如何通过消息或调试日志快速识别长时间的 GC 暂停
Java 垃圾回收（gc）暂停由 Neo4j 中的 MonitorGc 进程监控，并记录在 $NEO4J_HOME/logs/debug.log（对于 Neo4j v2.3.x 及更早版本，则为 $NEO4J_HOME/data/graph.db/messages.log）。要快速查找其中最长的 10 次暂停，
在 Neo4j 3.0 到 Neo4j 3.3 之间，可以运行
$ grep -n -i blocked debug.log | sort -r -n -k 11 | head -10其输出将类似于
7830:2016-12-05 19:04:23.865+0000 WARN [o.n.k.i.c.MonitorGc] GC Monitor: Application threads blocked for 707ms. 3283:2016-12-02 13:25:52.264+0000 WARN [o.n.k.i.c.MonitorGc] GC Monitor: Application threads blocked for 579ms. 6360:2016-12-02 15:38:35.502+0000 WARN [o.n.k.i.c.MonitorGc] GC Monitor: Application threads blocked for 381ms. 6349:2016-12-02 15:02:33.156+0000 WARN [o.n.k.i.c.MonitorGc] GC Monitor: Application threads blocked for 285ms. 6361:2016-12-02 15:50:45.560+0000 WARN [o.n.k.i.c.MonitorGc] GC Monitor: Application threads blocked for 274ms. 6920:2016-12-05 16:57:47.170+0000 WARN [o.n.k.i.c.MonitorGc] GC Monitor: Application threads blocked for 251ms.
示例 grep 命令会对 debug.log 执行不区分大小写的搜索（由 -i 参数实现），查找字符串 'blocked' 并返回相应的行号（由 -n 参数实现）。随后，这些结果会按照表示时间的字段进行排序（分别通过 -k 11 和 -k 10 实现），并以数值顺序而非 ASCII 顺序排序（-n 参数），随后按逆时间顺序呈现（-r 参数）。最后，head -10 只会在排序后显示前 10 条结果。
对于 Neo4j 3.4 及以上版本，运行
$ grep -n "VM stop-the-world" debug.log | sort -t= -nrk2 | head -10其输出将类似于
27675:2019-04-09 10:39:27.115+0000 WARN [o.n.k.i.c.VmPauseMonitorComponent] Detected VM stop-the-world pause: {pauseTime=516, gcTime=508, gcCount=1}
2671:2019-04-08 08:03:12.921+0000 WARN [o.n.k.i.c.VmPauseMonitorComponent] Detected VM stop-the-world pause: {pauseTime=483, gcTime=487, gcCount=1}
27885:2019-04-09 11:01:07.033+0000 WARN [o.n.k.i.c.VmPauseMonitorComponent] Detected VM stop-the-world pause: {pauseTime=371, gcTime=422, gcCount=1}
16302:2019-04-09 09:53:31.066+0000 WARN [o.n.k.i.c.VmPauseMonitorComponent] Detected VM stop-the-world pause: {pauseTime=338, gcTime=372, gcCount=1}
14283:2019-04-08 16:54:03.108+0000 WARN [o.n.k.i.c.VmPauseMonitorComponent] Detected VM stop-the-world pause: {pauseTime=272, gcTime=358, gcCount=1}
27904:2019-04-09 11:26:29.463+0000 WARN [o.n.k.i.c.VmPauseMonitorComponent] Detected VM stop-the-world pause: {pauseTime=346, gcTime=355, gcCount=1}
4006:2019-04-08 12:14:33.514+0000 WARN [o.n.k.i.c.VmPauseMonitorComponent] Detected VM stop-the-world pause: {pauseTime=352, gcTime=349, gcCount=1}
14519:2019-04-08 20:37:12.126+0000 WARN [o.n.k.i.c.VmPauseMonitorComponent] Detected VM stop-the-world pause: {pauseTime=253, gcTime=345, gcCount=1}
3983:2019-04-08 11:17:23.039+0000 WARN [o.n.k.i.c.VmPauseMonitorComponent] Detected VM stop-the-world pause: {pauseTime=301, gcTime=322, gcCount=1}
16110:2019-04-09 07:19:25.626+0000 WARN [o.n.k.i.c.VmPauseMonitorComponent] Detected VM stop-the-world pause: {pauseTime=318, gcTime=314, gcCount=1}
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