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  1. <?xml version="1.0" encoding="UTF-8"?>
  2. <chapter id="time">
  3.   <?dbhtml filename="time.html"?>
  4.  
  5.   <title>Time management</title>
  6.  
  7.   <para>Time is one of the dimensions in which kernel, as well as the whole
  8.   system, operates. It is of special importance to many kernel subsytems.
  9.   Knowledge of time makes it possible for the scheduler to preemptively plan
  10.   threads for execution. Different parts of the kernel can request execution
  11.   of their callback function with some specified delay. A good example of such
  12.   kernel code is the synchronization subsystem which uses this functionality
  13.   to implement timeouting versions of synchronization primitives.</para>
  14.  
  15.   <section>
  16.     <title>System clock</title>
  17.  
  18.     <para>Every hardware architecture supported by HelenOS must support some
  19.     kind of a device that can be programmed to yield periodic time signals
  20.     (i.e. clock interrupts). Some architectures have external clock that is
  21.     merely programmed by the kernel to interrupt the processor multiple times
  22.     in a second. This is the case of ia32 and amd64 architectures<footnote>
  23.         <para>When running in uniprocessor mode.</para>
  24.       </footnote>, which use i8254 or a compatible chip to achieve the
  25.     goal.</para>
  26.  
  27.     <para>Other architectures' processors typically contain two registers. The
  28.    first register is usually called a compare or a match register and can be
  29.    set to an arbitrary value by the operating system. The contents of the
  30.    compare register then stays unaltered until it is written by the kernel
  31.    again. The second register, often called a counter register, can be also
  32.    written by the kernel, but the processor automatically increments it after
  33.    every executed instruction or in some fixed relation to processor speed.
  34.    The point is that a clock interrupt is generated whenever the values of
  35.    the counter and the compare registers match. Sometimes, the scheme of two
  36.    registers is modified so that only one register is needed. Such a
  37.    register, called a decrementer, then counts towards zero and an interrupt
  38.    is generated when zero is reached.</para>
  39.  
  40.    <para>In any case, the initial value of the decrementer or the initial
  41.    difference between the counter and the compare registers, respectively,
  42.    must be set accordingly to a known relation between the real time and the
  43.    speed of the decrementer or the counter register, respectively.</para>
  44.  
  45.    <para>The rest of this section will, for the sake of clarity, focus on the
  46.    two-register scheme. The decrementer scheme is very similar.</para>
  47.  
  48.    <para>The kernel must reinitialize one of the two registers after each
  49.    clock interrupt in order to schedule next interrupt. However this step is
  50.    tricky and must be done with caution. Imagine that the clock interrupt is
  51.    masked either because the kernel is servicing another interrupt or because
  52.    the processor locally disabled interrupts for a while. If the clock
  53.    interrupt occurs during this period, it will be pending until interrupts
  54.    are enabled again. In theory, that could happen arbitrary counter register
  55.    ticks later. Which is worse, the ideal time period between two non-delayed
  56.    clock interrupts can also elapse arbitrary number of times before the
  57.    delayed interrupt gets serviced. The architecture-specific part of the
  58.    clock interrupt driver must avoid time drifts caused by this by taking
  59.    proactive counter-measures.</para>
  60.  
  61.    <para>Let us assume that the kernel wants each clock interrupt be
  62.    generated every <constant>TICKCONST</constant> ticks. This value
  63.    represents the ideal number of ticks between two non-delayed clock
  64.    interrupts and has some known relation to real time. On each clock
  65.    interrupt, the kernel computes and writes down the expected value of the
  66.    counter register as it hopes to read it on the next clock interrupt. When
  67.    that interrupt comes, the kernel reads the counter register again and
  68.    compares it with the written down value. If the difference is smaller than
  69.    or equal to <constant>TICKCONST</constant>, then the time drift is none or
  70.    small and the next interrupt is scheduled earlier with a penalty of so
  71.    many ticks as is the value of the difference. However, if the difference
  72.    is bigger, then at least one clock signal was missed. In that case, the
  73.    missed clock signal is remembered in the special counter. If there are
  74.    more missed signals, each of them is recorded there. The next interrupt is
  75.    scheduled with respect to the difference similarily to the former case.
  76.    This time, the penalty is taken modulo <constant>TICKCONST</constant>. The
  77.    effect of missed clock signals is remedied in the generic clock interrupt
  78.    handler.</para>
  79.  </section>
  80.  
  81.  <section>
  82.    <title>Timeouts</title>
  83.  
  84.    <para>Kernel subsystems can register a callback function to be executed
  85.    with a specified delay. Such a registration is represented by a kernel
  86.    structure called <classname>timeout</classname>. Timeouts are registered
  87.    via <code>timeout_register</code> function. This function takes a pointer
  88.    to a timeout structure, a callback function, a parameter of the callback
  89.    function and a delay in microseconds as parameters. After the structure is
  90.    initialized with all these values, it is sorted into the processor's list
  91.     of active timeouts, according to the number of clock interrupts remaining
  92.     to their expiration and relatively to already listed timeouts.</para>
  93.  
  94.     <para>Timeouts can be unregistered via <code>timeout_unregister</code>.
  95.     This function can, as opposed to <code>timeout_register</code>, fail when
  96.     it is too late to remove the timeout from the list of active
  97.     timeouts.</para>
  98.  
  99.     <para>Timeouts are nearing their expiration in the list of active timeouts
  100.     which exists on every processor in the system. The expiration counters are
  101.     decremented on each clock interrupt by the generic clock interrupt
  102.     handler. Due to the relative ordering of timeouts in the list, it is
  103.     sufficient to decrement expiration counter only of the first timeout in
  104.     the list. Timeouts with expiration counter equal to zero are removed from
  105.     the list and their callback function is called with respective
  106.     parameter.</para>
  107.   </section>
  108.  
  109.   <section>
  110.     <title>Generic clock interrupt handler</title>
  111.  
  112.     <para>On each clock interrupt, the architecture specific part of the clock
  113.     interrupt handler makes a call to the generic clock interrupt handler
  114.     implemented by the <code>clock</code> function. The generic handler takes
  115.     care of several mission critical goals:</para>
  116.  
  117.     <itemizedlist>
  118.       <listitem>
  119.         <para>expiration of timeouts,</para>
  120.       </listitem>
  121.  
  122.       <listitem>
  123.         <para>updating time of the day counters for userspace and</para>
  124.       </listitem>
  125.  
  126.       <listitem>
  127.         <para>preemption of threads.</para>
  128.       </listitem>
  129.     </itemizedlist>
  130.  
  131.     <para>The <code>clock</code> function checks for expired timeouts and
  132.     decrements unexpired timeout expiration counters exactly one more times
  133.     than is the number of missed clock signals (i.e. at least once and
  134.     possibly more times, depending on the missed clock signals counter). The
  135.     time of the day counters are also updated one more times than is the
  136.     number of missed clock signals. And finally, the remaining timeslice of
  137.     the running thread is decremented with respect to this counter as well. By
  138.     considering its value, the kernel performs actions that would otherwise be
  139.     lost due to an occasional excessive time drift described in previous
  140.     paragraphs.</para>
  141.   </section>
  142.  
  143.   <section>
  144.     <title>Time source for userspace</title>
  145.  
  146.     <para>In HelenOS, userspace tasks don't communicate with the kernel in
  147.    order to read system time. Instead, a mechanism that shares kernel time of
  148.    the day counters with userspace address spaces is deployed. On the kernel
  149.    side, during system initialization, HelenOS allocates a frame of physical
  150.    memory and stores the time of the day counters there. The counters have
  151.    the following structure:</para>
  152.  
  153.    <itemizedlist>
  154.      <listitem>
  155.        <para>first 32-bit counter for seconds,</para>
  156.      </listitem>
  157.  
  158.      <listitem>
  159.        <para>32-bit counter for microseconds and</para>
  160.      </listitem>
  161.  
  162.      <listitem>
  163.        <para>second 32-bit counter for seconds.</para>
  164.      </listitem>
  165.    </itemizedlist>
  166.  
  167.    <para>One of the userspace tasks with capabilities of memory manager (e.g.
  168.    ns) asks the kernel to map this frame into its address space. Other,
  169.    non-privileged, tasks then use IPC to communicate read-only sharing of
  170.    this memory. Reading time in a userspace task is therefore just a matter
  171.    of reading memory.</para>
  172.  
  173.    <para>There are two interesting points about this. First, the counters are
  174.    32-bit even on 64-bit machines. The goal is to provide subsecond precision
  175.    with the possibility to span roughly 136 years. Note that a single 64-bit
  176.    microsecond counter could not be usually read atomically on 32-bit
  177.    platforms. Now the second point is that 32-bit platforms cannot atomically
  178.    read two 32-bit counters either. However, a generic protocol is used to
  179.    guarantee that sequentially read times will create a non-decreasing
  180.    sequence.</para>
  181.  
  182.    <para>The problematic part is updating both seconds and microseconds once
  183.    in a second. Seconds must be incremented and microseconds must be reset.
  184.    However, without any synchronization, the two kernel stores and the two
  185.    userspace reads can arbitrarily interleave. Furthemore, the reader has no
  186.    chance to detect that the counters were updated only from half. Therefore
  187.    three counters are used in HelenOS.</para>
  188.  
  189.    <para>If seconds need to be updated, the kernel increments the first
  190.    second counter, issues a write memory barrier operation, updates the
  191.    microsecond counter, issues another write memory barrier operation and
  192.    increments the second second counter. When only microseconds need to be
  193.    updated, no special action is taken by the kernel. On the other hand, the
  194.    userspace task must always read all three counters and in reversed order.
  195.    A read memory barrier operation must be issued between each two reads. A
  196.    non-atomic read is detected when the two second counters differ. The
  197.    userspace library solves this situation by returning zero instead of the
  198.    value read from the microsecond counter.</para>
  199.  </section>
  200. </chapter>