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|
package cache
import (
"crypto/rand"
"math"
"math/big"
insecurerand "math/rand"
"os"
"runtime"
"time"
)
// This is an experimental and unexported (for now) attempt at making a cache
// with better algorithmic complexity than the standard one, namely by
// preventing write locks of the entire cache when an item is added. As of the
// time of writing, the overhead of selecting buckets results in cache
// operations being about twice as slow as for the standard cache with small
// total cache sizes, and faster for larger ones.
//
// See cache_test.go for a few benchmarks.
type unexportedShardedCache struct {
*shardedCache
}
type shardedCache struct {
seed uint32
m uint32
cs []*cache
janitor *shardedJanitor
}
// djb2 with better shuffling. 5x faster than FNV with the hash.Hash overhead.
func djb33(seed uint32, k string) uint32 {
var (
l = uint32(len(k))
d = 5381 + seed + l
i = uint32(0)
)
// Why is all this 5x faster than a for loop?
if l >= 4 {
for i < l-4 {
d = (d * 33) ^ uint32(k[i])
d = (d * 33) ^ uint32(k[i+1])
d = (d * 33) ^ uint32(k[i+2])
d = (d * 33) ^ uint32(k[i+3])
i += 4
}
}
switch l - i {
case 1:
case 2:
d = (d * 33) ^ uint32(k[i])
case 3:
d = (d * 33) ^ uint32(k[i])
d = (d * 33) ^ uint32(k[i+1])
case 4:
d = (d * 33) ^ uint32(k[i])
d = (d * 33) ^ uint32(k[i+1])
d = (d * 33) ^ uint32(k[i+2])
}
return d ^ (d >> 16)
}
func (sc *shardedCache) bucket(k string) *cache {
return sc.cs[djb33(sc.seed, k)%sc.m]
}
func (sc *shardedCache) Set(k string, x interface{}, d time.Duration) {
sc.bucket(k).Set(k, x, d)
}
func (sc *shardedCache) Add(k string, x interface{}, d time.Duration) error {
return sc.bucket(k).Add(k, x, d)
}
func (sc *shardedCache) Replace(k string, x interface{}, d time.Duration) error {
return sc.bucket(k).Replace(k, x, d)
}
func (sc *shardedCache) Get(k string) (interface{}, bool) {
return sc.bucket(k).Get(k)
}
func (sc *shardedCache) Increment(k string, n int64) error {
return sc.bucket(k).Increment(k, n)
}
func (sc *shardedCache) IncrementFloat(k string, n float64) error {
return sc.bucket(k).IncrementFloat(k, n)
}
func (sc *shardedCache) Decrement(k string, n int64) error {
return sc.bucket(k).Decrement(k, n)
}
func (sc *shardedCache) Delete(k string) {
sc.bucket(k).Delete(k)
}
func (sc *shardedCache) DeleteExpired() {
for _, v := range sc.cs {
v.DeleteExpired()
}
}
// Returns the items in the cache. This may include items that have expired,
// but have not yet been cleaned up. If this is significant, the Expiration
// fields of the items should be checked. Note that explicit synchronization
// is needed to use a cache and its corresponding Items() return values at
// the same time, as the maps are shared.
func (sc *shardedCache) Items() []map[string]*Item {
res := make([]map[string]*Item, len(sc.cs))
for i, v := range sc.cs {
res[i] = v.Items()
}
return res
}
func (sc *shardedCache) Flush() {
for _, v := range sc.cs {
v.Flush()
}
}
type shardedJanitor struct {
Interval time.Duration
stop chan bool
}
func (j *shardedJanitor) Run(sc *shardedCache) {
j.stop = make(chan bool)
tick := time.Tick(j.Interval)
for {
select {
case <-tick:
sc.DeleteExpired()
case <-j.stop:
return
}
}
}
func stopShardedJanitor(sc *unexportedShardedCache) {
sc.janitor.stop <- true
}
func runShardedJanitor(sc *shardedCache, ci time.Duration) {
j := &shardedJanitor{
Interval: ci,
}
sc.janitor = j
go j.Run(sc)
}
func newShardedCache(n int, de time.Duration) *shardedCache {
max := big.NewInt(0).SetUint64(uint64(math.MaxUint32))
rnd, err := rand.Int(rand.Reader, max)
var seed uint32
if err != nil {
os.Stderr.Write([]byte("WARNING: go-cache's newShardedCache failed to read from the system CSPRNG (/dev/urandom or equivalent.) Your system's security may be compromised. Continuing with an insecure seed.\n"))
seed = insecurerand.Uint32()
} else {
seed = uint32(rnd.Uint64())
}
sc := &shardedCache{
seed: seed,
m: uint32(n),
cs: make([]*cache, n),
}
for i := 0; i < n; i++ {
c := &cache{
defaultExpiration: de,
items: map[string]*Item{},
}
sc.cs[i] = c
}
return sc
}
func unexportedNewSharded(defaultExpiration, cleanupInterval time.Duration, shards int) *unexportedShardedCache {
if defaultExpiration == 0 {
defaultExpiration = -1
}
sc := newShardedCache(shards, defaultExpiration)
SC := &unexportedShardedCache{sc}
if cleanupInterval > 0 {
runShardedJanitor(sc, cleanupInterval)
runtime.SetFinalizer(SC, stopShardedJanitor)
}
return SC
}
|
