multithreading - Does linux schedule a process or a thread?

Question

After reading this SO question I got a few doubts. Please help in understanding.

Scheduling involves deciding when to run a process and for what quantum of time.

1. Does linux kernel schedule a thread or a process? As process and thread are not differentiated inside kernel how a scheduler treats them?

2. How quantum for each thread is decided? a. If a quantum of time (say 100us) is decided for a process is that getting shared between all the threads of the process? or b. A quantum for each thread is decided by the scheduler?

Note: Questions 1 and 2 are related and may look the same but just wanted to be clear on how things are working posted them both here.

Answer 1

The Linux scheduler (on recent Linux kernels, e.g. 3.0 at least) is scheduling schedulable tasks or simply tasks.

A task may be :

• a single-threaded process (e.g. created by fork without any thread library)
• any thread inside a multi-threaded process (including its main thread), in particular Posix threads (pthreads)
• kernel tasks, which are started internally in the kernel and stay in kernel land (e.g. kworker, nfsiod, kjournald , kauditd, kswapd etc etc...)

In other words, threads inside multi-threaded processes are scheduled like non-threaded -i.e. single threaded- processes.

The low-level clone(2) syscall creates user-land schedulable tasks (and can be used both for creating fork-ed process or for implementation of thread libraries, like pthread). Unless you are a low-level thread library implementor, you don't want to use clone directly.

AFAIK, for multi-threaded processes, the kernel is (almost) not scheduling the process, but each individual thread inside (including the main thread).

^{Actually, there is some notion of thread groups and affinity in the scheduling, but I don't know them well}

These days, processors have generally more than one core, and each core is running a task (at some given instant) so you do have several tasks running in parallel.

CPU quantum times are given to tasks, not to processes