If your download manager is from a single/few IP address(es) or port(s), you can also consider using
tc to manage that. What you can do is create two "channels", one with a guaranteed large bandwidth and the other with a small guaranteed bandwidth, but with the ability to borrow from the other "channel". For example:
tc qdisc add dev eth1 root handle 1: htb default 10
tc class add dev eth1 parent 1: classid 1:1 htb rate 4mbit ceil 4mbit
tc class add dev eth1 parent 1:1 classid 1:10 htb rate 3mbit ceil 4mbit
tc class add dev eth1 parent 1:1 classid 1:11 htb rate 1mbit ceil 4mbit
tc filter add dev eth1 protocol ip parent 1:0 prio 1 u32 match ip src 10.2.1.100 flowid 1:11
tc filter add dev eth1 protocol ip parent 1:0 prio 1 u32 match ip dst 10.2.1.100 flowid 1:11
This will guarantee 1Mbit for the traffic to/from 10.2.1.100, but it allow to use up to 4MBit if available, and guarantees 3MBit for the traffic for all the other hosts, or more if available.
The only problem is that
tc documentation is not exactly good.