Lets talk about what is `radix tree`. Radix tree is a `compressed trie` where [trie](http://en.wikipedia.org/wiki/Trie) is a data structure which implements interface of an associative array and allows to store values as `key-value`. The keys are usually strings, but any other data type can be used as well. Trie is different from any `n-tree` in 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:
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:
```
@ -41,9 +41,9 @@ Lets talk about what is `radix tree`. Radix tree is a `compressed trie` where [t
+-----------+
```
So in this example, we can see the `trie` with keys, `go` and `cat`. The compressed trie or `radix tree` differs from `trie`, such that all intermediates nodes which have only one child are removed.
So in this example, we can see the `trie` with keys, `go` and `cat`. A compressed trie or `radix tree` differs from a `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 the integer key. 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):
Radix tree in linux kernel is the datastructure which maps values to the integer key. 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 {
@ -61,7 +61,7 @@ This structure presents the root of a radix tree and contains three fields:
The first structure 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) be sleep and wait for memory, (`__GFP_HIGHMEM` flag) is high memory can be used, (`GFP_ATOMIC` flag) is allocation process high-priority and can't sleep etc.
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.
The next structure is `rnode`:
@ -92,14 +92,14 @@ This structure contains information about the offset in a parent and height from
* `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 tree in the linux kernel also supports tags which are associated with the `tags` fields in the `radix_tree_node` structure. Tags allow to set individual bits on records which are stored in the radix 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 we know about radix tree structure, time to look on its API.
Now that we know about radix tree structure, it is time to look on its API.
We start from the datastructure intialization. There are two ways to initialize new radix tree. The first is to use `RADIX_TREE` macro:
We start from the datastructure intialization. There are two ways to initialize new radix tree. The first is to use `RADIX_TREE` macro:
```C
RADIX_TREE(name, gfp_mask);
@ -164,7 +164,7 @@ 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 returns associated record with this key. The second `radix_tree_gang_lookup` function have the following signature
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,
andars
9 years ago