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THREAD SCHEDULING (Approaches to Thread Scheduling ((:star:Dynamic…

- - - - -For some applications it is possible to provide language and system tools that permit the number of threads in the process to be altered dynamically
        ---this would allow the operating system to adjust the load to improve utilization
        -Both the operating system and the application are involved in making scheduling decisions
        -The scheduling responsibility of the operating system is primarily limited to processor allocation
        -This approach is superior to gang scheduling or dedicated processor assignment for applications that can take advantage of it
    - - processes are not assigned to a particular processor
        
        Simplest approach and carries over most directly from a uniprocessor environment
        
        :smiley:ADVANTAGE
        -load is distributed evenly across the processors
        -no centralized scheduler required
        -the global queue can be organized and accessed using any of the schemes discussed in Chapter 9
        
        :unamused:DISADVANTAGE
        -Central queue occupies a region of memory that must be accessed in a manner that enforces mutual exclusion
        ----can lead to bottlenecks
        -Preemptive threads are unlikely to resume execution on the same processor
        ----caching can become less efficient
        -If all threads are treated as a common pool of threads, it is unlikely that all of the threads of a program will gain access to processors at the same time
        ----the process switches involved may seriously
    - - a set of related thread scheduled to run on a set of processors at the same time, on a one-to-one basis
        
        Simultaneous scheduling of the threads that make up a single process
        
        smiley:ADVANTAGE
        -synchronization blocking may be reduced, less process
        -switching may be necessary, and performance will increase
        scheduling overhead may be reduced
        
        -Useful for medium-grained to fine-grained parallel applications whose performance severely degrades when any part of the application is not running while other parts are ready to run
        -Also beneficial for any parallel application
    - - When an application is scheduled, each of its threads is assigned to a processor that remains dedicated to that thread until the application runs to completion
        
        If a thread of an application is blocked waiting for I/O or for synchronization with another thread, then that thread’s processor remains idle
        ----there is no multiprogramming of processors
        
        Defense of this strategy:
        --in a highly parallel system, with tens or hundreds of processors, processor utilization is no longer so important as a metric for effectiveness or performance
        --the total avoidance of process switching during the lifetime of a program should result in a substantial speedup of that program