操作环境
SuSE11sp1
问题现象
执行su - test命令切换失败，提示"su: cannot set user id: Resource temporarily unavailable"
问题分析
猜测是test使用资源超过系统的限制（比如进程数、打开的文件句柄）
1、查看/etc/security/limits.conf文件，文件并无对test用户做特殊限制
2、ulimit -a，查看输出结果与步骤1中的配置一致。
3、pf -fu test | wc -l，查看test用户打开的进程数，小于上述通用配置nproc参数值。OK
4、lsof | awk '{print $3}' | sort | uniq -c,查看test用户打开的文件句柄数。小于上述通用配置的参数值nofile。OK
5、ps -eLF | grep 'test'|wc-l,查看test用户打开的线程数（Java里面的一个线程在Linux下会最终映射成操作系统的一个进程），等于当前通用配置的参数值nproc。找到了原因。因此需要修改nproc参数。
问题解决
1、调大/etc/security/limits.conf中nproc参数。不需要重启。
2、执行ulimit -a查看确认修改已生效。
3、重新执行su - test，切换成功。问题解决
知识拓展
1、/etc/security/limits.conf文件及内容介绍
# /etc/security/limits.conf
#
#This file sets the resource limits for the users logged in via PAM.
#It does not affect resource limits of the system services.
#
#Also note that configuration files in /etc/security/limits.d directory,
#which are read in alphabetical order, override the settings in this
#file in case the domain is the same or more specific.
#That means for example that setting a limit for wildcard domain here
#can be overriden with a wildcard setting in a config file in the
#subdirectory, but a user specific setting here can be overriden only
#with a user specific setting in the subdirectory.
#
#Each line describes a limit for a user in the form:
#
#<domain> <type> <item> <value>
#
#Where:
#<domain> can be:
# - a user name
# - a group name, with @group syntax
# - the wildcard *, for default entry
# - the wildcard %, can be also used with %group syntax,
# for maxlogin limit
#
#<type> can have the two values:
# - "soft" for enforcing the soft limits
# - "hard" for enforcing hard limits
#
#<item> can be one of the following:
# - core - limits the core file size (KB)
# - data - max data size (KB)
# - fsize - maximum filesize (KB)
# - memlock - max locked-in-memory address space (KB)
# - nofile - max number of open file descriptors
# - rss - max resident set size (KB)
# - stack - max stack size (KB)
# - cpu - max CPU time (MIN)
# - nproc - max number of processes
# - as - address space limit (KB)
# - maxlogins - max number of logins for this user
# - maxsyslogins - max number of logins on the system
# - priority - the priority to run user process with
# - locks - max number of file locks the user can hold
# - sigpending - max number of pending signals
# - msgqueue - max memory used by POSIX message queues (bytes)
# - nice - max nice priority allowed to raise to values: [-, ]
# - rtprio - max realtime priority
#
#<domain> <type> <item> <value>
# #* soft core
#* hard rss
#@student hard nproc
#@faculty soft nproc
#@faculty hard nproc
#ftp hard nproc
#@student - maxlogins # End of file
@users soft nofile
@users hard nofile
@root soft nofile
@root hard nofile
2、ulimit命令，设置限制,可以把命令加到profile文件里，也可以在/etc/security/limits.conf文件中定义限制。
常用参数如下
-a 显示所有限制
-n 打开文件数的上限
-u 进程数的上限
-c core文件大小的上限
-d 进程数据段大小的上限
-f shell所能创建的文件大小的上限
-m 驻留内存大小的上限
-s 堆栈大小的上限
-t 每秒可占用的CPU时间上限
-p 管道大小
-v 虚拟内存的上限