Thread Synchronization, Priority Inversion, and CPU Efficiency

Can the Priority Inversion Problem Happen with User-Level Threads?

The priority inversion problem occurs when a low-priority process is in its critical region and suddenly a high-priority process becomes ready and is scheduled. With user-level threads, it cannot happen that a low-priority thread is suddenly preempted to allow a high-priority thread to run. There is no preemption. With kernel-level threads, this problem can arise.

Does Round-Robin Scheduling Resolve Priority Inversion?

Round-robin scheduling can resolve the issue. Sooner or later, the low-priority process will run and eventually leave its critical region. With priority scheduling, the low-priority process never gets to run at all; with round-robin, it gets a normal time slice periodically, giving it a chance to leave its critical region.

Stacks in User-Level and Kernel-Level Threads

Each thread calls procedures on its own, so it must have its own stack for the local variables, return addresses, and so on. This is true for both user-level threads and kernel-level threads.

Using a Barrier to Synchronize Two Processes

If the program operates in phases where neither process may enter the next phase until both are finished with the current phase, it makes sense to use a barrier.

Thread Synchronization with Kernel Semaphores

With kernel threads, threads can synchronize using semaphores without issues. However, with user-level threads, blocking on a semaphore can cause the entire process to fail.

CPU Efficiency in Round-Robin Scheduling

For different quantum values, the CPU efficiency varies. The formula for CPU efficiency is dependent on the quantum value compared to the process run time and process switch time.

Minimizing Average Response Time with Shortest Job First

Shortest job first is the optimal strategy to minimize average response time. The order of job execution depends on the expected run times of the jobs.

Aging Algorithm Prediction of Run Times

Using the aging algorithm with a=1/2, the prediction of run times is based on previous runs. The prediction of the next time is calculated based on the algorithm's formula.