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Top 10 List of Week 08

1. Preemptive Scheduling
   Preemptive scheduling is used when a process switches from running state to ready state or from waiting state to ready state. The resources (mainly CPU cycles) are allocated to the process for the limited amount of time and then is taken away, and the process is again placed back in the ready queue if that process still has CPU burst time remaining. That process stays in ready queue till it gets next chance to execute.

2. Non-Preemptive Scheduling
   Non-preemptive Scheduling is used when a process terminates, or a process switches from running to waiting state. In this scheduling, once the resources (CPU cycles) is allocated to a process, the process holds the CPU till it gets terminated or it reaches a waiting state. In case of non-preemptive scheduling does not interrupt a process running CPU in middle of the execution. Instead, it waits till the process complete its CPU burst time and then it can allocate the CPU to another process.

3. Schedulers
   Schedulers are special system software which handle process scheduling in various ways. Their main task is to select the jobs to be submitted into the system and to decide which process to run. There are 3 types of schedulers: Long-Term Scheduler, Short-Term Scheduler, and Medium-Term Scheduler.

4. Long Term Scheduler
   It is also called a job scheduler. A long-term scheduler determines which programs are admitted to the system for processing. It selects processes from the queue and loads them into memory for execution. Process loads into the memory for CPU scheduling.

5. Short Term Scheduler
   It is also called as CPU scheduler. Its main objective is to increase system performance in accordance with the chosen set of criteria. It is the change of ready state to running state of the process. CPU scheduler selects a process among the processes that are ready to execute and allocates CPU to one of them.

6. Medium Term Scheduler
   Medium-term scheduling is a part of swapping. It removes the processes from the memory. It reduces the degree of multiprogramming. The medium-term scheduler is in-charge of handling the swapped out-processes.

7. Scheduling Policies
   There are several criteria for invoking the best scheduling policies for a system: CPU Utilization, Throughput, Wait Time, Response Time, and Turnaround Time. To fulfill those criteria, a scheduler has to use various policies or strategies, such as Fairness, FCFS (First Come First Served), and Round Robin.

8. Process
   Process means any program is in execution. Process control block controls the operation of any process. Process control block contains information about processes for example Process priority, process id, process state, CPU, register, etc. A process can creates other processes which are known as Child Processes. Process takes more time to terminate and it is isolated means it does not share memory with any other process. The process can have the following states like new, ready, running, waiting, terminated, suspended.

9. Thread
   Thread is the segment of a process means a process can have multiple threads and these multiple threads are contained within a process. A thread have 3 states: running, ready, and blocked. Thread takes less time to terminate as compared to process and like process threads do not isolate.

10. Deadline Scheduler
    Deadline Scheduler is n I/O scheduler for the Linux kernel and guarantee a start service time for a request. Deadline Scheduler imposes deadlines on all I/O operations in order to prevent wanted requests. Two deadline read and write queues (basically sorted by their deadline) are maintained. For every new request, the scheduler selects which queue will serve for it. Read queues are given high priority than write queues because during read operations the processes usually get blocked. Now, the deadline scheduler checks if the first request in the deadline queue has expired then, a batch of requests from the sorted queue is served. The scheduler serves both a batch of requests following the chosen request in the sorted queue. By default, the Expiration time of read request is of 500 ms and write requests of 5 seconds.