render_list doesnt work

I'm running pre-render for ajt style map

render_list -all --socket /var/run/renderd/renderd.sock -z 1 -Z 1 -n 15

I can see this in the syslog:

renderd[35121]: DEBUG: Got incoming connection, fd 7, number 1
renderd[35121]: DEBUG: Got incoming connection, fd 9, number 2
renderd[35121]: DEBUG: Got incoming connection, fd 10, number 3
renderd[35121]: DEBUG: Got incoming connection, fd 11, number 4
renderd[35121]: DEBUG: Got incoming connection, fd 12, number 5
renderd[35121]: DEBUG: Got incoming connection, fd 13, number 6
renderd[35121]: DEBUG: Got incoming connection, fd 17, number 7
renderd[35121]: DEBUG: Got incoming connection, fd 18, number 8
renderd[35121]: DEBUG: Got incoming connection, fd 19, number 9
renderd[35121]: DEBUG: Got incoming connection, fd 20, number 10
renderd[35121]: DEBUG: Got incoming connection, fd 21, number 11
renderd[35121]: DEBUG: Got incoming connection, fd 22, number 12
renderd[35121]: DEBUG: Got incoming connection, fd 23, number 13
renderd[35121]: DEBUG: Got incoming connection, fd 24, number 14
renderd[35121]: DEBUG: Got incoming connection, fd 25, number 15

But when looking on syslog and htop and even iotop it looks like the machine is doing nothing

What am I missing?