Performance analysis of LAS-based scheduling disciplines in a packet switched network

Abstract

The Least Attained Service (LAS) scheduling policy, when used for scheduling packets over the bottleneck link of an Internet path, can greatly reduce the average flow time for short flows while not significantly increasing the average flow time for the long flows that share the same bottleneck. No modification of the packet headers is required to implement the simple LAS policy. However, previous work has also shown that a drawback of the LAS scheduler is that, when link utilization is greater than 70%, long flows experience large jitter in their packet transfer times as compared to the conventional First-Come-First-Serve (FCFS) link scheduling. This paper proposes and evaluates new differentiated LAS scheduling policies that reduce the jitter for long flows that are identified as "priority" flows. To evaluate the new policies, we develop analytic models to estimate average flow transfer time as a function of flow size, and average packet transmission time as a function of position in the flow, for the single-bottleneck "dumbbell topology" used in many ns simulation studies. Models are developed for FCFS scheduling, LAS scheduling, and each of the new differentiated LAS scheduling policies at the bottleneck link. Over a wide range of configurations, the analytic estimates agree very closely with the ns estimates. Thus, the analytic models can be used instead of simulation for comparing the policies with respect to mean flow transfer time (as a function of flow size) and mean packet transfer time. Furthermore, an initial discrepancy between the analytic and simulation estimates revealed errors in the parameter values that are often specified in the widely used ns Web workload generator. We develop an improved Web workload specification, which is used to estimate the packet jitter for long flows (more accurately than with previous simulation workloads). Results for the scheduling policies show that a particular policy, LAS-log, greatly improves the mean flow transfer time for priority long flows while providing performance similar to LAS for the ordinary flows. Simulations show that the LAS-log policy also greatly reduces the jitter in packet delivery times for the priority flows.

EFD: An Efficient Low-Overhead Scheduler

“…Size-based scheduling has received a lot of attention from the research community with applications to Web servers [15], Internet traffic [3,14,16] or 3G networks [2,10]. The key idea is to favor short flows at the expense of long ones because short flows are in general related to interactive applications like Email, Web browsing or DNS requests/responses; unlike long flows which represent background traffic.…”

Stability of size-based scheduling disciplines in resource-sharing networks

“…While such information is typically available in web servers, it is usually nearly impossible to obtain in Internet routers. An alternative strategy which has therefore been advocated for scheduling data flows is the Least Attained Service first (LAS) discipline also known as ForegroundBackground Processor Sharing (FBPS) [22,27,29,30]. In case the service requirement distribution has a decreasing failure rate, it has been shown that LAS stochastically minimizes the number of jobs in the system among all strategies that use no knowledge of the remaining job sizes [31].…”

Fairness and scheduling in single server queues

“…For example, at an online shopping site the large requests are often the shopping cart transactions. The same tradeoff has appeared across diverse computer system application areas, e.g., process scheduling [14], routers [36,37], wireless networks [22], transport protocols [56], and beyond.…”

“…For example, Shortest Remaining Processing Time (SRPT) is well-known to minimize mean response time across general arrival processes and job size distributions [47] and, among non-preemptive policies, Shortest Job First (SJF) optimizes the mean response time [11]. As a result, designs that give priority to jobs with small sizes have been suggested for a variety of computer systems in recent years, such as [20,38,36,37,22,56,14]. However, the adoption of these new designs has been slowed by concerns about the unfairness of these policies.…”

“…In fact, this bound converges to 1 as ρ → 1, which signifies that the size of the smallest job that might be treated unfairly is increasing unboundedly as ρ increases. Note though that although we have focused the above discussion on SRPT, a wide variety of other common policies have been analyzed with respect to Definition 1 in [5,10,16,21,36,37,53]. The reader may find illustrations of the fairness behavior of FB, PSJF, and many other common policies in Figures 1 and 2.…”