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Synchronization (Semaphore (Implementation (Without Busy waiting…

- - - - With each semaphore there is an associated waiting queue
      - Operations
        
        block – place the process invoking the operation on the appropriate waiting queue
        
        wakeup – remove one of processes in the waiting queue and place it in the ready queue
- - - - Preemptive
        
        Allows preemption of process when running in kernel mode
        
        Preemption is the act of temporarily interrupting a task with the intention of resuming the task later
        
        Context switches
      - Non-preemptive
        
        Runs until exits kernel mode, blocks, or voluntarily yields CPU
    - - Assume that the load and store machine-language instructions are atomic; that is, cannot be interrupted
      - Both processes share two variables
        
        Boolean flag[2]
        
        int turn
      - Two process solution
      - Variable turn indicates whose turn it is to enter the critical section
      - Good algorithmic description of solving the problem
      - Flag array is used to indicate if a process is ready to enter the critical section. flag[i] = true implies that process Pi is ready
- - - - A data set is shared among a number of concurrent processes
        
        Readers – only read the data set; they do not perform any updates
        
        Writers – can both read and write
      - Problem – allow multiple readers to read at the same time
        
        First variation – no reader kept waiting unless writer has permission to use shared object
        
        Second variation – once writer is ready, it performs the write ASAP
    - - Semaphore mutex initialized to the value 1
      - n buffers, each can hold one item
      - Semaphore full initialized to the value 0
      - Semaphore empty initialized to the value n
    - - Handling
        
        Allow at most 4 philosophers to be sitting simultaneously at the table
        
        Allow a philosopher to pick up the forks only if both are available (picking must be done in a critical section)
        
        Use an asymmetric solution