A Localized Congestion Control Mechanism for PCI Express Advanced Switching Fabrics
Venkata Krishnan, David R. Mayhew · 2004
Even though there is a commonality in their physical and link layers, Advanced Switching (AS) is more than merely an extension of PCI Express. The role of PCI Express is primarily as a chip-interconnect. In contrast, AS functions as a true system fabric interconnect encompassing domains that are vastly different and indeed more sophisticated than that of PCI Express’. In such a setup, congestion management is crucial for optimal utilization of the fabric bandwidth. Congestion control in network fabrics has generally been based on end-to-end and link-by-link schemes for controlling packet injection. Though seemingly adequate, these schemes may not be effective in handling “transient ” congestion arising from intermittent traffic − for transient congestion can indeed occur in fabrics operating well below their saturation limit. In such scenarios, a link-by-link scheme does not prevent congestion from spreading while an end-to-end scheme may result in under-utilization of the fabric bandwidth. This paper details a congestion control mechanism called Status Based Flow Control (SBFC) that has been incorporated into AS and is specifically targeted for alleviating the transition congestion problem. The SBFC mechanism exploits the source-based path routing used in AS. It enables upstream switch nodes to modify the transmission of packets based on the congestion status of links in a downstream switch. Simulation studies show that the SBFC mechanism indeed permits optimal usage of the fabric bandwidth during periods of transient congestion and effectively complements the traditional end-to-end and link-by-link schemes for congestion control. 1