What's JVM-垃圾收集器与内存分配策略

原創

2021-07-16 10:59

收集方式	解释
Minor GC/Young GC	仅对新生代进行垃圾回收
Major GC/Old GC	只对老年代进行垃圾收集/对整堆进行垃圾收集
Mixed GC	对整个新生代和部分老年代的垃圾收集
Full GC	对整个Java堆和方法区的垃圾收集

"}}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"3.2.2. 标记-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":"🥭标记就是判断对象是否属于垃圾的过程，这个可由可达性分析算法进行标记，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":"🍍清除就是把标记过的区域清空。","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":"这个算法足够简单，但是它有两个缺点：","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":"执行效率不稳定","attrs":{}},{"type":"text","text":"，它的执行效率随着堆中需要回收的对象数量增加而下降。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","text":"第二个是","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"碎片化问题","attrs":{}},{"type":"text","text":"，过多的碎片会导致没有足够大的空间分配对象进而触发进一步垃圾收集。","attrs":{}}]}]}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"3.2.3. 标记-复制算法","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"为了解决标记清除算法的执行效率不稳定问题，引入了标记-复制算法。","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":"🥥标记还是判断对象是否需要回收。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"🥝复制则是把需要保留的对象复制到另一半区域。","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":"此算法会把堆分为两个区域，其中一个保留，另一个存放对象；每次垃圾回收就把需要存活的对象复制到另一半内存，然后清空整个内存半区，这样这一半就完全空闲，而且所有的存活对象都会整齐地排列在另一半中。下次同样这样操作。","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":"这种算法的缺点显而易见：每次只有一半内存得以使用，未免有点太浪费空间了。","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":"为了解决这个问题，可以把内存划分比例换一下，因为统计发现，绝大多数新生代对象撑不过第一轮垃圾回收。","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":"目前有一种更好的解决方案：把内存划分成两个较小的Survivor(以下简称S)和一个较大的Eden(以下简称E)。","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"每次只使用一个S和一个E来分配内存","attrs":{}},{"type":"text","text":"。垃圾回收时，把这个E和S上的存活对象移动到另一个S上，然后清空E和刚刚那个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":"凡事总有个例外，如果S不够容纳一次GC之后的存活对象，就需要老年代的部分区域来存放。这部分实现的安全性由JVM担保。","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":"由此可见这个算法主要用于新生代的GC。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"3.2.4. 标记-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":"🍅标记依旧是判断对象是否需要回收。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"🍆而整理则是把存活对象移动到一端，然后直接清除边界以外的内存区域即可。","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":"这个算法如果用在老年代上的话，估计不是很理想，因为老年代存活对象太多了。而且这个算法在移动对象时，必须暂停用户程序，就像JOJO里Dio喊出：The World！全世界冻结一样。然后搬运它需要搬运的对象。","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":"是否移动对象有弊有利。移动的好处在于内存空间整齐，方便新空间的申请，且加大系统吞吐量；弊端就是","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"会因为移动对象而产生过多的停顿","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":"还有一种中和式，就是在内存碎片多到无法容忍时进行整理，CMS便是这样的原理。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"3.3. 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根节点枚举","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里，固定可作为根节点的有全局性引用和执行上下文(栈帧中的本地变量表)。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"🥦迄今为止，","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"所有的根节点枚举都是需要暂停用户程序的","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":"🥬通过一种称为","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"OOPMap的数据结构，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":"因为在","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"类加载时会确定对象偏移量多少的位置上，数据是什么类型","attrs":{}},{"type":"text","text":"，加上即时编译时，也会记录栈和寄存器里哪些保存的是引用类型，所以最终可以","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"直接得到所有引用的位置","attrs":{}},{"type":"text","text":"。然后把它们加入到OOPMap，进而选出根节点。","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记录OOPMap具体过程：一个线程意味着一个栈，一个栈由多个栈帧组成，一个栈帧对应着一个方法，一个方法里面可能有多个安全点(见下面)。 GC发生时，程序首先运行到最近的一个安全点停下来，然后更新自己的OOPMap ，记下栈上哪些位置代表着引用。枚举根节点时，递归遍历每个栈帧的OOPMap，通过栈中记录的被引用对象的内存地址，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还可用作准确式GC(以下内容为","attrs":{}},{"type":"link","attrs":{"href":"https://my.oschina.net/u/1757225/blog/1583822","title":"","type":null},"content":[{"type":"text","text":"转载","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在进行GC的时候，会从一些已知的位置(GC Roots)开始扫描内存，扫描到一个数字就判断他是不是可能是指向GC堆中的一个指针(这里会涉及上下边界检查(GC堆的上下界是已知的)、对齐检查(通常分配空间的时候会有对齐要求，假如说是4字节对齐，那么不能被4整除的数字就肯定不是指针)，之类的)。然后一直递归的扫描下去，最后完成可达性分析。这种模糊的判断方法因为无法准确判断一个位置上是否是真的指向GC堆中的指针，所以被命名为保守式GC。这种可达性分析的方式因为不需要准确的判断出一个指针，所以效率快，但是也正因为这种特点，它存在下面两个明显的缺点：","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":"因为是模糊的检查，所以对于一些已经死掉的对象，很可能会被误认为仍有地方引用他们，GC也就自然不会回收他们，从而引起了无用的内存占用，就是典型的占着茅坑不拉屎，造成资源浪费。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"由于不知道疑似指针是否真的是指针，所以它们的值都不能改写；移动对象就意味着要修正指针。换言之，对象就不可移动了。有一种办法可以在使用保守式GC的同时支持对象的移动，那就是增加一个间接层，不直接通过指针来实现引用，而是添加一层“句柄”(handle)在中间，所有引用先指到一个句柄表里，再从句柄表找到实际对象。这样，要移动对象的话，只要修改句柄表里的内容即可。但是这样的话引用的访问速度就降低了。Sun 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VM用过这种全handle的设计，但效果实在算不上好。","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":"准确式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":"与保守式GC相对的就是准确式GC，何为准确式GC？就是我们准确的知道，某个位置上面是否是指针，对于Java来说，就是知道对于某个位置上的数据是什么类型的，这样就可以判断出所有的位置上的数据是不是指向GC堆的引用，包括栈和寄存器里的数据。","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":"网上看了下说是实现这种要求的方法有好几种，但是在java中实现的方式是：从我外部记录下类型信息，存成映射表，在HotSpot中把这种映射表称之为OOPMap，不同的虚拟机名称可能不一样。","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":"实现这种功能，需要虚拟机的解释器和JIT编译器支持，由他们来生成OOPMap。生成这样的映射表一般有两种方式：","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":"每次都遍历原始的映射表，循环的一个个偏移量扫描过去；这种用法也叫“解释式”；","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"为每个映射表生成一块定制的扫描代码（想像扫描映射表的循环被展开的样子），以后每次要用映射表就直接执行生成的扫描代码；这种用法也叫“编译式”。总而言之，GC开始的时候，就通过OOPMap这样的一个映射表知道，在对象内的什么偏移量上是什么类型的数据，而且特定的位置记录下栈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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支持的最大虚拟地址空间为47位，也就是128TB，和最大物理地址空间46位，也就是64TB。而在64位机中，Linux保留了高18位，如果我们可以缩减堆地址的话，那就会有部分高位怎么也用不到。","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位机，这样第43-46位就可为我们所用(实际上目前ZGC支持到了16TB，这个是后话)。","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位这四种便可以实现把引用状态信息保存在指针中。","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":"染色指针有三大优势：","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":"某个Region一旦被移走就立刻可用，因为移走之后指向这个位置的指针被标记为Remapped，接下来通过“自愈”技术修复即可。所以下一次访问这里的指针会自动重定向到新的位置。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","text":"是一种可扩展的结构，方便日后提高性能。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":3,"align":null,"origin":null},"content":[{"type":"text","text":"可以大幅减少内存屏障使用的数量。因为ZGC不需要记录跨代引用，其一是它可以把这些信息记录在指针里，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":"但是有一个问题，Java作为一个进程，直接修改内存指针，OS是否支持？事实上，因为虚拟地址到物理地址的映射关系，加上ZGC使用了空间换时间，使得这个问题得以解决。所谓空间换时间，只是把三(如果算上原本的地址就是四个)个地址映射到了同一个物理地址空间。","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在做虚拟->物理空间映射时，把地址后m位提出来，把前64-m位当成页表索引，找到n，再把n+m拼接到一起(不是加到一起)，作为物理地址，n的范围，m的值由OS和实际内存大小决定。","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，如果后m位一致，那么多个虚拟地址会映射到同一个物理地址的。","attrs":{}},{"type":"link","attrs":{"href":"https://juejin.cn/post/6845166890763943943","title":"","type":null},"content":[{"type":"text","text":"详见","attrs":{}}]},{"type":"text","text":"。所以ZGC利用这一点，把四个标志位不同的虚拟地址提前申请了，让它们都映射到同一个物理地址，解决了OS不支持的问题。","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位标示位看成内存分段符，算上原本的，四位标识位，有0000，0001，0010，0100四种，所以它们在后42位相同的情况下，有0+4TB，4+4TB，8+4TB，16+4TB的内存空间起始间隔。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent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