Real-time analysis of priority-preemptive NoCs with arbitrary buffer sizes and router delays

Abstract

Nowadays available multiprocessor platforms predominantly use a Network-on-Chip (NoC) architecture as an interconnect medium, due to its good scalability and performance. During the last decade, NoCs received a significant amount of attention from the real-time community. One promising category of approaches suggests to employ already existing hardware features called virtual channels, and dedicate them, exclusively, to individual communication traffic flows. In this way, NoCs become more amenable to the real-time analysis, which is an essential requirement for providing both safe and tight worst-case analysis methods, and consequently deriving real-time guarantees. In this manuscript, we present the approach which falls in the aforementioned category. Specifically, we propose a novel method for the worst-case analysis of the NoC traffic, assuming the existence of per-flow dedicated virtual channels. Compared to the state-of-the-art techniques, our approach yields substantially tighter upper-bounds on the worst-case traversal times (WCTTs) of communication traffic flows. By employing the proposed method, resource over-provisioning can be mitigated to a large extent, and significant designcost reductions can be achieved. Moreover, we implemented a cycle-accurate simulator of the assumed NoC architecture, and used it to assess the tightness of derived WCTT bounds. Finally, we reached an interesting conclusion that bigger virtual channel buffers do not necessarily lead to better results, and in many cases can be counter-productive, which is a very important finding for system designers.

Graph-Based Approach for Buffer-Aware Timing Analysis of Heterogeneous Wormhole NoCs Under Bursty Traffic

“…We first notice that our approach gives similar results to [9]. To further quantify the similarity of the results, we subtracted the tightness ratio obtained by the two approaches on each bound to obtain what we call "tightness difference", denoted ∆τ .…”

“…• Flows' priority assignment follows a rate monotonic policy; • Each router supports 4 Virtual Channels with no prioritysharing and no VC-sharing, i.e., one flow per VC; • To compare our results to the ones in [9], we performed the analysis for different buffer sizes (2, 100 and 1000000 flits, the latest being large enough to assume buffer size is infinite).…”

“…Thus, we expect BATA and G-BATA to give the exact same results for the worstcase delay bounds, which we checked was the case. We then plotted comparative graphs on Figure 21 and computed the average tightness of our approach (Table III), using results from simulations performed by Nikolić et al [9]. The average tightness ratio for G-BATA approach with buffer size 2, 100 and infinite are 64%, 67% and 71% respectively.…”

“…where τ G−BAT A is the tightness ratio of the bound yielded by G-BATA, and τ ST is the tightness ratio of the bound yielded by the method of [9]. The tightness difference ∆τ is positive when G-BATA gives the tighter bound and negative otherwise.…”

“…• considering specific assumptions, such as: (i) distinct priorities and unique virtual channel assignment for each traffic flow in a router [6] [4] [9]; (ii) a priority-share policy, but with a number of Virtual Channels (VC) at least equal to the number of traffic priority levels like in [7] [8] [12] [10] or the maximum number of contentions along the NoC [16]; • ignoring the buffer backpressure phenomena, such as in [7], [11], [13], [14], [17]; • ignoring the flows serialization phenomena 2 along the flow path by conducting an iterative response time computation, commonly used in Scheduling Theory and CPA, which generally leads to pessimistic delay bounds. Hence, to cope with these identified limitations, we have proposed in [5] a timing analysis using Network Calculus [18] and referred as Buffer-Aware Worst-case Timing Analysis (BATA) from this point on.…”