Effects and Implications of File Size/Service Time Correlation on Web Server Scheduling Policies Dong Lu* + Peter Dinda* Yi Qiao* Huanyuan Sheng* *Northwestern University + Ask Jeeves, Inc.

2 Outline Quick review of size-based scheduling Motivation and approach Correlation between file size and service time: a measurement study Performance of SRPT scheduling under real workload Domain-based scheduling

3 Quick Review of Size-based Scheduling SRPT –Shortest Remaining Processing Time –Assuming perfect knowledge of service times FSP –Fair Sojourn Protocol –Assuming perfect knowledge of service times Typical non-size-based scheduling –Processor Sharing (PS) –First Come First Serve (FCFS)

4 SRPT Always serve the job with minimum remaining processing time first, preemptive scheduling –Performance: Minimum mean response time [Schrage, Operations Research, 1968] –Fairness: performance gains of SRPT over PS do not usually come at the expense of large jobs, in other words, it is fair for heavy-tail job size distribution [Bansal and Harchol-Balter, Sigmetrics ‘01]

5 FSP Combined SRPT with PS, preemptive scheduling. [Friedman, et al, Sigmetrics ‘03] –SRPT + the longer a job stay in the queue, the higher its priority –Performance: Mean response time is close to that of SRPT –Fairness: Fairer than PS

6 Outline Quick review of size-based scheduling Motivation and approach Correlation between file size and service time: a measurement study Performance of SRPT scheduling under real workload Domain-based scheduling

7 Motivation Current implementation of SRPT and FSP –Use file size as service time (sorting jobs using file size) Is file size a good estimator of service time? What is the performance of SRPT and FSP using file size as service time? And how to improve? Service time: the time needed to send requested data in the absence of other requests in the system

8 Trace-driven Simulation Simulator: –C++ –Supports G/G/n/m queuing model –Driven by enhanced web server traces –Validation Little’s law Repeat the simulations in the FSP paper [Friedman, et al, Sigmetrics ‘03] Compare with available theoretical results [Bansal and Harchol-Balter, Sigmetrics ‘01]

9 Scheduling Policies Studied SRPT: Ideal SRPT SRPT-FS: File size as service time SRPT-D: Domain-estimated service time FSP: Ideal FSP FSP-FS: File size as service time FSP-D: Domain-estimated service time PS: Processor sharing

10 Outline Quick review of size-based scheduling Motivation and approach Correlation between file size and service time: a measurement study Performance of SRPT-FS and FSP-FS scheduling under real workload Domain-based scheduling

11 Correlation is Weak on a Typical Web Server Measurement on departmental web server: Scatter plot of file size versus service time (log-log scale) R ≈ 0.14 Service time File Size Request from the whole Internet

12 Correlation is Weak on Web Cache Servers Measurement on 10 Squid web cache servers: – Correlation Coefficient R Between File size and Service time P[R>x]

13 Main reason for the weak correlation End-to-end path diversity Web Server Client 1 Client 2 Client 3 Client 4

14 Outline Quick review of size-based scheduling Motivation and approach Correlation between file size and service time: a measurement study Performance of SRPT-FS and FSP-FS scheduling under real workload Domain-based scheduling

15 Mean Response Time Much Worse Than Expected Simulation driven by web server trace. G/G/1/m. Pareto arrivals (rate controlled to tune the load). Load on the queue Mean Response Time (millisec) PS SRPT-FS FSP-FS Ideal SRPT and FSP

16 Mean Queue Length Much Worse Than Expected Simulation driven by web server trace. G/G/1/m. Pareto arrivals (rate controlled to tune the load). Load on the queue Mean Queue Length FSP-FS SRPT-FS PS Ideal SRPT and FSP

17 Requirements For A Better Service Time Estimator Low overhead –Passive measurement –Low computation complexity –Low / adjustable memory usage Effective –Approximate the correct ordering of the service times. High correlation.

18 Outline Quick review of size-based scheduling Motivation and approach Correlation between file size and service time: a measurement study Performance of SRPT-FS and FSP-FS scheduling under real workload Domain-based scheduling

19 Domain-based estimator Divide Internet into smaller “domains” by leveraging CIDR (Classless Inter-domain Routing) Hosts in the same domain are likely to share same/similar routes to web server, and thus similar throughput Web Server

20 Supporting Facts Statistical Internet stability and locality –Routing stability [Paxson, Sigcomm 1996] –TCP throughput locality and stability [Balakrishnan, et al, Sigmetrics 1997]; [Seshan, et al, USITS 1997]; [Myers, et al, Infocom 1999] Classless Inter-domain Routing –implies that routes from machines in the domain to a server outside the domain will share many hops.

21 Algorithm Use high order k bits of client IP address to classify clients into 2 k domains For each domain, calculate R = F/S –R: representative service rate –F: sum of file sizes delivered to domain –S: sum of corresponding service times For each request, first extract its domain, then service time can be estimated as B/R –B: requested file size –R: representative service rate obtained before

22 Higher Correlation Can Be Achieved Correlation Coefficient R Bits used to define a domain

23 Much Lower Service Times Can Be Achieved Bits used to define a domain Mean Response time (milisec) PS FSP-D SRPT-FS FSP-FS SRPT-D SRPT and FSP

24 Much Lower Queue Lengths Can Be Achieved Bits used to define a domain Mean queue length FSP-D FSP-FS SRPT-FS PS SRPT-D SRPT and FSP

25 Conclusions File size may not be a good estimator of service time for many regimes File size-based SRPT and FSP can perform worse than PS in these regimes Domain-based scheduling brings the benefits of size-based scheduling to these regimes

26 For more information Prescience Lab at Northwestern University –

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