1,108 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
A Lagrangian sub-grid model for the dispersion of clouds of tracers
No full textTurbulence models are expected to satisfy the conflicting requirements of accuracy and computational efficiency. Here we discuss a new model that was recently developed in order to accurately and efficiently describe the dynamic of a clouds of tracers particles in Large Eddy Simulations of homogeneous and isotropic turbulent flows. The models incorporates the multi-scale nature of time and space turbulent velocity correlations that are essential in order to correctly reproduce the relative dispersion of multi-particle clouds. The model can be seen as an off-grid solver for the Eulerian velocity field at the positions of a given number of Lagrangian tracers that self-consistently move with it. Extensions to non homogeneous and isotropic turbulence as well as to the dynamics of particles will be discussed
I.M. Panagiotopoulos as an autobiographer: the adventure of the hero and the personal myth of the author
No full textThe present doctoral dissertation refers to the complex and prominent image of I.M. Panagiotopoulos as a human being, which emerges through his exuberant and diverse work. This dissertation explains man with work and work with man and highlights a special aspect of I.M. Panagiotopoulos that of the pedagogue, through his pedagogical texts, his presence in the classroom and, in general, his pedagogical action. It refers to his childhood, repulsed mostly to the unconscious and to the deeper layers of his psyche. Also, the object of this study is the novel writing of I.M. Panagiotopoulos which istreated as a consequence of the denaturation of his obstructed erotic energy. Next, the two dominant impulses, Love and Death that meet in art and in life, are explored, with the impulses of Death predominant, a fact that leads to their narcissistic character, as well as to their conflicting nature. The anthropogeographical and metaphysical elements that constitute the concept and ontology of Greece are then highlighted. Then, the pedagogical work of I.M. Panagiotopoulos, his positions,contradictions and his proposals for Education is examined. I.M. Panagiotopoulos fought for a school that will offer Liberal Education, within a democratic regime that defines the equality of opportunity in learning. In addition, our national language is presented during its historical course and its expressive and iconographic boldness is highlighted. Finally, this thesis codifies the evolutionary stages of a single hero and creates a typology of his inner adventure, which is the adventure of the hero's desire. It also traces the author's personal myth on a general level, the correspondences and the dialectical relationship between the author and his work, in such a one-way way, that the work defines and composes the author and not vice versa.Η παρούσα διδακτορική διατριβή αναφέρεται στη σύνθετη κα εξέχουσα εικόνα του Ι.Μ. Παναγιωτόπουλου ως ανθρώπου, η οποία ξεπροβάλλει μέσα από το πληθωρικό και πολύμορφο έργο του. Εξηγεί τον άνθρωπο με το έργο και το έργο με τον άνθρωπο. Αναδεικνύει μια ξεχωριστή πλευρά του Ι.Μ. Παναγιωτόπουλου αυτή του Παιδαγωγού, μέσα από τα παιδαγωγικά του κείμενα, από την παρουσία του στην τάξη και γενικότερα από την παιδαγωγική του δράση. Αναφέρεται στην παιδική του ηλικία απωθημένη ως επί το πλείστον στο ασυνείδητο και στα βαθύτερα στρώματα του ψυχισμού του. Επίσης, αντικείμενο της μελέτης είναι η μυθιστορηματική γραφή του Ι.Μ. Παναγιωτόπουλου που αντιμετωπίζεται ως απόρροια μετουσίωσης της παρεμποδισμένης ερωτικής του ενέργειας. Ακολούθως, διερευνώνται οι δύο κυρίαρχες ενορμήσεις, του Έρωτα και του Θανάτου, που ανταμώνουν στην τέχνη και στη ζωή, με κυρίαρχες μάλιστα τις ενορμήσεις του Θανάτου, γεγονός που οδηγεί στον ναρκισσικό χαρακτήρα τους, καθώς και στη συγκρουσιακή τους φύση. Στη συνέχεια, αναδεικνύονται τα ανθρωπογεωγραφικά και μεταφυσικά στοιχεία που συνιστούν την έννοια και την οντολογία της Ελλάδας. Κατόπιν, εξετάζεται το Παιδαγωγικό του έργο και παρουσιάζονται οι θέσεις, οι αντιθέσεις και οι προτάσεις του Ι.Μ. Παναγιωτόπουλου για την Παιδεία. Ο Ι.Μ. Παναγιωτόπουλος αγωνίστηκε για ένα σχολείο, το οποίο θα προσφέρει Αγωγή φιλελεύθερη, μέσα σ’ ένα δημοκρατικό καθεστώς που ορίζει την ισότητα των ευκαιριών στη μάθηση. Επιπλέον, παρουσιάζεται η εθνική μας γλώσσα κατά την ιστορική της διαδρομή και αναδεικνύεται η εκφραστική και εικονοπλαστική της τόλμη. Τέλος, η παρούσα διατριβή κωδικοποιεί τα εξελικτικά στάδια ενός ενιαίου ήρωα και δημιουργεί μια τυπολογία της εσωτερικής του περιπέτειας, που είναι η περιπέτεια της επιθυμίας του ήρωα. Επίσης, ανιχνεύονται ο προσωπικός μύθος του συγγραφέα σε ένα γενικό επίπεδο, οι αντιστοιχίες και η διαλεκτική σχέση ανάμεσα στον συγγραφέα και στο έργο του, με τέτοιο μονοδρομικό τρόπο, ώστε το έργο να καθορίζει και να συνθέτει τον συγγραφέα και όχι αντίθετα
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
Methods and software for significant indicators determination of the natural language texts author profile
No full textMethods for the formation and optimization of author profiles are presented. The author profile is an image – a vector in a multidimensional space, which components are author’s texts measurements by a number of methods based on 4-grams, stemming, recurrence analysis and formal stochastic grammar. The author’s profile is a model of his language, including vocabulary, sentence syntax features. A comparative analysis of each of the methods effectiveness is carried out. By means of the genetic algorithm, a reduced profile of the author is formed. Insignificant indicators are excluded, which allows to reduce their number by 20%. The reduced author’s profile contains attributes that are significant for this author and is an effective attribution of a particular author.Prombles in programming 2023; 3: 22-2
Information about the Golden Horde in the Saltuk-name of the Ottoman author Ebul-Khayri Rumi
No full textThe purpose of the study is to analyze information on the history of the Golden Horde from the multi-volume work of Ebul-Khayri Rumi “Saltuk-name”, an Ottoman author of the 15th century.
Research materials. Ottoman historical works contain quite original information about the Golden Horde and the Tatar khanates. Their information allows us to reveal previously unknown pages of the history of the Golden Horde period. One of such important and early works is the three-volume work “Saltuk-name” by Ebul-Khayri Rumi, an Ottoman author of the 15th century, describing the legendary history of Sary-Saltuk, who was engaged in the spread of Islam, including in the lands of the Golden Horde, primarily in Crimea. In Russian-language historiography, this author is little known and his information has not been used for scientific research. The relevance of the article is explained by the need to introduce into scientific circulation the information of the “Saltuk-name” about the Golden Horde.
Results and scientific novelty. The study is the first attempt at a systematic analysis of Ebul-Khayri Rumi’s Saltuk-name in Russian. The novelty of the study is the introduction of specific information from the Ottoman historical work and its interpretation into scientific circulation. This study provides an important impetus for the introduction of information from the works of Ottoman authors who wrote about the Golden Horde into active scientific circulation
Measurements of fluid flow in weld pools
No full textUnderstanding the fluid flow in weld pools contributes significantly toward controlling the heat distribution in the base material and the mass distribution of molten base and additive materials. Currently, most investigations focus primarily on numerical models, due to the experimental difficulties associated with the challenging environment in and around the weld pool. Numerical simulations based on computational fluid dynamics are currently addressing the dynamic behaviour of weld pools, such as melting, solidification, temperature and velocity fields; these results provide information about conditions inside the weld pool that is impossible to gain experimentally. However, there is still a shortage of experimental validation to support these models. In this thesis, experimental methods are applied to study the fluid flow of the weld pool in both arc and laser generated weld pools. The general conclusions from the experimental study provides some suggestions for understanding the fluid flow in the weld pool, such as flow asymmetry, rotational flow and interface instabilities. For example, the interface instabilities can affect the energy and mass distribution. Describing these instabilities numerically requires three dimensional flow models. Moreover, the current experimental results question the validity of works where the surfactant concentration is assumed to be constant in the weld pool without consideration of dynamic diffusion and convection. This work has focused on experimental measurement of the fluid dynamics of weld pools, and current results provide a number of validation cases for further numerical simulations.Materials Science & EngineeringMechanical, Maritime and Materials Engineerin
Microstructural Evolution during High-Frequency Post Weld Impact Treatments for High-Strength Steels
No full textMaterial Science and EngineeringMechanical, Maritime and Materials Engineerin
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