In a multi-channel program environment, there is a critical resource, which refers to the resources that must be mutually exclusive when the multi-process exists. That is, a number of processes are not allowed to access at a moment, only a single process access. We call the fragments of these programs as critical regions or critical segments. Its existence is to effectively prevent competitive conditions from ensuring maximizing sharing data. These concurrent processes must have a good solution to prevent the following: Multiple processes are in critical regions, and the process of process outside the critical area is blocked, and some processes have endless waiting outside the critical regions. In addition, these programs cannot make any assumptions for the speed and number of CPUs. It is only a good solution to satisfy these conditions. The loop process of accessing critical resources can be described in this way: Repeat Entry Section Critical Sections; EXIT Section Remainder Sectioni; Until False To implement process mutual exclusion, you can use software methods, or set a special synchronization mechanism in the system to coordinate multiple Process, but all synchronization mechanisms should follow four criteria: 1. Idle allows the critical resources to be idle, allowing a process requesting to enter the critical area to enter the critical area, and effectively utilize resources. 2. Busy is waiting for the process that has entered the critical area, meaning that the corresponding critical resources are being accessed, so other processs that are ready to enter the incoming district must wait to ensure multi-process mutual exclusion. 3. Limited Waiting for the process of accessing critical resources, should ensure that the process can enter the critical regions in a valid time, prevent death and other states. 4. Let the right to wait when the process cannot enter the critical area, should immediately release the processor to prevent the process from being waiting. Early Solution Mutual Exclusion Problem has software methods and hardware methods, such as: strict rotation, Peterson solutions, TSL instructions, SWAP instructions can achieve process mutual exclusion, but they have a certain defect, here is not One by one detailed description, while the semaphore mechanism proposed by Kijkstra is better solves the mutual exclusion problem.
