PATH Mac OS 8 Developer Documentation
> Operating System Services
> Multiprocessing Services
>
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Adding Multitasking Capability to Applications Using Multiprocessing Services
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PATHMac OS 8 Developer Documentation
> Operating System Services
> Multiprocessing Services
>
|
|
Adding Multitasking Capability to Applications Using Multiprocessing Services
|
In addition to notification mechanisms, you can also specify critical regions in a multitasking environment. A critical region is a section of code that can be accessed by only one task at a time. For example, say part of a task's job is to search a data tree and modify it. If multiple tasks were allowed to search and try to modify the tree at the same time, the tree would quickly become corrupted. An easy way to avoid the problem is to form a critical region around the tree searching and modification code. When a task tries to enter the critical region it can do so only if no other task is currently in it, thus preserving the integrity of the tree.
Critical regions differ from semaphores in that critical regions can handle recursive entries and code that has multiple entry points. For example, if three functions func1 , func2 , and func3 access some common resource (such as the tree described above), but never call each other, then you can use a semaphore to synchronize access. However, suppose func3 calls func1 internally. In that case, func3 would obtain the semaphore, but when it calls func1 , it will deadlock. Synchronizing using a critical region instead allows the same task to enter multiple times, so func1 can enter the region again when called from func3 .
Because critical regions introduce forced linearity into task execution, improper use can create bottlenecks that reduce performance. For example, if the tree search described above constituted the bulk of a task's work, then the tasks would spend most of their time just trying to get permission to enter the critical region, at great cost to overall performance. A better approach in this case might be to use different critical regions to protect subtrees. You can then have one task search one part of the tree while others were simultaneously working on other parts of the tree.