A fair arbitration for Network-on-Chip routing with odd-even turn model

Abstract

With the increasing of Network-on-Chip size, deterministic routing algorithms suffer from a poor performance under heavy traffic. Therefore, routing schemes with varied candidate paths and fair selection strategy are required to relieve congestion. We propose an alterable priority arbitration for the fair granting of packet transmission from all possible directions permitted by the odd-even turn model. To achieve fairness, the proposed arbitration strategy alters the priority orders based on an optimized pointer, its granting decisions are made according to fixed mechanism under varied priority orders. Under a cycle-accurate simulator based on System C, the proposed alterable priority arbiter is verified to achieve better fairness than the RR arbiter. Simulation results show that the average packet latency of the odd-even turn model is reduced by 26.46%, and the saturation throughput is improved by 29.79% with the introduction of proposed arbitration scheme. Furthermore, the proposed arbiter achieves ignorable increasing of the hardware implementation overhead.

Introduction

With the rapid development of Integrated Circuits technology, highly scalable Network-on-Chip (NoC) has replaced conventional bus connection as an effective solution to communication issues on highly integrated chips [1], [2]. Routing algorithm influences the performance of NoC significantly, which determines the paths of packets transferred in the network [3], [4]. For classic deterministic XY routing [5], the routing path is determined by given source-destination pair [6], [7]. As a minimal routing, XY routing provides the feature of deadlock-freedom. However, deterministic routings result in disability to avoid congested regions under heavy traffic, which impacts the transmitting of packets. To improve the network performance, routing algorithms with varied candidate paths are required [8].

The adaptiveness degree of a routing algorithm is defined as the number of possible shortest paths for packets to traverse from the source to the destination [9]. Routing algorithms with higher adaptiveness degree provide more candidate paths for transferred packets. Furthermore, by employing proper path selection strategy, effective routing scheme with better traffic distribution can be achieved to relieve congestion [10]. Since conventional adaptive routing algorithms are prone to deadlock [11], [12], Virtual Channels(VCs) are introduced for deadlock-free adaptive routings [13]. Nevertheless, the applied VCs would involve extra hardware overhead and complex control logic on routers. As a sort of partial adaptive routing, turn models prohibit the minimal number of turns based on analyzing possible turns and loops for packet traversal, in this case, turn models achieve both adaptiveness and deadlock-freedom without introducing VCs [14]. Turn models such as west-first(WF), north-last(NL) and negative-first(NF) prohibit certain turns for global routing, which are vulnerable to highly unevenness of adaptiveness degree and traffic distribution [15]. For odd-even(OE) turn model [16], turns are restricted according to the parity of the node's column, instead of prohibiting certain turns globally. As a result, OE turn model solves the problem of unevenness. In addition, OE turn model is proved to achieve more candidate paths for routing selection than other turn models do [17].

As the odd columns and even columns appear alternatively in the network, conventional OE turn model is limited to a relatively determined routing path [16]. To improve the number of candidate paths, proper selection strategy among all possible paths is recommended, instead of determining the transmission merely according to column's parity. In this paper, arbitration scheme is employed as the selection strategy, the packet's traversing is determined by the granting result for all turns permitted by OE turn model.

Fairness is one of the most important evaluations for the performance of an arbiter [18]. As arbitration determines the packet transmission on every router, to balance the network load, each possible adjacent router should be granted as next hop to receive packet with equal probability [19], [20]. The Round Robin(RR) arbiter [21] is a conventional arbitration that can grant requests under different priorities fairly, while its fairness is limited by the pointer scheme. In this paper, to improve the fairness of path selection, an arbitration that alters priority orders according to an optimized pointer is proposed.

The remainder of this paper is organized as follows. The related works are reviewed in Section 2. The mechanism of the proposed alterable priority arbiter is introduced in Section 3. The realization of routing applying the proposed arbitration as path selection strategy is described in Section 4. Simulation results are presented in Section 5. The conclusions of this paper are drawn in Section 6.