20. Suppose we run program P in a computer system using the Linux operating syst

Question

20. Suppose we run program P in a computer system using the Linux operating system. We use the Linux time command
to collect some time measurements while P is running. The output from time is,

real.       53m27 .589s
user      24m41 .850s
system 1m3 .337s

a. What was the execution time of P?
b. What was the user CPU time of P?
c. What was the system CPU time of P?
d. How much time during the execution of P did the system spend not executing the instructions of P nor OS in-
structions on behalf of P?
e. List and describe one factor or component of the time from exercise d, i.e., name one thing the system might have
been doing during this time period

Explanation / Answer

**The total execution time of a program means the total time of the CPU used for executing the commands of the program or the kernal commands(System calls) on behalf of the program.
When the normal instructions are performed the user CPU time is utilized and when system calls are executed system CPU time is utilized.
Thus Total Execution time is the sum of the user time and system time.

a)Execution time of P= User CPU time+System CPU time
                     =24m41.850s+1m3.337s
                     =25m45.187s.

b)User CPU time of P is the time spent by CPU for performing program instructions.
i.e 24m41.850s

c)System CPU time of P is the time spent by CPU for performing system calls on behalf of P.
i.e 1m3.337s

d)The Real time represents the actual time elapsed by the clock from the starting of Execution of P to the Completion of Execution of P. Which could include the Actual exection time of P and the Waiting of P.
When P is executed on single CPU the real time will always be greater than the actual execution time.

So,during the execution of P, the time of system spend not executing the instructions of P nor OS instructions on behalf of P will be
real time-Execution time
i.e 53m27.589s - 25m45.187s.
    =3207.589s- 1545.187s
    =1662.402s
    =27m 42.402s

e)
----The P must have beeen waiting for the I/O resources to be available, during which the CPU may be executing other process which is ready to Executed. During this the CPU loads another program in to memory and execute it.

----P may have been send to sleep as a high priority task is requested to the CPU.

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