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 <?xml version="1.0" encoding="UTF-8"?>
 <chapter id="scheduling">
   <?dbhtml filename="scheduling.html"?>
   <title>Scheduling</title>
   <para>One of the key aims of the operating system is to create and support
   the impression that several activities are executing contemporarily. This is
   true for both uniprocessor as well as multiprocessor systems. In the case of
   multiprocessor systems, the activities are trully happening in parallel. The
   scheduler helps to materialize this impression by planning threads on as
   many processors as possible and, where this means reaches its limits, by
   quickly switching among threads executing on a single processor.</para>
   <section>
     <title>Contexts</title>
     <para>The term context refers to the set of processor resources that
     define the current state of the computation or the environment and the
     kernel understands it in several more or less narrow sences:</para>
     <itemizedlist>
       <listitem>
         <para>synchronous register context,</para>
       </listitem>
       <listitem>
         <para>asynchronous register context,</para>
       </listitem>
       <listitem>
         <para>FPU context and</para>
       </listitem>
       <listitem>
         <para>memory management context.</para>
       </listitem>
     </itemizedlist>
     <para>The most narrow sence refers to the the synchronous register
     context. It includes all the preserved registers as defined by the
     architecture. To highlight some, the program counter and stack pointer
     take part in the synchronous register context. These are the registers
     that must be preserved across a procedure call and during synchronous
     context switches.</para>
     <para>The next type of the context understood by the kernel is the
     asynchronous register context. On an interrupt, the interrupted execution
     flow's state must be guaranteed to be eventually completely restored.
     Therefore the interrupt context includes, among other things, the scratch
     registers as defined by the architecture. As a special optimization and if
     certain conditions are met, it need not include the architecture's
     preserved registers. The condition mentioned in the previous sentence is
     that the low-level assembly language interrupt routines don't modify the
     preserved registers. The handlers usually call a higher-level C routine.
     The preserved registers are then saved on the stack by the compiler
     generated code of the higher-level function. In HelenOS, several
     architectures can be compiled with this optimization.</para>
     <para>Although the kernel does not do any floating point
     arithmetics<footnote>
         <para>Some architectures (e.g. ia64) inevitably use a fixed set of
         floating point registers to carry out its normal operations.</para>
       </footnote>, it must protect FPU context of userspace threads against
     destruction by other threads. Moreover, only a fraction of userspace
     programs use the floating point unit. HelenOS contains a generic framework
     for switching FPU context only when the switch is forced.</para>
     <para>The last member of the context family is the memory management
     context. It includes memory management registers that identify address
     spaces on hardware level (i.e. ASIDs and page tables pointers).</para>
     <section>
       <title>Synchronous context switches</title>
       <para>The scheduler, but also other pieces of the kernel, make heavy use
       of synchronous context switches, because it is a natural vehicle not
       only for changes in control flow, but also for switching between two
       kernel stacks. Two functions figure in a synchronous context switch
       implementation: <code>context_save</code> and
       <code>context_restore</code>. Note that these two functions break the
       natural perception of the linear C code execution flow starting at
       function's entry point and ending on one of the function's exit
       points.</para>
       <para>When the <code>context_save</code> function is called, the
       synchronous context is saved in a memory structure passed to it. After
       executing <code>context_save</code>, the caller is returned 1 as a
       return value. The execution of instructions continues as normally until
       <code>context_restore</code> is called. For the caller, it seems like
       the call never returns<footnote>
           <para>Which might be a source of problems with variable liveliness
           after <code>context_restore</code>.</para>
         </footnote>. Nevertheless, a synchronous register context, which is
       saved in a memory structure passed to <code>context_restore,</code> is
       restored, thus transfering the control flow to the place of occurrence
       of the corresponding call to <code>context_save</code>. From the
       perspective of the caller of the corresponding
       <code>context_save</code>, it looks as though a return from
       <code>context_save</code>. However, this time a return value of 0 is
       returned.</para>
     </section>
   </section>
   <section>
     <title>Threads</title>
     <para>A thread is the basic executable entity with some code and stack.
     While the code, implemented by a C language function, can be shared by
     several threads, the stack is always private to each instance of the
     thread. Each thread belongs to exactly one task through which it shares
     address space with its sibling threads. Threads that execute purely in the
     kernel don't have any userspace memory allocated. However, when a thread
     has ambitions to run in userspace, it must be allocated a userspace stack.
     The distinction between the purely kernel threads and threads running also
     in userspace is made by refering to the former group as to kernel threads
     and to the latter group as to userspace threads. Both kernel and userspace
     threads are visible to the scheduler and can become a subject of kernel
     preemption and thread migration during times when preemption is
     possible.</para>
     <para>HelenOS userspace layer knows even smaller units of execution. Each
     userspace thread can make use of an arbitrary number of pseudo threads.
     These pseudo threads have their own synchronous register context,
     userspace code and stack. They live their own life within the userspace
     thread and the scheduler does not have any idea about them because they
     are completely implemented by the userspace library. This implies several
     things:</para>
     <itemizedlist>
       <listitem>
         <para>pseudothreads schedule themselves cooperatively within the time
         slice given to their userspace thread,</para>
       </listitem>
       <listitem>
         <para>pseudothreads share FPU context of their containing thread
         and</para>
       </listitem>
       <listitem>
         <para>all pseudothreads of one userspace thread block when one of them
         goes to sleep.</para>
       </listitem>
     </itemizedlist>
+  </section>
+  <section>
+    <title>Scheduler</title>
+    <section>
+      <title>Run queues</title>
+      <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
+      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
+      priority of threads linked in that run queue and the shorter is the time
+      in which those threads will execute. When kernel code wants to access
+      the run queue, it must first acquire its lock.</para>
+    </section>
+    <section>
+      <title>Scheduler operation</title>
+      <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
+      relinquish the processor for a while) or implicitly on a periodic basis
+      when the generic clock interrupt preempts the current thread. After its
+      invocation, the scheduler saves the synchronous register context of the
+      current thread and switches to its private stack. Afterwards, a new
+      thread is selected according to the scheduling policy. If there is no
+      suitable thread, the processor is idle and no thread executes on it.
+      Note that the act of switching to the private scheduler stack is
+      essential. If the processor kept running using the stack of the
+      preempted thread it could damage it because the old thread can be
+      migrated to another processor and scheduled there. In the worst case
+      scenario, two execution flows would be using the same stack. </para>
+      <para>The scheduling policy is implemented in function
+      <code>find_best_thread</code>. This function walks the processor run
+      queues from lower towards higher indices and looks for a thread. If the
+      visited run queue is empty, it simply searches the next run queue. If it
+      is known in advance that there are no ready threads waiting for
+      execution, <code>find_best_thread</code> interruptibly halts the
+      processor or busy waits until some threads arrive. This process repeats
+      until <code>find_best_thread</code> succeeds.</para>
+      <para>After the best thread is chosen, the scheduler switches to the
+      thread's task and memory management context. Finally, the saved
+      synchronous register context is restored and the thread runs. Each
+      scheduled thread is given a time slice depending on its priority (i.e.
+      run queue). The higher priority, the shorter timeslice. To summarize,
+      this policy schedules threads with high priorities more frequently but
+      gives them smaller time slices. On the other hand, lower priority
+      threads are scheduled less frequently, but run for longer periods of
-    <para></para>
+      time.</para>
+      <para>When a thread uses its entire time slice, it is preempted and put
+      back into the run queue that immediately follows the previous run queue
+      from which the thread ran. Threads that are woken up from a sleep are
+      put into the biggest priority run queue. Low priority threads are
+      therefore those that don't go to sleep so often and just occupy the
+      processor.</para>
+      <para>In order to avoid complete starvation of the low priority threads,
+      from time to time, the scheduler will provide them with a bonus of one
+      point priority increase. In other words, the scheduler will now and then
+      move the entire run queues one level up.</para>
+    </section>
+    <section>
+      <title>Processor load balancing</title>
+      <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
+      which one processor is heavily overloaded while others sit idle can
+      occur. HelenOS deploys special kernel threads to help to mitigate this
+      problem. Each processor is associated with one load balancing thread
+      called <code>kcpulb</code> that wakes up regularily to see whether its
+      processor is underbalanced or not. If yes, the thread attempts to
+      migrate threads from other overloaded processors to its own processor's
+      run queues. When the job is done or there is no need for load balancing,
+      the thread goes to sleep.</para>
+      <para>The balancing threads operate very gently and try to migrate low
+      priority threads first; one <code>kcpulb</code> never takes from one
+      processor twice in a row. The load balancing threads as well as threads
+      that were just stolen cannot be migrated. The <code>kcpulb</code>
+      threads are wired to their processors and cannot be migrated whatsoever.
+      The ordinary threads are protected only until they are
+      rescheduled.</para>
+    </section>
   </section>
 </chapter>

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