CICQ = Combined Input-crosspoint Queued Switch

CICQ has VOQ at the input and each crosspoint has a buffer (buffered crossbar, or BX). In the paper, the following are assumed:

  • $N\times N$ switch architecture
  • $N^2$ crosspoints, and thus $N^2$ buffers
  • There are 2 priorities (high, low) and they use different VOQ, thus there are $2N$ VOQs per input
  • Size-based scheduling (SB): First $\theta$ packets of a flow are marked high priority, others are low priority

VOQ to crosspoint queues are in PIFO (push-in first-out) operation

  • Packets are allowed to insert to the middle
  • Packets of low priority enqueued to the end
  • Packets of high priority enqueued to in front of the first low priority packet

pCICQ-1: First version of SB scheduling. Output scheduler (the one selects a crosspoint buffer to dequeue to the output at each cycle) maintains a 1-bit indicator for each crosspoint buffer. The indicator $PI_{i,j}$ is 1 iff the head of line packet at crosspoint buffer $i,j$ is high priority. The output scheduler always prefer those crosspoint buffer with $PI_{i,j} = 1$, i.e. high priority packets are dequeued sooner than low priority and low priority packets are outputted only if no high priority packets are there. The output selection can be round-robin, longest queue first, oldest HOL packet first or otherwise. The input scheduler (the one puts packet from VOQ to crosspoint buffer) simply prefer high-priority VOQ over low-priority one. Therefore, in pCICQ-1, the total additional requirement is $N^2$ bits for the $PI_{i,j}$. But it does not guarantee strict priority: Because an arriving packet need to wait 1 cycle to be seen by the output scheduler. Moreover, in case two high priority packets are from different input port but destined to the same output port, their arrival order may not be preserved at departure.

pCICQ-2: Improvement over pCICQ-1. For each output, there is an $ASH_j$ queue to hold the arrival sequence of high-priority packets. The $ASH_j$ queue holds the ID of the arrival input ports. Enqueue is done by the input ports upon high priority packet arrivals. Dequeue, however, is by the output scheduler. The output scheduler for output $j$ works with the $ASH_j$ queue. If the $ASH_j$ queue is empty, it is sure that there is no high priority packet and a low priority packet is dequeued by round robin or otherwise. If it is not empty, then $ASH_j$ is dequeued to find the input port of the oldest high priority packet $i$, and that packet is retrieved from buffer $i,j$. In this way, cross-input FIFO property for high priority packet is maintained.

Solution to strict priority is the following: There is a sequence controller that inputs to $ASH_j$. The input port i will assert a 1-bit indicator $R_{i,j}$ when it sees a high priority packet destined to output $j$. The sequence control will work every cycle to scan all $R_{i,j}$ and put the ID $i$ to $ASH_j$. Normally the input scheduler should then put the packets to respective crosspoint buffer but if the due time computed as $\textrm{length}(ASH_j)+\sum_{k=0}^{i-1}(R_{k,j})=0$, then this packet is the one that should be immediately sent out. So, instead of enqueued to the crosspoint buffer, it is delivered to the output by cut-through switching.

In pCICQ-2, the $ASH_j$ queue should be long enough to prevent overflow.

Bibliographic data

   title = "Size-based Flow Scheduling in a CICQ Switch",
   author = "Dinil Mon Divakaran and Fabienne Anhalt and Eitan Altman and Pascale Vicat-Blanc Primet",
   howpublished = "HPSR 2010 (submitted)",
   year = "2010",