Choosing traffic shaping discipline for a small ISP

[Ovi]

Hello

What is the best choice for traffic shaping when you have like 2500 
users, on a single router with 50 Mbits/s internet fiber connection?
I've tried IPFW+dummynet, but browsing and download is not stable 
(variations are high), and second problem, with 98% cpu idle (Xeon CPU) 
with firewall activated, not all bandwidth is used, even on the evening 
when most of users are online.

I've tried pf + altq HFSC which works well for up to 500 users, for more 
users CPU is high, and browsing is very slow.
It is possible with CBQ to share internet for like 2500 users and limit 
their bandwidth? Both IPFW and PF when altq discipline needs sum of 
bandwidth for all users not to exceed total bandwidth (50 Mbps in my 
case) or well 100%, but having like 2500 and not 100 users, I cannot use 
values like 0.2%.

I did not posed rules, I've used standard shaping rules that can be 
found in any howto/man page, my question is only, how discipline should 
I use to get similar performances to HTB (or TC ) on Linux, for like 
2000-2500 users.

Best Regards,
ovi