In windows, assume i have an application which consumes 20% of cpu during its highest usage. Will this 20% cpu usage depend on the current total cpu usage across system.
for e.g: when the system total cpu usage is 40%, the application uses 10% cpu. But when the system total cpu usage is 75%, the application uses 20% cpu. Is this possible ?
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According to Xcode, my app is using up about 23% of CPU:
This seems consistent with its CPU usage indicated by Activity Monitor:
Now if you look at the bottom section of the Activity Monitor screenshot, you'll see it's indicating about 5% User CPU usage, i.e. "The percentage of CPU capability that’s being used by apps you opened, or by the processes opened by those apps."
This looks incoherent. If the app is taking up 23% CPU, why is User CPU usage 5%?
An Apple Developer Tools Engineer answered my question in the Apple Developer Forums, I'm reposting below:
An app's CPU usage is measured in terms of "how much of a single CPU core does it use?". That's why the CPU usage of a process in this view can also go above 100%. E.g. 300% CPU usage would mean the process uses as much CPU cycles as 3 CPUs can provide (it might still be running on 6 CPUs for 50% each).
However, the split in System, User and Idle is measured in terms of total CPU cycles the system can provide. You can also see this difference in the Xcode view: The left-most number on the gauge is 1200, indicating that the maximum CPU usage you can have is 1200%. This indicates that the system you are measuring has 12 CPU cores available.
You can now take the 23% of a single CPU the app uses and divide it by 12 to arrive at the share of the system's CPUs the app uses: 23%/12 = 1.9%, which fits into the 5.13% of user CPU usage you see.
My inference from the above is, the number of requests has increased which has increased the CPU usage and so the response time also has increased.
Is my inference correct?
How can I make use of the CPU credits?
or is increasing the RAM size the only solution?
I am using cloud.elastic.co
his is a chart of the CPU utilization, and the CPUCredit in my t3.small instance:
According to the documentation, an instance will only use CPU credits when the CPU utilization is above the baseline. If the instance has more than one vCPU, the baseline performance will be shown at the original level.
If I understand correctly, the instance should use CPU credits only when utilization is above 20%. In the chart, it seems like CPU credits are consumed even when the utilization is lower. Why is that?
The graph in your answer shows the average CPU utilization per time period. For the calculation of CPU credit usage however it's relevant how the maximum CPU utilization per minute looks like. Therefore if you change the aggregation method of your CPU utilization from average to maximum you should see a graph which makes more sense.
This happened to be one of my class test question.
In a demand paging system, the CPU utilization is 20% and the paging disk utilization is 97.7%
If the CPU speed is increased, will the CPU usage be increased in this scenario?
Paging is effectively a bottleneck in this example. The amount of computation per unit time might increase slightly with a faster CPU but not in proportion to the increase in CPU speed (so the percentage utilization would decrease).
A quick and dirty estimation would use Amdahl's Law. In the example, 80% of the work is paging and 20% is CPU-limited, so an N-fold improvement in CPU performance would result in a speedup factor of 1/((1 - 0.2) + (0.2/N)).
A more realistic estimate would add an awareness of queueing theory to recognize that if the paging requests came in more frequently the utilization would actually increase even with a fixed buffer size. However, the increase in paging utilization is smaller than the increase in request frequency.
Without looking at the details of queueing theory, one can also simply see that the maximum potential improvement in paging is just over 2%. (If paging utilization was driven up to 100%: 100/97.7 or 1.0235.) Even at 100% paging utilization, paging would take 0.80/(100/97.7) of the original time, so clearly there is not much opportunity for improvement.
If a 10-fold CPU speed improvement drove paging utilization to effectively 100%, every second of work under the original system would use 781.6 milliseconds in paging (800 ms / (100/97.7)) and 20 milliseconds in the CPU (200 ms / 10). CPU utilization would decrease to 20 / (781.6 + 20) or about 2.5%.
How to calculate CPU service time on Windows and Linux OS for a running application? I believe this can be calculated as total time of running application multiply by % utilization of CPU but not sure. Also, What is CPU time and How CPU time is different than Service time?
The windows task manager can show the cpu time (might have to enable it in the menu). In linux running the application with time application gives you the cpu time after the application has finished and I guess top or htop can show it for a running application.
The cpu-time is the time used by the cpu(s) to process the instructions of the application. So for the given cpu-time the application used 100% of a CPU.
The usage of the CPU for a wall clock time intervall would be (sum of all cpu times)/(wall clock time) i.e if 10 application have 0.1s of cpu time in a frame of 1s the total utilization would be 100%.
CPU utilization for a given application would be (cpu time)/(wall clock time) for a single CPU or (cpu time)/(#CPUs * wall clock time) if it uses multiple CPUs.
So yes cpu-time would be wall-clock-time*%CPU utilization.
The diffence between CPU time and service time (called wall clock time above) is that service time is the time elapsed since the start of the application and the cpu time is the time it could/did actually use a CPU.