From 1cb307ea85c53b1329c3d43edc375c4a08a437f7 Mon Sep 17 00:00:00 2001 From: cposture Date: Sun, 28 Feb 2016 11:30:55 +0800 Subject: [PATCH] Translated Data Structures in the Linux Kernel --- ...123 Data Structures in the Linux Kernel.md | 202 ------------------ 1 file changed, 202 deletions(-) delete mode 100644 sources/tech/20151123 Data Structures in the Linux Kernel.md diff --git a/sources/tech/20151123 Data Structures in the Linux Kernel.md b/sources/tech/20151123 Data Structures in the Linux Kernel.md deleted file mode 100644 index edc4c1a5a7..0000000000 --- a/sources/tech/20151123 Data Structures in the Linux Kernel.md +++ /dev/null @@ -1,202 +0,0 @@ -【Translating By cposture 2016-02-26】 -Data Structures in the Linux Kernel -================================================================================ - -Radix tree --------------------------------------------------------------------------------- - -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: - -* [include/linux/radix-tree.h](https://github.com/torvalds/linux/blob/master/include/linux/radix-tree.h) -* [lib/radix-tree.c](https://github.com/torvalds/linux/blob/master/lib/radix-tree.c) - -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: - - -``` -               +-----------+ -               |           | -               |    " "    | - | | -        +------+-----------+------+ -        |                         | -        |                         | -   +----v------+            +-----v-----+ -   |           |            |           | -   |    g      |            |     c     | - | | | | -   +-----------+            +-----------+ -        |                         | -        |                         | -   +----v------+            +-----v-----+ -   |           |            |           | -   |    o      |            |     a     | - | | | | -   +-----------+            +-----------+ -                                  | -                                  | -                            +-----v-----+ -                            |           | -                            |     t     | - | | -                            +-----------+ -``` - -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. - -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): - -```C -struct radix_tree_root { - unsigned int height; - gfp_t gfp_mask; - struct radix_tree_node __rcu *rnode; -}; -``` - -This structure presents the root of a radix tree and contains three fields: - -* `height` - height of the tree; -* `gfp_mask` - tells how memory allocations will be performed; -* `rnode` - pointer to the child node. - -The first field we will discuss is `gfp_mask`: - -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. - -* `GFP_NOIO` - can sleep and wait for memory; -* `__GFP_HIGHMEM` - high memory can be used; -* `GFP_ATOMIC` - allocation process is high-priority and can't sleep; - -etc. - -The next field is `rnode`: - -```C -struct radix_tree_node { - unsigned int path; - unsigned int count; - union { - struct { - struct radix_tree_node *parent; - void *private_data; - }; - struct rcu_head rcu_head; - }; - /* For tree user */ - struct list_head private_list; - void __rcu *slots[RADIX_TREE_MAP_SIZE]; - unsigned long tags[RADIX_TREE_MAX_TAGS][RADIX_TREE_TAG_LONGS]; -}; -``` - -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: - -* `path` - offset in parent & height from the bottom; -* `count` - count of the child nodes; -* `parent` - pointer to the parent node; -* `private_data` - used by the user of a tree; -* `rcu_head` - used for freeing a node; -* `private_list` - used by the user of a tree; - -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. - -Now that we know about radix tree structure, it is time to look on its API. - -Linux kernel radix tree API ---------------------------------------------------------------------------------- - -We start from the datastructure initialization. There are two ways to initialize a new radix tree. The first is to use `RADIX_TREE` macro: - -```C -RADIX_TREE(name, gfp_mask); -```` - -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: - -```C -#define RADIX_TREE(name, mask) \ - struct radix_tree_root name = RADIX_TREE_INIT(mask) - -#define RADIX_TREE_INIT(mask) { \ - .height = 0, \ - .gfp_mask = (mask), \ - .rnode = NULL, \ -} -``` - -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. - -The second way is to define `radix_tree_root` structure by hand and pass it with mask to the `INIT_RADIX_TREE` macro: - -```C -struct radix_tree_root my_radix_tree; -INIT_RADIX_TREE(my_tree, gfp_mask_for_my_radix_tree); -``` - -where: - -```C -#define INIT_RADIX_TREE(root, mask) \ -do { \ - (root)->height = 0; \ - (root)->gfp_mask = (mask); \ - (root)->rnode = NULL; \ -} while (0) -``` - -makes the same initialziation with default values as it does `RADIX_TREE_INIT` macro. - -The next are two functions for inserting and deleting records to/from a radix tree: - -* `radix_tree_insert`; -* `radix_tree_delete`; - -The first `radix_tree_insert` function takes three parameters: - -* root of a radix tree; -* index key; -* data to insert; - -The `radix_tree_delete` function takes the same set of parameters as the `radix_tree_insert`, but without data. - -The search in a radix tree implemented in two ways: - -* `radix_tree_lookup`; -* `radix_tree_gang_lookup`; -* `radix_tree_lookup_slot`. - -The first `radix_tree_lookup` function takes two parameters: - -* root of a radix tree; -* index key; - -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 - -```C -unsigned int radix_tree_gang_lookup(struct radix_tree_root *root, - void **results, - unsigned long first_index, - unsigned int max_items); -``` - -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. - -And the last `radix_tree_lookup_slot` function will return the slot which will contain the data. - -Links ---------------------------------------------------------------------------------- - -* [Radix tree](http://en.wikipedia.org/wiki/Radix_tree) -* [Trie](http://en.wikipedia.org/wiki/Trie) - --------------------------------------------------------------------------------- - -via: https://github.com/0xAX/linux-insides/edit/master/DataStructures/radix-tree.md - -作者：[0xAX] -译者：[译者ID](https://github.com/译者ID) -校对：[校对者ID](https://github.com/校对者ID) - -本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创翻译，[Linux中国](http://linux.cn/) 荣誉推出 -