A Feedback Control Architecture and Design Methodology for ...

A Feedback Control Architecture and Design Methodology for ...

A Feedback Control Architecture and Design Methodology for Service Delay Guarantees in Web Servers Presentation by Amitayu Das 1 Introduction Response-time delay while browsing

Implications Loss for the website Loss of revenue for the hosting platform, too Reasons - Server end problem Network latency Were talking about Server end problem

2 Motivation Time-varying workload for limited resource Limited adaptation of web-server: best-effort Lack of enforcement of QoS guarantees at web-server Notion of service differentiation is not enforced at server end 3

Differential service Two classes: premium, basic (say) Best-effort model: no guarantee Absolute model: soft deadline How to decide the deadline? Depends on several things No overload => all classes receive satisfactory delay Overload => degradation in prioritized order

Proportional model: no fixed deadline, hence flexible Hybrid model: Gets the best of above two Flexibility with no overload, bounded delay for high priority classes on overload

4 Performance differentiation is better than previous two Web server mechanism Scenario Handle incoming TCP connection by assigning a server Multi-threaded/multi-process setup Multi-threaded setup is very costly in UNIX

HTTP 1.0: Excessive # of concurrent TCP connection HTTP 1.1: 5

for web server Persistent connection and problems with that Which is the bottleneck here? Service delay guarantees Connection delay:

Time b/w arrival and acceptance Processing delay:

6 Time b/w arrival and transferring response to client Connection delay (Ck(m)): average for class k (0 k N) within ((m-1)S, mS) Relative delay guarantee: Cj(m)/Cl(m) = Wj/Wl for all j and l (j l); Wj is the relative desired delay Absolute delay guarantee: Cj(m) Wj; for all classes j if there is a class l > j and Cl(m) Wl , which is desired (absolute) delay Hybrid delay guarantee: Wk represents both desired delay and relative delay The Feedback-Control Architecture for

Delay Guarantees 7 Delay controllers Controller: (Reference, Output, Error, Control Input) (VSk, Vk(m), Ek(m), Uk(m)) Absolute delay controller CAk: (Wk, Ck(m), VSK(m) Vk(m), Bk(m)) Relative delay controller CRk: (Wk/Wk-1, Ck(m)/Ck-1(m), VSK(m) Vk(m),

Bk-1(m)/Bk(m)) Hybrid delay controller - Switching condition: - 8 - C0 (m) > W0 + H, switch to CA; H is a threshold, to avoid

C0 (m) > W0 - H, switch to CR; thrashing b/w controllers Design of delay-controller Performance specification Stability Settling time (TS):measures efficiency of controller

Steady state error (ES): measures accuracy System Identification: establish dynamic model Root Locus: designs controller to meet performance specification 9 Architecture for system

identification System identification Estimated parameters

10 Model structure White noise input LSE (a1, a2, b1, b2) = (0.74, -0.37, 0.95, 0.12): relative delay (a1, a2, b1, b2) = (0.08, -0.2, 0.2, -0.05): absolute delay System identification results for relative delay

11 Results (relative delay) 12 Results (absolute delay) 13 Root Locus design g

= 0.3, r = 0.05 for relative delay controller g = -4.6, r = 0.3 for absolute delay controller 14 Evaluation of relative-delay guarantees 15 Evaluation of relative-delay guarantees

16 Evaluation of absolute-delay guarantees 17 What self-* about it? Proposes adaptive architecture Avoids laborious ad-hoc approaches for tuning and design iteration

18 Result with three classes 19 Last slide Questions?? 20

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