Overview: 4 Multitasking and Multithreading

Multitasking is the ability to have multiple processes running simultaneously on a single system. In actuality, PCs have only one processor and only one task can run at any given moment; the operating system must manage the processor and allocate time to all the different programs running at once.

A thread is a single entity that runs in the GEOS system. Threads may be event-driven or procedural. An event-driven thread runs objects and has a message queue (also called an event queue). The thread receives messages for its objects and is only active when the objects are handling the messages. If an event-driven thread never receives a message, it will never use the processor. Procedural threads execute sequential code such as functions or procedures in C. Procedural threads do not have a message queue and do not run objects.

Cooperative multitasking , used in some operating systems, requires all processes running to cooperate with the operating system. The system has a routine which each process must call periodically; this routine (called a Context Switch routine) checks if any other processes are waiting to run. If other processes are waiting, priorities are checked and the process with the highest priority takes over the processor.

GEOS uses preemptive multitasking , in which threads are given the illusion that they have nonstop access to the processor. A preemptive system periodically interrupts the thread running and checks for other waiting threads; if there are any, the system will switch context automatically. Programs do not have to call a context switch routine, a cumbersome requirement in many cases.

GEOS also maintains a knowledge of thread priority . Every thread (including the kernel and user interface) has a base priority and a current priority; the base priority rarely changes, while the current priority decreases with recent processor usage. GEOS maintains these priorities intelligently, making sure that no thread ever uses the CPU more than its share of the time.

Any application running in GEOS may have multiple threads. This is useful, for example, in a spreadsheet application, where complex and time-consuming calculations may be done by a background thread that wakes up only when calculations need to be made. Use of multiple threads can make the application appear to be extraordinarily fast; the user can continue to navigate menus and use other UI gadgetry while the application is also doing something else.

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