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Tasks and Threads

Mac OS X processes and POSIX threads (pthreads) are implemented on top of Mach tasks and threads, respectively. A thread is a point of control flow in a task. A task exists to provide resources for the threads it contains. This split is made to provide for parallelism and resource sharing.

A thread

• is a point of control flow in a task.

• has access to all of the elements of the containing task.

• executes (potentially) in parallel with other threads, even threads within the same task.

• has minimal state information for low overhead.

A task

• is a collection of system resources. These resources, with the exception of the address space, are referenced by ports. These resources may be shared with other tasks if rights to the ports are so distributed.

• provides a large, potentially sparse address space, referenced by virtual address. Portions of this space may be shared through inheritance or external memory management.

• contains some number of threads.

Note that a task has no life of its own—only threads execute instructions. When it is said that “task Y does X,” what is really meant is that “a thread contained within task Y does X.”

A task is a fairly expensive entity. It exists to be a collection of resources. All of the threads in a task share everything. Two tasks share nothing without an explicit action (although the action is often simple) and some resources (such as port receive rights) cannot be shared between two tasks at all.

A thread is a fairly lightweight entity. It is fairly cheap to create and has low overhead to operate. This is true because a thread has little state information (mostly its register state). Its owning task bears the burden of resource management. On a multiprocessor computer, it is possible for multiple threads in a task to execute in parallel. Even when parallelism is not the goal, multiple threads have an advantage in that each thread can use a synchronous programming style, instead of attempting asynchronous programming with a single thread attempting to provide multiple services.

A thread is the basic computational entity. A thread belongs to one and only one task that defines its virtual address space. To affect the structure of the address space or to reference any resource other than the address space, the thread must execute a special trap instruction that causes the kernel to perform operations on behalf of the thread or to send a message to some agent on behalf of the thread. In general, these traps manipulate resources associated with the task containing the thread. Requests can be made of the kernel to manipulate these entities: to create them, delete them, and affect their state.

Mach provides a flexible framework for thread–scheduling policies. Early versions of Mac OS X support both time-sharing and fixed-priority policies. A time-sharing thread’s priority is raised and lowered to balance its resource consumption against other time-sharing threads.

Fixed-priority threads execute for a certain quantum of time, and then are put at the end of the queue of threads of equal priority. Setting a fixed priority thread’s quantum level to infinity allows the thread to run until it blocks, or until it is preempted by a thread of higher priority. High priority real-time threads are usually fixed priority.

Mac OS X also provides time constraint scheduling for real-time performance. This scheduling allows you to specify that your thread must get a certain time quantum within a certain period of time.

Mach scheduling is described further in “Mach Scheduling and Thread Interfaces”.

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