nachos.threads
Class Condition

java.lang.Object
  nachos.threads.Condition

public class Condition
extends Object

An implementation of condition variables built upon semaphores.

A condition variable is a synchronization primitive that does not have a value (unlike a semaphore or a lock), but threads may still be queued.

• sleep(): atomically release the lock and relinkquish the CPU until woken; then reacquire the lock.
• wake(): wake up a single thread sleeping in this condition variable, if possible.
• wakeAll(): wake up all threads sleeping inn this condition variable.

Every condition variable is associated with some lock. Multiple condition variables may be associated with the same lock. All three condition variable operations can only be used while holding the associated lock.

In Nachos, condition variables are summed to obey Mesa-style semantics. When a wake() or wakeAll() wakes up another thread, the woken thread is simply put on the ready list, and it is the responsibility of the woken thread to reacquire the lock (this reacquire is taken core of in sleep()).

By contrast, some implementations of condition variables obey Hoare-style semantics, where the thread that calls wake() gives up the lock and the CPU to the woken thread, which runs immediately and gives the lock and CPU back to the waker when the woken thread exits the critical section.

The consequence of using Mesa-style semantics is that some other thread can acquire the lock and change data structures, before the woken thread gets a chance to run. The advance to Mesa-style semantics is that it is a lot easier to implement.

Constructor Summary

Condition(Lock conditionLock)
Allocate a new condition variable.

Method Summary

void	sleep() Atomically release the associated lock and go to sleep on this condition variable until another thread wakes it using wake().
void	wake() Wake up at most one thread sleeping on this condition variable.
void	wakeAll() Wake up all threads sleeping on this condition variable.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

Condition

public Condition(Lock conditionLock)

Allocate a new condition variable.

Parameters:

conditionLock - the lock associated with this condition variable. The current thread must hold this lock whenever it uses sleep(), wake(), or wakeAll().

Method Detail

sleep

public void sleep()

Atomically release the associated lock and go to sleep on this condition variable until another thread wakes it using wake(). The current thread must hold the associated lock. The thread will automatically reacquire the lock before sleep() returns.

This implementation uses semaphores to implement this, by allocating a semaphore for each waiting thread. The waker will V() this semaphore, so thre is no chance the sleeper will miss the wake-up, even though the lock is released before caling P().

wake

public void wake()

Wake up at most one thread sleeping on this condition variable. The current thread must hold the associated lock.

wakeAll

public void wakeAll()

Wake up all threads sleeping on this condition variable. The current thread must hold the associated lock.