Selection of Techniques and Metrics Raj Jain Washington University in Saint Louis Saint Louis, MO 63130 [email protected] These slides are available on-line at: http://www.cse.wustl.edu/~jain/cse567-06/ Washington University in St. Louis

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Overview Criteria for Selecting an Evaluation Technique ! Three Rules of Validation ! Selecting Performance Metrics ! Commonly Used Performance Metrics ! Utility Classification of Metrics ! Setting Performance Requirements !

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Criteria for Selecting an Evaluation Technique

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Three Rules of Validation !

Do not trust the results of a simulation model until they have been validated by analytical modeling or measurements.

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Do not trust the results of an analytical model until they have been validated by a simulation model or measurements.

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Do not trust the results of a measurement until they have been validated by simulation or analytical modeling.

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Selecting Performance Metrics

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Selecting Metrics !

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Include: " Performance Time, Rate, Resource " Error rate, probability " Time to failure and duration Consider including: " Mean and variance " Individual and Global Selection Criteria: " Low-variability " Non-redundancy " Completeness

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Case Study: Two Congestion Control Algorithms Service: Send packets from specified source to specified destination in order. ! Possible outcomes: " Some packets are delivered in order to the correct destination. " Some packets are delivered out-of-order to the destination. " Some packets are delivered more than once (duplicates). " Some packets are dropped on the way (lost packets). !

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Case Study (Cont) !

Performance: For packets delivered in order, " Time-rate-resource ⇒ ! Response time to deliver the packets ! Throughput: the number of packets per unit of time. ! Processor time per packet on the source end system. ! Processor time per packet on the destination end systems. ! Processor time per packet on the intermediate systems. " Variability of the response time ⇒ Retransmissions ! Response time: the delay inside the network

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Case Study (Cont) Out-of-order packets consume buffers ⇒ Probability of out-of-order arrivals. " Duplicate packets consume the network resources ⇒ Probability of duplicate packets " Lost packets require retransmission ⇒ Probability of lost packets " Too much loss cause disconnection ⇒ Probability of disconnect "

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Case Study (Cont) !

Shared Resource ⇒ Fairness

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Fairness Index Properties: " Always lies between 0 and 1. " Equal throughput ⇒ Fairness =1. " If k of n receive x and n-k users receive zero throughput: the fairness index is k/n.

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Case Study (Cont) "

Throughput and delay were found redundant ⇒ Use Power.

Variance in response time redundant with the probability of duplication and the probability of disconnection " Total nine metrics. "

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Commonly Used Performance Metrics !

Response time and Reaction time

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Response Time (Cont)

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Capacity

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Common Performance Metrics (Cont) Nominal Capacity: Maximum achievable throughput under ideal workload conditions. E.g., bandwidth in bits per second. The response time at maximum throughput is too high. ! Usable capacity: Maximum throughput achievable without exceeding a pre-specified response-time limit !

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Knee Capacity: Knee = Low response time and High throughput

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Common Performance Metrics (cont) ! ! !

Turnaround time = the time between the submission of a batch job and the completion of its output. Stretch Factor: The ratio of the response time with multiprogramming to that without multiprogramming. Throughput: Rate (requests per unit of time) Examples: " Jobs per second " Requests per second " Millions of Instructions Per Second (MIPS) " Millions of Floating Point Operations Per Second (MFLOPS) " Packets Per Second (PPS) " Bits per second (bps) " Transactions Per Second (TPS)

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Common Performance Metrics (Cont) !

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Efficiency: Ratio usable capacity to nominal capacity. Or, the ratio of the performance of an n-processor system to that of a one-processor system is its efficiency. Utilization: The fraction of time the resource is busy servicing requests. Average fraction used for memory.

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Common Performance Metrics (Cont) Reliability: " Probability of errors " Mean time between errors (error-free seconds). ! Availability: " Mean Time to Failure (MTTF) " Mean Time to Repair (MTTR) " MTTF/(MTTF+MTTR) !

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Utility Classification of Metrics

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Setting Performance Requirements !

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Examples: “ The system should be both processing and memory efficient. It should not create excessive overhead” “ There should be an extremely low probability that the network will duplicate a packet, deliver a packet to the wrong destination, or change the data in a packet.” Problems: Non-Specific Non-Measurable Non-Acceptable Non-Realizable Non-Thorough ⇒ SMART

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Case Study 3.2: Local Area Networks ! !

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Service: Send frame to D Outcomes: " Frame is correctly delivered to D " Incorrectly delivered " Not delivered at all Requirements: Speed " The access delay at any station should be less than one second. " Sustained throughput must be at least 80 Mbits/sec. Reliability: Five different error modes. " Different amount of damage " Different level of acceptability.

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Case Study (Cont) " " " "

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The probability of any bit being in error must be less than 1E-7. The probability of any frame being in error (with error indication set) must be less than 1%. The probability of a frame in error being delivered without error indication must be less than 1E-15. The probability of a frame being misdelivered due to an undetected error in the destination address must be less than 1E-18. The probability of a frame being delivered more than once (duplicate) must be less than 1E-5. The probability of losing a frame on the LAN (due to all sorts of errors) must be less than 1%.

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Case Study (Cont) !

Availability: Two fault modes – Network reinitializations and permanent failures " The mean time to initialize the LAN must be less than 15 milliseconds. " The mean time between LAN initializations must be at least one minute. " The mean time to repair a LAN must be less than one hour. (LAN partitions may be operational during this period.) " The mean time between LAN partitioning must be at least one-half a week.

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Summary of Part I

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Systematic Approach: Define the system, list its services, metrics, parameters, decide factors, evaluation technique, workload, experimental design, analyze the data, and present results Selecting Evaluation Technique: The life-cycle stage is the key. Other considerations are: time available, tools available, accuracy required, trade-offs to be evaluated, cost, and saleability of results.

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Summary (Cont) Selecting Metrics: " For each service list time, rate, and resource consumption " For each undesirable outcome, measure the frequency and duration of the outcome " Check for low-variability, non-redundancy, and completeness. ! Performance requirements: Should be SMART. Specific, measurable, acceptable, realizable, and thorough. !

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Exercise 3.1 What methodology would you choose: a. To select a personal computer for yourself? b. To select 1000 workstations for your company? c. To compare two spread sheet packages? d. To compare two data-flow architectures, if the answer was required: i. Yesterday? ii. Next quarter? iii. Next year? Washington University in St. Louis

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Homework #2 Read chapters 3 ! Submit answers to " Exercise 3.1 !

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Selection of Techniques and Metrics - Washington University in St ...

Case Study: Two Congestion Control Algorithms. ❑ Service: ... Some packets are delivered out-of-order to the ... Some packets are dropped on the way (lost.

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