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    <para>The last member of the context family is the memory management

    <para>The last member of the context family is the memory management

    context. It includes memory management registers that identify address

    context. It includes memory management registers that identify address

    spaces on hardware level (i.e. ASIDs and page tables pointers).</para>

    spaces on hardware level (i.e. ASIDs and page tables pointers).</para>

    <section>

    <section>

      <para>The scheduler, but also other pieces of the kernel, make heavy use

      <para>The scheduler, but also other pieces of the kernel, make heavy use

      of synchronous context switches, because it is a natural vehicle not

      of synchronous context switches, because it is a natural vehicle not

      only for changes in control flow, but also for switching between two

      only for changes in control flow, but also for switching between two

      kernel stacks. Two functions figure in a synchronous context switch

      kernel stacks. Two functions figure in a synchronous context switch

    threads are visible to the scheduler and can become a subject of kernel

    threads are visible to the scheduler and can become a subject of kernel

    preemption and thread migration during times when preemption is

    preemption and thread migration during times when preemption is

    possible.</para>

    possible.</para>

    <formalpara>

    <formalpara>

      <para>In each moment, a thread exists in one of six possible thread

      <para>In each moment, a thread exists in one of six possible thread

      states. When the thread is created and first readied into the

      states. When the thread is created and first readied into the

      scheduler's run queues or when a thread is migrated to a new processor,

      scheduler's run queues or when a thread is migrated to a new processor,

      it is put into the <constant>Entering</constant> state. After some time,

      it is put into the <constant>Entering</constant> state. After some time,

      state is not reached.<figure float="1">

      state is not reached.<figure float="1">

          <title>Transitions among thread states.</title>

          <title>Transitions among thread states.</title>

          <mediaobject id="thread_states" xreflabel="">

          <mediaobject id="thread_states" xreflabel="">

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              <imagedata fileref="images/thread_states.png" format="PNG" />

              <imagedata fileref="images/thread_states.png" format="PNG" />

            </imageobject>

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        </figure></para>

        </figure></para>

    </formalpara>

    </formalpara>

    <formalpara>

    <formalpara>

      <para>HelenOS userspace layer knows even smaller units of execution.

      <para>HelenOS userspace layer knows even smaller units of execution.

      Each userspace thread can make use of an arbitrary number of pseudo

      Each userspace thread can make use of an arbitrary number of pseudo

      threads. These pseudo threads have their own synchronous register

      threads. These pseudo threads have their own synchronous register

      context, userspace code and stack. They live their own life within the

      context, userspace code and stack. They live their own life within the

  <section>

  <section>

    <title>Scheduler</title>

    <title>Scheduler</title>

    <section>

    <section>

      <para>There is an array of several run queues on each processor. The

      <para>There is an array of several run queues on each processor. The

      current version of HelenOS uses 16 run queues implemented by 16 doubly

      current version of HelenOS uses 16 run queues implemented by 16 doubly

      linked lists. Each of the run queues is associated with thread priority.

      linked lists. Each of the run queues is associated with thread priority.

      The lower the run queue index in the array is, the higher is the

      The lower the run queue index in the array is, the higher is the

      in which those threads will execute. When kernel code wants to access

      in which those threads will execute. When kernel code wants to access

      the run queue, it must first acquire its lock.</para>

      the run queue, it must first acquire its lock.</para>

    </section>

    </section>

    <section>

    <section>

      <para>The scheduler is invoked either explicitly when a thread calls the

      <para>The scheduler is invoked either explicitly when a thread calls the

      <code>scheduler</code> function (e.g. goes to sleep or merely wants to

      <code>scheduler</code> function (e.g. goes to sleep or merely wants to

      relinquish the processor for a while) or implicitly on a periodic basis

      relinquish the processor for a while) or implicitly on a periodic basis

      when the generic clock interrupt preempts the current thread. After its

      when the generic clock interrupt preempts the current thread. After its

      point priority increase. In other words, the scheduler will now and then

      point priority increase. In other words, the scheduler will now and then

      move the entire run queues one level up.</para>

      move the entire run queues one level up.</para>

    </section>

    </section>

    <section>

    <section>

      <para>Normally, for the sake of cache locality, threads are scheduled on

      <para>Normally, for the sake of cache locality, threads are scheduled on

      one of the processors and don't leave it. Nevertheless, a situation in

      one of the processors and don't leave it. Nevertheless, a situation in

      which one processor is heavily overloaded while others sit idle can

      which one processor is heavily overloaded while others sit idle can

      occur. HelenOS deploys special kernel threads to help mitigate this

      occur. HelenOS deploys special kernel threads to help mitigate this