THREAD SCHEDULING

part of the OS (usually) that is responsible for sharing the available CPUs out between the various threads. How exactly the scheduler works depends on the individual platform, but various modern operating systems (notably Windows and Linux) use largely similar techniques that we'll describe here. We'll also mention some key varitions between the platforms.

Across platforms, thread scheduling1 tends to be based on at least the following criteria:

• a priority, or in fact usually multiple "priority" settings that we'll discuss below;
  
• a quantum, or number of allocated timeslices of CPU, which essentially determines the amount of CPU time a thread is allotted before it is forced to yield the CPU to another thread of the same or lower priority (the system will keep track of the remaining quantum at any given time, plus its default quantum, which could depend on thread type and/or system configuration);
  
• a state, notably "runnable" vs "waiting";
  
• metrics about the behaviour of threads, such as recent CPU usage or the time since it last ran (i.e. had a share of CPU), or the fact that it has "just received an event it was waiting for".