e9eda7e7c8
* cache: add sharded cache implementation Add Cache impl and a few tests. This cache is 256-way sharded, mainly so each shard has it's own lock. The main cache structure is a readonly jump plane into the right shard. This should remove the single lock contention on the main lock and provide more concurrent throughput - Obviously this hasn't been tested or measured. The key into the cache was made a uint32 (hash.fnv) and the hashing op is not using strings.ToLower anymore remove any GC in that code path. * here too * Minimum shard size * typos * blurp * small cleanups no defer * typo * Add freq based on Johns idea * cherry-pick conflict resolv * typo * update from early code review from john * add prefetch to the cache * mw/cache: add prefetch * remove println * remove comment * Fix tests * Test prefetch in setup * Add start of cache * try add diff cache options * Add hacky testcase * not needed * allow the use of a percentage for prefetch If the TTL falls below xx% do a prefetch, if the record was popular. Some other fixes and correctly prefetch only popular records.
129 lines
2.5 KiB
Go
129 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) uint32 {
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h := fnv.New32()
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h.Write(what)
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return h.Sum32()
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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[uint32]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 < 512 {
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ssize = 512
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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 uint32, 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 uint32) (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 uint32) {
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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[uint32]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 uint32, el interface{}) {
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l := s.Len()
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if l+1 > s.size {
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s.Evict()
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}
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s.Lock()
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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 uint32) {
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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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defer s.Unlock()
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key := -1
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for k := range s.items {
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key = int(k)
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break
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}
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if key == -1 {
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// empty cache
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return
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}
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delete(s.items, uint32(key))
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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 uint32) (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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