[[email protected] ~]# su - postgres
Last login: Mon Aug  7 09:44:30 EDT 2017 on pts/0
[[email protected] ~]$ ulimit -u
4096
[[email protected] ~]$ cat /etc/security/limits.
limits.conf  limits.d/    
[[email protected] ~]$ cat /etc/security/limits.conf 
# /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: [-20, 19]
#        - rtprio - max realtime priority
#
#<domain>      <type>  <item>         <value>
#

#*               soft    core            0
#*               hard    rss             10000
#@student        hard    nproc           20
#@faculty        soft    nproc           20
#@faculty        hard    nproc           50
#ftp             hard    nproc           0
#@student        -       maxlogins       4

# End of file
[
 
[email protected] ~]$ ulimit -u
4096
[[email protected] ~]$ cat /etc/security/limits.d/20-nproc.conf
# Default limit for number of user's processes to prevent
# accidental fork bombs.
# See rhbz #432903 for reasoning.

*          soft    nproc     4096
root       soft    nproc     unlimited
[[email protected]
 
 ~]$ cd/etc/security/limits.d/
-bash: cd/etc/security/limits.d/: No such file or directory
[[email protected] ~]$ cd/etc/security/limits.d/
-bash: cd/etc/security/limits.d/: No such file or directory
[[email protected] ~]$ cd /etc/security/limits.d/
[[email protected] limits.d]$ ls -lrt
total 4
-rw-r--r--. 1 root root 191 Jul 19  2016 20-nproc.conf
[
 
[email protected] limits.d]$ cat 20-nproc.conf 
# Default limit for number of user's processes to prevent
# accidental fork bombs.
# See rhbz #432903 for reasoning.

*          soft    nproc     4096
root       soft    nproc     unlimited
[[email protected] limits.d]$ ll
total 4
-rw-r--r--. 1 root root 191 Jul 19  2016 20-nproc.conf
[[email protected] limits.d]$ cat /etc/redhat-release 
Red Hat Enterprise Linux Server release 7.4 (Maipo)
[[email protected] limits.d]$ uname -a
Linux pgserver 3.10.0-693.el7.x86_64 #1 SMP Thu Jul 6 19:56:57 EDT 2017 x86_64 x86_64 x86_64 GNU/Linux
[[email protected] limits.d]$ 

參考文章:
How to set or change the default soft or hard limit for the number of user's processes?
https://access.redhat.com/solutions/406663