","text":"是一種可擴展的結構，方便日後提高性能。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":3,"align":null,"origin":null},"content":[{"type":"text","text":"可以大幅減少內存屏障使用的數量。因爲ZGC不需要記錄跨代引用，其一是它可以把這些信息記錄在指針裏，二是它不支持跨代引用。","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":"但是有一個問題，Java作爲一個進程，直接修改內存指針，OS是否支持？事實上，因爲虛擬地址到物理地址的映射關係，加上ZGC使用了空間換時間，使得這個問題得以解決。所謂空間換時間，只是把三(如果算上原本的地址就是四個)個地址映射到了同一個物理地址空間。","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":"首先，OS在做虛擬->物理空間映射時，把地址後m位提出來，把前64-m位當成頁表索引，找到n，再把n+m拼接到一起(不是加到一起)，作爲物理地址，n的範圍，m的值由OS和實際內存大小決定。","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":"所以可以知道，如果後m位一致，那麼多個虛擬地址會映射到同一個物理地址的。","attrs":{}},{"type":"link","attrs":{"href":"https://juejin.cn/post/6845166890763943943","title":"","type":null},"content":[{"type":"text","text":"詳見","attrs":{}}]},{"type":"text","text":"。所以ZGC利用這一點，把四個標誌位不同的虛擬地址提前申請了，讓它們都映射到同一個物理地址，解決了OS不支持的問題。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/c7/c7e812dd8d6464b80830cabe669ce284.jpeg","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":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如果把4位標示位看成內存分段符，算上原本的，四位標識位，有0000，0001，0010，0100四種，所以它們在後42位相同的情況下，有0+4TB，4+4TB，8+4TB，16+4TB的內存空間起始間隔。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent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