Anyone know how to control the queue length with HTB ?

I'm seeing large numbers of packets queued (>50), which
causes high latency. Reading the code it isn't immediately
clear to me how the HTB queues are bounded.

In fact, it isn't clear to me that there is queueing in
sch_htb.c. Is there a default leaf qdisc associated with
every HTB class ? Perhaps I need to define a non-default
leaf qdisc and set its queue length ?

Thanks in advance for your help.