$ su - testuser
su: cannot set user id: Resource temporarily unavailable
原因： 默认一般用户的最大允许进程数是1024，而smber用户进程数超过限制。
$ lsof -u testuser | wc -l
2546
更改：增大10倍，生产中实践没问题
vim /etc/security/limits.conf
# Default limit for number of user's processes to prevent
# accidental fork bombs.
# See rhbz #432903 for reasoning.
* soft nproc 10240
root soft nproc unlimited
