OpenJDK 11 是在 OpenJDK 8 之后的第一个长期支持版本，这一版本在JVM日志配置有了很大改动，主要是规范化，统一化了。在 OpenJDK 8 中，日志配置有很多状态位，让人摸不着头脑，并且比较难以维护与进一步迭代。在 OpenJDK 11终于将JVM日志相关的配置规范起来，统一配置。这篇文章会对于这些配置做一个基本的说明和解析。
一、JVM日志标签
JVM 日志和我们 java 代码中的日志，其实是类似。在 Java 代码中，我们一般使用 slf4j 记录日志，例如：
Logger logger = LogFactory.getLooger("core-logger"); logger.info("this is core logger log");
然后日志中就会输出类似于：
2020-02-05 10:50:52.670 INFO [core-logger] [22] [pool-13-thread-1]: this is core logger log
包括时间戳，日志级别，日志标签（core-logger），日志内容这些信息。JVM 日志也是包括这些元素。举个例子：
[0.182s][debug][jit,compilation] 1 3 java.lang.StringLatin1::hashCode (42 bytes) [0.183s][debug][jit,compilation] 2 3 java.lang.Object::<init> (1 bytes) [0.183s][debug][jit,compilation] 3 3 java.lang.String::hashCode (49 bytes)
可以看出，默认的 JVM 日志包括：
[启动经过时间][日志级别][日志标签，可能包含多个] 日志内容
其中一行日志，可能包含多个标签，之后关于JVM日志相关的配置，也是围绕着这些标签进行配置。大部分的标签是给JVM开发者用的，其中某些标签供我们使用 JVM 的人进行 JVM 参数调优以及代码调优。那么我们需要关心哪些标签呢？我个人按照功能，把我们需要关心的标签分为如下几大类
1. gc相关
gc日志有很多标签与组合，大部分以 gc 标签为开始，混合搭配其他一些标签。一般，有如下几个标签我们会经常用到：
标签gc
gc总体描述日志，一般设置info级别查看gc的发生时间，消耗时间还有内存大小。例如：Pause Young (Normal) (g1 Evacuation Pause) 3480M->1565M(5120M) 15.968ms 包含了gc类型，gc原因，收集内存大小，持续时间等信息
标签gc,age
gc 中 age 相关信息，age比较高的对象会进入老年代。如果是 trace 级别，会输出每一个 age 的所有对象占用总大小，以及比这个 age 低的所有 age 的大小，debug 级别只会输出最高级别的 age 以及期望大小，不是当前总大小，例如：
[2020-02-26T08:34:12.823+0000][debug][gc,age ] gc(1661) Desired survivor size 167772160 bytes, new threshold 6 (max threshold 6) [2020-02-26T08:34:12.823+0000][trace][gc,age ] age 1: 16125960 bytes, 16125960 total [2020-02-26T08:34:12.823+0000][trace][gc,age ] age 2: 16259512 bytes, 32385472 total [2020-02-26T08:34:12.823+0000][trace][gc,age ] age 3: 2435240 bytes, 34820712 total [2020-02-26T08:34:12.823+0000][trace][gc,age ] age 4: 17179320 bytes, 52000032 total [2020-02-26T08:34:12.823+0000][trace][gc,age ] age 5: 43986952 bytes, 95986984 total [2020-02-26T08:34:12.823+0000][trace][gc,age ] age 6: 20858328 bytes, 116845312 total
标签gc,alloc，gc,alloc,region
这两个参数仅对 g1 gc 有效 gc,alloc 在 gc 完成的时候，打印 trace 级别日志记录触发 gc 的线程是哪一个以及返回的 gc 结果地址;这个一般是在调试 gc 的时候才需要看这个日志。 gc,alloc,region 统计每次 gc 的 Regions 信息，打印 debug 级别日志。
[2020-02-28T02:14:02.694+0000][trace][gc,alloc ] sdk-27692-2-amqp-t-4: Successfully scheduled collection returning 0x00000007ffc00000 [2020-02-28T02:16:00.372+0000][debug][gc,alloc,region ] gc(7848) Mutator Allocation stats, regions: 677, wasted size: 63832B ( 0.0%)
标签gc,cpu 这个是大多数 gc 问题定位需要查看的日志， info 级别打印每次gc真正耗时：
[2020-02-28T01:59:46.406+0000][info ][gc,cpu ] gc(7841) User=0.10s Sys=0.00s Real=0.04s [2020-02-28T02:01:20.148+0000][info ][gc,cpu ] gc(7842) User=0.04s Sys=0.06s Real=0.04s
注意这个和 JFR 的统计可能会有差异，JFR 统计的 gc 时间是从开始 schedule gc 就认为开始 gc ，而这里的时间是从开始标记开始。
标签gc,ergo，gc,ergo,cset，gc,ergo,ihop，gc,ergo,refine
这是 Adaptive Size Policy 相关的日志，如果想详细学习算法，可以用 trace 级别，一般的 debug 级别信息就够了
[2020-02-28T01:59:46.367+0000][trace][gc,ergo,cset ] gc(7841) Start choosing CSet. pending cards: 26996 predicted base time: 13.34ms remaining time: 186.66ms target pause time: 200.00ms [2020-02-28T01:59:46.367+0000][trace][gc,ergo,cset ] gc(7841) Add young regions to CSet. eden: 676 regions, survivors: 6 regions, predicted young region time: 19.02ms, target pause time: 200.00ms [2020-02-28T01:59:46.367+0000][debug][gc,ergo,cset ] gc(7841) Finish choosing CSet. old: 0 regions, predicted old region time: 0.00ms, time remaining: 167.64 [2020-02-28T01:59:46.389+0000][debug][gc,ergo ] gc(7841) Running g1 Clear Card Table Task using 4 workers for 7 units of work for 895 regions. [2020-02-28T01:59:46.391+0000][debug][gc,ergo ] gc(7841) Running g1 Free Collection Set using 4 workers for collection set length 682 [2020-02-28T01:59:46.391+0000][trace][gc,ergo,refine ] gc(7841) Updating Refinement Zones: update_rs time: 6.800ms, update_rs buffers: 397, update_rs goal time: 19.998ms [2020-02-28T01:59:46.391+0000][debug][gc,ergo,refine ] gc(7841) Updated Refinement Zones: green: 572, yellow: 1716, red: 2860 [2020-02-28T02:01:20.108+0000][trace][gc,ergo,cset ] gc(7842) Start choosing CSet. pending cards: 25786 predicted base time: 12.87ms remaining time: 187.13ms target pause time: 200.00ms [2020-02-28T02:01:20.108+0000][trace][gc,ergo,cset ] gc(7842) Add young regions to CSet. eden: 677 regions, survivors: 5 regions, predicted young region time: 14.43ms, target pause time: 200.00ms [2020-02-28T02:01:20.108+0000][debug][gc,ergo,cset ] gc(7842) Finish choosing CSet. old: 0 regions, predicted old region time: 0.00ms, time remaining: 172.70 [2020-02-28T02:01:20.132+0000][debug][gc,ergo ] gc(7842) Running g1 Clear Card Table Task using 4 workers for 8 units of work for 903 regions. [2020-02-28T02:01:20.133+0000][debug][gc,ergo ] gc(7842) Running g1 Free Collection Set using 4 workers for collection set length 682 [2020-02-28T02:01:20.133+0000][trace][gc,ergo,refine ] gc(7842) Updating Refinement Zones: update_rs time: 6.303ms, update_rs buffers: 305, update_rs goal time: 19.997ms [2020-02-28T02:01:20.133+0000][debug][gc,ergo,refine ] gc(7842) Updated Refinement Zones: green: 572, yellow: 1716, red: 2860 [2020-02-28T02:04:36.095+0000][trace][gc,ergo,cset ] gc(7843) Start choosing CSet. pending cards: 26115 predicted base time: 12.85ms remaining time: 187.15ms target pause time: 200.00ms [2020-02-28T02:04:36.095+0000][trace][gc,ergo,cset ] gc(7843) Add young regions to CSet. eden: 676 regions, survivors: 6 regions, predicted young region time: 69.11ms, target pause time: 200.00ms [2020-02-28T02:04:36.095+0000][debug][gc,ergo,cset ] gc(7843) Finish choosing CSet. old: 0 regions, predicted old region time: 0.00ms, time remaining: 118.04 [2020-02-28T02:04:36.118+0000][debug][gc,ergo ] gc(7843) Running g1 Clear Card Table Task using 4 workers for 7 units of work for 894 regions. [2020-02-28T02:04:36.120+0000][debug][gc,ergo ] gc(7843) Running g1 Free Collection Set using 4 workers for collection set length 682 [2020-02-28T02:04:36.121+0000][trace][gc,ergo,refine ] gc(7843) Updating Refinement Zones: update_rs time: 6.929ms, update_rs buffers: 364, update_rs goal time: 19.997ms [2020-02-28T02:04:36.121+0000][debug][gc,ergo,refine ] gc(7843) Updated Refinement Zones: green: 572, yellow: 1716, red: 2860
标签gc,heap，gc,heap,region
gc,heap的 debug 级别会显示 gc 的时候堆的概况，对于 g1 gc gc,heap,region的 trace 级别，会打印每一个 region 的详细情况，这个一般供 gc 调试使用。
我们一般只需要关心gc,heap的日志就行了
这些标签是在
[2020-02-28T06:01:20.787+0000][debug][gc,heap ] gc(7922) Heap before gc invocations=7922 (full 0): garbage-first heap total 8388608K, used 4076387K [0x0000000600000000, 0x0000000800000000) [2020-02-28T06:01:20.787+0000][debug][gc,heap ] gc(7922) region size 4096K, 682 young (2793472K), 5 survivors (20480K) [2020-02-28T06:01:20.787+0000][debug][gc,heap ] gc(7922) Metaspace used 163068K, capacity 166731K, committed 169728K, reserved 1198080K[2020-02-28T06:01:20.787+0000][debug][gc,heap ] gc(7922) class space used 18180K, capacity 19580K, committed 20480K, reserved 1048576K [2020-02-28T06:01:20.787+0000][trace][gc,heap,region ] gc(7922) Heap Regions: E=young(eden), S=young(survivor), O=old, HS=humongous(starts), HC=humongous(continues), CS=collection set, F=free, A=archive, TAMS=top-at-mark-start (previous, next)[2020-02-28T06:01:20.787+0000][trace][gc,heap,region ] gc(7922) | 0|0x0000000600000000, 0x0000000600400000, 0x0000000600400000|100%| O| |TAMS 0x0000000600400000, 0x0000000600000000| Untracked [2020-02-28T06:01:20.787+0000][trace][gc,heap,region ] gc(7922) | 1|0x0000000600400000, 0x0000000600800000, 0x0000000600800000|100%| O| |TAMS 0x0000000600800000, 0x0000000600400000| Untracked [2020-02-28T06:01:20.787+0000][trace][gc,heap,region ] gc(7922) | 2|0x0000000600800000, 0x0000000600c00000, 0x0000000600c00000|100%| O| |TAMS 0x0000000600c00000, 0x0000000600800000| Untracked [2020-02-28T06:01:20.787+0000][trace][gc,heap,region ] gc(7922) | 3|0x0000000600c00000, 0x0000000601000000, 0x0000000601000000|100%| O| |TAMS 0x0000000601000000, 0x0000000600c00000| Untracked [2020-02-28T06:01:20.787+0000][trace][gc,heap,region ] gc(7922) | 4|0x0000000601000000, 0x0000000601400000, 0x0000000601400000|100%| O| |TAMS 0x0000000601400000, 0x0000000601000000| Untracked [2020-02-28T06:01:20.787+0000][trace][gc,heap,region ] gc(7922) | 5|0x0000000601400000, 0x0000000601800000, 0x0000000601800000|100%| O| |TAMS 0x0000000601800000, 0x0000000601400000| Untracked [2020-02-28T06:01:20.787+0000][trace][gc,heap,region ] gc(7922) | 6|0x0000000601800000, 0x0000000601c00000, 0x0000000601c00000|100%| O| |TAMS 0x0000000601c00000, 0x0000000601800000| Untracked
标签gc,humongous 如果你使用的是g1 gc，并且经常出现 Evacuation Failure 或者 Humongous Allocation ，并且不知道是什么原因的话，可以考虑看看这个标签相关的日志：
[2020-02-28T06:01:20.831+0000][debug][gc,humongous ] gc(7922) Live humongous region 219 object size 2160888 start 0x0000000636c00000 with remset 1 code roots 0 is marked 0 reclaim candidate 0 type array 0 [2020-02-28T06:01:20.831+0000][debug][gc,humongous ] gc(7922) Live humongous region 412 object size 2160888 start 0x0000000667000000 with remset 1 code roots 0 is marked 0 reclaim candidate 0 type array 0 [2020-02-28T06:01:20.831+0000][debug][gc,humongous ] gc(7922) Live humongous region 443 object size 3241320 start 0x000000066ec00000 with remset 1 code roots 0 is marked 0 reclaim candidate 0 type array 0 [2020-02-28T06:01:20.831+0000][debug][gc,humongous ] gc(7922) Live humongous region 489 object size 2160888 start 0x000000067a400000 with remset 2 code roots 0 is marked 0 reclaim candidate 0 type array 0 [2020-02-28T06:01:20.831+0000][debug][gc,humongous ] gc(7922) Live humongous region 490 object size 2160888 start 0x000000067a800000 with remset 1 code roots 0 is marked 0 reclaim candidate 0 type array 0 [2020-02-28T06:01:20.831+0000][debug][gc,humongous ] gc(7922) Live humongous region 499 object size 7292936 start 0x000000067cc00000 with remset 2 code roots 0 is marked 0 reclaim candidate 0 type array 0 [2020-02-28T06:01:20.831+0000][debug][gc,humongous ] gc(7922) Live humongous region 536 object size 2160888 start 0x0000000686000000 with remset 2 code roots 0 is marked 0 reclaim candidate 0 type array 0 [2020-02-28T06:01:20.831+0000][debug][gc,humongous ] gc(7922) Live humongous region 656 object size 2160888 start 0x00000006a4000000 with remset 1 code roots 0 is marked 0 reclaim candidate 0 type array 0 [2020-02-28T06:01:20.831+0000][debug][gc,humongous ] gc(7922) Live humongous region 768 object size 2160888 start 0x00000006c0000000 with remset 1 code roots 0 is marked 0 reclaim candidate 0 type array 0 [2020-02-28T06:01:20.831+0000][debug][gc,humongous ] gc(7922) Live humongous region 786 object size 2160888 start 0x00000006c4800000 with remset 1 code roots 0 is marked 0 reclaim candidate 0 type array 0
标签gc,metaspace，gc,metaspace,freelist，gc,metaspace,freelist,blocks查看 metaspace 相关的 gc 日志，gc,metaspace的 info 级别会输出每次 gc 涉及的 metaspace 内存变化，如果有变化，详细变化会通过gc,metaspace,freelist，gc,metaspace,freelist,blocks的 trace 级别输出。
[2020-02-28T04:32:13.123+0000][info ][gc,metaspace ] gc(7896) Metaspace: 163062K->163062K(1198080K) [2020-02-28T04:35:44.956+0000][trace][gc,metaspace,freelist ] SpaceManager::grow_and_allocate for 49 words 109 words used 19 words left [2020-02-28T04:35:44.956+0000][trace][gc,metaspace,freelist ] ChunkManager::free_chunks_get: free_list: 0x00007fddccb89770 chunks left: 433. [2020-02-28T04:35:44.956+0000][trace][gc,metaspace,freelist ] ChunkManager::chunk_freelist_allocate: 0x00007fddccb89770 chunk 0x00007fdc74221000 size 128 count 433 Free chunk total 255616 count 824 [2020-02-28T04:35:44.956+0000][trace][gc,metaspace,freelist,blocks] returning block at 0x00007fdd95575b68 size = 19 [2020-02-28T04:35:44.956+0000][trace][gc,metaspace,freelist ] SpaceManager::added chunk: [2020-02-28T04:35:44.956+0000][trace][gc,metaspace,freelist ] Metachunk: bottom 0x00007fdc74221000 top 0x00007fdc74221040 end 0x00007fdc74221400 size 128 (specialized) [2020-02-28T04:35:44.956+0000][trace][gc,metaspace,freelist ] Free chunk total 255616 count 824 [2020-02-28T04:36:35.367+0000][info ][gc,metaspace ] gc(7897) Metaspace: 163065K->163065K(1198080K)
标签gc,phases，gc,phases,ref，gc,phases,task，gc,ref，gc,start,gc,ref,start 这些标签与 gc 步骤相关，如果想学习 gc 算法，可以查看这些标签的日志，来了解 gc 的步骤以及原理
标签safepoint
我们知道只有到达 safepoint，我们才可以进行 gc，如果我们对这些 safepoint 感兴趣，可以查看这个标签的 debug 级别的日志
2. 类加载与运行时编译相关
标签class,preorder，class,init，class,load,class,unload 顾名思义，这是类初始化，类加载与类卸载的日志，info 级别的信息就以足够。 如果你想学习 JVM 类加载过程，可以查看 class 标签的 trace 级别日志。
[8.931s][debug][class,preorder ] com.fasterxml.jackson.core.PrettyPrinter source: file:/D:/Repositories/maven/com/fasterxml/jackson/core/jackson-core/2.10.0/jackson-core-2.10.0.jar [8.931s][info][class,init ] 2740 Initializing 'com/fasterxml/jackson/core/PrettyPrinter' (0x0000000801399220) [8.934s][info][class,load ] com.fasterxml.jackson.core.PrettyPrinter source: file:/D:/Repositories/maven/com/fasterxml/jackson/core/jackson-core/2.10.0/jackson-core-2.10.0.jar
标签jit,compilation 一般我们对于即时编译优化，只用看 jit 编译日志即可，对应的标签是jit,compilation，日志级别是 debug
[2020-02-28T03:01:51.619+0000][debug][jit,compilation] 153756 ! 4 jdk.internal.reflect.GeneratedConstructorAccessor161::newInstance (49 bytes) made zombie [2020-02-28T03:01:51.620+0000][debug][jit,compilation] 153219 4 io.lettuce.core.protocol.CommandArgs$IntegerArgument::encode (12 bytes) made zombie [2020-02-28T03:01:51.623+0000][debug][jit,compilation] 153192 4 io.lettuce.core.protocol.CommandArgs$StringArgument::writeString (60 bytes) made zombie [2020-02-28T03:01:54.911+0000][debug][jit,compilation] 157252 ! 4 jdk.internal.reflect.GeneratedConstructorAccessor161::newInstance (49 bytes)
3. 其他运行时相关
标签monitorinflation 同步锁相关日志, 一般查看 debug 级别的，可以用于定位死锁
[5.033s][debug][monitorinflation] Deflating object 0x0000000708310378 , mark 0x0000021cef446002 , type java.lang.ref.ReferenceQueue$Lock [5.033s][debug][monitorinflation] Inflating object 0x0000000708310378 , mark 0x0000021cf085c002 , type java.lang.ref.ReferenceQueue$Lock [5.035s][debug][monitorinflation] Deflating object 0x0000000708310378 , mark 0x0000021cf085c002 , type java.lang.ref.ReferenceQueue$Lock [5.035s][debug][monitorinflation] Inflating object 0x0000000708310378 , mark 0x0000021cef445e02 , type java.lang.ref.ReferenceQueue$Lock
标签biasedlocking 偏向锁相关日志，一般查看 info 级别即可，trace 级别显示更详细的偏向锁争用细节，可以用于学习偏向锁实现原理
[7.273s][info ][biasedlocking] Revoking bias by walking my own stack: [7.273s][info ][biasedlocking] Revoking bias of object 0x0000000711b1ca40, mark 0x000001c6d0acc905, type sun.net.www.protocol.jar.URLJarFile, prototype header 0x0000000000000105, allow rebias 0, requesting thread 0x000001c6d0acc800 [7.273s][info ][biasedlocking] Revoked bias of object biased toward live thread (0x000001c6d0acc800) [7.273s][trace][biasedlocking] mon_info->owner (0x00000007022634d8) != obj (0x0000000711b1ca40) [7.273s][trace][biasedlocking] mon_info->owner (0x0000000711b200d8) != obj (0x0000000711b1ca40) [7.273s][trace][biasedlocking] mon_info->owner (0x0000000711b200d8) != obj (0x0000000711b1ca40) [7.273s][trace][biasedlocking] mon_info->owner (0x0000000702970260) != obj (0x0000000711b1ca40) [7.273s][info ][biasedlocking] Revoked bias of currently-unlocked object
二、JVM日志配置
配置格式：
-Xlog[:[what][:[output][:[decorators][:output-options[,...]]]]]
如果不配置，默认的是：
-Xlog:all=warning:stdout:uptime,level,tags
这是一个冒号分割的配置，第一个冒号后面的就是what，第二个是output，第三个是decorators，第四个是逗号分割的output-options.没有配置的部分就是上面默认值的对应部分，例如下面的几组配置就是等价的：
-Xlog:all=warning与-Xlog::stdout与-Xlog::::uptime,level,tags与-Xlog:all=warning:stdout与-Xlog::stdout:uptime,level,tags与-Xlog:all=warning:stdout:uptime,level,tags-Xlog:gc*=info与-Xlog:gc*=info:stdout:uptime,level,tags-Xlog::file=/project/log/app.log::filecount=50,filesize=100M与-Xlog:all=warning:file=/project/log/app.log:uptime,level,tags:filecount=50,filesize=100M
