How to increase number of child proceses - linux

Question

cat /sys/fs/cgroup/pids/parent/pids.max = "max"
I created it following https://www.kernel.org/doc/html/latest/admin-guide/cgroup-v1/pids.html
Consider this Python Code demonstrating the problem:
from os import fork, getpid
from time import sleep
i=0
print( "pid = %d " % getpid())
with open("/proc/%d/limits" % getpid(), "r") as f:
print(f.read())
try:
while fork():
i+=1
except BaseException as e:
print(i)
print(e)
sleep(10)
print("done")
exit(1)
My output:
pid = 18091
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 999999 999999 processes
Max open files 1024 1048576 files
Max locked memory 67108864 67108864 bytes
Max address space unlimited unlimited bytes
Max file locks unlimited unlimited locks
Max pending signals 31412 31412 signals
Max msgqueue size 819200 819200 bytes
Max nice priority 0 0
Max realtime priority 0 0
Max realtime timeout unlimited unlimited us
10227
[Errno 11] Resource temporarily unavailable
done

Answer 1

In Linux you have the pid_max limit:
$ cat /proc/sys/kernel/pid_max
32768
However, if your Linux is running on systemd you might hit user-slice limits:
for root
$ cat /sys/fs/cgroup/pids/user.slice/user-0.slice/pids.max
for currently logged in user:
$ cat /sys/fs/cgroup/pids/user.slice/user-$(id -u).slice/pids.max
10813
The systemd equivalent would be:
$ systemd-analyze dump | sed -n "/-> Unit user-$(id -u).slice:/,/-> Unit /p"| grep -e "TasksMax="
TasksMax=10813
According to man logind.conf
UserTasksMax=
Sets the maximum number of OS tasks each user may run concurrently. > This controls the TasksMax= setting of the per-user slice unit, see systemd.resource-control(5) for details. If assigned the special value
"infinity", no tasks limit is applied. Defaults to 33%, which equals 10813 with the kernel's defaults on the host, but might be smaller in OS containers.
This limit is defined in /etc/systemd/logind.conf, even though it might be commented out, 33% out of 32768.
#UserTasksMax=33%
When you modify the UserTasksMax limit or increase sysctl kernel.pid_max you'll have to restart systemd-logind service:
service systemd-logind restart
On 64-bit system you should be able to increase the value up to 2^22, i.e.: 4194304
sysctl kernel.pid_max=4194304