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Performance
Measurement
COMP755 Advanced OS
Easily Measurable Values
- Length of an observation period.
- Number of jobs during the observation
period.
- Number of accesses for each device
- % utilization of the CPU
- Hardware service times
Example System
2 Disk 12 ms 3 Network 25 ms
1 CPU
4 CD 70 ms
The time values represent the average service time for that unit. A performance monitor was run for a 15 minute period. The above data was collected.
CPU utilization: 32.8%
requests to the CD: 7,
requests to the network: 22,
requests to the disk:25,
Transactions executed: 3,
Transaction Systems
- We will first analyze transaction systems.
- A transaction is typically a predefined
program that is executed when an input
arrives. The transaction usually performs
some processing and then sends output
back to the user.
- Transactions usually terminate after
performing their short function.
Arrival Rates
- The arrival rate is the number of accesses
to a device per time period. (i.e. a disk
might be used 34 times per second.)
- Arrival rates will be represented by the
Greek letter lambda, λi
- Let λ 0 represent the arrival rate of jobs into
the system.
- Arrival rates can be calculated from the
number of accesses divided by the length
of the observation period.
Example Arrival Rates The observation period is 15*60=900 seconds
CPU utilization: 59,040 65.
requests to the CD: 7,380 8.
requests to the network: 22,140 24.
requests to the disk: 25,830 28.
Transactions executed: 3,690 4.
The CPU access count is the sum of the device and transaction counts.
Visitation Ratios
- The visitation ratio is the number of times
an average program uses a device.
- The visitation ration can be calculated by
dividing the access rate of a device by the
job entry rate.
0
i i
V =
Example Visitation Rates
CPU utilization: 59,040 65.6 16
requests to the CD: 7,380 8.2 2
requests to the network: 22,140 24.6 6
requests to the disk: 25,830 28.7 7
Transactions executed: 3,690 4.
Device access λi V (^) i
Utilization
- The utilization of a device is the fraction of
time the device is busy.
- You can compute individual devices
utilization from the devices arrival rate and
the mean service time.
Note that utilization is always a number between zero and one.
i i i
ρ = λ * S
Example Utilization
CPU utilization: 59,040 65.6 16 0.005.
requests to the CD: 7,380 8.2 2 0.070.
requests to the network: 22,140 24.6 6 0.025.
requests to the disk: 25,830 28.7 7 0.012.
Transactions executed: 3,690 4.
Device access λi Vi S (^) i ρi
Saturation
- One device will typically saturate before the
other devices when the load increases. This
is the bottleneck device.
- A device saturates when its utilization is100%.
- From the service time of a device, you can
calculate the arrival rate that will cause it to
saturate.
i i
λ = 1 / S
- The busiest device saturates first.
Example Saturation
CPU utilization: 59,040 65.6 16 0.005.
requests to the CD: 7,380 8.2 2 0.070.
requests to the network: 22,140 24.6 6 0.025.
requests to the disk: 25,830 28.7 7 0.012.
Transactions executed: 3,690 4.
Device access λi Vi S (^) i ρi
The network is busiest and will saturate with
an arrival rate of 1/0.025 = 40.
