GEOS SDK TechDocs
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3.2 Context Switches
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3.4 Applications and Threads
When there is a context switch, GEOS must determine which thread will be executed next. It does this by examining the run queue and selecting the most important thread to run. A thread's importance is reflected in its priority: the lower the priority number, the more important the thread.
Each thread gets a base priority, a value from zero to 255. Applications generally have a priority between 128 and 191. Threads that are critical to quick system response (such as user input threads that manage such things as pull-down menus and dialog boxes) are given lower numbers. Higher numbers can be given to less time-critical threads such as those used for print spooling and other background activities. To provide faster response to the user, GEOS temporarily reduces an application's base priority by 32 (giving it a higher priority) when the user is interacting with it. When the user switches to interact with another application, the first application's base priority returns to normal, and the new one gets a reduced base priority number.
GEOS keeps track of a thread's recent CPU usage with a number that varies from zero to 60. Starting at zero, the number is incremented at every timer interrupt while the thread is running. Once each second, the recent CPU usage is halved, so that as a thread's CPU usage recedes into the past, its recent CPU usage number will diminish. The resulting number reflects how much time the thread has had control of the CPU and how long ago this time was.
Once each second, the current priority of each thread is recomputed by adding the base priority to the recent CPU usage. The resulting number is the one used in selecting a thread from the run queue.
When it is time for a context switch, GEOS selects the thread with the lowest current priority number from the run queue. If there is a tie, the selection is arbitrary. However, because recent CPU usage counts against a thread, two threads of equal priority will not stay that way. One will run, and its recent CPU usage (and thus its current priority number) will be increased. The other thread will therefore get its chance to run.
GEOS SDK TechDocs
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| 3.2 Context Switches
| 3.4 Applications and Threads