tools: LRU cache to use for pxar FUSE implementation.
Implements a cache with least recently used cache replacement policy. Internally the state is tracked by a HashMap (for fast access) and a doubly linked list (for the access order). Signed-off-by: Christian Ebner <c.ebner@proxmox.com>
This commit is contained in:
Christian Ebner
Dietmar Maurer
parent
f1d99e3f6a
commit
1685c2e341
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@ -23,6 +23,7 @@ pub mod async_io;
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pub mod borrow;
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pub mod daemon;
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pub mod fs;
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pub mod lru_cache;
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pub mod runtime;
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pub mod ticket;
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pub mod timer;
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@ -0,0 +1,219 @@
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//! Least recently used (LRU) cache
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//!
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//! Implements a cache with least recently used cache replacement policy.
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//! A HashMap is used for fast access by a given key and a doubly linked list
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//! is used to keep track of the cache access order.
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use std::collections::HashMap;
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/// Node of the doubly linked list storing key and value
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struct CacheNode<V> {
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// We need to additionally store the key to be able to remove it
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// from the HashMap when removing the tail.
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key: u64,
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value: V,
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prev: *mut CacheNode<V>,
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next: *mut CacheNode<V>,
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}
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impl<V> CacheNode<V> {
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fn new(key: u64, value: V) -> Self {
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Self {
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key,
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value,
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prev: std::ptr::null_mut(),
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next: std::ptr::null_mut(),
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}
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}
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}
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/// LRU cache instance.
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///
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/// # Examples:
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/// ```
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/// # use self::proxmox_backup::tools::lru_cache::LruCache;
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/// # fn main() -> Result<(), failure::Error> {
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/// let mut cache = LruCache::new(3);
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///
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/// assert_eq!(cache.get_mut(1), None);
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/// assert_eq!(cache.len(), 0);
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///
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/// cache.insert(1, 1);
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/// cache.insert(2, 2);
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/// cache.insert(3, 3);
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/// cache.insert(4, 4);
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/// assert_eq!(cache.len(), 3);
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///
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/// assert_eq!(cache.get_mut(1), None);
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/// assert_eq!(cache.get_mut(2), Some(&mut 2));
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/// assert_eq!(cache.get_mut(3), Some(&mut 3));
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/// assert_eq!(cache.get_mut(4), Some(&mut 4));
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///
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/// cache.remove(4);
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/// cache.remove(3);
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/// cache.remove(2);
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/// assert_eq!(cache.len(), 0);
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/// assert_eq!(cache.get_mut(2), None);
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///
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/// cache.insert(1, 1);
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/// assert_eq!(cache.get_mut(1), Some(&mut 1));
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///
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/// cache.clear();
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/// assert_eq!(cache.len(), 0);
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/// assert_eq!(cache.get_mut(1), None);
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/// # Ok(())
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/// # }
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/// ```
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pub struct LruCache<V> {
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map: HashMap<u64, *mut CacheNode<V>>,
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head: *mut CacheNode<V>,
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tail: *mut CacheNode<V>,
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capacity: usize,
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}
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impl<V> LruCache<V> {
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/// Create LRU cache instance which holds up to `capacity` nodes at once.
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pub fn new(capacity: usize) -> Self {
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Self {
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map: HashMap::with_capacity(capacity),
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head: std::ptr::null_mut(),
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tail: std::ptr::null_mut(),
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capacity,
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}
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}
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/// Clear all the entries from the cache.
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pub fn clear(&mut self) {
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// Dump all heap allocations, then dump all the pointers in the HashMap
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for node_ptr in self.map.values() {
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unsafe { Box::from_raw(*node_ptr) };
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}
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self.map.clear();
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// Reset head and tail pointers
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self.head = std::ptr::null_mut();
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self.tail = std::ptr::null_mut();
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}
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/// Insert or update an entry identified by `key` with the given `value`.
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/// This entry is placed as the most recently used node at the head.
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pub fn insert(&mut self, key: u64, value: V) {
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match self.get_mut(key) {
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// Key already exists and get_mut brings node to the front, so only update its value.
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Some(old_val) => *old_val = value,
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None => {
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// If we have more elements than capacity, delete the tail entry
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// (= oldest entry).
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if self.map.len() >= self.capacity {
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self.remove_tail();
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}
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self.insert_front(key, value);
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}
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}
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}
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/// Insert a key, value pair at the front of the linked list and it's pointer
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/// into the HashMap.
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fn insert_front(&mut self, key: u64, value: V) {
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// First create heap allocated `CacheNode` containing value.
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let mut node = Box::new(CacheNode::new(key, value));
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// Old head gets new heads next
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node.next = self.head;
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// Release ownership of node, rest can be handled with just the pointer.
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let node_ptr = Box::into_raw(node);
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// Update the prev for the old head
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if !self.head.is_null() {
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unsafe { (*self.head).prev = node_ptr };
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}
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// Update the head to the new node pointer
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self.head = node_ptr;
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// If there was no old tail, this node will be the new tail too
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if self.tail.is_null() {
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self.tail = node_ptr;
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}
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// finally insert the node pointer into the HashMap
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self.map.insert(key, node_ptr);
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}
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/// Remove the given `key` and its `value` from the cache.
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pub fn remove(&mut self, key: u64) -> Option<V> {
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// Remove node pointer from the HashMap and get ownership of the node
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let node_ptr = self.map.remove(&key)?;
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let node = unsafe { Box::from_raw(node_ptr) };
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// Update the previous node or otherwise the head
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if !node.prev.is_null() {
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unsafe { (*node.prev).next = node.next };
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} else {
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// No previous node means this was the head
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self.head = node.next;
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}
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// Update the next node or otherwise the tail
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if !node.next.is_null() {
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unsafe { (*node.next).prev = node.prev };
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} else {
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// No next node means this was the tail
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self.tail = node.prev;
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}
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Some(node.value)
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}
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/// Remove the least recently used node from the cache.
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fn remove_tail(&mut self) {
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if self.tail.is_null() {
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panic!("Called remove_tail on empty tail pointer!");
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}
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let old_tail = unsafe { Box::from_raw(self.tail) };
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self.tail = old_tail.prev;
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// Update next node for new tail
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if !self.tail.is_null() {
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unsafe { (*self.tail).next = std::ptr::null_mut() };
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}
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// Remove HashMap entry for old tail
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self.map.remove(&old_tail.key);
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}
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/// Get a mutable reference to the value identified by `key`.
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/// This will update the cache entry to be the most recently used entry.
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pub fn get_mut<'a>(&'a mut self, key: u64) -> Option<&'a mut V> {
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let node_ptr = self.map.get(&key)?;
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if *node_ptr == self.head {
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// node is already head, just return
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return Some(unsafe { &mut (*self.head).value });
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}
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// Update the prev node to point to next (or null if current node is tail)
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let mut node = unsafe { Box::from_raw(*node_ptr) };
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unsafe { (*node.prev).next = node.next };
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// Update the next node or otherwise the tail
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if !node.next.is_null() {
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unsafe { (*node.next).prev = node.prev };
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} else {
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// No next node means this was the tail
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self.tail = node.prev;
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}
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node.prev = std::ptr::null_mut();
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node.next = self.head;
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// update the head and release ownership of the node again
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let node_ptr = Box::into_raw(node);
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// Update current head
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unsafe { (*self.head).prev = node_ptr };
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// Update to new head
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self.head = node_ptr;
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Some(unsafe { &mut (*self.head).value })
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}
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/// Number of entries in the cache.
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pub fn len(&self) -> usize {
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self.map.len()
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}
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}
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