Garbage First G1收集器理解和原理分析

时间 2019-12-06

标签 garbage g1 收集理解原理分析繁體版

原文原文链接

G1简介

Garbage First Collector, 简称G1 Collector，是HotspotJDK1.7后提供的面向大内存(Heap区数G到数10G)、多核系统的收集器，可以实现软停顿目标收集而且具备高吞吐量, 具备更可预测的停顿时间。一些基本的垃圾收集的概念和术语能够参考我以前的一篇理解GC(垃圾回收算法和原理)。html

G1是一种并发、并行、部分Stop The World、使用Copying算法收集的分代的增量式收集器，
G1的全堆的操做，像global marking，是和应用(mutator)并发执行的，这样能够减小对mutator的暂停时间。清除阶段则使用多线程来提升吞吐量。
与Hotspot以前的Serial、Parallel、CMS等收集器不一样的是，G1将堆分为不少大小相等的Region, 每次收集时会判断各个Region的活性-即垃圾对象的占比，垃圾对象占比越多的Region回收的收益越大，而后G1会按照设置的停顿时间目标、前几回回收Region所用时间来估算要回收哪些Region,即用最小的时间获取最大的收益，这也是Garbage First名字的含义。
Garbage First Collector的使命是在将来替换CMS，而且在JDK1.9已经成为默认的收集器。java

为何须要G1

Hotspot以前已经携带了Serial, Paralel, CMS等收集器，为何还须要研发一个新的G1呢？垃圾收集的三个性能指标: footprint, max pause time, throughput彷佛像CAP同样不能同时知足。
在服务端更注重的是短停顿时间，也就是stop-the-world的时间，另一段时间内的总停顿时间也是一个衡量指标。
Mark-Sweep, Mark-Compact均须要和清理区域大小成比例的工做量，而Copying算法则须要通常是一半的空间用于存放每次copy的活对象。CMS的Initial Marking和Remarking两个STW阶段在Heap区愈来愈大的状况下须要的时间越长，而且因为内存碎片，须要压缩的话也会形成较长停顿时间。因此须要一种高吞吐量的短暂停时间的收集器，而无论堆内存多大。linux

G1的实现方式

Region

避免长暂停时间，能够考虑将堆分红多个部分，一次收集其中一部分，这样的方式又叫作增量收集(incremental collection), 分代收集也能够当作一种特殊的增量收集。
G1收集器将堆内存划分为一系列大小相等的Region区域，Region大小在1MB到32MB在启动时肯定，G1一样也使用分代收集策略，将堆分为Eden, Survivior, Old等，只不过是按照逻辑划分的，每一个Region逻辑上属于一个分代区域，而且在物理上不连续，当一个Old的Region收集完成后会变成新可用Region并可能成为下一个Eden Region。当申请的对象大于Region大小的一半时，会被放入一个Humongous Region(巨型区域)中。当一个Region中是空的时，称为可用Region或新Region。

图片中E指Eden, S是Survivor, H指Humongous, O是Old, 空白区域是可用分区。git

CardTable

由于G1只回收一部分Region, 因此回收的时候须要知道哪些其余Region的对象引用着本身Region的对象，由于采用的copying算法须要移动对象，因此要更新引用为对象的新地址，在普通的分代收集中也是如此，分代收集中年轻代收集须要老年代到年轻代的引用的记录，一般叫作remembered set(简称RS)。CardTable是一种remembered set, 一个card表明一个范围的内存，目前采用512bytes表示一个card，cardtable就是一个byte数组，每一个Region有本身的cardtable。维护remembered set须要mutator线程在可能修改跨Region的引用的时候通知collector, 这种方式一般叫作write barrier(和GC中的Memory Barrier不一样), 每一个线程都会有本身的remembered set log,至关于各自的修改的card的缓冲buffer,除此以外还有全局的buffer, mutator本身的remember set buffer满了以后会放入到全局buffer中，而后建立一个新的buffer。

只有来自其余Region的引用须要记录在RS中，因此Region内部的引用和null都不须要记录RS。github

收集过程分类

Marking

G1收集器的标记阶段负责标记处存活的对象、而且计算各个Region的活跃度等。
G1使用了一种Snaphot-At-The-Beginning简称SATB的标记算法, 记录标记开始时的对象图的快照，以后并发收集过程当中的新申请的对象都认为是存活对象, 当堆使用比例超过InitiatingHeapOccupancyPercent后开始marking阶段，使用SATB记录marking开始阶段的对象图快照，。
G1使用bitmap标记处哪些位置已经完成标记了，一个bitmap的bit表示8bytes, 咱们使用两个marking bitmap，一个previous、一个next, previous marking bitmap表示已经完成标记的部分，标记完成后会交换previous和next
标记阶段分为几个步骤。算法

Initial Marking Phase

标记周期的最开始是清除next marking bitmap，是并发执行的。而后开始initial marking phase, 会暂停全部线程，标记出全部能够直接从GC roots能够到达的对象，这是在Young GC的暂停收集阶段顺带进行的。数组

Root Region Scan Phase

找出全部的GC Roots的Region, 而后从这些Region开始标记可到达的对象，是一个并发阶段。多线程

Concurrent Marking Phase

这个阶段G1经过tracing找出整个堆全部的可到达的对象。这个阶段是并发执行的。并发

Remark Phase

Remark是一个STW阶段，G1将全部的SATB buffer处理完成。oracle

Cleanup Phase

marking的最后一个阶段，G1统计各个Region的活跃性，彻底没有存活对象的Region直接放入空闲可用Region列表中，而后会找出mixed GC的Region候选列表。

收集过程

和通常的分代式收集不一样，G1中除了普通的Young GC，还有Mixed GC。

Young Garbage Collection

当Eden区域没法申请新的对象时（满了），就会进行Young GC, Young GC将Eden和Survivor区域的Region(称为Collection Set, CSet)中的活对象Copy到一些新Region中(即新的Survivor)，当对象的GC年龄达到阈值后会Copy到Old Region中。因为采起的是Copying算法，因此就避免了内存碎片的问题，再也不须要单独的压缩。

Mixed Garbage Collection

当old区Heap的对象占总Heap的比例超过InitiatingHeapOccupancyPercent以后，就会开始ConcurentMarking, 完成了Concurrent Marking后，G1会从Young GC切换到Mixed GC, 在Mixed GC中，G1能够增长若干个Old区域的Region到CSet中。
Mixed GC的次数根据候选的Old CSet和每次回收的

Full GC

和CMS同样，G1的一些收集过程是和应用程序并发执行的，因此可能尚未回收完成，是因为申请内存的速度比回收速度快，新的对象就占满了全部空间，在CMS中叫作Concurrent Mode Failure, 在G1中称为Allocation Failure，也会降级为一个STW的fullgc。

Floating Garbage

G1使用一种Snapshot-At-The-Begining的方式记录活对象，也就是那一时刻(整个堆concurrent marking开始的时候)的内存的Object graph, 可是在以后这里面的对象可能会变成Garbage, 叫作floating garbage 只能等到下一次收集回收掉。

总结

G1的特色有，将Heap分为大小相等的Region，逻辑分代，Marking的大部分是并发的，STW中大部分采起多线程并行执行，采用Copying进行多线程并行收集。

G1常见的调节参数

-Xmx -Xms

和其余收集器同样，是配置堆的最大大小和初始大小

1	-XX:MaxGCPauseMillis=200

GC最大暂停时间，默认200ms

1	-XX:InitiatingHeapOccupancyPercent=45

开始一个标记周期的堆占用比例阈值，默认45%，注意这里是整个堆，不一样于CMS中的Old堆比例。

1	-XX:G1HeapRegionSize=n

设置每一个Region的大小，这里须要是1MB到32MB的2的指数的大小。

G1在生产环境中的使用实践和日志分析

在JDK1.8上从CMS切换到G1, 只须要更换启动参数便可。

1	-XX:+UseG1GC

从gc监控和gc日志来看，没有明显大的变化，整体上出现的Full gc更少了。下面是一段GC日志。

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// JVM、机器等信息

Java HotSpot(TM) 64-Bit Server VM (25.45-b02) for linux-amd64 JRE (1.8.0_45-b14), built on Apr 10 2015 10:07:45 by "java_re" with gcc 4.3.0 20080428 (Red Hat 4.3.0-8)

// 内存信息

Memory: 4k page, physical 8059416k(2486228k free), swap 2096440k(2096440k free)

// 启动时参数

CommandLine flags: -XX:+DisableExplicitGC -XX:ErrorFile=/xxx/xxx -XX:G1LogLevel=finest -XX:+G1PrintRegionLivenessInfo -XX:+G1SummarizeConcMark -XX:+HeapDumpOnOutOfMemoryError -XX:HeapDumpPath=/var/xxx/logs/xxxx/xxxx.heaperr.log.201706051331 -XX:InitialHeapSize=128950656 -XX:MaxHeapSize=2063210496 -XX:+PrintAdaptiveSizePolicy -XX:+PrintFlagsFinal -XX:+PrintGC -XX:+PrintGCApplicationConcurrentTime -XX:+PrintGCApplicationStoppedTime -XX:+PrintGCDateStamps -XX:+PrintGCDetails -XX:+PrintGCTimeStamps -XX:+PrintHeapAtGC -XX:+PrintTenuringDistribution -XX:+UnlockDiagnosticVMOptions -XX:+UnlockExperimentalVMOptions -XX:+UseCompressedClassPointers -XX:+UseCompressedOops -XX:+UseG1GC

// 由于启动参数中Xms和Xmx不相等，因此G1根据其余参数判断须要进行扩容，进行了heap扩容

0.038: [G1Ergonomics (Heap Sizing) expand the heap, requested expansion amount: 130023424 bytes, attempted expansion amount: 130023424 bytes]

2017-06-05T13:31:19.764+0800: 0.779: Application time: 0.4628680 seconds

2017-06-05T13:31:19.764+0800: 0.779: Total time for which application threads were stopped: 0.0001855 seconds, Stopping threads took: 0.0000637 seconds

2017-06-05T13:31:19.892+0800: 0.907: Application time: 0.1276311 seconds

2017-06-05T13:31:19.892+0800: 0.907: Total time for which application threads were stopped: 0.0001424 seconds, Stopping threads took: 0.0000359 seconds

2017-06-05T13:31:20.176+0800: 1.191: Application time: 0.2839383 seconds

// GC前打印的Heap信息

{Heap before GC invocations=0 (full 0):

garbage-first heap total 126976K, used 14336K [0x0000000085000000, 0x00000000851003e0, 0x0000000100000000)

region size 1024K, 14 young (14336K), 0 survivors (0K)

Metaspace used 8432K, capacity 8590K, committed 8704K, reserved 1056768K

class space used 899K, capacity 921K, committed 1024K, reserved 1048576K

// Young GC的清理暂停

2017-06-05T13:31:20.176+0800: 1.191: [GC pause (G1 Evacuation Pause) (young)

Desired survivor size 1048576 bytes, new threshold 15 (max 15)

// 选择要Collect的Region, 构成Collection Set，CSet

1.191: [G1Ergonomics (CSet Construction) start choosing CSet, _pending_cards: 0, predicted base time: 10.00 ms, remaining time: 190.00 ms, target pause time: 200.00 ms]

// 选择了 14个Eden, 0个Survivor

1.191: [G1Ergonomics (CSet Construction) add young regions to CSet, eden: 14 regions, survivors: 0 regions, predicted young region time: 356.52 ms]

1.191: [G1Ergonomics (CSet Construction) finish choosing CSet, eden: 14 regions, survivors: 0 regions, old: 0 regions, predicted pause time: 366.52 ms, target pause time: 200.00 ms]

, 0.0055898 secs]

[Parallel Time: 4.2 ms, GC Workers: 4]

[GC Worker Start (ms): 1191.5 1191.5 1193.8 1195.6

Min: 1191.5, Avg: 1193.1, Max: 1195.6, Diff: 4.2]

[Ext Root Scanning (ms): 1.3 1.3 0.0 0.0

Min: 0.0, Avg: 0.7, Max: 1.3, Diff: 1.3, Sum: 2.6]

[Update RS (ms): 0.0 0.0 0.0 0.0

Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]

[Processed Buffers: 0 0 0 0

Min: 0, Avg: 0.0, Max: 0, Diff: 0, Sum: 0]

[Scan RS (ms): 0.0 0.0 0.0 0.0

Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]

[Code Root Scanning (ms): 0.4 0.1 0.0 0.0

Min: 0.0, Avg: 0.1, Max: 0.4, Diff: 0.4, Sum: 0.5]

[Object Copy (ms): 2.4 2.7 1.7 0.0

Min: 0.0, Avg: 1.7, Max: 2.7, Diff: 2.7, Sum: 6.7]

[Termination (ms): 0.1 0.1 0.1 0.0

Min: 0.0, Avg: 0.1, Max: 0.1, Diff: 0.1, Sum: 0.3]

[Termination Attempts: 1 3 1 1

Min: 1, Avg: 1.5, Max: 3, Diff: 2, Sum: 6]

[GC Worker Other (ms): 0.0 0.0 0.0 0.0

Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.1]

[GC Worker Total (ms): 4.2 4.2 1.8 0.0

Min: 0.0, Avg: 2.5, Max: 4.2, Diff: 4.2, Sum: 10.2]

[GC Worker End (ms): 1195.6 1195.6 1195.6 1195.6

Min: 1195.6, Avg: 1195.6, Max: 1195.6, Diff: 0.0]

[Code Root Fixup: 0.1 ms]

[Code Root Purge: 0.0 ms]

[Clear CT: 0.1 ms]

[Other: 1.1 ms]

[Choose CSet: 0.0 ms]

[Ref Proc: 0.8 ms]

[Ref Enq: 0.0 ms]

[Redirty Cards: 0.1 ms]

[Parallel Redirty: 0.0 0.0 0.0 0.0

Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]

[Redirtied Cards: 885 0 0 0

Min: 0, Avg: 221.2, Max: 885, Diff: 885, Sum: 885]

[Humongous Reclaim: 0.0 ms]

[Humongous Total: 0]

[Humongous Candidate: 0]

[Humongous Reclaimed: 0]

[Free CSet: 0.0 ms]

[Young Free CSet: 0.0 ms]

[Non-Young Free CSet: 0.0 ms]

[Eden: 14.0M(14.0M)->0.0B(18.0M) Survivors: 0.0B->2048.0K Heap: 14.0M(124.0M)->4016.5K(124.0M)]

Heap after GC invocations=1 (full 0):

garbage-first heap total 126976K, used 4016K [0x0000000085000000, 0x00000000851003e0, 0x0000000100000000)

region size 1024K, 2 young (2048K), 2 survivors (2048K)

Metaspace used 8432K, capacity 8590K, committed 8704K, reserved 1056768K

class space used 899K, capacity 921K, committed 1024K, reserved 1048576K

}

[Times: user=0.01 sys=0.00, real=0.00 secs]

2017-06-05T13:31:20.182+0800: 1.197: Total time for which application threads were stopped: 0.0059362 seconds, Stopping threads took: 0.0000589 seconds

// G1 请求开始并发标记周期, 缘由是内存占用比例超过了阈值

[G1Ergonomics (Concurrent Cycles) request concurrent cycle initiation, reason: occupancy higher than threshold, occupancy: 63963136 bytes, allocation request: 0 bytes, threshold: 58510530 bytes (45.00 %), source: end of GC]

, 0.0152145 secs]

[Parallel Time: 10.3 ms, GC Workers: 4]

[GC Worker Start (ms): 5509.1 5509.1 5514.1 5519.3

Min: 5509.1, Avg: 5512.9, Max: 5519.3, Diff: 10.2]

[Ext Root Scanning (ms): 0.4 0.3 0.0 0.0

Min: 0.0, Avg: 0.2, Max: 0.4, Diff: 0.4, Sum: 0.7]

[Update RS (ms): 1.5 1.8 0.0 0.0

Min: 0.0, Avg: 0.8, Max: 1.8, Diff: 1.8, Sum: 3.3]

[Processed Buffers: 9 6 0 0

Min: 0, Avg: 3.8, Max: 9, Diff: 9, Sum: 15]

[Scan RS (ms): 0.6 0.3 0.0 0.0

Min: 0.0, Avg: 0.2, Max: 0.6, Diff: 0.6, Sum: 0.9]

[Code Root Scanning (ms): 0.0 0.0 0.0 0.0

Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]

[Object Copy (ms): 2.5 2.4 0.0 0.0

Min: 0.0, Avg: 1.2, Max: 2.5, Diff: 2.5, Sum: 4.9]

[Termination (ms): 5.3 5.3 5.2 0.0

Min: 0.0, Avg: 3.9, Max: 5.3, Diff: 5.3, Sum: 15.7]

[Termination Attempts: 1 1 1 1

Min: 1, Avg: 1.0, Max: 1, Diff: 0, Sum: 4]

[GC Worker Other (ms): 0.0 0.0 0.0 0.0

Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]

[GC Worker Total (ms): 10.2 10.2 5.2 0.0

...

// 开始并发周期

5.696: [G1Ergonomics (Concurrent Cycles) initiate concurrent cycle, reason: concurrent cycle initiation requested]

// Young GC中顺带执行的initial-mark

2017-06-05T11:26:22.512+0800: 5.696: [GC pause (G1 Evacuation Pause) (young) (initial-mark)

...

// root region scan

2017-06-05T11:26:22.538+0800: 5.722: [GC concurrent-root-region-scan-start]

2017-06-05T11:26:22.540+0800: 5.724: [GC concurrent-root-region-scan-end, 0.0015848 secs]

// concurrent mark

2017-06-05T11:26:22.540+0800: 5.724: [GC concurrent-mark-start]

2017-06-05T11:26:22.540+0800: 5.724: Total time for which application threads were stopped: 0.0281922 seconds, Stopping threads took: 0.0000381 seconds

2017-06-05T11:26:22.582+0800: 5.766: [GC concurrent-mark-end, 0.0416350 secs]

2017-06-05T11:26:22.582+0800: 5.766: Application time: 0.0417029 seconds

// remark

2017-06-05T11:26:22.582+0800: 5.766: [GC remark 5.766: [Finalize Marking, 0.0002957 secs] 5.766: [GC ref-proc, 0.0039387 secs] 5.770: [Unloading 5.770: [System Dictionary Unloading, 0.0000263 secs] 5.770: [Parallel Unloading, 0.0046529 secs] 5.775: [Deallocate Metadata, 0.0000266 secs], 0.0049240 secs], 0.0094765 secs]

[Times: user=0.03 sys=0.00, real=0.01 secs]

2017-06-05T11:26:22.591+0800: 5.775: Total time for which application threads were stopped: 0.0096170 seconds, Stopping threads took: 0.0000400 seconds

2017-06-05T11:26:22.592+0800: 5.776: Application time: 0.0004581 seconds

// cleanup

2017-06-05T11:26:22.592+0800: 5.776: [GC cleanup

// 统计出了各个Region的收集效率

### PHASE Post-Marking @ 5.776

### HEAP reserved: 0x0000000085000000-0x0000000100000000 region-size: 1048576

###

### type address-range used prev-live next-live gc-eff remset code-roots

### (bytes) (bytes) (bytes) (bytes/ms) (bytes) (bytes)

### OLD 0x0000000085000000-0x0000000085100000 1048576 1048576 245296 0.0 4216 408

### OLD 0x0000000085100000-0x0000000085200000 1048576 1048576 154904 0.0 21376 5424

### OLD 0x0000000085200000-0x0000000085300000 1048576 1048576 1024728 0.0 30904 8520

### OLD 0x0000000085300000-0x0000000085400000 1048576 1048576 669968 0.0 5072 576

### OLD 0x0000000085400000-0x0000000085500000 1048576 1048576 6888 0.0 3136 16

### OLD 0x0000000085500000-0x0000000085600000 1048576 1048576 0 0.0 3136 16

### OLD 0x0000000085600000-0x0000000085700000 1048576 1048576 0 0.0 3136 16

### OLD 0x0000000085700000-0x0000000085800000 1048576 1048576 0 0.0 3136 16

### OLD 0x0000000085800000-0x0000000085900000 1048576 1048576 0 0.0 3136 16

### OLD 0x0000000085900000-0x0000000085a00000 1048576 1048576 0 0.0 3136 16

### OLD 0x0000000085a00000-0x0000000085b00000 1048576 1048576 0 0.0 3136 16

### OLD 0x0000000085b00000-0x0000000085c00000 1048576 1048576 0 0.0 3136 16

### OLD 0x0000000085c00000-0x0000000085d00000 1048576 1048576 0 0.0 3136 16

### OLD 0x0000000085d00000-0x0000000085e00000 1048576 1048576 501136 0.0 11088 2016

...

### OLD 0x000000008a000000-0x000000008a100000 524296 524296 2576 0.0 3136 16

### FREE 0x000000008a100000-0x000000008a200000 0 0 0 0.0 3136 16

### SURV 0x000000008a200000-0x000000008a300000 1048576 1048576 1048576 0.0 4168 16

### SURV 0x000000008a300000-0x000000008a400000 1048576 1048576 1048576 0.0 3824 16

### SURV 0x000000008a400000-0x000000008a500000 1048576 1048576 1048576 0.0 3824 16

### SURV 0x000000008a500000-0x000000008a600000 1048576 1048576 1048576 0.0 3824 16

### SURV 0x000000008a600000-0x000000008a700000 1048576 1048576 1048576 0.0 7368 16

### SURV 0x000000008a700000-0x000000008a800000 1048576 1048576 1048576 0.0 9088 16

### FREE 0x000000008a800000-0x000000008a900000 0 0 0 0.0 3136 16

### FREE 0x000000008a900000-0x000000008aa00000 0 0 0 0.0 3136 16

### EDEN 0x000000008c900000-0x000000008ca00000 1048576 1048576 1048576 0.0 3136 16

### EDEN 0x000000008ca00000-0x000000008cb00000 1048576 1048576 1048576 0.0 3136 16

### EDEN 0x000000008cb00000-0x000000008cc00000 1048576 1048576 1048576 0.0 3136 16

### SUMMARY capacity: 124.00 MB used: 82.50 MB / 66.53 % prev-live: 82.50 MB / 66.53 % next-live: 44.85 MB / 36.17 % remset: 0.64 MB code-roots: 0.02 MB

2017-06-05T11:26:22.595+0800: 5.779: Application time: 0.0020252 seconds

2017-06-05T11:26:22.595+0800: 5.779: [GC concurrent-cleanup-start]

2017-06-05T11:26:22.595+0800: 5.779: [GC concurrent-cleanup-end, 0.0000586 secs]

5.899: [G1Ergonomics (Concurrent Cycles) do not request concurrent cycle initiation, reason: still doing mixed collections, occupancy: 65011712 bytes, allocation request: 0 bytes, threshold: 58510530 bytes (45.00 %), source: end of GC]

// 开始mixed GC，由于有Concurrent Mark阶段添加的old region候选

5.899: [G1Ergonomics (Mixed GCs) start mixed GCs, reason: candidate old regions available, candidate old regions: 29 regions, reclaimable: 15030208 bytes (11.56 %), threshold: 5.00 %]

, 0.0136069 secs]

[Parallel Time: 9.0 ms, GC Workers: 4]

[GC Worker Start (ms): 5885.7 5885.7 5889.8 5889.8

Min: 5885.7, Avg: 5887.7, Max: 5889.8, Diff: 4.1]

[Ext Root Scanning (ms): 0.3 0.3 0.0 0.0

Min: 0.0, Avg: 0.2, Max: 0.3, Diff: 0.3, Sum: 0.7]

[Update RS (ms): 3.0 3.0 0.0 0.0

Min: 0.0, Avg: 1.5, Max: 3.0, Diff: 3.0, Sum: 6.0]

[Processed Buffers: 9 7 0 0

...

// 收集够足够的内存后，取消 mixed GC回退到了 Young GC。

5.920: [G1Ergonomics (Mixed GCs) do not continue mixed GCs, reason: reclaimable percentage not over threshold, candidate old regions: 17 regions, reclaimable: 6121168 bytes (4.71 %), threshold: 5.00 %]

, 0.0069783 secs]

Garbage First G1收集器 理解和原理分析

G1简介

为何须要G1

G1的实现方式

Region

CardTable

收集过程分类

Marking

Initial Marking Phase

Root Region Scan Phase

Concurrent Marking Phase

Remark Phase

Cleanup Phase

收集过程

Young Garbage Collection

Mixed Garbage Collection

Full GC

Floating Garbage

总结

G1常见的调节参数

推荐使用G1的场景

G1在生产环境中的使用实践和日志分析

参考

Garbage First G1收集器理解和原理分析