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  <para>One of the key aims of the operating system is to create and support
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  <para>One of the key aims of the operating system is to create and support
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  the impression that several activities are executing contemporarily. This is
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  the impression that several activities are executing contemporarily. This is
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  true for both uniprocessor as well as multiprocessor systems. In the case of
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  true for both uniprocessor as well as multiprocessor systems. In the case of
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  multiprocessor systems, the activities are trully happening in parallel. The
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  multiprocessor systems, the activities are trully happening in parallel. The
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  scheduler helps to materialize this impression by planning threads on as
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  scheduler helps to materialize this impression by planning threads on as
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  many processors as possible and, where this means reaches its limits, by
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  many processors as possible and, where this means reaching its limits, by
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  quickly switching among threads executing on a single processor.</para>
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  quickly switching among threads executing on a single processor.</para>
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  <section>
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  <section>
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    <title>Contexts</title>
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    <title>Contexts</title>
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    <para>The term context refers to the set of processor resources that
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    <para>The term <emphasis>context</emphasis> refers to the set of processor
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    define the current state of the computation or the environment and the
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    resources that define the current state of the computation or the
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    kernel understands it in several more or less narrow sences:</para>
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    environment and the kernel understands it in several more or less narrow
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    sences:</para>
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    <itemizedlist>
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    <itemizedlist>
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      <listitem>
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      <listitem>
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        <para>synchronous register context,</para>
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        <para>synchronous register context,</para>
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      </listitem>
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      </listitem>
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  </section>
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  </section>
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  <section>
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  <section>
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    <title>Threads</title>
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    <title>Threads</title>
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    <para>A thread is the basic executable entity with some code and stack.
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    <para>A thread is the basic executable entity with a code and a stack.
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    While the code, implemented by a C language function, can be shared by
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    While the code, implemented by a C language function, can be shared by
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    several threads, the stack is always private to each instance of the
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    several threads, the stack is always private to each instance of the
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    thread. Each thread belongs to exactly one task through which it shares
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    thread. Each thread belongs to exactly one task through which it shares
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    address space with its sibling threads. Threads that execute purely in the
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    address space with its sibling threads. Threads that execute purely in the
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    kernel don't have any userspace memory allocated. However, when a thread
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    kernel don't have any userspace memory allocated. However, when a thread
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    has ambitions to run in userspace, it must be allocated a userspace stack.
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    has ambitions to run in userspace, it must be allocated a userspace stack.
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    The distinction between the purely kernel threads and threads running also
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    The distinction between the purely kernel threads and threads running also
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    in userspace is made by refering to the former group as to kernel threads
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    in userspace is made by refering to the former group as to kernel threads
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    and to the latter group as to userspace threads. Both kernel and userspace
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    and to the latter group as to userspace threads. Both kernel and userspace
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    threads are visible to the scheduler and can become a subject of kernel
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    threads are visible to the scheduler and can become a subject of kernel
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    preemption and thread migration during times when preemption is
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    preemption and thread migration anytime when preemption is not
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    possible.</para>
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    disabled.</para>
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    <formalpara>
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    <formalpara>
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      <title>Thread States</title>
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      <title>Thread States</title>
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      <para>In each moment, a thread exists in one of six possible thread
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      <para>In each moment, a thread exists in one of six possible thread
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      states. When the thread is created and first readied into the
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      states. When the thread is created and first inserted into the
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      scheduler's run queues or when a thread is migrated to a new processor,
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      scheduler's run queues or when a thread is migrated to a new processor,
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      it is put into the <constant>Entering</constant> state. After some time,
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      it is put into the <constant>Entering</constant> state. After some time
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      the scheduler picks up the thread and starts executing it. A thread
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      elapses, the scheduler picks up the thread and starts executing it. A
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      being currently executed on a processor is in the
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      thread being currently executed on a processor is in the
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      <constant>Running</constant> state. From there, the thread has three
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      <constant>Running</constant> state. From there, the thread has three
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      possibilities. It either runs until it is preemtped, in which case the
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      possibilities. It either runs until it is preemtped, in which case the
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      state changes to <constant>Ready</constant>, goes to the
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      state changes to <constant>Ready</constant>, goes to the
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      <constant>Sleeping</constant> state by going to sleep or enters the
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      <constant>Sleeping</constant> state by going to sleep or enters the
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      <constant>Exiting</constant> state when it reaches termination. When the
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      <constant>Exiting</constant> state when it reaches termination. When the
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      essential. If the processor kept running using the stack of the
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      essential. If the processor kept running using the stack of the
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      preempted thread it could damage it because the old thread can be
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      preempted thread it could damage it because the old thread can be
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      migrated to another processor and scheduled there. In the worst case
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      migrated to another processor and scheduled there. In the worst case
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      scenario, two execution flows would be using the same stack.</para>
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      scenario, two execution flows would be using the same stack.</para>
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      <para>The scheduling policy is implemented in function
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      <para>The scheduling policy is implemented in the function
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      <code>find_best_thread</code>. This function walks the processor run
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      <code>find_best_thread</code>. This function walks the processor run
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      queues from lower towards higher indices and looks for a thread. If the
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      queues from lower towards higher indices and looks for a thread. If the
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      visited run queue is empty, it simply searches the next run queue. If it
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      visited run queue is empty, it simply searches the next run queue. If it
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      is known in advance that there are no ready threads waiting for
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      is known in advance that there are no ready threads waiting for
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      execution, <code>find_best_thread()</code> interruptibly halts the
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      execution, <code>find_best_thread()</code> interruptibly halts the