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OutlineUse of Feedback Control Theory in computing systems for QoS management
To apply theory to govern component execution in the scope of the TAPAS project, addressing the goal of developing Trusted and QoS-aware middleware services for Application Hosting
• Control Theory• Feedback Control• Middleware for QoS Control• Conclusions
Motivation
Focus
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Dynamic Systems and Control Theory
• Example: in an e-business server, neither the resource requirements nor the arrival rate of service requests are known a priori
• Problem: difficulty meeting performance guarantees, failure may result in loss of customers and financial damage
• Solution: a well developed analytic foundation for performance control in physical systems based on Control Theory
Subject: soft real-time applications executing in open and unpredictable environments
Application over Internet: Dynamic System living in an ever changing environment, it evolves over the time and nothing is known a priori
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Control Theory
• Success: due to its robustness in the face of modelling errors and external disturbances, which reduces the need for accurate system models
• Recent results: Control Theory in Computing Systems– Network flow controllers (TCP/IP), C. Hollot et al. (U.Mass)– Lotus Notes admission control, N. Gandhi et al. (IBM)– QoS in Caching, Y. Lu et al. (U.Virginia)– Performance guarantees for Apache , T. Abdelzaher et al.
Control Theory
The objective in a Control System is to make some output behave in a desired way by manipulating some input, employing differential equations as a fundamental modelling tool
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Control SystemENVIRONMENT
CONTROLLER
CONTROLLEDSYSTEM
manipulation observation
CONTROL SYSTEM
Control System
Represented by a controlled system in combination with a controller, their interactions are observations and manipulations performed by the controller on controlled system.
• Example: In a water heater, if the temperature drops below a set value, a constant heat source is switched on, to be switched off again when the temperature rises above a set maximum
input output
controlinformation
BOILER
cold water hot waterwater heater
thermostat
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Control System Strategies
Feed forward control strategy manipulation through control actions is determined based on observation of the input to the controlled system A priori workload knowledge, a combination of pre run time analysis
and admission control algorithms ensure that the system is not overloaded and that the desired performance is achieved
Feedback control strategy measurements of the output delivered are compared with a desired behaviour (reference) and the difference (error) is used to decide on the control actions to be taken Applied for behaviour optimisation, it regulates control actions
according to continuous performance feedback
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Feedback Control System
ACTUATOR
reference
control input
controlled variable
manipulatedvariable
CONTROLLED SYSTEM
+-
error
controlalgorithm
CONTROLLER
sampleSENSOR
FEEDBACK CONTROL LOOP
The Sensor measures the controlled variable and feeds the sample back to the Controller
The Controller compares the performance reference with controlled variable to get the current error, and it calls a control algorithm to compute the control input, new value of the manipulated variable based on the error
The Actuator changes the manipulated variable based on the newly computed control input
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Feedback Control
Design challenges:• To develop a general methodology for converting specifications
of a computing system into feedback loops with known set points and feedback control parameters.
• To find a convenient interface between the service software and the middleware control loops that manage its performance.
• To design appropriate software performance sensors and actuators.
Continuous monitoring of the controlled system, comparing the actual behaviour result against a specification of the expected behaviour and adjusting the performances accordingly
Feedback Control
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Feedback Control
Potential applications:
Performance-assured services (e-commerce, online trading)
Services differentiation Contractual satisfaction guarantees Overload control
E-COMMERCE
PRICING
NETWORK QoS CLIENTHETEROGENEITY
MULTIMEDIA
SERVER OVERLOAD
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Feedback Control Related works:
AMIDST: architecture to support QoS-aware m/w, focusing on the QoS-control system. An architectural framework, based on models from Control Theory, observes and manipulates the state of the m/w platform that supports distributed applications
CONTROLWARE: m/w architecture for QoS guarantees in distributed environments. It implements a new paradigm for QoS control, suitable for system operating in highly uncertain environments or when accurate system load and resource models are not available
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Middleware for QoS ControlDiagram of a model
SENSOR
INTERPRETER
COMPARATOR
DECIDER
TRANSLATOR
ACTUATOR
APPLICATIONS
MIDDLEWARE PLATFORM
COMPUTING & COMMUNICATION RESOURCES
observation
measurement(QoS state)
agreedQoS difference
control strategy
control action
input output
probe probe
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Middleware for QoS Control
Adaptation based middleware architecture demands three principal stages of run time support:
• probing the performance of QoS parameters
• instantiating the initial middleware configuration
• adapting to on-the-fly variations
Middleware for QoS Control monitors the output and reconfigures itself to provide the application with a stable execution environment
TAPAS Project: Trusted and QoS-aware middleware services for Application Hosting
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Conclusions
Feedback Control Theory offers a valid solution to achieve performance guarantees for soft real-time applications
Successfully applied by the engineering community in controlling a vast variety of physical systems
Recent results prove that Control Theory can also be efficiently applied to the control of software performance
Theoretical basis for the design of a QoS aware middleware
Applying Feedback Control to QoS management
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Selected bibliography- MODERN CONTROL SYSTEMS, R.Dorf, R.Bishop, Prentice
Hall
- ControlWare: A Middleware Architecture for Feedback Control of Software Performance, R. Zhang, C.Lu, Tarek F. Abdelzaher, John A. Stankovic (Department of Computer Science University of Virginia - 2002)
- SWiFT: A Feedback Control and Dynamic Reconfiguration Toolkit A.Goel, D.Steere, C.Pu, J.Walpole,(Department of Computer Science and Engineering Oregon Graduate Institute, Portland - 1999)
- AMIDST: A QoS-Control Architecture for Object Middleware, L.Bergmans, A.van Halteren, L.Ferreira Pires, M.van Sinderen and M.Aksit, (CTIT University of Twente, The Netherlands - 2000)
