231 research outputs found
Scheduling in Stochastic Resource-Sharing Systems
No full textIn this thesis we study queueing models that arise in the context of resource sharing in communication networks. We determine scheduling policies that (asymptotically) optimize the performance of the system, and evaluate policies that share the resources among the users in a fair manner. Chapter 1 gives an overview of several concepts related to resource-sharing systems and introduces the queueing models and main techniques used throughout the thesis. We describe the work-conserving single-server system, the linear bandwidthsharing network, and the parallel two-server model. In the latter two models the speed at which the system works depends on the scheduling decision taken and on the types of users presently in the system. These models can therefore be seen as extensions of the single-server model. The stochastic evolution of the numbers of users is determined by the scheduling policy, which specifies how the resources are shared among all users. We use sample-path techniques and stochastic dynamic programming tools in order to characterize policies that minimize the holding cost. Since the original stochastic model is not always tractable, we also investigate two limiting regimes. One of them is the heavy-traffic regime where we let the offered load approach the maximum capacity. In the other regime we consider the queuelength processes under the so-called fluid scaling. In Chapter 2 we focus on the single-server system in heavy traffic and analyze a generalization of the DPS policy. More specifically, we consider phase-type distributed service requirements and allow customers to have different weights in various phases of their service. In our main result we establish a state-space collapse for the scaled steady-state queue length vector in heavy traffic. The result shows that in the limit, the queue length vector is the product of an exponentially distributed random variable and a deterministic vector. The proof consists in showing that the joint probability generating function of the queue lengths satisfies a partial differential equation that allows a closed-form solution after passing to the heavy-traffic limit. Our result has several interesting consequences for the standard DPS queue. We derive that, conditioned on the number of customers, the remaining service requirements of the various customers are independent and distributed according to the forward recurrence times. In addition, we show that the scaled holding cost stochastically reduces as more preference is given according to the mean forward recurrence times of the service requirements. In Chapters 3–7 we study the linear bandwidth-sharing network. This network provides a natural modeling framework for the dynamic flow-level interaction among data transfers in wired communication networks. It models the bandwidth sharing of data traffic that traverses multiple links and the cross traffic it meets on its route. Size-based scheduling policies, such as SRPT and LAS, are popular mechanisms for reducing the number of users in single-server systems by favoring smaller requests over larger ones. In Chapter 3 we prove that straightforward extensions of such policies may cause instability effects in the linear network and will therefore certainly not yield optimal performance. For networks with sufficiently many nodes, instability phenomena may in fact arise at arbitrarily low traffic loads. In Chapters 4–6 we turn to finding policies that minimize the holding cost in a linear network. In Chapter 4 we restrict the search to the class of non-anticipating policies and assume exponentially distributed service requirements. We show that simple priority rules are optimal for certain settings of the parameters of the service requirements. For the remaining cases we prove that, in the case of a two-node linear network, an average-cost optimal policy is characterized by "switching curves", i.e., the policy dynamically switches between several priority rules. Since an exact characterization of these curves is not possible in general, we study in Chapter 5 the related fluid control problem. We show that the optimal fluid control can be explicitly characterized by linear switching curves. In most cases these curves provide asymptotically fluid-optimal policies in the original stochastic model as well. For some scenarios however, fluid-based switching curves may result in a policy that is unstable. In that case, the diffusion scaling is appropriate and efficient switchingcurve policies have a square-root shape. The class of weighted a-fair bandwidth-sharing policies are commonly accepted to model the dynamic flow-level bandwidth-allocation as realized by packet-based protocols. Through numerical experiments we evaluate the performance of these policies by comparing them to asymptotically optimal policies. In Chapter 4 we find that the gap between a-fair policies and optimal policies is not that large provided the system load is moderate. In addition, the performance under a-fair policies is quite insensitive to a, as long as this value is not too small. In Chapter 5 we observe that weighted a-fair policies can approach the optimal performance when choosingthe weights appropriately. In Chapter 6 we consider a linear network with generally distributed service requirements and allow anticipating policies. We focus on policies that allocate the capacity across the classes such that stability of the system is guaranteed. Motivated by the size-based scheduling results for single-server systems, we then prioritize within a class the large requests over the small ones. These size-based scheduling policies are proven to be asymptotically optimal in a heavy-traffic setting for service requirements with bounded support. In addition, we show that these policies may outperform a-fair policies, which are non-anticipating, by an arbitrarily large factor when the load is sufficiently high. In Chapter 7 we first focus on a multi-class queueing system with general interarrival times and service requirements, and give sufficient conditions in order to compare sample-path wise the workload and the number of users under different policies. This allows us to evaluate the performance of the system under various policies in terms of stability and the mean holding cost. We then apply this framework to the linear network under weighted a-fair policies. We obtain stability results and, in the case of exponentially distributed service requirements, establish monotonicity of the mean holding cost with respect to the fairness parameter a and the relative weights. In addition, we investigate the monotonicity properties in a heavytraffic regime and perform numerical experiments. Furthermore, for a single-server system with two user classes we obtain that under DPS and GPS the mean holding cost is monotone with respect to their relative weights. This result is in line with the monotonicity result for DPS under a heavy-traffic setting as obtained in Chapter 2. In Chapter 8 we focus on a parallel two-servermodel with two classes of users with exponentially distributed service requirements. The study of this model is motivated by scheduling questions in wireless cellular communication networks. It may model for example the power control of two interfering base stations. For certain choices of the parameters of the service requirements we give an exact characterization of an optimal policy. For the remaining cases we study the related deterministic fluid control model for which we show that the optimal control is described by a switching curve. Using similar techniques as in Chapter 5, we prove that policies characterized by either linear or exponential switching curves are asymptotically fluid-optimal in the original stochastic model. For a moderately-loaded system, we numerically compare these fluid-based policies with Max-Weight and threshold-based policies, which are known to be optimal in a heavy-traffic setting. We observe that the fluidbased and the threshold-based policies perform well, while significant performance gains can be achieved over Max-Weight policies
Resonant Air Quality Sensor
No full textIn this Thesis report the modeling, design, simulation, fabrication and measurements done at the Resonant Air Quality Sensor is described. The sensor is based on a resonant system, made in a silicon wafer with MEMS technology. By depositing the particles to be measured on top of the mass, the total mass of these particles can be measured by measuring the change in resonant frequency of the system. This sensor is made in the DIMES Cleanroom, and the fabrication process used is based on the existing DIMES BICMOS5 process, extended with a by the author designed MEMS module. The mean resonant frequency of the clean device is measured to be 498 kHz, with a quality factor of 1400. The mass sensitivity of the sensor is measured to by 920Hz/ng.MicroelectronicsElectrical EngineeringElectrical Engineering, Mathematics and Computer Scienc
Energy-aware capacity scaling in virtualized environments with performance guarantees
No full textWe investigate the trade-off between performance and power consumption in servers hosting virtual machines running IT services. The performance behavior of such servers is modeled through Generalized Processor Sharing (GPS) queues enhanced with a green speed-scaling mechanism that controls the processing capacity to use depending on the number of active virtual machines. When the number of virtual machines grows large, we show that the stochastic evolution of our model converges to a system of ordinary differential equations for which we derive a closed-form formula for its unique stationary point. This point is a function of the capacity and the shares that characterize the GPS mechanism. It allows us to show that speed-scaling mechanisms can provide large reduction in power consumption having only small performance degradation in terms of the delays experienced in the virtual machines. In addition, we derive the optimal choice for the shares of the GPS discipline, which turns out to be non-trivial. Finally, we show how our asymptotic analysis can be applied to the dimensioning and service partitioning in data-centers. Experimental results show that our asymptotic formulas are accurate even when the number of virtual machines is small
The Fluid Dynamics of the Freeboard in a Fluidized Bed Reactor
No full textMechanical Maritime and Materials Engineerin
The chemistry in the freeboard of a coal-fired pressurized fluized bed combustor
No full textApplied Science
Evaluatie van criteria voor het beoordelen van simulatieruns
No full textHet doel van deze afstudeeropdracht was om de in theoretische voorstudies voorgestelde veiligheidsindicatoren voor het beoordelen van simulatieruns te valideren met behulp van expertmeningen. Na observatie van simulatieruns en interviews met experts is gekozen om de Closest point of Approach (CPA) indicator en de Reserved Control Indicator (RCI) te valideren. Deze validatie is uitgevoerd door, voor een groot aantal runs, het verloop van de indicatorwaarden tijdens een run te vergelijken met een meting van de Mental Load met behulp van de Rating Scale Mental Effort enquête van Zijlstra. Uit de validatie van de indicatoren is gebleken dat, op basis van de gebruikte methode, geen duidelijke correlatie lijkt te bestaan tussen de expertbeoordeling (uitkomst RSME enquête) en de simulatordata (verloop indicator). Het verschillende stuurgedrag van de loodsen had invloed op het verloop van de indicatoren tijdens een run. Bovendien bleken de te beoordelen situaties te complex te zijn om door de indicatoren alleen te kunnen worden beoordeeld. In plaats van het beoordelen van een gehele run is daarom getracht een analyse uit te voeren van onderdelen van de run. Hiertoe is het intermitterende stuur-/ordergedrag van twee verschillende loodsen onderzocht. Op basis van het verloop van de versnelling van het schip bleek een gegeven order in te delen in een van de volgende drie controlmodes: orders voor koerscorrectie, orders voor windcorrectie en onduidelijke orders. Tevens leken de twee loodsen verschillende strategieën te hanteren bij het manoeuvreren van het schip. Als er in vervolgonderzoek beter inzicht verkregen kan worden in de frequentie, volgorde, aard en grootte van de orders, zou er een oordeel gevormd kunnen worden over het verloop van een simulatierun.Biomechanical EngineeringMechanical, Maritime and Materials Engineerin
An assessment of the relation between monitored settlements and the face stability of a slurry-TBM
No full textDuring the boring of a tunnel in soft soils with a slurry TBM, support pressure is used to achieve equilibrium at the face of the TBM. When this equilibrium is not reached, when the face support pressure is too low or too high, settlements will occur. In this research settlement and pore water pressure measurements are used to monitor the behavior of the soil and estimate the stability of the tunnel face. During boring of the tunnel, the exact stability of the face is not known. The TBM driver has to rely on the provided stratigraphy data, the advised face support pressures range provided by the geotechnical engineers and the experience of the tunnel boring team. Monitoring is not yet used to determine the face stability during construction. To do so, field data from a case study at RijnlandRoute is compared with analytical and numerical models. Sensitivity of the measurement equipment, and of both the analytical (DIN) and numerical model (Plaxis 3D) with respect to soil parameters, are considered. It has been found that the strength parameters of the layer in which the face is located have the highest influence on the minimum face support pressure. For the maximum face support pressure this is the volumetric weight of the entire soil profile above the face. For comparing the (soft soil) field data with numerical results, a Plaxis 3D model is built, and it is determined that HSsmall is a suitable constitutive model to capture the interaction between face stability, tunneling operations and soil behavior. It is shown that the tail void injection influences the settlements above and in front of the cutter head, but this influence is discarded and replaced by a wished in place lining, to simplify the numerical model. A scenario analysis on the sensitivity of soil parameters shows settlements do not vary significantly between the characteristic low and high values. In this analysis correlation of parameters is taken into account. The failure mechanism for the minimum face support pressure coincides with the active cave-in failure mechanism found in literature. For the maximum face support pressure the failure mechanism found in Plaxis 3D does not coincide with the expected hydraulic fracturing failure mode. The continuum representation of the soil in Plaxis 3D does not allow a hydraulic fracturing like failure mechanism to develop. Instead, a blow-out approximately 10 m. in front of the cutterhead occurs. This behaviour better resembles the failure mechanism of a EPB TBM. The field data gathered from the case study shows a thrust wave in front of the cutter head in both settlement and pore water pressure measurements. This thrust wave reaches up to 20 to 40 meters in front of the TBM. The heave induced by the thrust wave reduces the amount of settlements after the TBM passage. Excess pore pressures, induced by a high thrust wave, affect the face stability negatively. The excess pore pressure mainly depends on the advance rate. The higher the advance rate, the less time the pore pressures have to dissipate, leading to an increase in excess pore pressure. The accuracy of the settlements measurement devices is 0.8 mm., and of the spade cells 1.0 kPa. In general, the field data shows settlement curves corresponding to the Peck (1969) Gaussian curve (in lateral and longitudinal direction). Comparing the case study settlements with the numerically generated settlement curves show similar trends. The field data shows lower settlements than the numerical results. This can be due to the presence of excess pore pressures, the accuracy of the TBM data or its interpretation. A method to increase the accuracy, which is expected to result in a better fit with the numerical results, was found in a late phase of the research. This method takes into account the settlements which are induced by the thrust wave in front of the TBM. Comparing the numerical and analytically determined limit support pressures, it is found that the minimum face support pressure are similar in both methods. As similar failure mechanisms are found, numerical modelling seems a reliable way of determining the face stability. However, due to the limited range of applied support pressures available from the TBM data set, limit states could not be fully analyzed. To assure the reliability of a numerical model to determine the actual face stability based on surface settlements during the construction phase, additional research must be done. It is suggested to extend the methods used in this research with physical modelling. For maximum face support pressures, the analytical and numerical models do not coincide. Hydraulic fracturing cannot be modelled in Plaxis 3D. It is recommended not to use numerical modelling to determine the face stability based on settlements at face pressures higher than the face pressures at which an equilibrium is achieved.RijnlandRouteGeo-Engineerin
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