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<?xml version="1.0" encoding="UTF-8"?>
<chapter id="time">
  <?dbhtml filename="time.html"?>
 
  <title>Time Management</title>
 
  <para>Time is one of the dimensions in which kernel, as well as the whole
  system, operates. It is of special importance to many kernel subsytems.
  Knowledge of time makes it possible for the scheduler to preemptively plan
  threads for execution. Different parts of the kernel can request execution
  of their callback function with some specified delay. A good example of such
  kernel code is the synchronization subsystem which uses this functionality
  to implement timeouting versions of synchronization primitives.</para>
 
  <section>
    <title>System Clock</title>
 
    <para>Every hardware architecture supported by HelenOS must support some
    kind of a device that can be programmed to yield periodic time signals
    (i.e. clock interrupts). Some architectures have external clock that is
    merely programmed by the kernel to interrupt the processor multiple times

    <title>Timeouts</title>
 
    <para>Kernel subsystems can register a callback function to be executed
    with a specified delay. Such a registration is represented by a kernel
    structure called <classname>timeout</classname>. Timeouts are registered
    via <code>timeout_register</code> function. This function takes a pointer
    to a timeout structure, a callback function, a parameter of the callback
    function and a delay in microseconds as parameters. After the structure is
    initialized with all these values, it is sorted into the processor's list
    of active timeouts, according to the number of clock interrupts remaining
    to their expiration and relatively to already listed timeouts.</para>
 
    <para>Timeouts can be unregistered via <code>timeout_unregister</code>.
    This function can, as opposed to <code>timeout_register</code>, fail when
    it is too late to remove the timeout from the list of active
    timeouts.</para>
 
    <para>Timeouts are nearing their expiration in the list of active timeouts
    which exists on every processor in the system. The expiration counters are

    the list and their callback function is called with respective
    parameter.</para>
  </section>
 
  <section>
    <title>Generic Clock Interrupt Handler</title>
 
    <para>On each clock interrupt, the architecture specific part of the clock
    interrupt handler makes a call to the generic clock interrupt handler
    implemented by the <code>clock</code> function. The generic handler takes
    care of several mission critical goals:</para>
 
    <itemizedlist>
      <listitem>
        <para>expiration of timeouts,</para>

      <listitem>
        <para>preemption of threads.</para>
      </listitem>
    </itemizedlist>
 
    <para>The <code>clock</code> function checks for expired timeouts and
    decrements unexpired timeout expiration counters exactly one more times
    than is the number of missed clock signals (i.e. at least once and
    possibly more times, depending on the missed clock signals counter). The
    time of the day counters are also updated one more times than is the
    number of missed clock signals. And finally, the remaining timeslice of

    lost due to an occasional excessive time drift described in previous
    paragraphs.</para>
  </section>
 
  <section>
    <title>Time Source for Userspace</title>
 
    <para>In HelenOS, userspace tasks don't communicate with the kernel in
    order to read the system time. Instead, a mechanism that shares kernel
    time of the the day counters with userspace address spaces is deployed. On
    the kernel side, during system initialization, HelenOS allocates a frame

Subversion Repositories HelenOS-doc

(root)/design/trunk/src/ch_time.xml – Rev 127 → 131

Rev 127		Rev 131
Line 1...		Line 1...
1	<?xml version="1.0" encoding="UTF-8"?>	1	<?xml version="1.0" encoding="UTF-8"?>
2	<chapter id="time">	2	<chapter id="time">
3	<?dbhtml filename="time.html"?>	3	<?dbhtml filename="time.html"?>
4		4
5	<title>Time management</title>	5	<title>Time Management</title>
6		6
7	<para>Time is one of the dimensions in which kernel, as well as the whole	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.	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	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	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	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	12	kernel code is the synchronization subsystem which uses this functionality
13	to implement timeouting versions of synchronization primitives.</para>	13	to implement timeouting versions of synchronization primitives.</para>
14		14
15	<section>	15	<section>
16	<title>System clock</title>	16	<title>System Clock</title>
17		17
18	<para>Every hardware architecture supported by HelenOS must support some	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	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	20	(i.e. clock interrupts). Some architectures have external clock that is
21	merely programmed by the kernel to interrupt the processor multiple times	21	merely programmed by the kernel to interrupt the processor multiple times
Line 82...		Line 82...
82	<title>Timeouts</title>	82	<title>Timeouts</title>
83		83
84	<para>Kernel subsystems can register a callback function to be executed	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	85	with a specified delay. Such a registration is represented by a kernel
86	structure called <classname>timeout</classname>. Timeouts are registered	86	structure called <classname>timeout</classname>. Timeouts are registered
87	via <code>timeout_register()</code> function. This function takes a pointer	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	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	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	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	91	of active timeouts, according to the number of clock interrupts remaining
92	to their expiration and relatively to already listed timeouts.</para>	92	to their expiration and relatively to already listed timeouts.</para>
93		93
94	<para>Timeouts can be unregistered via <code>timeout_unregister()</code>.	94	<para>Timeouts can be unregistered via <code>timeout_unregister</code>.
95	This function can, as opposed to <code>timeout_register()</code>, fail when	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	96	it is too late to remove the timeout from the list of active
97	timeouts.</para>	97	timeouts.</para>
98		98
99	<para>Timeouts are nearing their expiration in the list of active timeouts	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	100	which exists on every processor in the system. The expiration counters are
Line 105...		Line 105...
105	the list and their callback function is called with respective	105	the list and their callback function is called with respective
106	parameter.</para>	106	parameter.</para>
107	</section>	107	</section>
108		108
109	<section>	109	<section>
110	<title>Generic clock interrupt handler</title>	110	<title>Generic Clock Interrupt Handler</title>
111		111
112	<para>On each clock interrupt, the architecture specific part of the clock	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	113	interrupt handler makes a call to the generic clock interrupt handler
114	implemented by the <code>clock()</code> function. The generic handler takes	114	implemented by the <code>clock</code> function. The generic handler takes
115	care of several mission critical goals:</para>	115	care of several mission critical goals:</para>
116		116
117	<itemizedlist>	117	<itemizedlist>
118	<listitem>	118	<listitem>
119	<para>expiration of timeouts,</para>	119	<para>expiration of timeouts,</para>
Line 126...		Line 126...
126	<listitem>	126	<listitem>
127	<para>preemption of threads.</para>	127	<para>preemption of threads.</para>
128	</listitem>	128	</listitem>
129	</itemizedlist>	129	</itemizedlist>
130		130
131	<para>The <code>clock()</code> function checks for expired timeouts and	131	<para>The <code>clock</code> function checks for expired timeouts and
132	decrements unexpired timeout expiration counters exactly one more times	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	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	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	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	136	number of missed clock signals. And finally, the remaining timeslice of
Line 139...		Line 139...
139	lost due to an occasional excessive time drift described in previous	139	lost due to an occasional excessive time drift described in previous
140	paragraphs.</para>	140	paragraphs.</para>
141	</section>	141	</section>
142		142
143	<section>	143	<section>
144	<title>Time source for userspace</title>	144	<title>Time Source for Userspace</title>
145		145
146	<para>In HelenOS, userspace tasks don't communicate with the kernel in	146	<para>In HelenOS, userspace tasks don't communicate with the kernel in
147	order to read the system time. Instead, a mechanism that shares kernel	147	order to read the system time. Instead, a mechanism that shares kernel
148	time of the the day counters with userspace address spaces is deployed. On	148	time of the the day counters with userspace address spaces is deployed. On
149	the kernel side, during system initialization, HelenOS allocates a frame	149	the kernel side, during system initialization, HelenOS allocates a frame