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Chapter 4. User level kernel implementation
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4.3.8 Sleeping a thread
The thread sleeping facility is used to suspend the execution of threads for a predetermined
length of time. The sleep facility is built on top of the task timer service, which generates upcall
events at specied times.
The timer handler() upcall routine and thread sleep() routine share a sorted queue,
thread sleep q. Sleeping threads are placed on this queue, sorted in the order of wakeup
time. When timer handler() is called, expired threads on the thread sleep q are placed on
the ready queue and executed.
int thread_sleep( unsigned long sleep_time );
puts the caller thread to sleep for the specied number of clock ticks. The
number of clock ticks per second is a value that is interactively set at kernel boot time.
thread sleep()
4.4 Semaphore management
Semaphores provide a way to synchronize actions and communicate events between threads. A
common use for semaphores is to provide mutual exclusion around a piece of sensitive code.
Other uses include controlling producer/consumer type problems between threads. For example,
the interprocess communication system uses semaphores to synchronize message passing events
between threads.
The user level semaphore library, sem.c and global sem.c, provides two types of semaphores
through the same function call interface. Lightweight semaphores, local to an address space
can be created for exclusive use between threads in the same address space (used by the local
IPC implementation). Global semaphores can also be created, which allow threads in remote
address spaces to synchronize between each other (used by the global IPC implementation).
The global semaphore implementation requires special hooks into the kernel, and is therefore
slightly more costly in terms of performance. A parameter in the sem create() call allows the
caller to specify whether a local or global semaphore should be created.