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Translated Data Structures in the Linux Kernel
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【Translating By cposture 2016-02-26】
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Data Structures in the Linux Kernel
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================================================================================
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Radix tree
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--------------------------------------------------------------------------------
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As you already know linux kernel provides many different libraries and functions which implement different data structures and algorithms. In this part we will consider one of these data structures - [Radix tree](http://en.wikipedia.org/wiki/Radix_tree). There are two files which are related to `radix tree` implementation and API in the linux kernel:
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* [include/linux/radix-tree.h](https://github.com/torvalds/linux/blob/master/include/linux/radix-tree.h)
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* [lib/radix-tree.c](https://github.com/torvalds/linux/blob/master/lib/radix-tree.c)
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Lets talk about what a `radix tree` is. Radix tree is a `compressed trie` where a [trie](http://en.wikipedia.org/wiki/Trie) is a data structure which implements an interface of an associative array and allows to store values as `key-value`. The keys are usually strings, but any data type can be used. A trie is different from an `n-tree` because of its nodes. Nodes of a trie do not store keys; instead, a node of a trie stores single character labels. The key which is related to a given node is derived by traversing from the root of the tree to this node. For example:
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```
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+-----------+
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+------+-----------+------+
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+----v------+ +-----v-----+
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| g | | c |
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+-----------+ +-----------+
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+----v------+ +-----v-----+
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| o | | a |
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+-----------+ +-----------+
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+-----v-----+
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+-----------+
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```
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So in this example, we can see the `trie` with keys, `go` and `cat`. The compressed trie or `radix tree` differs from `trie` in that all intermediates nodes which have only one child are removed.
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Radix tree in linux kernel is the datastructure which maps values to integer keys. It is represented by the following structures from the file [include/linux/radix-tree.h](https://github.com/torvalds/linux/blob/master/include/linux/radix-tree.h):
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```C
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struct radix_tree_root {
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unsigned int height;
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gfp_t gfp_mask;
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struct radix_tree_node __rcu *rnode;
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};
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```
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This structure presents the root of a radix tree and contains three fields:
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* `height` - height of the tree;
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* `gfp_mask` - tells how memory allocations will be performed;
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* `rnode` - pointer to the child node.
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The first field we will discuss is `gfp_mask`:
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Low-level kernel memory allocation functions take a set of flags as - `gfp_mask`, which describes how that allocation is to be performed. These `GFP_` flags which control the allocation process can have following values: (`GF_NOIO` flag) means sleep and wait for memory, (`__GFP_HIGHMEM` flag) means high memory can be used, (`GFP_ATOMIC` flag) means the allocation process has high-priority and can't sleep etc.
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* `GFP_NOIO` - can sleep and wait for memory;
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* `__GFP_HIGHMEM` - high memory can be used;
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* `GFP_ATOMIC` - allocation process is high-priority and can't sleep;
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etc.
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The next field is `rnode`:
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```C
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struct radix_tree_node {
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unsigned int path;
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unsigned int count;
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union {
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struct {
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struct radix_tree_node *parent;
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void *private_data;
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};
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struct rcu_head rcu_head;
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};
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/* For tree user */
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struct list_head private_list;
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void __rcu *slots[RADIX_TREE_MAP_SIZE];
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unsigned long tags[RADIX_TREE_MAX_TAGS][RADIX_TREE_TAG_LONGS];
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};
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```
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This structure contains information about the offset in a parent and height from the bottom, count of the child nodes and fields for accessing and freeing a node. This fields are described below:
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* `path` - offset in parent & height from the bottom;
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* `count` - count of the child nodes;
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* `parent` - pointer to the parent node;
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* `private_data` - used by the user of a tree;
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* `rcu_head` - used for freeing a node;
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* `private_list` - used by the user of a tree;
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The two last fields of the `radix_tree_node` - `tags` and `slots` are important and interesting. Every node can contains a set of slots which are store pointers to the data. Empty slots in the linux kernel radix tree implementation store `NULL`. Radix trees in the linux kernel also supports tags which are associated with the `tags` fields in the `radix_tree_node` structure. Tags allow individual bits to be set on records which are stored in the radix tree.
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Now that we know about radix tree structure, it is time to look on its API.
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Linux kernel radix tree API
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---------------------------------------------------------------------------------
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We start from the datastructure initialization. There are two ways to initialize a new radix tree. The first is to use `RADIX_TREE` macro:
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```C
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RADIX_TREE(name, gfp_mask);
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````
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As you can see we pass the `name` parameter, so with the `RADIX_TREE` macro we can define and initialize radix tree with the given name. Implementation of the `RADIX_TREE` is easy:
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```C
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#define RADIX_TREE(name, mask) \
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struct radix_tree_root name = RADIX_TREE_INIT(mask)
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#define RADIX_TREE_INIT(mask) { \
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.height = 0, \
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.gfp_mask = (mask), \
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.rnode = NULL, \
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}
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```
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At the beginning of the `RADIX_TREE` macro we define instance of the `radix_tree_root` structure with the given name and call `RADIX_TREE_INIT` macro with the given mask. The `RADIX_TREE_INIT` macro just initializes `radix_tree_root` structure with the default values and the given mask.
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The second way is to define `radix_tree_root` structure by hand and pass it with mask to the `INIT_RADIX_TREE` macro:
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```C
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struct radix_tree_root my_radix_tree;
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INIT_RADIX_TREE(my_tree, gfp_mask_for_my_radix_tree);
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```
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where:
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```C
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#define INIT_RADIX_TREE(root, mask) \
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do { \
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(root)->height = 0; \
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(root)->gfp_mask = (mask); \
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(root)->rnode = NULL; \
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} while (0)
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```
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makes the same initialziation with default values as it does `RADIX_TREE_INIT` macro.
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The next are two functions for inserting and deleting records to/from a radix tree:
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* `radix_tree_insert`;
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* `radix_tree_delete`;
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The first `radix_tree_insert` function takes three parameters:
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* root of a radix tree;
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* index key;
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* data to insert;
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The `radix_tree_delete` function takes the same set of parameters as the `radix_tree_insert`, but without data.
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The search in a radix tree implemented in two ways:
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* `radix_tree_lookup`;
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* `radix_tree_gang_lookup`;
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* `radix_tree_lookup_slot`.
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The first `radix_tree_lookup` function takes two parameters:
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* root of a radix tree;
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* index key;
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This function tries to find the given key in the tree and return the record associated with this key. The second `radix_tree_gang_lookup` function have the following signature
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```C
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unsigned int radix_tree_gang_lookup(struct radix_tree_root *root,
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void **results,
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unsigned long first_index,
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unsigned int max_items);
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```
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and returns number of records, sorted by the keys, starting from the first index. Number of the returned records will not be greater than `max_items` value.
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And the last `radix_tree_lookup_slot` function will return the slot which will contain the data.
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Links
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---------------------------------------------------------------------------------
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* [Radix tree](http://en.wikipedia.org/wiki/Radix_tree)
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* [Trie](http://en.wikipedia.org/wiki/Trie)
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--------------------------------------------------------------------------------
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via: https://github.com/0xAX/linux-insides/edit/master/DataStructures/radix-tree.md
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作者：[0xAX]
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译者：[译者ID](https://github.com/译者ID)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创翻译，[Linux中国](http://linux.cn/) 荣誉推出
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198
translated/tech/20151123 Data Structures in the Linux Kernel.md
Normal file
198
translated/tech/20151123 Data Structures in the Linux Kernel.md
Normal file
@ -0,0 +1,198 @@
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Linux内核数据结构
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================================================================================
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基数树 Radix tree
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--------------------------------------------------------------------------------
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正如你所知道的，Linux内核提供了许多不同的库和函数，它们实现了不同的数据结构和算法。在这部分，我们将研究其中一种数据结构——[基数树 Radix tree](http://en.wikipedia.org/wiki/Radix_tree)。在Linux内核中，有两个与基数树实现和API相关的文件：
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* [include/linux/radix-tree.h](https://github.com/torvalds/linux/blob/master/include/linux/radix-tree.h)
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* [lib/radix-tree.c](https://github.com/torvalds/linux/blob/master/lib/radix-tree.c)
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让我们讨论什么是`基数树`吧。基数树是一种`压缩的字典树`，而[字典树](http://en.wikipedia.org/wiki/Trie)是实现了关联数组接口并允许以`键值对`方式存储值的一种数据结构。该键通常是字符串，但能够使用任何数据类型。字典树因为它的节点而与`n叉树`不同。字典树的节点不存储键；相反，字典树的一个节点存储单个字符的标签。与一个给定节点关联的键可以通过从根遍历到该节点获得。举个例子：
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```
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+-----------+
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+------+-----------+------+
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+----v------+ +-----v-----+
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+-----------+ +-----------+
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+----v------+ +-----v-----+
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+-----------+ +-----------+
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+-----v-----+
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+-----------+
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```
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因此在这个例子中，我们可以看到一个有着两个键`go`和`cat`的`字典树`。压缩的字典树或者`基数树`和`字典树`不同于所有只有一个孩子的中间节点都被删除。
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Linu内核中的基数树是映射值到整形键的一种数据结构。[include/linux/radix-tree.h](https://github.com/torvalds/linux/blob/master/include/linux/radix-tree.h)文件中的以下结构体表示了基数树：
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```C
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struct radix_tree_root {
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unsigned int height;
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gfp_t gfp_mask;
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struct radix_tree_node __rcu *rnode;
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};
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```
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这个结构体表示了一个基数树的根，并包含了3个域成员：
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* `height` - 树的高度;
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* `gfp_mask` - 告诉如何执行动态内存分配;
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* `rnode` - 孩子节点指针.
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我们第一个要讨论的域是`gfp_mask`：
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底层内核内存动态分配函数以一组标志作为` gfp_mask `，用于描述如何执行动态内存分配。这些控制分配进程的`GFP_`标志拥有以下值：(`GF_NOIO`标志)意味着睡眠等待内存，(`__GFP_HIGHMEM`标志)意味着高端内存能够被使用，(`GFP_ATOMIC`标志)意味着分配进程拥有高优先级并不能睡眠等等。
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* `GFP_NOIO` - 睡眠等待内存
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* `__GFP_HIGHMEM` - 高端内存能够被使用;
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* `GFP_ATOMIC` - 分配进程拥有高优先级并且不能睡眠;
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等等。
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下一个域是`rnode`：
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```C
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struct radix_tree_node {
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unsigned int path;
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unsigned int count;
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union {
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struct {
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struct radix_tree_node *parent;
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void *private_data;
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};
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struct rcu_head rcu_head;
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};
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/* For tree user */
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struct list_head private_list;
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void __rcu *slots[RADIX_TREE_MAP_SIZE];
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unsigned long tags[RADIX_TREE_MAX_TAGS][RADIX_TREE_TAG_LONGS];
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};
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```
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这个结构体包含的信息有父节点中的偏移以及到底端(叶节点)的高度、孩子节点的个数以及用于访问和释放节点的域成员。这些域成员描述如下：
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* `path` - 父节点中的偏移和到底端(叶节点)的高度
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* `count` - 孩子节点的个数;
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* `parent` - 父节点指针;
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* `private_data` - 由树的用户使用;
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* `rcu_head` - 用于释放节点;
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* `private_list` - 由树的用户使用;
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`radix_tree_node`的最后两个成员——`tags`和`slots`非常重要且令人关注。Linux内核基数树的每个节点都包含一组存储指向数据指针的slots。Linux内核基数树实现的空slots存储`NULL`值。Linux内核中的基数树也支持与`radix_tree_node`结构体的`tags`域相关联的标签。标签允许在基数树存储的记录中设置各个位。
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既然我们了解了基数树的结构，那么该是时候看一下它的API了。
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Linux内核基数树API
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---------------------------------------------------------------------------------
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我们从结构体的初始化开始。有两种方法初始化一个新的基数树。第一种是使用`RADIX_TREE`宏：
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```C
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RADIX_TREE(name, gfp_mask);
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````
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正如你所看到的，我们传递`name`参数，所以使用`RADIX_TREE`宏，我们能够定义和初始化基数树为给定的名字。`RADIX_TREE`的实现是简单的：
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```C
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#define RADIX_TREE(name, mask) \
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struct radix_tree_root name = RADIX_TREE_INIT(mask)
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#define RADIX_TREE_INIT(mask) { \
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.height = 0, \
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.gfp_mask = (mask), \
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.rnode = NULL, \
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}
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```
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在`RADIX_TREE`宏的开始，我们使用给定的名字定义`radix_tree_root`结构体实例，并使用给定的mask调用`RADIX_TREE_INIT`宏。`RADIX_TREE_INIT`宏只是初始化`radix_tree_root`结构体为默认值和给定的mask而已。
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第二种方法是亲手定义`radix_tree_root`结构体，并且将它和mask传给`INIT_RADIX_TREE`宏：
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```C
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struct radix_tree_root my_radix_tree;
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INIT_RADIX_TREE(my_tree, gfp_mask_for_my_radix_tree);
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```
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where:
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```C
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#define INIT_RADIX_TREE(root, mask) \
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do { \
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(root)->height = 0; \
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(root)->gfp_mask = (mask); \
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(root)->rnode = NULL; \
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} while (0)
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```
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和`RADIX_TREE_INIT`宏所做的初始化一样，初始化为默认值。
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接下来是用于从基数树插入和删除数据的两个函数：
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* `radix_tree_insert`;
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* `radix_tree_delete`;
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第一个函数`radix_tree_insert`需要3个参数：
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* 基数树的根;
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* 索引键;
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* 插入的数据;
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`radix_tree_delete`函数需要和`radix_tree_insert`一样的一组参数，但是没有data。
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基数树的搜索以两种方法实现：
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* `radix_tree_lookup`;
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* `radix_tree_gang_lookup`;
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* `radix_tree_lookup_slot`.
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第一个函数`radix_tree_lookup`需要两个参数：
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* 基数树的根;
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* 索引键;
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这个函数尝试在树中查找给定的键，并返回和该键相关联的记录。第二个函数`radix_tree_gang_lookup`有以下的函数签名：
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```C
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unsigned int radix_tree_gang_lookup(struct radix_tree_root *root,
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void **results,
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unsigned long first_index,
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unsigned int max_items);
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```
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和返回记录的个数，(results指向的数据)按键排序并从第一个索引开始。返回的记录个数将不会超过`max_items`。
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最后一个函数`radix_tree_lookup_slot`将会返回包含数据的slot。
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链接
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---------------------------------------------------------------------------------
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||||
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||||
* [Radix tree](http://en.wikipedia.org/wiki/Radix_tree)
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||||
* [Trie](http://en.wikipedia.org/wiki/Trie)
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||||
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||||
--------------------------------------------------------------------------------
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via: https://github.com/0xAX/linux-insides/edit/master/DataStructures/radix-tree.md
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作者：[0xAX]
|
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译者：[cposture](https://github.com/cposture)
|
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校对：[校对者ID](https://github.com/校对者ID)
|
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|
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创翻译，[Linux中国](http://linux.cn/) 荣誉推出
|
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|
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