5 research outputs found
Constructing Matrix Exponential Distributions by Moments and Behavior around Zero
This paper deals with moment matching of matrix exponential (ME) distributions used to approximate general probability density functions (pdf). A simple and elegant approach to this problem is applying Padé approximation to the moment generating function of the ME distribution. This approach may, however, fail if the resulting ME function is not a proper probability density function; that is, it assumes negative values. As there is no known, numerically stable method to check the nonnegativity of general ME functions, the applicability of Padé approximation is limited to low-order ME distributions or special cases. In this paper, we show that the Padé approximation can be extended to capture the behavior of the original pdf around zero and this can help to avoid representations with negative values and to have a better approximation of the shape of the original pdf. We show that there exist cases when this extension leads to ME function whose nonnegativity can be verified, while the classical approach results in improper pdf. We apply the ME distributions resulting from the proposed approach in stochastic models and show that they can yield more accurate results
Improved Load Balancing in Large Scale Systems using Attained Service Time Reporting
Our interest lies in load balancing jobs in large scale systems consisting of
multiple dispatchers and FCFS servers. In the absence of any information on job
sizes, dispatchers typically use queue length information reported by the
servers to assign incoming jobs. When job sizes are highly variable, using only
queue length information is clearly suboptimal and performance can be improved
if some indication can be provided to the dispatcher about the size of an
ongoing job. In a FCFS server measuring the attained service time of the
ongoing job is easy and servers can therefore report this attained service time
together with the queue length when queried by a dispatcher.
In this paper we propose and analyse a variety of load balancing policies
that exploit both the queue length and attained service time to assign jobs, as
well as policies for which only the attained service time of the job in service
is used. We present a unified analysis for all these policies in a large scale
system under the usual asymptotic independence assumptions. The accuracy of the
proposed analysis is illustrated using simulation.
We present extensive numerical experiments which clearly indicate that a
significant improvement in waiting (and thus also in response) time may be
achieved by using the attained service time information on top of the queue
length of a server. Moreover, the policies which do not make use of the queue
length still provide an improved waiting time for moderately loaded systems