深入理解java虚拟机GC垃圾回收-虚拟机及垃圾收集器日志参数总结
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前言
阅读分析虚拟机和垃圾收集器的日志是处理Java虚拟机内存问题必备的基础技能,垃圾收集器日志是一系列人为设定的规则,没有任何的“业界标准”可言,每个收集器的日志格式都可能不一样。在JDK 9以前, HotSpot并没有提供统一的日志处理框架,虚拟机各个功能模块的日志开关分布在不同的参数上,日志级别、循环日志大小、输出格式、重定向等设置在不同功能上都要单独解决。直到JDK 9,这种混乱不堪的局面才终于消失, HotSpot所有功能的日志都收归到了“-Xlog”参数上,这个参数的能力也相应被极大拓展了。
一、虚拟机及垃圾收集器日志
-Xlog[:[selector][:[output][:[decorators][:output-options]]]]
命令行中最关键的参数是选择器(Selector),它由标签(Tag)和日志级别(Level)共同组成。标签可理解为虚拟机中某个功能模块的名字,它告诉日志框架用户希望得到虚拟机哪些功能的日志输出。垃圾收集器的标签名称为“gc”,垃圾收集器日志只是HotSpot众多功能日志的其中一项,全部支持的功能模块标签名如下所示:
add, age, alloc, annotation, aot, arguments, attach, barrier, biasedlocking, blocks, bot, breakpoint, bytecode
日志级别从低到高,共有Trace, Debug, Info, Warning, Error, Off六种级别,日志级别决定了输出信息的详细程度,默认级别为Info, HotSpot的日志规则与Log4j、 SLF4j这类Java日志框架大体上是一致的。另外,还可以使用修饰器(Decorator)来要求每行日志输出都附加上额外的内容,支持附加在日志行上的信息包括:
- time:当前日期和时间。
- uptime:虚拟机启动到现在经过的时间,以秒为单位。
- timemillis:当前时间的毫秒数,相当于System.currentTimeMillis()的输出。
- uptimemillis:虚拟机启动到现在经过的毫秒数。
- timenanos:当前时间的纳秒数,相当于System.nanoTime()的输出。
- uptimenanos:虚拟机启动到现在经过的纳秒数。
- pid:进程ID。
- tid:线程ID。
- level:日志级别。
- tags:日志输出的标签集。
如果不指定,默认值是uptime、 level、 tags这三个,此时日志输出类似于以下形式:
[3.080s][info][gc,cpu] GC(5) User=0.03s Sys=0.00s Real=0.01s
二、垃圾收集器日志分析样例
下面举几个例子,展示在JDK 9统一日志框架前、后是如何获得垃圾收集器过程的相关信息,以下均以JDK 9的G1收集器( JDK 9下默认收集器就是G1,所以命令行中没有指定收集器)为例。
1)查看GC基本信息,在JDK 9之前使用-XX: +PrintGC, JDK 9后使用-Xlog: gc:
bash-3.2$ java -Xlog:gc GCTest
[0.222s][info][gc] Using G1
[2.825s][info][gc] GC(0) Pause Young (G1 Evacuation Pause) 26M->5M(256M) 355.623ms
[3.096s][info][gc] GC(1) Pause Young (G1 Evacuation Pause) 14M->7M(256M) 50.030ms
[3.385s][info][gc] GC(2) Pause Young (G1 Evacuation Pause) 17M->10M(256M) 40.576ms
2)查看GC详细信息,在JDK 9之前使用-XX: +PrintGCDetails,在JDK 9之后使用-X-log: gc*,用通配符*将GC标签下所有细分过程都打印出来,如果把日志级别调整到Debug或者Trace(基于版面篇幅考虑,例子中并没有),还将获得更多细节信息:
bash-3.2$ java -Xlog:gc* GCTest
[0.233s][info][gc,heap] Heap region size: 1M
[0.383s][info][gc ] Using G1
[0.383s][info][gc,heap,coops] Heap address: 0xfffffffe50400000, size: 4064 MB, Compressed Oops mode: Non-zero ba
0xfffffffe50000000, Oop shift amount: 3
[3.064s][info][gc,start ] GC(0) Pause Young (G1 Evacuation Pause)
gc,task ] GC(0) Using 23 workers of 23 for evacuation
[3.420s][info][gc,phases ] GC(0) Pre Evacuate Collection Set: 0.2ms
[3.421s][info][gc,phases ] GC(0) Evacuate Collection Set: 348.0ms
gc,phases ] GC(0) Post Evacuate Collection Set: 6.2ms
[3.421s][info][gc,phases ] GC(0) Other: 2.8ms
gc,heap ] GC(0) Eden regions: 24->0(9)
[3.421s][info][gc,heap ] GC(0) Survivor regions: 0->3(3)
[3.421s][info][gc,heap ] GC(0) Old regions: 0->2
[3.421s][info][gc,heap ] GC(0) Humongous regions: 2->1
[3.421s][info][gc,metaspace ] GC(0) Metaspace: 4719K->4719K(1056768K)
[3.421s][info][gc ] GC(0) Pause Young (G1 Evacuation Pause) 26M->5M(256M) 357.743ms
[3.422s][info][gc,cpu ] GC(0) User=0.70s Sys=5.13s Real=0.36s
[3.648s][info][gc,start ] GC(1) Pause Young (G1 Evacuation Pause)
[3.648s][info][gc,task ] GC(1) Using 23 workers of 23 for evacuation
[3.699s][info][gc,phases ] GC(1) Pre Evacuate Collection Set: 0.3ms
gc,phases ] GC(1) Evacuate Collection Set: 45.6ms
gc,phases ] GC(1) Post Evacuate Collection Set: 3.4ms
gc,phases ] GC(1) Other: 1.7ms
gc,heap ] GC(1) Eden regions: 9->0(10)
[3.699s][info][gc,heap ] GC(1) Survivor regions: 3->2(2)
[3.699s][info][gc,heap ] GC(1) Old regions: 2->5
[3.700s][info][gc,heap ] GC(1) Humongous regions: 1->1
[3.700s][info][gc,metaspace ] GC(1) Metaspace: 4726K->4726K(1056768K)
[3.700s][info][gc ] GC(1) Pause Young (G1 Evacuation Pause) 14M->7M(256M) 51.872ms
[3.700s][info][gc,cpu ] GC(1) User=0.56s Sys=0.46s Real=0.05s
3)查看GC前后的堆、方法区可用容量变化,在JDK 9之前使用-XX: +PrintHeapAtGC, JDK 9之后使用-Xlog: gc+heap=debug:
bash-3.2$ java -Xlog:gc+heap=debug GCTest
[0.113s][info][gc,heap] Heap region size: 1M
[0.113s][debug][gc,heap] Minimum heap 8388608 Initial heap 268435456 Maximum heap 4261412864
[2.529s][debug][gc,heap] GC(0) Heap before GC invocations=0 (full 0):
[2.529s][debug][gc,heap] GC(0) garbage-first heap total 262144K, used 26624K [0xfffffffe50400000, 0xfffffffe5050
0xffffffff4e400000)
[2.529s][debug][gc,heap] GC(0) region size 1024K, 24 young (24576K), 0 survivors (0K)
[2.530s][debug][gc,heap] GC(0) Metaspace used 4719K, capacity 4844K, committed 5120K, reserved 1056768K
[2.530s][debug][gc,heap] GC(0) class space used 413K, capacity 464K, committed 512K, reserved 1048576K
[2.892s][info ][gc,heap] GC(0) Eden regions: 24->0(9)
[2.892s][info ][gc,heap] GC(0) Survivor regions: 0->3(3)
[2.892s][info ][gc,heap] GC(0) Old regions: 0->2
[2.892s][info ][gc,heap] GC(0) Humongous regions: 2->1
[2.893s][debug][gc,heap] GC(0) Heap after GC invocations=1 (full 0):
[2.893s][debug][gc,heap] GC(0) garbage-first heap total 262144K, used 5850K [0xfffffffe50400000, 0xfffffffe50500
[2.893s][debug][gc,heap] GC(0) region size 1024K, 3 young (3072K), 3 survivors (3072K)
[2.893s][debug][gc,heap] GC(0) Metaspace used 4719K, capacity 4844K, committed 5120K, reserved 1056768K
[2.893s][debug][gc,heap] GC(0) class space used 413K, capacity 464K, committed 512K, reserved 1048576K
4)查看GC过程中用户线程并发时间以及停顿的时间,在JDK 9之前使用-XX: +PrintGCApplicationConcurrentTime以及-XX: +PrintGCApplicationStoppedTime, JDK 9之后使用-Xlog:safepoint:
bash-3.2$ java -Xlog:safepoint GCTest
[1.376s][info][safepoint] Application time: 0.3091519 seconds
[1.377s][info][safepoint] Total time for which application threads were stopped: 0.0004600 seconds, Stopping thr
0.0002648 seconds
[2.386s][info][safepoint] Application time: 1.0091637 seconds
[2.387s][info][safepoint] Total time for which application threads were stopped: 0.0005217 seconds, Stopping thr
0.0002297 seconds
5)查看收集器Ergonomics机制(自动设置堆空间各分代区域大小、收集目标等内容,从Parallel收集器开始支持)自动调节的相关信息。在JDK 9之前使用-XX: +PrintAdaptive-SizePolicy, JDK 9之后使用-Xlog: gc+ergo*=trace:
bash-3.2$ java -Xlog:gc+ergo*=trace GCTest [0.122s][debug][gc,ergo,refine] Initial Refinement Zones: green: 23,
69, red: 115, min yellow size: 46
[0.142s][debug][gc,ergo,heap ] Expand the heap. requested expansion amount:268435456B expansion amount:268435456
[2.475s][trace][gc,ergo,cset ] GC(0) Start choosing CSet. pending cards: 0 predicted base time: 10.00ms remainin
190.00ms target pause time: 200.00ms
[2.476s][trace][gc,ergo,cset ] GC(0) Add young regions to CSet. eden: 24 regions, survivors: 0 regions, predicte
region time: 367.19ms, target pause time: 200.00ms
[2.476s][debug][gc,ergo,cset ] GC(0) Finish choosing CSet. old: 0 regions, predicted old region time: 0.00ms, ti
remaining: 0.00
[2.826s][debug][gc,ergo ] GC(0) Running G1 Clear Card Table Task using 1 workers for 1 units of work for 24 regi
[2.827s][debug][gc,ergo ] GC(0) Running G1 Free Collection Set using 1 workers for collection set length 24
[2.828s][trace][gc,ergo,refine] GC(0) Updating Refinement Zones: update_rs time: 0.004ms, update_rs buffers: 0,
goal time: 19.999ms
6)查看熬过收集后剩余对象的年龄分布信息,在JDK 9前使用-XX: +PrintTenuring-Distribution,JDK 9之后使用-Xlog: gc+age=trace:
bash-3.2$ java -Xlog:gc+age=trace GCTest
[2.406s][debug][gc,age] GC(0) Desired survivor size 1572864 bytes, new threshold 15 (max threshold 15)
[2.745s][trace][gc,age] GC(0) Age table with threshold 15 (max threshold 15)
[2.745s][trace][gc,age] GC(0) - age 1: 3100640 bytes, 3100640 total
[4.700s][debug][gc,age] GC(5) Desired survivor size 2097152 bytes, new threshold 15 (max threshold 15)
[4.810s][trace][gc,age] GC(5) Age table with threshold 15 (max threshold 15)
[4.810s][trace][gc,age] GC(5) - age 1: 2658280 bytes, 2658280 total
[4.810s][trace][gc,age] GC(5) - age 2: 1527360 bytes, 4185640 total
三、JDK 9前后日志参数变化
四、垃圾收集器参数总结
结尾
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