The behavior of robots in the electronic mail business web sites was described. The analysis aimed at identifying, characterizing and distinguishing two categories of robots called Crawlers and ShopBots. Crawlers seek...
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The behavior of robots in the electronic mail business web sites was described. The analysis aimed at identifying, characterizing and distinguishing two categories of robots called Crawlers and ShopBots. Crawlers seek out information relevant to a specific pre-defined set of subjects whereas ShopBots search for prices of items in various electronic-trailers and summarize findings in a single page to the user. The performance model was used for assessing the impact of robot workloads on the consumption of the site resources.
Recent efforts to add new services to the Internet have increased the interest in software-based routers that are easy to extend and evolve. This paper describes our experiences implementing a software-based router, w...
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Recent efforts to add new services to the Internet have increased the interest in software-based routers that are easy to extend and evolve. This paper describes our experiences implementing a software-based router, with a particular focus on the main difficulty we encountered: how to schedule the router's CPU cycles. The scheduling decision is complicated by the desire to differentiate the level of service for different packet flows, which leads to two fundamental conflicts: (1) assigning processor shares in a way that keeps the processes along the forwarding path in balance while meeting QoS promises, and (2) adjusting the level of batching in a way that minimizes overhead while meeting QoS promises.
We consider the problem of designing a virtual topology to minimize electronic routing, that is, grooming traffic, in wavelength routed optical rings. We present a new framework consisting of a sequence of bounds, bot...
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We consider the problem of designing a virtual topology to minimize electronic routing, that is, grooming traffic, in wavelength routed optical rings. We present a new framework consisting of a sequence of bounds, both upper and lower, in which each successive bound is at least as strong as the previous one. The successive bounds take larger amounts of computation to evaluate, and the number of bounds to be evaluated for a given problem instance is only limited by the computational power available. The bounds are based on decomposing the ring into sets of nodes arranged in a path, and adopting the locally optimal topology within each set. Our approach can be applied to many virtual topology problems on rings. The upper bounds we obtain also provide a useful series of heuristic solutions.
The increasing complexity of modern superscalar microprocessors makes the evaluation of new designs and techniques much more difficult. Fast and accurate methods for simulating program execution on realistic and hypot...
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The increasing complexity of modern superscalar microprocessors makes the evaluation of new designs and techniques much more difficult. Fast and accurate methods for simulating program execution on realistic and hypothetical processor models are of great interest to many computer architects and compiler writers. There are many existing techniques, from profile based runtime estimation to complete cycle-level simulations. Many researchers choose to sacrifice the speed of profiling for the accuracy obtainable by cycle-level simulators. This paper presents a technique that provides accurate performance predictions, while avoiding the complexity associated with a complete processor emulator. The approach augments a fast in-order simulator with a time-stamping algorithm that provides a very good estimate of program running time. This algorithm achieves an average accuracy that is within 7.5% of a cycle-level out-of-order simulator in approximately 41% of the running time on the eight SPECInt95 integer benchma rks.
Soft Modems use the main processor to execute modem functions traditionally performed by hardware on the modem card. To function correctly, soft modems require that ongoing signal processing computations be performed ...
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Soft Modems use the main processor to execute modem functions traditionally performed by hardware on the modem card. To function correctly, soft modems require that ongoing signal processing computations be performed on the host CPU in a timely manner. Thus, signal processing is a commonly occurring background real-time application-one running on systems that were not designed to support predictable real-time execution. This paper presents a detailed study of the performance characteristics and resource requirements of a popular soft modem. Understanding these requirements should inform the efforts of those designing and building operating systems needing to support soft modems. Furthermore, we believe that the conclusions of this study also apply to other existing and upcoming soft devices, such as soft Digital Subscriber Line (DSL) cards. We conclude that (1) signal processing in an interrupt handler is not only unnecessary but also detrimental to the predictability of other computations in the system and (2) a realtime scheduler can provide predictability for the soft modem while minimizing its impact on other computations in the system.
In this paper we develop a new predictive flow control scheme and analyze its performance. This scheme controls the non-real-time traffic based on predicting the real-time traffic. The goal of the work is to operate t...
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In this paper we develop a new predictive flow control scheme and analyze its performance. This scheme controls the non-real-time traffic based on predicting the real-time traffic. The goal of the work is to operate the network in a low congestion, high throughput regime. We provide a rigorous analysis of the performance of our flow control method and show that the algorithm has attractive and useful properties. From our analysis we obtain an explicit condition that gives us design guidelines on how to choose a predictor. We learn that it is especially important to take the queueing effect into account in developing the predictor. We also provide numerical results comparing different predictors that use varying degrees of information from the network.
Exact as well as approximate analytical solutions for quantitative performance models of computersystems are usually obtained by performing a series of arithmetical operations on the input parameters of the model. Ho...
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Exact as well as approximate analytical solutions for quantitative performance models of computersystems are usually obtained by performing a series of arithmetical operations on the input parameters of the model. However, especially during early phases of system design and implementation, not all the parameter values are usually known exactly. In related research contributions, intervals have been proposed as a means to capture parameter uncertainties. Furthermore, methods to adapt existing solution algorithms to parameter intervals have been discussed. In this paper we present the adaptation of an existing performance model to parameter intervals. The approximate solution of a queueing network modelling an Enterprise JavaBeans server implementation is adapted to interval arithmetic in order to represent the uncertainty in some of the parameters of the model. A new interval splitting method is applied to obtain reasonable tight performance measure intervals. Monotonicity properties of intermediate comp utation results are exploited to achieve a more efficient interval solution. In addition, parts of the original solution algorithm are modified to increase the efficiency of the corresponding interval arithmetical solution.
In this paper we study the performance trade-offs between conventional cellular and multi-hop ad-hoc wireless networks. We compare through simulations the performance of the two network models in terms of raw network ...
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In this paper we study the performance trade-offs between conventional cellular and multi-hop ad-hoc wireless networks. We compare through simulations the performance of the two network models in terms of raw network capacity, end-to-end throughput, end-to-end delay, power consumption, per-node fairness (for throughput, delay, and power), and impact of mobility on the network performance. The simulation results show that while adhoc networks perform better in terms of throughput, delay, and power, they suffer from unfairness and poor network performance in the event of mobility. We discuss the trade-offs involved in the performance of the two network models, identify the specific reasons behind them, and argue that the trade-offs preclude the adoption of either network model as a clear solution for future wireless communication systems. Finally, we present a simple hybrid wireless network model that has the combined advantages of cellular and ad-hoc wireless networks but does not suffer from the disadvan tages of either.
In this paper, we explore the use of fixed point methods to evaluate the performance of a large population of TCP flows traversing a network of routers implementing active queue management (AQM) such as RED (random ea...
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In this paper, we explore the use of fixed point methods to evaluate the performance of a large population of TCP flows traversing a network of routers implementing active queue management (AQM) such as RED (random early detection). Both AQM routers that drop and that mark packets are considered along with infinite and finite duration TCP flows. In the case of finite duration flows, we restrict ourselves to networks containing one congested router. In all cases, we formulate a fixed point problem with the router average queue lengths as unknowns. Once these are obtained, other metrics such as router loss probability, TCP flow throughput, TCP flow end-to-end loss rates, average round trip time, and average session duration are easily obtained. Comparison with simulation for a variety of scenarios shows that the model is accurate in its predictions (mean errors less than 5%). Last, we establish monotonicity properties exhibited by the solution for a single congested router that explains several interesting observations, such as TCP SACK suffers higher loss than TCP Reno.
The Shortest-Remaining-Processing-Time (SRPT) scheduling policy has long been known to be optimal for minimizing mean response time (sojourn time). Despite this fact, SRPT scheduling is rarely used in practice. It is ...
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The Shortest-Remaining-Processing-Time (SRPT) scheduling policy has long been known to be optimal for minimizing mean response time (sojourn time). Despite this fact, SRPT scheduling is rarely used in practice. It is believed that the performance improvements of SRPT over other scheduling policies stem from the fact that SRPT unfairly penalizes the large jobs in order to help the small jobs. This belief has led people to instead adopt "fair" scheduling policies such as Processor-Sharing (PS), which produces the same expected slowdown for jobs of all sizes. This paper investigates formally the problem of unfairness in SRPT scheduling as compared with PS scheduling. The analysis assumes an M/G/1 model, and emphasizes job size distributions with a heavy-tailed property, as are characteristic of empirical workloads. The analysis shows that the degree of unfairness under SRPT is surprisingly small. The M/G/1/SRPT and M/G/1/UPS queues are also analyzed under overload and closed-form expressions for m ean response time as a function of job size are proved in this setting.
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