What's the semantic of "max user processes" (ulimit)

Question

Hello everyone,
I don't understand the behaviour of ulimit, have a look at this shell dump:
$ ps hax | wc -l
120
$ ps h -u myUsername | wc -l
35
$ ulimit -Su 78
$ ps h -u myUsername | wc -l
bash: fork: retry: No child processes
bash: fork: retry: No child processes
bash: fork: retry: No child processes
bash: fork: retry: No child processes
bash: fork: Resource temporarily unavailable
$ ulimit -Su 80
$ ps h -u myUsername | wc -l
35
$
If I am currently running 35 processes, one would assume that >35 more should be sufficient to start two more processes - but obviously it isn't.
In this case, I needed at least "ulimit -Su 80" to start ps and wc.
If I set the "max user processes" limit to 79, the first four error messages change from "bash: fork: retry: No child processes" to "bash: fork: retry: Resource temporarily unavailable"
Can somebody explain this bevahiour to me, please? Why am I not allowed to create new processes, although the limits should be sufficient?

Answer 1

Apologies for the necro-bump but I hope this will help others.
ulimit -u does not give you the max number of processes rather it displays the max number of threads.  To list the number of threads a user is currently using:
ps -L -u myusername | wc -l
The output from the above two commands can now be compared and allow you to set the appropriate ulimit.