levelDBソース分析-Skiplist
ここでは主にlevelDBにおけるSkipListの実現について紹介します.SkipListの紹介については「SkipList」を参照してください.ここでは引用しません.
levelDBでの使用:
levelDBのMemtableにはコアのデータ構造Skiplistがあり、具体的に実現されるコードは少し違っていますが、基本原理は一致しています. levelDBのSkiplistはテンプレートクラスと定義されています.
コードプログラミングの技術:内部クラスまたは友達クラスの定義を使用することができます.
levelDBでの使用:
levelDBのMemtableにはコアのデータ構造Skiplistがあり、具体的に実現されるコードは少し違っていますが、基本原理は一致しています. levelDBのSkiplistはテンプレートクラスと定義されています.
// description:
// Thread safety
// Writes require external synchronization, most likely a mutex.
// Reads require a guarantee that the SkipList will not be destroyed while the read is in progress. Apart from that, reads progress without any internal locking or synchronization.
//
// Invariants:
// (1) Allocated nodes are never deleted until the SkipList is destroyed. This is trivially guaranteed by the code since we never delete any skip list nodes.
// (2) The contents of a Node except for the next/prev pointers are immutable after the Node has been linked into the SkipList. Only Insert() modifies the list, and it is careful to initialize a node and use release-stores to publish the nodes in one or more lists.
template<typename Key, class Comparator>
class SkipList {
private:
struct Node;
public:
// Create a new SkipList object that will use "cmp" for comparing keys, and will allocate memory using "*arena". Objects allocated in the arena must remain allocated for the lifetime of the skiplist object.
explicit SkipList(Comparator cmp, Arena* arena);
// Insert key into the list.
// REQUIRES: nothing that compares equal to key is currently in the list.
void Insert(const Key& key); // key Skiplist
// Returns true iff an entry that compares equal to key is in the list.
bool Contains(const Key& key) const; // Skiplist key
private:
enum { kMaxHeight = 12 }; // level
// Immutable after construction
Comparator const compare_; // key ， （ ， key， ）
Arena* const arena_; // Arena used for allocations of nodes // levelDB Arena
Node* const head_; // Skiplist
// Modified only by Insert(). Read racily by readers, but stale
// values are ok.
port::AtomicPointer max_height_; // Height of the entire list // Skiplist
inline int GetMaxHeight() const { // Skiplist
return reinterpret_cast<intptr_t>(max_height_.NoBarrier_Load());
}
// Read/written only by Insert().
Random rnd_; // ， level
Node* NewNode(const Key& key, int height); // level=height， key
int RandomHeight(); // level
bool Equal(const Key& a, const Key& b) const { return (compare_(a, b) == 0); } // 2 key
// Return true if key is greater than the data stored in "n"
bool KeyIsAfterNode(const Key& key, Node* n) const; // key Node n key， key
// Return the earliest node that comes at or after key.
// Return NULL if there is no such node.
// If prev is non-NULL, fills prev[level] with pointer to previous
// node at "level" for every level in [0..max_height_-1].
Node* FindGreaterOrEqual(const Key& key, Node** prev) const; // key Node key Node
// Return the latest node with a key < key, return head_ if there is no such node.
Node* FindLessThan(const Key& key) const; // key Node
// Return the last node in the list.
// Return head_ if list is empty.
Node* FindLast() const; // Skiplist Node
// No copying allowed
SkipList(const SkipList&); //
void operator=(const SkipList&);
};
// Implementation details follow
template<typename Key, class Comparator>
struct SkipList<Key,Comparator>::Node { // Skiplist Node
explicit Node(const Key& k) : key(k) { }
Key const key;
// Accessors/mutators for links. Wrapped in methods so we can add the appropriate barriers as necessary.
Node* Next(int n) {
assert(n >= 0);
// Use an 'acquire load' so that we observe a fully initialized version of the returned Node.
return reinterpret_cast<Node*>(next_[n].Acquire_Load());
}
void SetNext(int n, Node* x) {
assert(n >= 0);
// Use a 'release store' so that anybody who reads through this pointer observes a fully initialized version of the inserted node.
next_[n].Release_Store(x);
}
// No-barrier variants that can be safely used in a few locations.
Node* NoBarrier_Next(int n) {
assert(n >= 0);
return reinterpret_cast<Node*>(next_[n].NoBarrier_Load());
}
void NoBarrier_SetNext(int n, Node* x) {
assert(n >= 0);
next_[n].NoBarrier_Store(x);
}
private:
// Array of length equal to the node height. next_[0] is lowest level link.
port::AtomicPointer next_[1]; // forward
};
template<typename Key, class Comparator>
typename SkipList<Key,Comparator>::Node* SkipList<Key,Comparator>::NewNode(const Key& key, int height) { // Node （ key level ）
char* mem = arena_->AllocateAligned(sizeof(Node) + sizeof(port::AtomicPointer) * (height - 1));
return new (mem) Node(key); // new
}
template<typename Key, class Comparator>
SkipList<Key,Comparator>::SkipList(Comparator cmp, Arena* arena) //
: compare_(cmp),
arena_(arena),
head_(NewNode(0 /* any key will do */, kMaxHeight)), // key
max_height_(reinterpret_cast<void*>(1)),
rnd_(0xdeadbeef) {
for (int i = 0; i < kMaxHeight; i++) {
head_->SetNext(i, NULL); //
}
}
template<typename Key, class Comparator>
int SkipList<Key,Comparator>::RandomHeight() { // (Skiplist )
// Increase height with probability 1 in kBranching
static const unsigned int kBranching = 4;
int height = 1;
while (height < kMaxHeight && ((rnd_.Next() % kBranching) == 0)) { //? ？ ？
height++;
}
assert(height > 0);
assert(height <= kMaxHeight);
return height;
}
template<typename Key, class Comparator>
bool SkipList<Key,Comparator>::KeyIsAfterNode(const Key& key, Node* n) const { // Return true if key is greater than the key stored in "n"
// NULL n is considered infinite，NULL （ NIL）
return (n != NULL) && (compare_(n->key, key) < 0);
}
template<typename Key, class Comparator>
typename SkipList<Key,Comparator>::Node* SkipList<Key,Comparator>::FindGreaterOrEqual(const Key& key, Node** prev) const { //
Node* x = head_;
int level = GetMaxHeight() - 1;
while (true) {
Node* next = x->Next(level);
if (KeyIsAfterNode(key, next)) { // key next ， true， next NULL
// Keep searching in this list
x = next;
} else {
if (prev != NULL) prev[level] = x; // level ，x >=key ， ，
if (level == 0) {
return next;
} else {
// Switch to next list( low level link list)
level--;
}
}
}
}
template<typename Key, class Comparator>
typename SkipList<Key,Comparator>::Node* SkipList<Key,Comparator>::FindLessThan(const Key& key) const { // Return the latest node with a key < key, return head_ if there is no such node.
Node* x = head_;
int level = GetMaxHeight() - 1;
while (true) {
assert(x == head_ || compare_(x->key, key) < 0); //
Node* next = x->Next(level);
if (next == NULL || compare_(next->key, key) >= 0) { // level
// key> key ， next ，level--， level=0
if (level == 0) {
return x;
} else {
// Switch to next list
level--; //
}
} else {
x = next;
}
}
}
template<typename Key, class Comparator>
typename SkipList<Key,Comparator>::Node* SkipList<Key,Comparator>::FindLast() const { // level ， level ， level=0
Node* x = head_;
int level = GetMaxHeight() - 1;
while (true) {
Node* next = x->Next(level);
if (next == NULL) {
if (level == 0) {
return x;
} else {
// Switch to next list
level--;
}
} else {
x = next;
}
}
}
template<typename Key, class Comparator>
void SkipList<Key,Comparator>::Insert(const Key& key) { // key
// TODO(opt): We can use a barrier-free variant of FindGreaterOrEqual()
// here since Insert() is externally synchronized.
Node* prev[kMaxHeight];
Node* x = FindGreaterOrEqual(key, prev); // prev level
// Our data structure does not allow duplicate insertion
assert(x == NULL || !Equal(key, x->key));
int height = RandomHeight();
if (height > GetMaxHeight()) {
for (int i = GetMaxHeight(); i < height; i++) {
prev[i] = head_;
}
//fprintf(stderr, "Change height from %d to %d
", max_height_, height);
// It is ok to mutate max_height_ without any synchronization
// with concurrent readers. A concurrent reader that observes
// the new value of max_height_ will see either the old value of
// new level pointers from head_ (NULL), or a new value set in
// the loop below. In the former case the reader will
// immediately drop to the next level since NULL sorts after all
// keys. In the latter case the reader will use the new node.
max_height_.NoBarrier_Store(reinterpret_cast<void*>(height));
}
x = NewNode(key, height); // Node
for (int i = 0; i < height; i++) { // level ， level
// NoBarrier_SetNext() suffices since we will add a barrier when we publish a pointer to "x" in prev[i].
x->NoBarrier_SetNext(i, prev[i]->NoBarrier_Next(i));
prev[i]->SetNext(i, x);
}
}
template<typename Key, class Comparator>
bool SkipList<Key,Comparator>::Contains(const Key& key) const { // Skiplist key
Node* x = FindGreaterOrEqual(key, NULL); // key
if (x != NULL && Equal(key, x->key)) { // ， ，
return true;
} else {
return false;
}
}
// Iteration over the contents of a skiplist
template<typename Key, class Comparator>
class SkipList<Key,Comparator>::Iterator { // Skiplist
public:
// Initialize an iterator over the specified list.
// The returned iterator is not valid.
explicit Iterator(const SkipList* list);
// Returns true iff the iterator is positioned at a valid node.
bool Valid() const;
// Returns the key at the current position.
// REQUIRES: Valid()
const Key& key() const;
// Advances to the next position.
// REQUIRES: Valid()
void Next();
// Advances to the previous position.
// REQUIRES: Valid()
void Prev();
// Advance to the first entry with a key >= target
void Seek(const Key& target);
// Position at the first entry in list.
// Final state of iterator is Valid() iff list is not empty.
void SeekToFirst();
// Position at the last entry in list.
// Final state of iterator is Valid() iff list is not empty.
void SeekToLast();
private:
const SkipList* list_;
Node* node_;
// Intentionally copyable copy ，
};
template<typename Key, class Comparator>
inline SkipList<Key,Comparator>::Iterator::Iterator(const SkipList* list) { // ， iterator
list_ = list;
node_ = NULL;
}
template<typename Key, class Comparator>
inline bool SkipList<Key,Comparator>::Iterator::Valid() const { // Returns true iff the iterator is positioned at a valid node.
return node_ != NULL;
}
template<typename Key, class Comparator>
inline const Key& SkipList<Key,Comparator>::Iterator::key() const { // Returns the key at the current position.
assert(Valid());
return node_->key;
}
template<typename Key, class Comparator>
inline void SkipList<Key,Comparator>::Iterator::Next() { // Advances to the next position.
assert(Valid());
node_ = node_->Next(0); // level 0
}
template<typename Key, class Comparator>
inline void SkipList<Key,Comparator>::Iterator::Prev() { // Advances to the previous position.
// Instead of using explicit "prev" links, we just search for the
// last node that falls before key.
assert(Valid());
node_ = list_->FindLessThan(node_->key); // ， head_， NULL
if (node_ == list_->head_) {
node_ = NULL;
}
}
template<typename Key, class Comparator>
inline void SkipList<Key,Comparator>::Iterator::Seek(const Key& target) { // Advance to the first entry with a key >= target
node_ = list_->FindGreaterOrEqual(target, NULL);
}
//
template<typename Key, class Comparator>
inline void SkipList<Key,Comparator>::Iterator::SeekToFirst() { // Position at the first entry in list.
node_ = list_->head_->Next(0);
}
template<typename Key, class Comparator>
inline void SkipList<Key,Comparator>::Iterator::SeekToLast() { // Position at the last entry in list.
node_ = list_->FindLast(); // ， ， null
if (node_ == list_->head_) {
node_ = NULL;
}
}コードプログラミングの技術:内部クラスまたは友達クラスの定義を使用することができます.