/etc/security/limits.conf not applied

I have /etc/security/limits.conf, that seems not been applied:

a soft nofile 1048576 # default: 1024
a hard nofile 2097152
a soft noproc 262144  # default 128039
a hard noproc 524288  

Where a is my username, when I run ulimit -Hn and ulimit -Sn, it shows:

4096
1024

There's only one other file in the /etc/security/limits.d that the content is:

scylla  -  core     unlimited
scylla  -  memlock  unlimited
scylla  -  nofile   200000
scylla  -  as       unlimited
scylla  -  nproc    8096

I tried also append those values to /etc/security/limits.conf then restarting, and do this:

echo '
session required pam_limits.so
' | sudo tee -a /etc/pam.d/common-session

but it didn't work. My OS is Ubuntu 17.04.