Linux - threads and process scheduling priorities

Question

if we create pthreads (pthread_create) or processes (fork) with default scheduling policies on linux, will the scheduler treats the processes and threads with same priority

Answer 1

It's true that Linux kernel (from version 2.6.23 and on) schedules tasks, which are either threads or (single-threaded) processes.

In addition to the accepted answer of paxdiablo, I am adding this one in order to add one more generic informative view about different ways of thread scheduling.

In general, threads can be either user-space threads or kernel-space threads. User-space threads are usually implemented by a library. Thus, kernel knows almost nothing about them (kernel knows only about the process they belong to) and they are handled in user-space. Contrariwise, kernel threads are implemented by kernel and they are completely handled by the kernel. You can take a generic view from the following image.

As you can see, the left picture show some user-space threads, where kernel possesses only information about processes (the so-called process control blocks - PCBs). All the resources-info regarding the threads are kept inside the process (in a thread table) and handled by the corresponding threads library in user space. In the right picture, you can see kernel threads, where both process table and threads table are kept in kernel.

An example of kernel threads are LinuxThreads, which have now been replaced by the more modern NPTL library. An example of user-space threads is the library GNU Portable Threads

Now, as far as scheduling is concerned, as it is expected, user-space threads are scheduled in a different way to kernel threads :

• When user-space threads are used, the scheduler is scheduling processes. So, it selects a specific process and allocates the allowable time quantum. Then, the thread scheduler inside the process is responsible to select how the scheduling between the threads will be made. Since user-space threads are not stopped by interrupts, the selected thread will tend to consume the whole quantum of the process, until it has completed its task. So, the hypothetical scheduling in this case would be something like:

  P1(T1), P2(T1), P1(T1), P2(T1 - T1 completes its task and yields), P2(T2 - for the remaining time quantum, P1(T1), P2(T2), P1(T1), ...

• When kernel threads are used, then kernel schedules threads. The kernel show no interest to which process the thread belongs, but it allocates one time quantum to each thread equally. So, in this case the hypothetical scheduling would be :

  P1(T1), P2(T1), P2(T2), P1(T1), P2(T2), P1(T1), ...

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Note that there is also another category, the hybrid threads, where some user-space threads are translated into one kernel thread. However, it is more complex and requires a more thorough analysis.