Thread Scheduling

Thread scheduler, 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.

Note that we'll continue to talk about a single thread scheduler. On multiprocessor systems, there is generally some kind of scheduler per processor, which then need to be coordinated in some way. (On some systems, switching on different processors is staggered to avoid contention on shared scheduling tables.) Unless otherwise specified, we'll use the term thread scheduler to refer to this overall system of coordinated per-CPU schedulers.

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".

Most systems use what we might dub priority-based round-robin scheduling to some extent. The general principles are:

****a thread of higher priority (which is a function of base and local priorities) will preempt a thread of lower priority;

****otherwise, threads of equal priority will essentially take turns at getting an allocated slice or quantum of CPU;

****there are a few extra "tweaks" to make things work