问题
组内某业务的几个相关接口均超时，上阿里云查日志一看是Dubbo调用超时，查看网络情况未发现异常，直接上Provider的机器查看占用：
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# top
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
7020 root 20 0 2538892 164144 11856 S 90.3 8.7 61:23.54 java
11022 root 20 0 2560528 241340 11920 S 0.3 12.8 311:23.23 java
26805 root 20 0 32612 4036 2472 S 0.3 0.2 24:50.95 AliYunDunUpdate
26838 root 10 -10 134120 14524 5924 S 0.3 0.8 343:05.22 AliYunDun
1 root 20 0 43280 3300 2108 S 0.0 0.2 2:16.82 systemd
2 root 20 0 0 0 0 S 0.0 0.0 0:01.78 kthreadd
3 root 20 0 0 0 0 S 0.0 0.0 1:30.68 ksoftirqd/0
5 root 0 -20 0 0 0 S 0.0 0.0 0:00.00 kworker/0:0H
7 root rt 0 0 0 0 S 0.0 0.0 0:00.00 migration/0
8 root 20 0 0 0 0 S 0.0 0.0 0:00.00 rcu_bh
9 root 20 0 0 0 0 S 0.0 0.0 65:15.85 rcu_sched
10 root rt 0 0 0 0 S 0.0 0.0 2:14.65 watchdog/0
12 root 20 0 0 0 0 S 0.0 0.0 0:00.00 kdevtmpfs
13 root 0 -20 0 0 0 S 0.0 0.0 0:00.00 netns
14 root 20 0 0 0 0 S 0.0 0.0 0:00.00 khungtaskd
15 root 0 -20 0 0 0 S 0.0 0.0 0:00.00 writeback
16 root 0 -20 0 0 0 S 0.0 0.0 0:00.00 kintegrityd
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查询到7020这个进程有异常，在继续查看具体异常线程
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# top -Hp 7020
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
23328 root 20 0 2538892 164144 11856 S 90.0 8.7 0:00.00 java
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找到了当前异常进程下的异常线程后使用jstack查看详细情况
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# jsack -l 6377 > error.log
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# printf "%x\n" 23328
5b20
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然后从jstack里查询该线程信息
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# grep '18e9' error.log --color
"http-bio-6379-exec-200" #8869954 daemon prio=5 os_prio=0 tid=0x00007f74a81f6800 nid=0x5b20 waiting for monitor entry [0x00007f742457f000]
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最后从jstack文件定位到堆栈信息
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"http-bio-7020-exec-200" #8869954 daemon prio=5 os_prio=0 tid=0x00007f74a81f6800 nid=0x5b20 waiting for monitor entry [0x00007f742457f000]
java.lang.Thread.State: BLOCKED (on object monitor)
at org.apache.log4j.Category.callAppenders(Category.java:204)
- waiting to lock <0x00000000800371d0> (a org.apache.log4j.spi.RootLogger)
at org.apache.log4j.Category.forcedLog(Category.java:391)
at org.apache.log4j.Category.log(Category.java:856)
at org.slf4j.impl.Log4jLoggerAdapter.info(Log4jLoggerAdapter.java:368)
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结论
在log4j 1.x版本中，logger.info等日志记录方法是同步的，大量的日志导致线程阻塞在callAppenders()这个方法
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public void callAppenders(LoggingEvent event) {
int writes = 0;
for(Category c = this; c != null; c=c.parent) {
// Protected against simultaneous call to addAppender, removeAppender,...
synchronized(c) {
if(c.aai != null) {
writes += c.aai.appendLoopOnAppenders(event);
}
if(!c.additive) {
break;
}
}
}
if(writes == 0) {
repository.emitNoAppenderWarning(this);
}
}
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临时解决办法先把日志打印级别调高至error，后续在考虑优化方案。
优化方案
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升级log4j 1.x至log4j 2.x版本，同时修改相应配置
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<!-- https://mvnrepository.com/artifact/org.apache.logging.log4j/log4j-core -->
<dependency>
<groupId>org.apache.logging.log4j</groupId>
<artifactId>log4j-core</artifactId>
<version>2.11.2</version>
</dependency>
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-
使用log4j 桥接类
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如果是使用Spring Boot开发的话，就不会出现此问题，Spring Boot默认使用的时logback是不会有同步问题
参考：log4j平稳升级到log4j2
https://www.cnblogs.com/hujunzheng/p/9937097.html
