031dfede90
* pkg/cache: fix race in Add() and Evict() This fixes a race in Add() when the shard is at max capacity and the key being added is already stored. Previously, the shard would evict a random value - when all it needed to do was replace an existing value. There was a race in how Evict() picked which key to remove, which would cause concurrent calls to Evict() to remove the same key. Additionally, this commit removes a lot of the lock contention and a race around Add() and Evict() by changing them to immediately hold the write lock. Previously, they would check conditions with the read lock held and not re-check those conditions once the write lock was acquired (this is a race). * pkg/cache: code review comments * pkg/cache: simplify Add() logic
125 lines
2.5 KiB
Go
125 lines
2.5 KiB
Go
// Package cache implements a cache. The cache hold 256 shards, each shard
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// holds a cache: a map with a mutex. There is no fancy expunge algorithm, it
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// just randomly evicts elements when it gets full.
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package cache
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import (
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"hash/fnv"
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"sync"
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)
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// Hash returns the FNV hash of what.
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func Hash(what []byte) uint64 {
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h := fnv.New64()
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h.Write(what)
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return h.Sum64()
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}
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// Cache is cache.
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type Cache struct {
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shards [shardSize]*shard
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}
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// shard is a cache with random eviction.
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type shard struct {
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items map[uint64]interface{}
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size int
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sync.RWMutex
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}
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// New returns a new cache.
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func New(size int) *Cache {
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ssize := size / shardSize
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if ssize < 4 {
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ssize = 4
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}
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c := &Cache{}
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// Initialize all the shards
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for i := 0; i < shardSize; i++ {
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c.shards[i] = newShard(ssize)
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}
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return c
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}
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// Add adds a new element to the cache. If the element already exists it is overwritten.
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func (c *Cache) Add(key uint64, el interface{}) {
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shard := key & (shardSize - 1)
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c.shards[shard].Add(key, el)
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}
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// Get looks up element index under key.
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func (c *Cache) Get(key uint64) (interface{}, bool) {
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shard := key & (shardSize - 1)
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return c.shards[shard].Get(key)
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}
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// Remove removes the element indexed with key.
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func (c *Cache) Remove(key uint64) {
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shard := key & (shardSize - 1)
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c.shards[shard].Remove(key)
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}
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// Len returns the number of elements in the cache.
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func (c *Cache) Len() int {
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l := 0
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for _, s := range c.shards {
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l += s.Len()
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}
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return l
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}
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// newShard returns a new shard with size.
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func newShard(size int) *shard { return &shard{items: make(map[uint64]interface{}), size: size} }
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// Add adds element indexed by key into the cache. Any existing element is overwritten
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func (s *shard) Add(key uint64, el interface{}) {
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s.Lock()
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if len(s.items) >= s.size {
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if _, ok := s.items[key]; !ok {
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for k := range s.items {
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delete(s.items, k)
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break
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}
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}
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}
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s.items[key] = el
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s.Unlock()
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}
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// Remove removes the element indexed by key from the cache.
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func (s *shard) Remove(key uint64) {
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s.Lock()
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delete(s.items, key)
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s.Unlock()
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}
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// Evict removes a random element from the cache.
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func (s *shard) Evict() {
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s.Lock()
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for k := range s.items {
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delete(s.items, k)
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break
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}
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s.Unlock()
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}
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// Get looks up the element indexed under key.
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func (s *shard) Get(key uint64) (interface{}, bool) {
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s.RLock()
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el, found := s.items[key]
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s.RUnlock()
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return el, found
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}
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// Len returns the current length of the cache.
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func (s *shard) Len() int {
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s.RLock()
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l := len(s.items)
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s.RUnlock()
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return l
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}
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const shardSize = 256
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