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CPU time (or process time) is the amount of time that a central processing unit (CPU) was used for processing instructions of a computer program or operating system. CPU time is measured in clock ticks or seconds. Sometimes it is useful to convert CPU time into a percentage of the CPU capacity, giving the CPU usage.
For the portion of the time required for CPU cycles, the process is being executed and is occupying the CPU. During the time required for I/O cycles, the process is not using the processor. Instead, it is either waiting to perform Input/Output, or is actually performing Input/Output. An example of this is reading from or writing to a file on disk.
A "maximum execution time" is also calculated for each process to represent the time the process would have expected to run on an "ideal processor". This is the time the process has been waiting to run, divided by the total number of processes. When the scheduler is invoked to run a new process:
Waiting time and response time increase as the process's computational requirements increase. Since turnaround time is based on waiting time plus processing time, longer processes are significantly affected by this. Overall waiting time is smaller than FIFO, however since no process has to wait for the termination of the longest process.
Without scheduling, the processor would give attention to jobs based on when they arrived in the queue, which is usually not optimal. As part of the scheduling, the processor gives a priority level to different processes running on the machine. When two processes are requesting service at the same time, the processor performs the jobs for the ...
Further, a "cumulative clock rate" measure is sometimes assumed by taking the total cores and multiplying by the total clock rate (e.g. a dual-core 2.8 GHz processor running at a cumulative 5.6 GHz). There are many other factors to consider when comparing the performance of CPUs, like the width of the CPU's data bus , the latency of the memory ...
Round-robin algorithm is a pre-emptive algorithm as the scheduler forces the process out of the CPU once the time quota expires. For example, if the time slot is 100 milliseconds, and job1 takes a total time of 250 ms to complete, the round-robin scheduler will suspend the job after 100 ms and give other jobs their time on the CPU.
Least slack time (LST) scheduling is an algorithm for dynamic priority scheduling. It assigns priorities to processes based on their slack time. Slack time is the amount of time left after a job if the job was started now. This algorithm is also known as least laxity first.