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Performance engineering

Performance Engineering

Performance Metrics

Prof. Jerry Breecher

Performance Metrics


All about numbers
All About Numbers

Performance Metrics


Performance metrics
PERFORMANCE METRICS

In this section, a few of the metrics of performance are described. Along with these metrics are some simple equations that connect together the various quantities. We will continue to talk about metrics in a later section. In fact, when we get to talking about queues, we’ll be spending lots of time looking at the interrelations of these concepts.

Arrivals ------> SYSTEM ------> Departures

T is the length of TIME we observed the system.

A is the number of request ARRIVALS observed.

C is the number of request DEPARTURES observed.

W the ACCUMULATED TIME for all requests within the system - time spent both waiting for and using resources. It is

Srequests[ Tdeparture – Tarrival]

Performance Metrics


Performance metrics1
PERFORMANCE METRICS

Arrivals ------> RESOURCE ------> Departures

B is the length of time that the resource was observed to be BUSY.

Note that the pictures may be equivalent; a system may have only one resource. A request is presented to a system ( in which case it might be called a "job") or is presented to a particular resource.

Performance Metrics


Performance metrics2

Metric

Description

Formula

Units

Arrival Rate

How many requests arrive per time.

Y = A / T

Requests/second

Throughput (Departure Rate)

How many requests complete per time.

X = C / T

Requests/second

Utilization

Fraction of time the resource is in use.

U = B / T

Number in range 0 - 1.

Service Requirement

Time needed by a resource to handle a request.

S = B / C

Seconds

Requests in system

Average number of requests in the system.

N = W / T

Dimensionless

Residence time

Average time a request spends in the system.

R = W / C

Seconds

PERFORMANCE METRICS

Performance Metrics


Performance metrics3
PERFORMANCE METRICS

Example:

Requests

3 |

|

2 | ----------------- -----------------

|

1 | ----------------- ----------------- -----------------

|

+----------------------------------------------------------------------------

0 1 2 3 4 5 6 7 8 9 10

Time ( seconds )

Note that requests are serviced

simultaneously - it's a multiprocessor?

What are:

T Time interval =

A Arrivals =

C Completions =

B Time spent servicing =

requests

Y = A / T Arrival rate =

X = C / T Throughput =

U = B / T Utilization =

S = B / C Service time/request =

Performance Metrics


Performance metrics4
PERFORMANCE METRICS

UTILIZATION LAW U = X S

Proof: B / T = C / T * B / C

Example:

The server of dinners in the Cafeteria keeps busy 75% of the time between 12:00 and 1:00. During this time 90 people are served a dinner. How long does it take to serve each customer?

Performance Metrics


Performance metrics5
PERFORMANCE METRICS

Requests

3 | ---------

| | 3 |

2 | ------------------------

| | 2 | 3 |

1 | ----------------------------------------------

| | 1 | 2 | 3 |

+---------------------------------------------------------------------------------

0 1 2 3 4 5 6 7 8 9 10

Time ( seconds )

Y = A / T Arrival rate =

X = C / T Throughput =

R = W / C Residence Time =

U = B / T Utilization =

S = B / C Service time/request =

N = W / T Average number of =

requests in the system.

T Time interval =

A Arrivals =

C Completions =

B Time spent servicing =

requests

W Accumulated time =

within system

Performance Metrics


Performance metrics6
PERFORMANCE METRICS

LITTLE’S LAW N = X R

Proof: W / T = C / T * W / C

Example:

On an average day in the Cafeteria, four people are waiting in the sandwich line. Each of the two sandwich makers can produce a sandwich in one minute. How long must the diners wait in line until they are handed a sandwich? How many sandwiches can be produced in one hour?

Performance Metrics


Performance metrics7
PERFORMANCE METRICS

Example:

One day I was at the Weston Toll Booth on the Mass Pike. There were 10 collectors working; when I joined the line, there were 8 cars ahead of me, and when my turn came it took me 12 seconds to pay my toll.

What information can be derived from the above paragraph? What information can NOT be deduced?

Example:

Recently it was reported that the current U.S. Marriage rate is 6.8 per 100 adults and the divorce rate is 3.6 per 1000 adults. It was also reported that 48% of all adults are currently married.

Develop a simple model/picture in order to make some sense of these numbers.

Performance Metrics


Performance metrics8
PERFORMANCE METRICS

 Some more practice problems:

Example:

Packets arrive at a gateway at a rate of 125 packets/second. The gateway takes an average of 2 milliseconds to forward them.

What is the Throughput, Service Time, and Utilization.

Example:

A system has a CPU, Disk A, and Disk B. The throughputs of these devices are XCPU = 35.48, XA = 15.70, XB = 19.6. We also know that the Queue lengths are QCPU = 8.88, QA = 3.19, QB = 1.40.

What are the device response times for these devices?

Example:

On the campus, there are a number of graduate students who have varying degree and course requirements. Build a model using duration of time until their degree, as well as the fraction of time in school.

What is the service time, residence time, of a graduate student? What is the throughput of the CS Graduate School?

Performance Metrics


Performance metrics9
PERFORMANCE METRICS

SUMMARY OF PERFORMANCE METRICS

T is the length of TIME we observed the system.

A is the number of request ARRIVALS observed.

C is the number of request DEPARTURES observed.

W is the ACCUMULATED TIME for all requests within the system - time spent both waiting for and using resources.

B is the length of time that the resource was observed to be BUSY.

Arrival Rate Y = A / T

Throughput (Departure Rate) X = C / T

Utilization U = B / T

Service Requirement S = B / C

Requests in system N = W / T

Residence time R = W / C

UTILIZATION LAW U = X S

LITTLE’S LAW N= X R

Performance Metrics


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