What are the benefits and drawbacks of thread masking in a multiprocessor system?

1 What is thread masking?

Thread masking is a way of controlling the interruptibility of a thread on a multiprocessor system. Interrupts are signals that notify the processor of some events or requests, such as input/output, timers, or exceptions. When an interrupt occurs, the processor may suspend the current thread and switch to another thread that handles the interrupt. Thread masking allows a thread to specify which interrupts it wants to receive and which ones it wants to ignore. By doing so, the thread can avoid being interrupted by irrelevant or unwanted events, and focus on its critical tasks.

2 How does thread masking work?

Thread masking works by setting or clearing some bits in a special register called the interrupt mask register (IMR). The IMR indicates which interrupts are enabled and which are disabled for the current thread. Each bit in the IMR corresponds to a different type of interrupt, such as keyboard, mouse, disk, or network. When an interrupt occurs, the processor checks the IMR to see if the corresponding bit is set or cleared. If the bit is set, the interrupt is enabled and the processor handles it. If the bit is cleared, the interrupt is disabled and the processor ignores it.

3 What are the benefits of thread masking?

Thread masking can have several benefits for concurrent programs on a multiprocessor system. One benefit is that it can improve the performance of the program by reducing the overhead of context switching and interrupt handling. For example, if a thread is performing some intensive computation, it can mask out all the interrupts that are not related to its task, and avoid being interrupted by other threads or devices. This can save time and resources, and increase the throughput of the program. Another benefit is that it can improve the correctness of the program by preventing race conditions and deadlocks. For example, if a thread is accessing some shared data or resources, it can mask out all the interrupts that could interfere with its operation, and ensure that it completes its transaction without being preempted by another thread. This can prevent data corruption, inconsistency, or deadlock.

4 What are the drawbacks of thread masking?

Thread masking can also have some drawbacks and challenges for concurrent programs on a multiprocessor system. One drawback is that it can reduce the responsiveness and fairness of the system. For example, if a thread masks out all the interrupts for a long time, it can delay or starve other threads or devices that need attention or service. This can degrade the quality of service, user experience, or system functionality. Another drawback is that it can increase the complexity and difficulty of programming and debugging. For example, if a thread masks out some interrupts that are necessary for its execution, it can cause errors, exceptions, or hangs. Moreover, thread masking can introduce subtle dependencies and interactions between threads and interrupts, which can make the program logic and behavior hard to understand and verify.

5 How to use thread masking in OS?

Thread masking is usually provided by the operating system (OS) as a system call or a library function. The OS is responsible for managing the IMR and ensuring that the thread masking operations are atomic and consistent. The OS may also impose some restrictions or policies on how and when a thread can use thread masking, to prevent abuse or misuse of the technique. For example, the OS may limit the duration or frequency of thread masking, or require some privileges or permissions for thread masking. To use thread masking in OS, a thread typically needs to invoke the system call or the library function with the desired IMR value as an argument. The OS then updates the IMR accordingly, and returns the previous IMR value to the thread. The thread can then restore the previous IMR value when it finishes its critical section or task.