23-01-2013, 10:50 AM
Multiple Priority Levels in Buffered Crossbars
ABSTRACT:
A significant advantage of buffered crossbar (combined input-crosspoint queueing
- CICQ) switches is that they can directly operate on variable-size packets, thus saving the costs
and inefficiencies of packet segmentation and reassembly (SAR). However, in order to support
multiple priority levels, separate queues per priority are needed at each crosspoint, in order to
prevent HOL blocking and buffer hogging; these queues are expensive because they each need a
size of at least one maximum-size packet. In this paper we propose a scheme that uses only two
queues per crosspoint to effectively support multiple priorities. We adaptively adjust the priority
levels of the two queues so that most traffic goes through the ``lower'' queue, while the ``upper''
queue remains usually available for higher priority packets to overtake the former. Through
simulation, and assuming 8 priority levels, we compare our scheme to an ideal system that uses 8
queues per crosspoint. For realistic traffic, the two systems perform almost identically, although
ours uses 4 times less memory in the crossbar. Even under a highly irregular traffic pattern
Bursts60, our system will not increase the average delay of any priority level by more than 75
percent compared to the ideal system.