交大资工蔡文能计概UNIX.PPT

* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * The Critical Section Problem n processes all competing to use some shared data Each process has a code segment, called critical section (CS), in which the shared data is accessed. Problem – ensure that when one process is executing in its critical section, no other process is allowed to execute in its critical section Even with multiple CPUs Each process must request the permission to enter its CS The critical section problem is to design a protocol that the processes can use so that their action will not depend on the order in which their execution is interleaved (possibly on many processors) Semaphore, Monitor : provided by OS Test-and-Set instruction : Hardware /wiki/Critical_section Semaphore Synchronization tool (provided by the OS) that does not require busy waiting Formally, a semaphore is comprised of: An integer variable: value Two atomic operations: P( ) and V( ) P( ) also known as wait( ) While value = 0, sleepDecrement value and return V( ) also known as signal( ) Increments valueIf there are any threads sleeping waiting for value to become non-zero, wakeup at least 1 thread /wiki/Semaphore_(programming) Java: notify( ) notifyAll( ) /wiki/Producers-consumers_problem Monitors A monitor is a set of programmer-defined operations that are provided mutual exclusion within the monitor (the monitor construct prohibits concurrent access to all procedures defined within the monitor) Type of condition: x.wait and x.signal Signal-and-Wait: P=wait Q to leave the monitor or another condition Signal-and-Continue: Q=wait P to leave the monitor or other condition P x.signal Q(suspended) associated with condition x resume Java uses Monitors for Mutual Exclusion: wait( ), notify( ), notifyAll( ) A Monitor is essentially a class with private methods, plus a queue. /wiki/Operating_System_Des