arXiv subjects

Harsha Honnappa

Publications and source records attributed to Harsha Honnappa.

44 records · Page 3Linked to original sources

arXiv2017

Conditioned Functional Limits and Applications to Queues

We consider a renewal process that is conditioned on the number of events in a fixed time horizon. We prove that a centered and scaled version of this process converges to a Brownian bridge, as the number of events grows large, which relies on first establishing a functional strong law of large numbers result to determine the centering. These results are consistent with the asymptotic behavior of a conditioned Poisson process. We prove the limit theorems over triangular arrays of exchangeable random variables, obtained by conditionning a sequence of independent and identically distributed renewal processes. We construct martingale difference sequences with respect to these triangular arrays, and use martingale convergence results in our proofs. To illustrate how these results apply to performance analysis in queueing, we prove that the workload process of a single server queue with conditioned renewal arrival process can be approximated by a reflected diffusion having the sum of a Brownian Bridge and Brownian motion as input to its regulator mapping.

math.PR↗

arXiv2017

Asymptotically optimal appointment schedules with customer no-shows

We consider the problem of scheduling appointments for a finite customer population to a service facility with customer no-shows, to minimize the sum of customer waiting time and server overtime costs. Since appointments need to be scheduled ahead of time we refer to this problem as an optimization problem rather than a dynamic control one. We study this optimization problem in fluid and diffusion scales and identify asymptotically optimal schedules in both scales. In fluid scale, we show that it is optimal to schedule appointments so that the system is in critical load; thus heavy-traffic conditions are obtained as a result of optimization rather than as an assumption. In diffusion scale, we solve this optimization problem in the large horizon limit. Our explicit stationary solution of the corresponding Brownian Optimization Problem translates the customer-delay versus server-overtime tradeoff to a tradeoff between the state of a reflected Brownian motion in the half-line and its local time at zero. Motivated by work on competitive ratios, we also consider a reference model in which an oracle provides the decision maker with the complete randomness information. The difference between the values of the scheduling problem for the two models, to which we refer as the stochasticity gap (SG), quantifies the degree to which it is harder to design a schedule under uncertainty than when the stochastic primitives (i.e., the no-shows and service times) are known in advance. In the fluid scale, the SG converges to zero, but in the diffusion scale it converges to a positive constant that we compute.

math.OC↗

arXiv2017

Transitory Queueing Networks

Queueing networks are notoriously difficult to analyze sans both Markovian and stationarity assumptions. Much of the theoretical contribution towards performance analysis of time-inhomogeneous single class queueing networks has focused on Markovian networks, with the recent exception of work in Liu and Whitt (2011) and Mandelbaum and Ramanan (2010). In this paper, we introduce transitory queueing networks as a model of inhomogeneous queueing networks, where a large, but finite, number of jobs arrive at queues in the network over a fixed time horizon. The queues offer FIFO service, and we assume that the service rate can be time-varying. The non-Markovian dynamics of this model complicate the analysis of network performance metrics, necessitating approximations. In this paper we develop fluid and diffusion approximations to the number-in-system performance metric by scaling up the number of external arrivals to each queue, following Honnappa et al. (2014). We also discuss the implications for bottleneck detection in tandem queueing networks.

math.PR↗

arXiv2017

On Gaussian Limits and Large Deviations for Queues Fed by High Intensity Randomly Scattered Traffic

We study a single server FIFO queue that offers general service. Each of n customers enter the queue at random time epochs that are inde- pendent and identically distributed. We call this the random scattering traffic model, and the queueing model RS/G/1. We study the workload process associated with the queue in two different settings. First, we present Gaussian process approximations in a high intensity asymptotic scale and characterize the transient distribution of the approximation. Second, we study the rare event paths of the workload by proving a large deviations principle in the same high intensity regime. We also obtain exact asymptotics for the Gaussian approximations developed prior. This analysis significantly extends and simplifies recent work in [1] on uniform population acceleration asymptotics to the queue length and workload in the RS/G/1 queue.

math.PR↗

arXiv2017

Rare Events of Transitory Queues

We study the rare event behavior of the workload process in a transitory queue, where the arrival epochs (or points) of a finite number of jobs are assumed to be the ordered statistics of independent and identically distributed (i.i.d.) random variables. The service times (or marks) of the jobs are assumed to be i.i.d. random variables with a general distribution, that are jointly independent of the arrival epochs. Under the assumption that the service times are strictly positive, we derive the large deviations principle (LDP) satisfied by the workload process. The analysis leverages the connection between ordered statistics and self-normalized sums of exponential random variables to establish the LDP. This paper presents the first analysis of rare events in transitory queueing models, supplementing prior work that has focused on fluid and diffusion approximations.

math.PR↗

arXiv2014

On Transitory Queueing

We introduce a framework and develop a theory of transitory queueing models. These are models that are not only non-stationary and time-varying but also have other features such as the queueing system operates over finite time, or only a finite population arrives. Such models are relevant in many real-world settings, from queues at post-offces, DMV, concert halls and stadia to out-patient departments at hospitals. We develop fluid and diffusion limits for a large class of transitory queueing models. We then introduce three specific models that fit within this framework, namely, the Delta(i)/GI/1 model, the conditioned G/GI/1 model, and an arrival model of scheduled traffic with epoch uncertainty. We show that asymptotically these models are distributionally equivalent, i.e., they have the same fluid and diffusion limits. We note that our framework provides the first ever way of analyzing the standard G/GI/1 model when we condition on the number of arrivals. In obtaining these results, we provide generalizations and extensions of the Glivenko-Cantelli and Donskers Theorem for empirical processes with triangular arrays. Our analysis uses the population acceleration technique that we introduce and develop. This may be useful in analysis of other non-stationary and non-ergodic queuing models.

math.PR↗

arXiv2014

A queueing model with independent arrivals, and its fluid and diffusion limits

We introduce the Δ(i)/GI/1 queue, a new queueing model. In this model, customers from a given population independently sample a time to arrive from some given distribution F. Thus, the arrival times are an ordered statistics, and the inter-arrival times are differences of consecutive ordered statistics. They are served by a single server which provides service according to a general distribution G, with independent service times. The exact model is analytically intractable. Thus, we develop fluid and diffusion limits for the various stochastic processes, and performance metrics. The fluid limit of the queue length is observed to be a reflected process, while the diffusion limit is observed to be a function of a Brownian motion and a Brownian bridge process, and is given by a 'netput' process and a directional derivative of the Skorokhod reflected fluid netput in the direction of a diffusion refinement of the netput process. We also observe what may be interpreted as a transient Little's law. Sample path analysis reveals various operating regimes where the diffusion limit switches between a free diffusion, a reflected diffusion process and the zero process, with possible discontinuities during regime switches. The weak convergence is established in the M1 topology, and it is also shown that this is not possible in the J1 topology.

math.PR↗

arXiv2011

Strategic Arrivals into Queueing Networks: The Network Concert Queueing Game

Queueing networks are typically modelled assuming that the arrival process is exogenous, and unaffected by admission control, scheduling policies, etc. In many situations, however, users choose the time of their arrival strategically, taking delay and other metrics into account. In this paper, we develop a framework to study such strategic arrivals into queueing networks. We start by deriving a functional strong law of large numbers (FSLLN) approximation to the queueing network. In the fluid limit derived, we then study the population game wherein users strategically choose when to arrive, and upon arrival which of the K queues to join. The queues start service at given times, which can potentially be different. We characterize the (strategic) arrival process at each of the queues, and the price of anarchy of the ensuing strategic arrival game. We then extend the analysis to multiple populations of users, each with a different cost metric. The equilibrium arrival profile and price of anarchy are derived. Finally, we present the methodology for exact equilibrium analysis. This, however, is tractable for only some simple cases such as two users arriving at a two node queueing network, which we then present.

cs.GT↗