Why fork: retry: Resource temporarily unavailable?

Question

linux

I run a resource-intensive program and the server always stops responding to commands, I set the following limits:

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) 386628
max locked memory       (kbytes, -l) 64
max memory size         (kbytes, -m) unlimited
open files                      (-n) 1000000
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) 1000000
virtual memory          (kbytes, -v) unlimited
file locks                      (-x) unlimited

That is, there are not even enough resources to reboot. The problem is definitely not on the side of RAM and CPU, but somewhere in the maximum number of open streams or files, but how to understand how much the program consumes? How to solve the issue?