fiber: A User Space Threading Library

Fiber is a lightweight thread library with M:N mapping between user-level thread (ULT) and Linux native thread (or kernel-level thread, KLT).

Features

• Preemptive user-level threads
• Familiar threading concepts are available
  • Mutexes
  • Condition variables

Implementation Details

The preemptive scheduler is implemented through timer and signal functions. In k_thread_exec_func() function, a timer is initiated through the following:

setitimer(ITIMER_PROF, &timeslice, NULL)

When the timer expires, signal SIGPROF is sent to the process. sigaction() would invoke the scheduling routine schedule() to run, which chooses a thread from a run queue to run. The scheduler maintains a run queue. The new created threads are pushed into the end of the queue. The first thread in the head of the queue is the thread currently running. Each time the scheduler receives signal SIGPROF, it interrupts the running thread at the head by pushing this thread into the end of the queue. In addition, it swaps the context between this thread and next thread which is the new head of the queue.

A userspace program/process may not create a kernel thread. Instead, it could create a native thread using pthread_create, which invokes the clone system call to do so. Inside Fiber, clone system call is used for creating kernel-level threads. With a kernel that understands threads, we use clone, but we still have to create the new thread's stack. The kernel does not create/assign a stack for a new thread. The clone system call accepts a child_stack argument. Therefore, fiber_create must allocate a stack for the new thread and pass that to clone:

    /* invoke the clone system call to create a native thread */
    if (-1 == clone((int (*)(void *)) k_thread_exec_func,
                    (char *) stack + _THREAD_STACK,
                    SIGCHLD | CLONE_SIGHAND | CLONE_VM | CLONE_PTRACE, NULL)) {
        perror("Failed to invoke clone system call.");
        ...
    }

Only a process or main thread is assigned its initial stack by the kernel, usually at a high memory address. Thus, if the process does not use threads, normally, it just uses that pre-assigned stack. But, if a thread is created, i.e., the native thread, the starting process/thread must pre-allocate the area for the proposed thread with malloc.

License

fiber is released under the MIT License. Use of this source code is governed by a MIT License that can be found in the LICENSE file.