Design and implementation of a fair credit-based bandwidth sharing scheme for buses

Fair arbitration in the access to hardware shared resources is fundamental to obtain low worst-case execution time (WCET) estimates in the context of critical real-time systems, for which performance guarantees are essential. Several hardware mechanisms exist for managing arbitration in those resources (buses, memory controllers, etc.). They typically attain fairness in terms of the number of slots each contender (e.g., core) gets granted access to the shared resource. However, those policies may lead to unfair bandwidth allocations for workloads with contenders issuing short requests and contenders issuing long requests. We propose a Credit-Based Arbitration (CBA) mechanism that achieves fairness in the cycles each core is granted access to the resource rather than in the number of granted slots. Furthermore, we implement CBA as part of a LEON3 4-core processor for the Space domain in an FPGA proving the feasibility and good performance characteristics of the design by comparing it against other arbitration schemes.The research leading to these results has received funding from the European Community’s Seventh Framework Programme [FP7/2007-2013] under the PROXIMA Project (www.proxima-project.eu), grant agreement no 611085. This work has also been partially supported by the Spanish Ministry of Science and Innovation under grant TIN2015-65316-P and the HiPEAC Network of Excellence. Mladen Slijepcevic is funded by the Obra Social Fundaci´on la Caixa under grant Doctorado “la Caixa” - Severo Ochoa. Carles Hernández is jointly funded by the Spanish Ministry of Economy and Competitiveness (MINECO) and FEDER funds through grant TIN2014-60404-JIN. Jaume Abella has been partially supported by the MINECO under Ramon y Cajal postdoctoral fellowship number RYC-2013-14717.Peer Reviewe