NGL02-2: Ethernet Adaptive Link Rate (ALR): Analysis of a Buffer Threshold Policy

Abstract

Rapidly increasing energy use by computing and communications equipment is a significant problem that needs to be addressed. Ethernet network interface controllers (NICs) consume hundreds of millions of US$ in electricity per year. Most Ethernet links are underutilized and link power consumption can be reduced by operating at lower data rates. An output buffer threshold policy to change link data rate in response to utilization is investigated. Analytical and simulation models are developed to evaluate the performance of Adaptive Link Rate (ALR) with respect to mean packet delay and time spent in low data rate with Poisson traffic and 100 Mb/s network traces as inputs. A Markov model of a state-dependent service rate queue with rate transitions only at service completion is developed. For the traffic traces, it is found that a link can operate at 10 Mb/s for over 99% of the time yielding energy savings with no user-perceivable increase in packet delay.

Real-time performance analysis of Adaptive Link Rate

“…Furthermore, we acknowledge that our measured switching times are much longer than 1 ms, which is assumed in previous research [8] [9][10], but our switching time between 1 Gbps and 100 Mbps is well within the time projected by [3] [20]. Unfortunately, this lengthy time is due to limitations of the current PHY technology and IEEE standards.…”

“…In this example, the two communicating devices are a network interface card (NIC) and a switch, denoted as node A and node B, respectively. Without loss of generality, we assume the control policy is the simple dual buffer threshold policy proposed in [8]. The algorithm for switching from a higher link rate to a lower link rate using the MAC handshake protocol is as follows: 1) During link initialization, ALR capability is advertised during auto negotiation.…”

“…Most previous research focused heavily on addressing control policy challenges through simulation models [1] [8][9] [10]. Due to the absence of a real-time ALR capable system, these efforts did not consider the implementation challenges for MAC and PHY synchronization.…”

“…However, both the ALR and active/idle mechanisms introduce several challenges, such as control policy development, which has been the focus of much research. A transceiver buffer occupancy-based dual threshold policy was first proposed by Gunaratne et al [8]. In this policy, if the link was operating at a high link rate and the output buffer occupancy fell below a low threshold value, the link rate was decreased.…”

“…To ensure proper synchronization, we begin with an existing two-way MAC layer handshaking protocol [8]. This protocol begins with a request from the initiating device to switch the link rate (ALR request) and ends with a positive acknowledgment (ACK) or negative acknowledgement (NACK) from the recipient device to indicate whether or not the link rate can be switched.…”