How To Fix fork failed: Resource temporarily unavailable

Root Cause

There is a misbehaving service or process running, consuming more resources than expected.
The system was not able to create new processes, because of the limits set for nproc in /etc/security/limits.conf.
The system ran out of memory and new processes were unable to start because they could not allocate memory.
There is not an available ID to assign to the new process. A unique value less than kernel.pid_max must be available.

You can run the below command to find the number of processes opened for every user:

ps --no-headers auxwwwm | cut -f1 -d' ' | sort | uniq -c | sort -n

Compare to /etc/security/limits.conf or /etc/security/limits.d/*

Check the total number of threads and processes running on the server:

ps -eLf | wc -l

Compare to kernel.pid_max:

sysctl kernel.pid_max

Set /etc/security/limits.conf

* soft nproc 65535
* hard nproc 65535

* should be username

sar -r 1 10

Check commit%

echo 65535 > /proc/sys/kernel/pid_max

sysctl fs.file-nr

Explain:

<in_use> <unused_but_allocated> <maximum>

su - <username>
ulimit -Hn

lsof -u <username> 2>/dev/null | wc -l

/etc/sysctl.conf:

fs.file-max = 204708