{"type":"doc","content":[{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"每日一句","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":1,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":"每一日你所付出的代價都比前一日高，因爲你的生命又消短了一天，所以每一日你都要更用心。","attrs":{}}]}],"attrs":{}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/11/118b55534b7619141f818a1221e7befa.gif","alt":null,"title":"","style":[{"key":"width","value":"25%"},{"key":"bordertype","value":"border"}],"href":"","fromPaste":false,"pastePass":false}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"前提概要","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":" 對於JVM而言，最難能夠掌握就是GC回收部分的研究和探索。而對於虛擬機而言根據不同的區域以及範圍和方案分爲Minor GC、Major GC和Full GC等，此處暫時不描述Mixed GC，後續章節會詳細做專題探究。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"GC的引入","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","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":" 堆內存劃分爲Eden、Survivor（2）和Tenured/Old空間。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"發生在年輕代的GC——Minor GC","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/c9/c9c811b48d047237d2d120e3f06256ec.png","alt":"image.png","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","marks":[{"type":"size","attrs":{"size":10}}],"text":"其中Minor GC如下圖所示","attrs":{}}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":"虛擬機給每個對象定義一個對象年齡（Age）計數器。對象在Eden生並經過第一次Minor GC後仍然存活，並且能被Survivor容納的話，將被移動到Survivor空間中，並將對象年齡設爲 1。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":"對象在Survivor區中每熬過一次Minor GC，年齡就增加1歲，當它的年齡增加到一定程度（默認爲15歲）時，就會被晉升到老年代中。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":"對象晉升老年代的年齡閾值，可以通過參數-XX:MaxTenuringThreshold來設置","attrs":{}},{"type":"text","marks":[{"type":"size","attrs":{"size":10}}],"text":"。","attrs":{}}]}]}],"attrs":{}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/b0/b03e78be7a71e218ca97bc6cd6493396.png","alt":"image.png","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","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":"從年輕代空間（包括Eden和Survivor區域）回收內存被稱爲Minor GC。這一定義既清晰又易於理解，因爲Java對象大多都具備朝生夕滅的特性，所以Minor GC非常頻繁，一般回收速度也比較快。","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","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":"JVM無法爲一個新的對象分配空間時會觸發Minor GC，比如當Eden區滿了。所以分配率越高，越頻繁執行Minor GC。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":"內存池被填滿的時候，其中的內容全部會被複制，指針會從0開始跟蹤空閒內存。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":"Eden和Survivor區進行了標記和複製操作，取代了經典的標記、掃描、壓縮、清理操作。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":"所以Eden和Survivor區不存在內存碎片，寫指針總是停留在所使用內存池的頂部。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":"執行Minor GC操作時，不會影響到方法區","attrs":{}},{"type":"text","marks":[{"type":"size","attrs":{"size":10}}],"text":"。","attrs":{}},{"type":"text","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":"從方法區到年輕代的引用被當成GCRoots，從年輕代到方法區的引用在標記階段被直接忽略掉","attrs":{}},{"type":"text","marks":[{"type":"size","attrs":{"size":10}}],"text":"。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":"質疑常規的認知，所有的Minor GC都會觸發 “全世界的暫停（stop-the-world）”，停止應用程序的線程。對於大部分應用程序，停頓導致的延遲都是可以忽略不計的。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":"其中的真相就是，大部分Eden區中的對象都能被認爲是垃圾，永遠也不會被複制到Survivor區或者老年代空間。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":"如果正好相反，Eden區大部分新生對象不符合GC條件，Minor GC執行時暫停的時間將會長很多。","attrs":{}}]}]}],"attrs":{}},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"Minor GC觸發機制：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":1,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":"當年輕代滿時就會觸發Minor GC，這裏的年輕代滿指的是Eden代滿，Survivor滿不會引發GC。","attrs":{}}]},{"type":"horizontalrule","attrs":{}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"發生在老年代的GC——Major GC/Full GC","attrs":{}}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":"Major GC：主要是清理老年代：MajorGC的速度一般會比Minor GC慢 10倍以上","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":"Full GC：主要是清理整個堆空間—包括年輕代和老年代。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":"首先，許多Major GC是由Minor GC觸發，所以很多情況下將這兩種GC分離是不太可能的。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":"另一方面，許多現代垃圾收集機制會清理部分老年代空間，所以使用“cleaning”一詞只是部分正確。","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","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":" 這使得我們不用去關心到底是叫Major GC還是Full GC，大家應該關注當前的GC是否停止了所有應用程序的線程，還是能夠併發的處理而不用停掉應用程序的線程。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":" 老年代GC（Major GC/Full GC）：指發生在老年代的GC，出現了Major GC，經常會伴隨至少一次的Minor GC（但非絕對的，ParallelScavenge收集器的收集策略裏就有直接進行Major GC的策略選擇過程）","attrs":{}},{"type":"text","marks":[{"type":"size","attrs":{"size":10}}],"text":" 。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"Full GC觸發機制：","attrs":{}}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":"（1）調用System.gc時，系統建議執行Full GC，但是不必然執行","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":"（2）老年代空間不足","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":"（3）方法區空間不足：當永久代/元數據空間滿時也會引發Full GC，會導致Class、Method元信息的卸載。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":"（4）通過Minor GC後進入老年代的平均大小大於老年代的可用內存","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":"（5）由Eden區、survivor space1（From Space）區向survivor space2（To Space）區複製時，對象大小大於To Space可用內存，則把該對象轉存到老年代，且老年代的可用內存小於該對象大小","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","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":" Full GC定義是相對明確的，就是針對整個新生代、老生代、元空間（metaspace，java8以上版本取代perm gen）的全局範圍的GC","attrs":{}},{"type":"text","marks":[{"type":"size","attrs":{"size":10}}],"text":"；","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":" Minor GC和Major GC是俗稱，在Hotspot JVM實現的Serial GC, Parallel GC, CMS, G1 GC中大致可以對應到某個Young GC和Old GC算法組合","attrs":{}},{"type":"text","marks":[{"type":"size","attrs":{"size":10}}],"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":"第一次嘗試通過 jstat 輸出：","attrs":{}}]},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"my-precious: me$ jstat -gc -t 4235 1s\n複製代碼","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"Time S0C S1C S0U S1U EC EU OC OU MC MU CCSC CCSU YGC YGCT FGC FGCT GCT \n 5.7 34048.0 34048.0 0.0 34048.0 272640.0 194699.7 1756416.0 181419.9 18304.0 17865.1 2688.0 2497.6 3 0.275 0 0.000 0.275\n 6.7 34048.0 34048.0 34048.0 0.0 272640.0 247555.4 1756416.0 263447.9 18816.0 18123.3 2688.0 2523.1 4 0.359 0 0.000 0.359\n 7.7 34048.0 34048.0 0.0 34048.0 272640.0 257729.3 1756416.0 345109.8 19072.0 18396.6 2688.0 2550.3 5 0.451 0 0.000 0.451\n 8.7 34048.0 34048.0 34048.0 34048.0 272640.0 272640.0 1756416.0 444982.5 19456.0 18681.3 2816.0 2575.8 7 0.550 0 0.000 0.550\n 9.7 34048.0 34048.0 34046.7 0.0 272640.0 16777.0 1756416.0 587906.3 20096.0 19235.1 2944.0 2631.8 8 0.720 0 0.000 0.720\n10.7 34048.0 34048.0 0.0 34046.2 272640.0 80171.6 1756416.0 664913.4 20352.0 19495.9 2944.0 2657.4 9 0.810 0 0.000 0.810\n11.7 34048.0 34048.0 34048.0 0.0 272640.0 129480.8 1756416.0 745100.2 20608.0 19704.5 2944.0 2678.4 10 0.896 0 0.000 0.896\n12.7 34048.0 34048.0 0.0 34046.6 272640.0 164070.7 1756416.0 822073.7 20992.0 19937.1 3072.0 2702.8 11 0.978 0 0.000 0.978\n13.7 34048.0 34048.0 34048.0 0.0 272640.0 211949.9 1756416.0 897364.4 21248.0 20179.6 3072.0 2728.1 12 1.087 1 0.004 1.091\n14.7 34048.0 34048.0 0.0 34047.1 272640.0 245801.5 1756416.0 597362.6 21504.0 20390.6 3072.0 2750.3 13 1.183 2 0.050 1.233\n15.7 34048.0 34048.0 0.0 34048.0 272640.0 21474.1 1756416.0 757347.0 22012.0 20792.0 3200.0 2791.0 15 1.336 2 0.050 1.386\n16.7 34048.0 34048.0 34047.0 0.0 272640.0 48378.0 1756416.0 838594.4 22268.0 21003.5 3200.0 2813.2 16 1.433 2 0.050 1.484","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}}],"text":"這個片段是 JVM 啓動後第17秒提取的。基於該信息，我們可以得出這樣的結果，運行了12次 Minor GC、2次 Full GC，時間總跨度爲50毫秒。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}}],"text":"java -XX:+PrintGCDetails -XX:+UseConcMarkSweepGC eu.plumbr.demo.GarbageProducer","attrs":{}}]},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"3.157: [GC (Allocation Failure) 3.157: [ParNew: 272640K->34048K(306688K), 0.0844702 secs] 272640K->69574K(2063104K), 0.0845560 secs] [Times: user=0.23 sys=0.03, real=0.09 secs] \n4.092: [GC (Allocation Failure) 4.092: [ParNew: 306688K->34048K(306688K), 0.1013723 secs] 342214K->136584K(2063104K), 0.1014307 secs] [Times: user=0.25 sys=0.05, real=0.10 secs] \n... cut for brevity ...\n11.292: [GC (Allocation Failure) 11.292: [ParNew: 306686K->34048K(306688K), 0.0857219 secs] 971599K->779148K(2063104K), 0.0857875 secs] [Times: user=0.26 sys=0.04, real=0.09 secs] \n12.140: [GC (Allocation Failure) 12.140: [ParNew: 306688K->34046K(306688K), 0.0821774 secs] 1051788K->856120K(2063104K), 0.0822400 secs] [Times: user=0.25 sys=0.03, real=0.08 secs] \n12.989: [GC (Allocation Failure) 12.989: [ParNew: 306686K->34048K(306688K), 0.1086667 secs] 1128760K->931412K(2063104K), 0.1087416 secs] [Times: user=0.24 sys=0.04, real=0.11 secs] \n13.098: [GC (CMS Initial Mark) [1 CMS-initial-mark: 897364K(1756416K)] 936667K(2063104K), 0.0041705 secs] [Times: user=0.02 sys=0.00, real=0.00 secs] \n13.102: [CMS-concurrent-mark-start]\n13.341: [CMS-concurrent-mark: 0.238/0.238 secs] [Times: user=0.36 sys=0.01, real=0.24 secs] \n13.341: [CMS-concurrent-preclean-start]\n13.350: [CMS-concurrent-preclean: 0.009/0.009 secs] [Times: user=0.03 sys=0.00, real=0.01 secs] \n13.350: [CMS-concurrent-abortable-preclean-start]\n13.878: [GC (Allocation Failure) 13.878: [ParNew: 306688K->34047K(306688K), 0.0960456 secs] 1204052K->1010638K(2063104K), 0.0961542 secs] [Times: user=0.29 sys=0.04, real=0.09 secs] \n14.366: [CMS-concurrent-abortable-preclean: 0.917/1.016 secs] [Times: user=2.22 sys=0.07, real=1.01 secs] \n14.366: [GC (CMS Final Remark) [YG occupancy: 182593 K (306688 K)]14.366: [Rescan (parallel) , 0.0291598 secs]14.395: [weak refs processing, 0.0000232 secs]14.395: [class unloading, 0.0117661 secs]14.407: [scrub symbol table, 0.0015323 secs]14.409: [scrub string table, 0.0003221 secs][1 CMS-remark: 976591K(1756416K)] 1159184K(2063104K), 0.0462010 secs] [Times: user=0.14 sys=0.00, real=0.05 secs] \n14.412: [CMS-concurrent-sweep-start]\n14.633: [CMS-concurrent-sweep: 0.221/0.221 secs] [Times: user=0.37 sys=0.00, real=0.22 secs] \n14.633: [CMS-concurrent-reset-start]\n14.636: [CMS-concurrent-reset: 0.002/0.002 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}}],"text":" 在點頭同意這個結論之前，讓我們看看來自同一個 JVM 啓動收集的垃圾收集日誌的輸出。顯然- XX ： + PrintGCDetails 告訴我們一個不同且更詳細的故事：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}}],"text":" 基於這些信息，我們可以看到12次 Minor GC 後開始有些和上面不一樣了。沒有運行兩次Full GC，這不同的地方在於單個GC在永久代中不同階段運行了兩次：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}}],"text":" 最初的標記階段，用了0.0041705秒也就是4ms左右。這個階段會暫停“全世界（ stop-the-world）”的事件，停止所有應用程序的線程，然後開始標記。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}}],"text":" 並行執行標記和清洗階段。這些都是和應用程序線程並行的。 最後 Remark 階段，花費了0.0462010秒約46ms。這個階段會再次暫停所有的事件。 並行執行清理操作。正如其名，此階段也是並行的，不會停止其他線程。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}}],"text":" 所以，正如我們從垃圾回收日誌中所看到的那樣，實際上只是執行了 Major GC 去清理老年代空間而已，而不是執行了兩次 Full GC。","attrs":{}}]}]}
☕【JVM技術之旅】徹底弄清楚Minor GC和Major GC及Full GC
{"type":"doc","content":[{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"每日一句","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":1,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":"每一日你所付出的代價都比前一日高，因爲你的生命又消短了一天，所以每一日你都要更用心。","attrs":{}}]}],"attrs":{}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/11/118b55534b7619141f818a1221e7befa.gif","alt":null,"title":"","style":[{"key":"width","value":"25%"},{"key":"bordertype","value":"border"}],"href":"","fromPaste":false,"pastePass":false}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"前提概要","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":" 對於JVM而言，最難能夠掌握就是GC回收部分的研究和探索。而對於虛擬機而言根據不同的區域以及範圍和方案分爲Minor GC、Major GC和Full GC等，此處暫時不描述Mixed GC，後續章節會詳細做專題探究。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"GC的引入","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","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":" 堆內存劃分爲Eden、Survivor（2）和Tenured/Old空間。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"發生在年輕代的GC——Minor GC","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/c9/c9c811b48d047237d2d120e3f06256ec.png","alt":"image.png","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","marks":[{"type":"size","attrs":{"size":10}}],"text":"其中Minor GC如下圖所示","attrs":{}}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":"虛擬機給每個對象定義一個對象年齡（Age）計數器。對象在Eden生並經過第一次Minor GC後仍然存活，並且能被Survivor容納的話，將被移動到Survivor空間中，並將對象年齡設爲 1。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":"對象在Survivor區中每熬過一次Minor GC，年齡就增加1歲，當它的年齡增加到一定程度（默認爲15歲）時，就會被晉升到老年代中。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":"對象晉升老年代的年齡閾值，可以通過參數-XX:MaxTenuringThreshold來設置","attrs":{}},{"type":"text","marks":[{"type":"size","attrs":{"size":10}}],"text":"。","attrs":{}}]}]}],"attrs":{}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/b0/b03e78be7a71e218ca97bc6cd6493396.png","alt":"image.png","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","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":"從年輕代空間（包括Eden和Survivor區域）回收內存被稱爲Minor GC。這一定義既清晰又易於理解，因爲Java對象大多都具備朝生夕滅的特性，所以Minor GC非常頻繁，一般回收速度也比較快。","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","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":"JVM無法爲一個新的對象分配空間時會觸發Minor GC，比如當Eden區滿了。所以分配率越高，越頻繁執行Minor GC。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":"內存池被填滿的時候，其中的內容全部會被複制，指針會從0開始跟蹤空閒內存。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":"Eden和Survivor區進行了標記和複製操作，取代了經典的標記、掃描、壓縮、清理操作。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":"所以Eden和Survivor區不存在內存碎片，寫指針總是停留在所使用內存池的頂部。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":"執行Minor GC操作時，不會影響到方法區","attrs":{}},{"type":"text","marks":[{"type":"size","attrs":{"size":10}}],"text":"。","attrs":{}},{"type":"text","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":"從方法區到年輕代的引用被當成GCRoots，從年輕代到方法區的引用在標記階段被直接忽略掉","attrs":{}},{"type":"text","marks":[{"type":"size","attrs":{"size":10}}],"text":"。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":"質疑常規的認知，所有的Minor GC都會觸發 “全世界的暫停（stop-the-world）”，停止應用程序的線程。對於大部分應用程序，停頓導致的延遲都是可以忽略不計的。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":"其中的真相就是，大部分Eden區中的對象都能被認爲是垃圾，永遠也不會被複制到Survivor區或者老年代空間。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":"如果正好相反，Eden區大部分新生對象不符合GC條件，Minor GC執行時暫停的時間將會長很多。","attrs":{}}]}]}],"attrs":{}},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"Minor GC觸發機制：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":1,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":"當年輕代滿時就會觸發Minor GC，這裏的年輕代滿指的是Eden代滿，Survivor滿不會引發GC。","attrs":{}}]},{"type":"horizontalrule","attrs":{}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"發生在老年代的GC——Major GC/Full GC","attrs":{}}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":"Major GC：主要是清理老年代：MajorGC的速度一般會比Minor GC慢 10倍以上","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":"Full GC：主要是清理整個堆空間—包括年輕代和老年代。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":"首先，許多Major GC是由Minor GC觸發，所以很多情況下將這兩種GC分離是不太可能的。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":"另一方面，許多現代垃圾收集機制會清理部分老年代空間，所以使用“cleaning”一詞只是部分正確。","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","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":" 這使得我們不用去關心到底是叫Major GC還是Full GC，大家應該關注當前的GC是否停止了所有應用程序的線程，還是能夠併發的處理而不用停掉應用程序的線程。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":" 老年代GC（Major GC/Full GC）：指發生在老年代的GC，出現了Major GC，經常會伴隨至少一次的Minor GC（但非絕對的，ParallelScavenge收集器的收集策略裏就有直接進行Major GC的策略選擇過程）","attrs":{}},{"type":"text","marks":[{"type":"size","attrs":{"size":10}}],"text":" 。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"Full GC觸發機制：","attrs":{}}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":"（1）調用System.gc時，系統建議執行Full GC，但是不必然執行","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":"（2）老年代空間不足","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":"（3）方法區空間不足：當永久代/元數據空間滿時也會引發Full GC，會導致Class、Method元信息的卸載。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":"（4）通過Minor GC後進入老年代的平均大小大於老年代的可用內存","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":"（5）由Eden區、survivor space1（From Space）區向survivor space2（To Space）區複製時，對象大小大於To Space可用內存，則把該對象轉存到老年代，且老年代的可用內存小於該對象大小","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","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":" Full GC定義是相對明確的，就是針對整個新生代、老生代、元空間（metaspace，java8以上版本取代perm gen）的全局範圍的GC","attrs":{}},{"type":"text","marks":[{"type":"size","attrs":{"size":10}}],"text":"；","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}},{"type":"strong","attrs":{}}],"text":" Minor GC和Major GC是俗稱，在Hotspot JVM實現的Serial GC, Parallel GC, CMS, G1 GC中大致可以對應到某個Young GC和Old GC算法組合","attrs":{}},{"type":"text","marks":[{"type":"size","attrs":{"size":10}}],"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":"第一次嘗試通過 jstat 輸出：","attrs":{}}]},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"my-precious: me$ jstat -gc -t 4235 1s\n複製代碼","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"Time S0C S1C S0U S1U EC EU OC OU MC MU CCSC CCSU YGC YGCT FGC FGCT GCT \n 5.7 34048.0 34048.0 0.0 34048.0 272640.0 194699.7 1756416.0 181419.9 18304.0 17865.1 2688.0 2497.6 3 0.275 0 0.000 0.275\n 6.7 34048.0 34048.0 34048.0 0.0 272640.0 247555.4 1756416.0 263447.9 18816.0 18123.3 2688.0 2523.1 4 0.359 0 0.000 0.359\n 7.7 34048.0 34048.0 0.0 34048.0 272640.0 257729.3 1756416.0 345109.8 19072.0 18396.6 2688.0 2550.3 5 0.451 0 0.000 0.451\n 8.7 34048.0 34048.0 34048.0 34048.0 272640.0 272640.0 1756416.0 444982.5 19456.0 18681.3 2816.0 2575.8 7 0.550 0 0.000 0.550\n 9.7 34048.0 34048.0 34046.7 0.0 272640.0 16777.0 1756416.0 587906.3 20096.0 19235.1 2944.0 2631.8 8 0.720 0 0.000 0.720\n10.7 34048.0 34048.0 0.0 34046.2 272640.0 80171.6 1756416.0 664913.4 20352.0 19495.9 2944.0 2657.4 9 0.810 0 0.000 0.810\n11.7 34048.0 34048.0 34048.0 0.0 272640.0 129480.8 1756416.0 745100.2 20608.0 19704.5 2944.0 2678.4 10 0.896 0 0.000 0.896\n12.7 34048.0 34048.0 0.0 34046.6 272640.0 164070.7 1756416.0 822073.7 20992.0 19937.1 3072.0 2702.8 11 0.978 0 0.000 0.978\n13.7 34048.0 34048.0 34048.0 0.0 272640.0 211949.9 1756416.0 897364.4 21248.0 20179.6 3072.0 2728.1 12 1.087 1 0.004 1.091\n14.7 34048.0 34048.0 0.0 34047.1 272640.0 245801.5 1756416.0 597362.6 21504.0 20390.6 3072.0 2750.3 13 1.183 2 0.050 1.233\n15.7 34048.0 34048.0 0.0 34048.0 272640.0 21474.1 1756416.0 757347.0 22012.0 20792.0 3200.0 2791.0 15 1.336 2 0.050 1.386\n16.7 34048.0 34048.0 34047.0 0.0 272640.0 48378.0 1756416.0 838594.4 22268.0 21003.5 3200.0 2813.2 16 1.433 2 0.050 1.484","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}}],"text":"這個片段是 JVM 啓動後第17秒提取的。基於該信息，我們可以得出這樣的結果，運行了12次 Minor GC、2次 Full GC，時間總跨度爲50毫秒。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}}],"text":"java -XX:+PrintGCDetails -XX:+UseConcMarkSweepGC eu.plumbr.demo.GarbageProducer","attrs":{}}]},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"3.157: [GC (Allocation Failure) 3.157: [ParNew: 272640K->34048K(306688K), 0.0844702 secs] 272640K->69574K(2063104K), 0.0845560 secs] [Times: user=0.23 sys=0.03, real=0.09 secs] \n4.092: [GC (Allocation Failure) 4.092: [ParNew: 306688K->34048K(306688K), 0.1013723 secs] 342214K->136584K(2063104K), 0.1014307 secs] [Times: user=0.25 sys=0.05, real=0.10 secs] \n... cut for brevity ...\n11.292: [GC (Allocation Failure) 11.292: [ParNew: 306686K->34048K(306688K), 0.0857219 secs] 971599K->779148K(2063104K), 0.0857875 secs] [Times: user=0.26 sys=0.04, real=0.09 secs] \n12.140: [GC (Allocation Failure) 12.140: [ParNew: 306688K->34046K(306688K), 0.0821774 secs] 1051788K->856120K(2063104K), 0.0822400 secs] [Times: user=0.25 sys=0.03, real=0.08 secs] \n12.989: [GC (Allocation Failure) 12.989: [ParNew: 306686K->34048K(306688K), 0.1086667 secs] 1128760K->931412K(2063104K), 0.1087416 secs] [Times: user=0.24 sys=0.04, real=0.11 secs] \n13.098: [GC (CMS Initial Mark) [1 CMS-initial-mark: 897364K(1756416K)] 936667K(2063104K), 0.0041705 secs] [Times: user=0.02 sys=0.00, real=0.00 secs] \n13.102: [CMS-concurrent-mark-start]\n13.341: [CMS-concurrent-mark: 0.238/0.238 secs] [Times: user=0.36 sys=0.01, real=0.24 secs] \n13.341: [CMS-concurrent-preclean-start]\n13.350: [CMS-concurrent-preclean: 0.009/0.009 secs] [Times: user=0.03 sys=0.00, real=0.01 secs] \n13.350: [CMS-concurrent-abortable-preclean-start]\n13.878: [GC (Allocation Failure) 13.878: [ParNew: 306688K->34047K(306688K), 0.0960456 secs] 1204052K->1010638K(2063104K), 0.0961542 secs] [Times: user=0.29 sys=0.04, real=0.09 secs] \n14.366: [CMS-concurrent-abortable-preclean: 0.917/1.016 secs] [Times: user=2.22 sys=0.07, real=1.01 secs] \n14.366: [GC (CMS Final Remark) [YG occupancy: 182593 K (306688 K)]14.366: [Rescan (parallel) , 0.0291598 secs]14.395: [weak refs processing, 0.0000232 secs]14.395: [class unloading, 0.0117661 secs]14.407: [scrub symbol table, 0.0015323 secs]14.409: [scrub string table, 0.0003221 secs][1 CMS-remark: 976591K(1756416K)] 1159184K(2063104K), 0.0462010 secs] [Times: user=0.14 sys=0.00, real=0.05 secs] \n14.412: [CMS-concurrent-sweep-start]\n14.633: [CMS-concurrent-sweep: 0.221/0.221 secs] [Times: user=0.37 sys=0.00, real=0.22 secs] \n14.633: [CMS-concurrent-reset-start]\n14.636: [CMS-concurrent-reset: 0.002/0.002 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}}],"text":" 在點頭同意這個結論之前，讓我們看看來自同一個 JVM 啓動收集的垃圾收集日誌的輸出。顯然- XX ： + PrintGCDetails 告訴我們一個不同且更詳細的故事：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}}],"text":" 基於這些信息，我們可以看到12次 Minor GC 後開始有些和上面不一樣了。沒有運行兩次Full GC，這不同的地方在於單個GC在永久代中不同階段運行了兩次：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}}],"text":" 最初的標記階段，用了0.0041705秒也就是4ms左右。這個階段會暫停“全世界（ stop-the-world）”的事件，停止所有應用程序的線程，然後開始標記。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}}],"text":" 並行執行標記和清洗階段。這些都是和應用程序線程並行的。 最後 Remark 階段，花費了0.0462010秒約46ms。這個階段會再次暫停所有的事件。 並行執行清理操作。正如其名，此階段也是並行的，不會停止其他線程。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"size","attrs":{"size":10}}],"text":" 所以，正如我們從垃圾回收日誌中所看到的那樣，實際上只是執行了 Major GC 去清理老年代空間而已，而不是執行了兩次 Full GC。","attrs":{}}]}]}
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