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co_sched - preemptive multitasking in userspace based on a SIGALRM signal

Description:
    This is a small experiment, an application, which starts 3 sorting
    routines, execution of each is preempted by SIGALRM signal, simulating
    an OS timer interrupt.  Each routine is an execution context, which
    can do a voluntary scheduling (calling schedule() directly) or be
    preempted by a timer, and in that case nonvoluntary scheduling occurs.

    The default time slice is 10ms, that means that each 10ms SIGALRM fires
    and next context is scheduled by round robin algorithm.

Usage:
    [roman@pb co_sched]$ make
    gcc  -O2 -Wall -o co_sched co_sched.c co_sched.S
    [roman@pb co_sched]$ ./co_sched
    ~~~~ let's rock!
    [1] test_sort: begin init, a=0x7ffff5210010
    [1] test_sort: start sort
    [2] test_sort: begin init, a=0x7ffff05c4010
    [2] test_sort: start sort
    [3] test_sort: begin init, a=0x7fffeb978010
    [3] test_sort: start sort
    [2] test_sort: end
    [3] test_sort: end
    [1] test_sort: end
    ~~~~ i am done