I tested this on different GNU/Linux installations:
perl -e 'while(1){open($a{$b++}, "<" ,"/dev/null") or die $b;print " $b"}'
System A and D
The first limit I hit is 1024. It is easily raised by putting this into /etc/security/limits.conf:
* hard nofile 1048576
and then run:
ulimit -n 1048576
echo 99999999 | sudo tee /proc/sys/fs/file-max
Now the test goes to 1048576.
However, it seems I cannot raise it above 1048576. If I put 1048577 in limits.conf it is simply ignored.
What is causing that?
System B
On system B I cannot even get to 1048576:
echo 99999999 | sudo tee /proc/sys/fs/file-max
/etc/security/limits.conf:
* hard nofile 1048576
Here I get:
$ ulimit -n 65537
bash: ulimit: open files: cannot modify limit: Operation not permitted
$ ulimit -n 65536
#OK
Where did that limit come from?
System C
This system also has the 1048576 limit in limits.conf and 99999999 in /proc/sys/fs/file-max.
But here the limit is 4096:
$ ulimit -n 4097
-bash: ulimit: open files: cannot modify limit: Operation not permitted
$ ulimit -n 4096
# OK
How do I raise that to (at least) 1048576?
(Note to self: Don't do: echo 18446744073709551616 | sudo tee /proc/sys/fs/file-max
)
Best Answer
Check that
/etc/ssh/sshd_config
contains:and that
/etc/pam.d/sshd
contains:Still no answer to why 1048576 is max.
The 1048576 seems to be per process. So by having multiple processes this limit can be overcome.