Shulou(Shulou.com)06/01 Report--

1. What is process scheduling

Whether in batch systems or time-sharing systems, there are generally more user processes than processors, which will cause them to compete with each other for processors. In addition, the system process also needs to use a processor. This requires the process scheduler to dynamically assign the processor to a process in the ready queue according to a certain strategy to make it execute.

2. Processor scheduling classification

High-level, intermediate-level, and low-level scheduling jobs often go through the following three levels of scheduling from submission to completion:

Advanced scheduling: (High-Level Scheduling), also known as job scheduling, decides to call the backup process into memory to run.

Low-level scheduling: (Low-Level Scheduling), also known as process scheduling, decides to get CPU for a process that is ready to queue.

Intermediate scheduling: (Intermediate-Level Scheduling), also known as the introduction of virtual memory, in the internal and external memory exchange area for process exchange.

3. Short process first

Shortest CPU runtime priority scheduling algorithm (SCBF--Shortest CPU Burst First)

The algorithm selects the next process with the shortest CPU execution period from the ready queue and allocates processors to it.

For example, there are four processes P1, P2, P3, and P4 in the ready queue, and their next execution

The periods are 16, 12, 4 and 3 unit time respectively, as shown in the following figure:

The turnover time of P1, P2, P3 and P4 was 35, 19, 7 and 3, respectively, and the average turnaround time was 16.

Although the algorithm can achieve better scheduling performance, it is difficult to know the next CPU execution period accurately, and it can only be predicted according to the execution history of each process.

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