How is fairness of thread scheduling ensured across processes?

Question

Every process has at least one thread of execution and I read somewhere that modern Operating Systems only schedule Thread and not process.

So if there are two processes running in the system - P1 with 1 thread and P2 with 100 threads, how will OS scheduling algorithm ensure that both P1 and P2 get approximately same amount of CPU time? If OS blindly schedules threads, P2 will get 100 times more CPU time than P1.

Does it also take into account which Process a particular thread belong to? Otherwise, it seems too easy for a process to hog all the CPU by creating more threads.

Answer 1

Does it also take into account which Process a particular thread belong to? Otherwise, it seems too easy for a process to hog all the CPU by creating more threads.

Wrong question. Consider two jobs that are trying to solve the exact same problem by doing the same work and are perfectly identical except for one thing -- one uses dozens of threads, the other uses dozens of processes. Why should the one that uses dozens of processes get more CPU time than the one that uses dozens of threads?

Your notion of fairness is not really a sensible one.

Instead, scheduling is more designed around trying to get as much work done as possible per unit time. The assumption is that everything the computer is doing is useful and it benefits competing tasks to have other tasks competing with them finish as quickly as possible too.

This is actually all you need the vast majority of the time. But occasionally you have special situations where this doesn't work. One is ultra-high-priority tasks like keeping video or audio flowing or keeping a user interface responsive. Another is ultra-low-priority tasks where there's an enormous amount of work you want done and you don't want the system to be slow for a long time while you're working on it. Priorities are used for this, and generally the system allows higher-priority threads to interrupt lower-priority ones to keep responsiveness.

Answer 2

In general, "fair thread scheduling" attempts to give each thread an equal amount of CPU time (regardless of how much CPU time all threads in a process get); and "fair process scheduling" attempts to give each process the same amount of CPU time (e.g. by giving threads belonging to different processes unequal amounts of CPU time). These are mutually exclusive - you can't have both (unless each process has the same number of threads).

Note that it's all a broken joke anyway. For example, if one thread gets 10 ms of time on a CPU that is running slow due to thermal throttling (and/or because another logical CPU in the same core is busy) and another thread gets 10 ms of time on a CPU that is running faster than normal (e.g. due to "turbo-boost" and/or because the other logical CPU in the core is not being used); then these threads have received an equal amount of CPU time but have not received anything that could be considered "fair" (because one thread might be able to get 20 times as much work done than the other).

Note that it's all unwanted anyway. For example, for a good OS threads would be given a priority to indicate how important the work they do is, and you don't want a high priority thread (doing very important work) to get the same "fair share" of CPU time as a low priority thread (doing irrelevant/unimportant work). For cases where two threads have equal priority you might (in theory) want them to get an "equal" amount of CPU time; but in practice this isn't common and threads block and unblock so often that it isn't worth caring about; and in practice it can lead to "two half finished jobs instead of one completed job and one unstarted job" scenarios that increases the average amount of time a job (e.g. request for work) takes to complete.

Answer 3

If the thread is the basic unit of scheduling (a generally safe assumption these days) then the process scheduler is the one to decide who to allocate the CPUs. How (and whether) it takes thread usage into account is entirely system specific. AND the behavior ma depends upon the type of process. For example, in VMS (and adopted in Windoze) realtime processes are treated differently than other types of processes.

In the VMS-type scheduling, a process with more threads gets more CPU by design. Better for an application to use more threads and for it to use more processes.

Keep in mind that a system may impose limits on the number of threads in a process.