Linux – /etc/security/limits.conf soft nproc limit appears to be ignored

My MongoDB database was running into problems under load, with the following errors spamming the logs:

[initandlisten] pthread_create failed: errno:11 Resource temporarily unavailable
[initandlisten] can't create new thread, closing connection

I've come to the conclusion that I need to raise the "ulimit -u" or "Max processes" setting which were at 1024, and the usage could have been exceeding that given the web frontends launched (not sure how to check this).

I edited /etc/security/limits.conf to add the last two lines (the first two were already there):

*                soft    nofile          350000
*                hard    nofile          350000
*                soft    nproc           30000
*                hard    nproc           30000

Then I rebooted the system (BTW should I have done that, or should a mongod service restart be enough?)

After reboot, reviewing the process limits for mongod process it seems the soft limit has been ignored:

$ cat /proc/2207/limits
Limit                     Soft Limit           Hard Limit           Units
Max cpu time              unlimited            unlimited            seconds
Max file size             unlimited            unlimited            bytes
Max data size             unlimited            unlimited            bytes
Max stack size            8388608              unlimited            bytes
Max core file size        0                    unlimited            bytes
Max resident set          unlimited            unlimited            bytes
Max processes             1024                 30000                processes
Max open files            350000               350000               files
Max locked memory         65536                65536                bytes
Max address space         unlimited            unlimited            bytes
Max file locks            unlimited            unlimited            locks
Max pending signals       273757               273757               signals
Max msgqueue size         819200               819200               bytes
Max nice priority         0                    0
Max realtime priority     0                    0
Max realtime timeout      unlimited            unlimited            us

$ whoami
mongod


$ ulimit -a
core file size          (blocks, -c) 0
data seg size           (kbytes, -d) unlimited
scheduling priority             (-e) 0
file size               (blocks, -f) unlimited
pending signals                 (-i) 273757
max locked memory       (kbytes, -l) 64
max memory size         (kbytes, -m) unlimited
open files                      (-n) 350000
pipe size            (512 bytes, -p) 8
POSIX message queues     (bytes, -q) 819200
real-time priority              (-r) 0
stack size              (kbytes, -s) 8192
cpu time               (seconds, -t) unlimited
max user processes              (-u) 1024
virtual memory          (kbytes, -v) unlimited
file locks                      (-x) unlimited

I expected that "Max processes" both hard and soft limits will be at 30000 as per the /etc/security/limits.conf file, but only the hard one is.

What am I doing wrong?

I'm running Amazon Linux on AWS EC2.

bash-4.1$ cat /etc/*-release
Amazon Linux AMI release 2012.09