Design Issues of Variable-Packet-Size, Multiple-Priority Buffered Crossbars
Nikos Chrysos · 2003
Buffered crossbars provide several advantages over unbuffered ones; one of them is that they can directly switch variable-size packets. The first part of this report considers several issues that arise when designing a variable-packet-size buffered crossbar with considerable Round-Trip Time: (1) we observe that an acknowledgment semantics for credits reduces the backpressure communication overhead and the storage of credits within the crossbar chip; (2) we show that cut-through can be implemented at the crosspoints and we discuss for a reasonable crosspoint buffer size under cut-through and store&forward; (3) we present a novel architecture for scheduling the operations at the contention points. The second part is concerned with the crosspoint queuing organization when handling multiple priorities. In buffered crossbars, separate buffers/queues (or lanes) per priority at each crosspoint are required to prevent HOL blocking and buffer hogging. In this report we propose a buffered crossbar that effectively supports multiple priorities with only two such lanes per crosspoint, even with no internal speed-up. Our method maps packets to crosspoint lanes using adaptive criteria, and dynamically changes the effective priority of crosspoint queues. Through simulations we show that when eight priorities are supported, under a typical, uniform traffic pattern, our method will not increase the average delay of any priority by more