> ## Content Index > Fetch the complete content index at: https://blog.hacking.pub/llms.txt > Use this file to discover other available public pages before exploring further. # 进程上下文切换 - URL: https://blog.hacking.pub/2016/10/31/jin-cheng-shang-xia-wen-qie-huan/ - Published: 2016-10-31T08:48:19.000Z - Updated: 2016-10-31T10:27:00.000Z - Author: chainhelen #### 问题 今天发现测试服务器的ssh登录很卡，几乎很难登录进去，即便登录进去了，看日志的时候也会卡死终端。 #### 查找原因 使用ssh -v 参数查看，一直卡在sent地方 ``` $ ssh -v test OpenSSH_6.9p1 Ubuntu-2, OpenSSL 1.0.2d 9 Jul 2015 debug1: Reading configuration data /home/ch/.ssh/config debug1: /home/ch/.ssh/config line 1: Applying options for test debug1: Reading configuration data /etc/ssh/ssh_config debug1: /etc/ssh/ssh_config line 19: Applying options for * debug1: Connecting to *.*.*.* [*.*.*.*] port 22. debug1: Connection established. debug1: key_load_public: No such file or directory debug1: identity file /home/ch/.ssh/id_rsa_test type -1 debug1: key_load_public: No such file or directory debug1: identity file /home/ch/.ssh/id_rsa_test-cert type -1 debug1: Enabling compatibility mode for protocol 2.0 debug1: Local version string SSH-2.0-OpenSSH_6.9p1 Ubuntu-2 debug1: Remote protocol version 2.0, remote software version OpenSSH_6.6.1p1 Ubuntu-2ubuntu2.3 debug1: match: OpenSSH_6.6.1p1 Ubuntu-2ubuntu2.3 pat OpenSSH_6.6.1* compat 0x04000000 debug1: Authenticating to 120.24.180.89:22 as 'root' debug1: SSH2_MSG_KEXINIT sent ``` top -o %CPU (有版本的top是没有o参数的)了一下，发现php的进程占用了99.9%的CPU。好不容易敲了reboot执行成功了，但是重启过后的，仅仅是稍微好了一些而已。 ``` $ vmstat 1 procs -----------memory---------- ---swap-- -----io---- -system-- ------cpu----- r b swpd free buff cache si so bi bo in cs us sy id wa st 2 0 0 79320 32996 393880 0 0 0 68 4855 18998 0 1 99 0 0 1 0 0 79296 32996 393888 0 0 0 0 4864 19101 0 0 99 0 1 1 0 0 79236 32996 393888 0 0 0 0 4849 19057 1 0 99 0 0 1 0 0 79208 32996 393888 0 0 0 0 4863 19181 0 0 100 0 0 1 0 0 79180 32996 393888 0 0 0 0 4809 18997 1 0 97 0 1 1 0 0 79152 32996 393888 0 0 0 0 4814 18920 3 0 97 0 0 ``` 可以看到cs过高，这东西就是上下文切换。说明上下文切换（cs）特别频繁 ``` $ pidstat -w 1 Average: UID PID cswch/s nvcswch/s Command Average: 0 3 18.81 0.00 ksoftirqd/0 Average: 0 6 0.20 0.00 kworker/u16:0 Average: 0 7 39.47 0.00 rcu_sched Average: 0 10 0.41 0.00 watchdog/0 Average: 0 16 181.80 0.00 kworker/0:1 Average: 0 125 0.61 0.00 jbd2/xvda1-8 Average: 0 836 0.20 0.00 kworker/u17:1 Average: 0 843 1.64 0.00 php5-fpm Average: 65534 1012 0.20 0.00 nscd Average: 106 1037 6578.53 0.00 redis-server Average: 0 1085 0.20 0.00 AliYunDunUpdate Average: 0 1104 28.63 0.00 AliYunDun Average: 0 1149 1.43 0.00 apache2 Average: 0 1163 28.63 0.00 AliHids Average: 103 1479 1.43 0.00 ntpd Average: 0 1894 7.16 0.00 PM2 Average: 0 1965 142.33 0.00 mongod Average: 0 2006 2.86 0.00 node Average: 0 6815 2.86 0.00 node Average: 0 7068 6682.21 6474.64 php Average: 0 7775 2.86 0.00 node Average: 0 7780 2.86 0.00 node Average: 0 7785 2.86 0.00 node Average: 33 8127 2.86 0.00 nginx Average: 33 8128 2.86 0.00 nginx Average: 33 8129 2.86 0.00 nginx Average: 0 14255 180.16 5.93 sshd Average: 0 14545 1.43 195.09 pidstat ``` 结果可以看出php进程的cswch(自愿的上下文切换)和nvcswch(非自愿的上下文切换) #### 暴力解决 直接kill掉，就正常了。重新手动启动php。 #### 实际原因 不停地测试不同php进程，终于找到是这个死循环造成的，usleep时间太短，上下文切换频繁。 ![php_code](http://image.hacking.pub/s/p91ltefvvkedygl/php_deamed.png?dl=0) #### 参考 > [用十条命令在一分钟内检查Linux服务器性能](http://www.infoq.com/cn/news/2015/12/linuxperformance/?ref=blog.hacking.pub) > [进程上下文频繁切换导致load average过高](http://www.361way.com/linux-context-switch/5131.html?ref=blog.hacking.pub) > [进程上下文切换 – 残酷的性能杀手（上）](http://www.cnblogs.com/zhiranok/archive/2012/08/13/context%5Fswitch%5F1.html?ref=blog.hacking.pub) > [Linux vmstat命令实战详解](http://www.cnblogs.com/ggjucheng/archive/2012/01/05/2312625.html?ref=blog.hacking.pub)