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1 | <?xml version="1.0" encoding="UTF-8"?> |
1 | <?xml version="1.0" encoding="UTF-8"?> |
2 | <chapter id="ds"> |
2 | <chapter id="ds"> |
3 | <?dbhtml filename="ds.html"?> |
3 | <?dbhtml filename="ds.html"?> |
4 | 4 | ||
5 | <title>Data structures</title> |
5 | <title>Data structures</title> |
6 | 6 | ||
7 | <para>There is lots of data that either flows through various HelenOS |
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 |
8 | subsystems or is stored directly by them. Each subsystem uses its own data |
9 | structures to represent the data. These data structures need to be kept |
9 | structures to represent the data. These data structures need to be kept |
10 | somewhere. In order to work efficiently, HelenOS, and especially its kernel, |
10 | somewhere. In order to work efficiently, HelenOS, and especially its kernel, |
11 | deploys several house keeping data types that are designed to faciliate |
11 | deploys several house keeping data types that are designed to faciliate |
12 | managing other data structures. Most of them serve like generic |
12 | managing other data structures. Most of them serve like generic |
13 | containers.</para> |
13 | containers.</para> |
14 | 14 | ||
15 | <section> |
15 | <section> |
16 | <title>Lists</title> |
16 | <title>Lists</title> |
17 | 17 | ||
18 | <indexterm> |
18 | <indexterm> |
19 | <primary>linked list</primary> |
19 | <primary>linked list</primary> |
20 | </indexterm> |
20 | </indexterm> |
21 | 21 | ||
22 | <para>HelenOS uses doubly-circularly-linked lists to bind related data |
22 | <para>HelenOS uses doubly-circularly-linked lists to bind related data |
23 | together. Lists are composed of an independent sentinel node called head |
23 | together. Lists are composed of an independent sentinel node called head |
24 | and links that are always part of the object that is to be put into the |
24 | and links that are always part of the object that is to be put into the |
25 | list. Adding items to a list thus doesn't require any further memory |
25 | list. Adding items to a list thus doesn't require any further memory |
26 | allocations. Head and each link then contains forward and backward |
26 | allocations. Head and each link then contains forward and backward |
27 | pointer. An empty list is composed of a sole head whose both pointers |
27 | pointer. An empty list is composed of a sole head whose both pointers |
28 | reference the head itself. The expense of two times bigger memory |
28 | reference the head itself. The expense of two times bigger memory |
29 | consumption as compared to memory consumption of singly linked lists is |
29 | consumption as compared to memory consumption of singly linked lists is |
30 | justified by constant insertion and removal times at random positions |
30 | justified by constant insertion and removal times at random positions |
31 | within the list.</para> |
31 | within the list.</para> |
32 | 32 | ||
33 | <para>Lists are frequently used to implement FIFO behaviour (e.g. |
33 | <para>Lists are frequently used to implement FIFO behaviour (e.g. |
34 | scheduler run queues or synchronization wait queues). Contrary to the FIFO |
34 | scheduler run queues or synchronization wait queues). Contrary to the FIFO |
35 | type, which is also supported by HelenOS, they don't take up any unused |
35 | type, which is also supported by HelenOS, they don't take up any unused |
36 | space and are more general. On the other hand, they are slower than |
36 | space and are more general. On the other hand, they are slower than |
37 | in-array FIFOs and can be hardly used to implement buffers.</para> |
37 | in-array FIFOs and can be hardly used to implement buffers.</para> |
38 | </section> |
38 | </section> |
39 | 39 | ||
40 | <section> |
40 | <section> |
41 | <title>FIFO queues</title> |
41 | <title>FIFO queues</title> |
42 | 42 | ||
43 | <indexterm> |
43 | <indexterm> |
44 | <primary>FIFO queue</primary> |
44 | <primary>FIFO queue</primary> |
45 | </indexterm> |
45 | </indexterm> |
46 | 46 | ||
47 | <para>FIFO queues are implemented as either statically or dynamically |
47 | <para>FIFO queues are implemented as either statically or dynamically |
48 | allocated arrays<footnote> |
48 | allocated arrays<footnote> |
49 | <para>Depending on the array size.</para> |
49 | <para>Depending on the array size.</para> |
50 | </footnote> of some generic type with two indices. The first index |
50 | </footnote> of some generic type with two indices. The first index |
51 | points to the head of the FIFO queue and the other points to the tail |
51 | points to the head of the FIFO queue and the other points to the tail |
52 | thereof. There can be as many items in the FIFO as is the number of |
52 | thereof. There can be as many items in the FIFO as is the number of |
53 | elements in the array and no more. The indices are taken modulo size of |
53 | elements in the array and no more. The indices are taken modulo size of |
54 | the queue because as a consequence of insertions and deletions, the tail |
54 | the queue because as a consequence of insertions and deletions, the tail |
55 | can have numericaly lower index than the head.</para> |
55 | can have numericaly lower index than the head.</para> |
56 | 56 | ||
57 | <para>FIFO queues are used, for example, in ASID management code to store |
57 | <para>FIFO queues are used, for example, in ASID management code to store |
58 | inactive ASIDs or in userspace keyboard driver to buffer read |
58 | inactive ASIDs or in userspace keyboard driver to buffer read |
59 | characters.</para> |
59 | characters.</para> |
60 | 60 | ||
61 | <figure> |
61 | <figure> |
62 | <mediaobject id="fifo" xreflabel=""> |
62 | <mediaobject id="fifo" xreflabel=""> |
63 | <imageobject role="eps"> |
63 | <imageobject role="pdf"> |
64 | <imagedata fileref="images.vector/fifo.eps" format="EPS" /> |
64 | <imagedata fileref="images.vector/fifo.pdf" format="PDF" /> |
65 | </imageobject> |
65 | </imageobject> |
66 | 66 | ||
67 | <imageobject role="html"> |
67 | <imageobject role="html"> |
68 | <imagedata fileref="images/fifo.png" format="PNG" /> |
68 | <imagedata fileref="images/fifo.png" format="PNG" /> |
69 | </imageobject> |
69 | </imageobject> |
70 | 70 | ||
71 | <imageobject role="fop"> |
71 | <imageobject role="fop"> |
72 | <imagedata fileref="images.vector/fifo.svg" format="SVG" /> |
72 | <imagedata fileref="images.vector/fifo.svg" format="SVG" /> |
73 | </imageobject> |
73 | </imageobject> |
74 | </mediaobject> |
74 | </mediaobject> |
75 | 75 | ||
76 | <title>FIFO queue showing the wrap around the end of the array.</title> |
76 | <title>FIFO queue showing the wrap around the end of the array.</title> |
77 | </figure> |
77 | </figure> |
78 | </section> |
78 | </section> |
79 | 79 | ||
80 | <section id="hashtables"> |
80 | <section id="hashtables"> |
81 | <title>Hash tables</title> |
81 | <title>Hash tables</title> |
82 | 82 | ||
83 | <indexterm> |
83 | <indexterm> |
84 | <primary>hash table</primary> |
84 | <primary>hash table</primary> |
85 | </indexterm> |
85 | </indexterm> |
86 | 86 | ||
87 | <para>The kernel, as well as userspace, provides hash table data type |
87 | <para>The kernel, as well as userspace, provides hash table data type |
88 | which uses separate chaining. The hash table type is very generic in that |
88 | which uses separate chaining. The hash table type is very generic in that |
89 | it forces the user to supply methods for computing the hash index, |
89 | it forces the user to supply methods for computing the hash index, |
90 | comparing items against a set of keys and the item removal callback |
90 | comparing items against a set of keys and the item removal callback |
91 | function. Besides these virtual operations, the hash table is composed of |
91 | function. Besides these virtual operations, the hash table is composed of |
92 | a dynamically allocated array of list heads that represent each chain, |
92 | a dynamically allocated array of list heads that represent each chain, |
93 | number of chains and the maximal number of keys.</para> |
93 | number of chains and the maximal number of keys.</para> |
94 | </section> |
94 | </section> |
95 | 95 | ||
96 | <section> |
96 | <section> |
97 | <title>Bitmaps</title> |
97 | <title>Bitmaps</title> |
98 | 98 | ||
99 | <indexterm> |
99 | <indexterm> |
100 | <primary>bitmap</primary> |
100 | <primary>bitmap</primary> |
101 | </indexterm> |
101 | </indexterm> |
102 | 102 | ||
103 | <para>Several bitmap operations such as clearing or setting consecutive |
103 | <para>Several bitmap operations such as clearing or setting consecutive |
104 | bit sequences as well as copying portions of one bitmap into another one |
104 | bit sequences as well as copying portions of one bitmap into another one |
105 | are supported.</para> |
105 | are supported.</para> |
106 | </section> |
106 | </section> |
107 | 107 | ||
108 | <section> |
108 | <section> |
109 | <title>B+trees</title> |
109 | <title>B+trees</title> |
110 | 110 | ||
111 | <indexterm> |
111 | <indexterm> |
112 | <primary>B-tree</primary> |
112 | <primary>B-tree</primary> |
113 | </indexterm> |
113 | </indexterm> |
114 | 114 | ||
115 | <para>HelenOS makes use of a variant of B-tree called B+tree. B+trees, in |
115 | <para>HelenOS makes use of a variant of B-tree called B+tree. B+trees, in |
116 | HelenOS implementation, are 3-4-5 balanced trees. They are characteristic |
116 | HelenOS implementation, are 3-4-5 balanced trees. They are characteristic |
117 | by the fact that values are kept only in the leaf-level nodes and that |
117 | by the fact that values are kept only in the leaf-level nodes and that |
118 | these nodes are linked together in a list. This data structure has |
118 | these nodes are linked together in a list. This data structure has |
119 | logaritmic search, insertion and deletion times and, thanks to the |
119 | logaritmic search, insertion and deletion times and, thanks to the |
120 | leaf-level list, provides fantastic means of walking the nodes containing |
120 | leaf-level list, provides fantastic means of walking the nodes containing |
121 | data. Moreover, B+trees can be used for easy storing, resizing and merging |
121 | data. Moreover, B+trees can be used for easy storing, resizing and merging |
122 | of disjunctive intervals.</para> |
122 | of disjunctive intervals.</para> |
123 | 123 | ||
124 | <figure> |
124 | <figure> |
125 | <mediaobject id="btree" xreflabel=""> |
125 | <mediaobject id="btree" xreflabel=""> |
126 | <imageobject role="eps"> |
126 | <imageobject role="pdf"> |
127 | <imagedata fileref="images.vector/btree.eps" format="EPS" /> |
127 | <imagedata fileref="images.vector/btree.pdf" format="PDF" /> |
128 | </imageobject> |
128 | </imageobject> |
129 | 129 | ||
130 | <imageobject role="html"> |
130 | <imageobject role="html"> |
131 | <imagedata fileref="images/btree.png" format="PNG" /> |
131 | <imagedata fileref="images/btree.png" format="PNG" /> |
132 | </imageobject> |
132 | </imageobject> |
133 | 133 | ||
134 | <imageobject role="fop"> |
134 | <imageobject role="fop"> |
135 | <imagedata fileref="images.vector/btree.svg" format="SVG" /> |
135 | <imagedata fileref="images.vector/btree.svg" format="SVG" /> |
136 | </imageobject> |
136 | </imageobject> |
137 | </mediaobject> |
137 | </mediaobject> |
138 | 138 | ||
139 | <title>B+tree containing keys ranging from 1 to 12.</title> |
139 | <title>B+tree containing keys ranging from 1 to 12.</title> |
140 | </figure> |
140 | </figure> |
141 | </section> |
141 | </section> |
142 | </chapter> |
142 | </chapter> |