it's my first post here.
I'm currently learning Modern Operating Systems and I'm stuck at this question : A computer system has enough room to hold five programs in its main memory. These programs are idle waiting for I/O half of the time. What fraction of the CPU time is wasted?
The answer is 1/32, but why ?
The answer is 1/32, but why ?
The sentence "These programs are idle waiting for I/O half of the time" is ambiguous. Let's look at a few different ways of interpreting this sentence and see if they match the expected answer:
a) "Each of the 5 programs spends 50% of the total time waiting for IO". In this case, while one program is waiting for IO the CPU could be being used by other programs; and all programs combined could use 100% of CPU time with no time wasted. In fact, you'd be able to use 100% of CPU time with only 2 programs (the 1st program uses the CPU while the 2nd program waits for IO, then the 2nd program uses the CPU while the 1st task waits for IO, then ...). This can't be the intended meaning of "These programs are idle waiting for I/O half of the time" because the answer (possibly zero CPU time wasted) doesn't match the expected answer.
b) "All of the programs are idle waiting for I/O at the same time, for half the time". This can't be the intended meaning of the question because the answer would obviously be "50% of CPU time is wasted" and doesn't match the expected answer.
c) "Each program spends half of the time available to it waiting for IO". In this case, the first program has 100% of CPU time available to it but spends 50% of the time using the CPU and waits for IO for the other 50% of the time, leaving 50% of CPU time available for the next program; then the 2nd program uses 50% of the remaining CPU time (25% of total time) using the CPU and 50% of the remaining CPU time (25% of total time) waiting for IO, leaving 25% of CPU time available for the next program; then the third program uses 50% of the remaining CPU time (12.5% of total time) using the CPU and 50% of the remaining CPU time (12.5% of total time) waiting for IO, leaving 12.5% of CPU time available to the next programs, then...
In this case, the remaining time is halved by each program, so you get a "negative power of 2" sequence (1/2, 1/4, 1/8, 1/16, 1/32) that arrives at an answer that matches the expected answer.
Because we get the right answer for this interpretation, we can assume that this is what "These programs are idle waiting for I/O half of the time" was supposed to mean.
Related
I'm trying to understand how to compute the CPU utilisation for audio and video use cases.
In real time audio applications, this is what I typically do:
if an application takes 4ms to process 28ms of audio data, I say that the CPU utilisation is 14.28% (4/28).
How should this be done for applications like resize/crop? let's say I'm resizing an image from 162*122 to 128*128 size image at 1FPS, and it takes 11ms.. What would be the CPU utilisation?
CPU utilization is quite complicated, and strongly depends on stuff like:
The CPU itself
The algorithms utilized for the task
Other tasks running alongside the CPU
CPU utilization is also strongly related to the process scheduling of your PC, hence the operating system used, so most operating systems will expose some kind of API for CPU utilization diagnostics, but such API is highly platform-dependent.
But how does CPU utilization calculations work anyway?
The most simple way in which CPU utilization is calculated is taking a (for example) 1 second period, in which you observe how long the CPU has been idling (not executing any processes), and divide that by the time interval you selected. For example, if the CPU did useful calculations for 10 milliseconds, and you were observing for 500ms, this would mean that the CPU utilization is 2%.
Answering your question / TL; DR
You can apply this principle in your program. For the case you provided (processing video), this could be done in more or less the same way: you calculate how long it takes to calculate one frame, and divide that by the length of a frame (1 / FPS). Of course, this could be done for a longer period of time, to get a more accurate reading, in the following way: you track how much time it takes to process, for example, 2 seconds of video, and divide that by 2. Then, you'll have your CPU utilization.
NOTE: if you aren't able to process the frame in time, for example, your video is 10FPS (0.1ms), and processing one frame takes 0.5ms, then your CPU utilization will be seemingly 500%, but obviously you can't utilize more than 100% of your CPU, so you should just cap the CPU utilization at 100%.
I've read in many places that a simple and decent way to get the % of CPU utilization is by this formula:
CPU utilization = 1 - p^n
where:
p - blocked time
n - number of processes
But i can't find an explanation for it. Seems it has to do with statistics, but i can't wrap my head around it.
My starting point is: if i have 2 processes with 50% wait time, then the formula would yield 1 - 1/4 = 75% CPU utilization. But my broken logic begs the question: if one process is blocked on I/O and the other is swapped in to run when the first is blocked(whatever the burst is), that means that while one waits, the second runs and their wait time overlap. Isn't that 100% CPU utilization? I think this is true only when the first half of the programs is guaranteed to run without IO need.
Question is: How is that formula taking into account every other possibility?
You need to think in terms of probabilities. If the probability of each of the cores to be idle (waiting for IO) is 0.5 then the probability of the CPU to be in idle is the probability of all of the cores to be in idle at the same time. That is 0.5 * 0.5 = 0.25 and so the probability the CPU is doing work is 1 - 0.25 = 0.75 = 75%
CPU utilisation is given as 1 - probability of CPU to being in the idle state
and CPU remain in the idle state when all the process loaded in the main memory is blocked time(I/O)
So if n process has wait time of 50% the probability that all the process are in
block(I/O) state
My question is why do we want to have CPU's operation overlap with that of the I/O processing. I have been thinking about optimization and such but yet to arrive at a conclusion.
If anyone is able to answer this question, it will be great. :D
I/O is generally very slow compared to the operating frequency of the CPU.
Suppose you have a 1GHz CPU that's capable of executing one instruction every clock cycle. That means the CPU is able to execute one instruction every nanosecond.
Now let's assume you want to fetch some data from your hard drive. Disk operations often take place in the milisecond scale, and we'll assume your drives are fast enough to fetch the data in only 1ms.
If the CPU just sit around and wait for the disk to fetch the data, the CPU will waste 1 million nanoseconds doing nothing, whereas it could be executing 1 million instructions for another task. When a program has a lot of IO access, those wasted cycles stacks up and become noticeable if you let the CPU wait and do nothing. This is why it's a good idea to overlap computation with IO so CPU cycles aren't wasted.
This is also why your computer becomes super unresponsive when your main memory is full, and the CPU has to page frequently to the disk. Your CPU cannot perform any useful task unless the data it needs has been retrieved from the disk into the main memory, so it must sit around and wait for the IOs to complete.
How does a long term scheduler decide which job is I/O bound and which one is CPU bound?
I heard that by using cpu burst we can distinguish between I/O bound and CPU bound jobs, but how is the CPU burst calculated without processing the program?
Generally, the CPU scheduler assigns time slices to processes/threads and switches between them whenever a) the time slice has run out or b) the process/thread blocks for I/O.
An I/O-bound job will be blocking for I/O very often, while a process/thread that always makes use of his full time slice can be assumed to be CPU-bound. So by distinguishing whether a process/thread blocks at the end of the time slice or by calling some wait_for_io_completion() function, you can effectively characterize those types of processes.
Note, that in real life, things get more complicated, because most of the time applications are not either I/O-bound or CPU-bound but switch roles all the time. This is why scheduling is about heuristics and not about correct solutions, because you cannot (always) predict the future.
CPU bound uses more of its time doing computations than I/O bound.
answered by tumaini kami david
Answers. Generally, the CPU scheduler assigns time slices to processes/threads and switches between them whenever a) the time slice has run out or b) the process/thread blocks for I/O. ... CPU bound uses more of its time doing computations than I/O bound.strong text
IO BOUND PROCESS :
Io bound process spends more time doing io than computations,many short cpu burst.
COU BOUND PROCESS :
process spends more time doing computations;few very long cpu bursts.
if i run an application with the performance test, the "cpu monitor" show me some informations like process ID/Name or CPU Time. But in which unit of time does it measure ?
An example: if i get 05.04 , what does mean for me
Best Regards
Plagiarized from http://en.wikipedia.org/wiki/CPU_time -
CPU time (or CPU usage, process time) is the amount of time for which a central processing unit (CPU) was used for processing instructions of a computer program, as opposed to, for example, waiting for input/output (I/O) operations.
The CPU time is often measured in clock ticks or seconds. CPU time is also mentioned as percentage of the CPU's capacity at any given time on multi-tasking environment. That helps in figuring out how a CPU’s computational power is being shared among multiple computer programs.