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52 | jermar | 1 | <?xml version="1.0" encoding="UTF-8"?> |
59 | jermar | 2 | <chapter id="ds"> |
3 | <?dbhtml filename="ds.html"?> |
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52 | jermar | 4 | |
59 | jermar | 5 | <title>Data structures</title> |
52 | jermar | 6 | |
59 | jermar | 7 | <para>There is lots of data that either flows through various HelenOS |
8 | subsystems or is stored directly by them. Each subsystem uses its own data |
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9 | structures to represent the data. These data structures need to be kept |
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10 | somewhere. In order to work efficiently, HelenOS, and especially its kernel, |
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11 | deploys several house keeping data types that are designed to faciliate |
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12 | managing other data structures. Most of them serve like generic |
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13 | containers.</para> |
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52 | jermar | 14 | |
59 | jermar | 15 | <section> |
16 | <title>Lists</title> |
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52 | jermar | 17 | |
73 | bondari | 18 | <indexterm> |
19 | <primary>linked list</primary> |
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20 | </indexterm> |
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21 | |||
59 | jermar | 22 | <para>HelenOS uses doubly-circularly-linked lists to bind related data |
23 | together. Lists are composed of an independent sentinel node called head |
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24 | and links that are always part of the object that is to be put into the |
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25 | list. Adding items to a list thus doesn't require any further memory |
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26 | allocations. Head and each link then contains forward and backward |
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27 | pointer. An empty list is composed of a sole head whose both pointers |
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28 | reference the head itself. The expense of two times bigger memory |
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29 | consumption as compared to memory consumption of singly linked lists is |
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30 | justified by constant insertion and removal times at random positions |
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31 | within the list.</para> |
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52 | jermar | 32 | |
59 | jermar | 33 | <para>Lists are frequently used to implement FIFO behaviour (e.g. |
34 | scheduler run queues or synchronization wait queues). Contrary to the FIFO |
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35 | type, which is also supported by HelenOS, they don't take up any unused |
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36 | space and are more general. On the other hand, they are slower than |
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37 | in-array FIFOs and can be hardly used to implement buffers.</para> |
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38 | </section> |
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39 | |||
40 | <section> |
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41 | <title>FIFO queues</title> |
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42 | |||
73 | bondari | 43 | <indexterm> |
44 | <primary>FIFO queue</primary> |
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45 | </indexterm> |
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46 | |||
59 | jermar | 47 | <para>FIFO queues are implemented as either statically or dynamically |
48 | allocated arrays<footnote> |
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49 | <para>Depending on the array size.</para> |
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50 | </footnote> of some generic type with two indices. The first index |
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51 | points to the head of the FIFO queue and the other points to the tail |
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52 | thereof. There can be as many items in the FIFO as is the number of |
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53 | elements in the array and no more. The indices are taken modulo size of |
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54 | the queue because as a consequence of insertions and deletions, the tail |
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55 | can have numericaly lower index than the head.</para> |
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56 | |||
57 | <para>FIFO queues are used, for example, in ASID management code to store |
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58 | inactive ASIDs or in userspace keyboard driver to buffer read |
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59 | characters.</para> |
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61 | jermar | 60 | |
62 | jermar | 61 | <figure> |
73 | bondari | 62 | <mediaobject id="fifo" xreflabel=""> |
63 | <imageobject role="html"> |
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64 | <imagedata fileref="images/fifo.png" format="PNG" /> |
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65 | </imageobject> |
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66 | |||
67 | <imageobject role="fop"> |
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68 | <imagedata fileref="images.vector/fifo.svg" format="SVG" /> |
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69 | </imageobject> |
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70 | </mediaobject> |
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71 | |||
72 | <title>FIFO queue showing the wrap around the end of the array.</title> |
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62 | jermar | 73 | </figure> |
59 | jermar | 74 | </section> |
75 | |||
76 | <section> |
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77 | <title>Hash tables</title> |
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78 | |||
73 | bondari | 79 | <indexterm> |
80 | <primary>hash table</primary> |
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81 | </indexterm> |
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82 | |||
59 | jermar | 83 | <para>The kernel, as well as userspace, provides hash table data type |
84 | which uses separate chaining. The hash table type is very generic in that |
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85 | it forces the user to supply methods for computing the hash index, |
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86 | comparing items against a set of keys and the item removal callback |
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87 | function. Besides these virtual operations, the hash table is composed of |
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88 | a dynamically allocated array of list heads that represent each chain, |
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89 | number of chains and the maximal number of keys.</para> |
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90 | </section> |
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91 | |||
92 | <section> |
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93 | <title>Bitmaps</title> |
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94 | |||
73 | bondari | 95 | <indexterm> |
96 | <primary>bitmap</primary> |
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97 | </indexterm> |
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98 | |||
59 | jermar | 99 | <para>Several bitmap operations such as clearing or setting consecutive |
100 | bit sequences as well as copying portions of one bitmap into another one |
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101 | are supported.</para> |
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102 | </section> |
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103 | |||
104 | <section> |
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105 | <title>B+trees</title> |
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106 | |||
73 | bondari | 107 | <indexterm> |
108 | <primary>B-tree</primary> |
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109 | </indexterm> |
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110 | |||
59 | jermar | 111 | <para>HelenOS makes use of a variant of B-tree called B+tree. B+trees, in |
112 | HelenOS implementation, are 3-4-5 balanced trees. They are characteristic |
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113 | by the fact that values are kept only in the leaf-level nodes and that |
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114 | these nodes are linked together in a list. This data structure has |
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115 | logaritmic search, insertion and deletion times and, thanks to the |
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116 | leaf-level list, provides fantastic means of walking the nodes containing |
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117 | data. Moreover, B+trees can be used for easy storing, resizing and merging |
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118 | of disjunctive intervals.</para> |
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60 | jermar | 119 | |
62 | jermar | 120 | <figure> |
73 | bondari | 121 | <mediaobject id="btree" xreflabel=""> |
122 | <imageobject role="html"> |
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123 | <imagedata fileref="images/btree.png" format="PNG" /> |
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124 | </imageobject> |
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62 | jermar | 125 | |
73 | bondari | 126 | <imageobject role="fop"> |
127 | <imagedata fileref="images.vector/btree.svg" format="SVG" /> |
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128 | </imageobject> |
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129 | </mediaobject> |
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130 | |||
131 | <title>B+tree containing keys ranging from 1 to 12.</title> |
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62 | jermar | 132 | </figure> |
59 | jermar | 133 | </section> |
134 | </chapter> |