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Copy pathobjcache.go
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315 lines (270 loc) · 8.72 KB
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/*
* Minio Cloud Storage, (C) 2016, 2017 Minio, Inc.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*
*/
// Package objcache implements in memory caching methods.
package objcache
import (
"bytes"
"errors"
"io"
"runtime/debug"
"sync"
"time"
)
const (
// NoExpiry represents caches to be permanent and can only be deleted.
NoExpiry = time.Duration(0)
// DefaultExpiry represents three days time duration when individual entries will be expired.
DefaultExpiry = time.Duration(3 * 24 * time.Hour)
// defaultBufferRatio represents default ratio used to calculate the
// individual cache entry buffer size.
defaultBufferRatio = uint64(10)
// defaultGCPercent represents default garbage collection target percentage.
defaultGCPercent = 50
)
var (
// ErrKeyNotFoundInCache - key not found in cache.
ErrKeyNotFoundInCache = errors.New("Key not found in cache")
// ErrCacheFull - cache is full.
ErrCacheFull = errors.New("Not enough space in cache")
// ErrExcessData - excess data was attempted to be written on cache.
ErrExcessData = errors.New("Attempted excess write on cache")
)
// buffer represents the in memory cache of a single entry.
// buffer carries value of the data and last accessed time.
type buffer struct {
value []byte // Value of the entry.
lastAccessed time.Time // Represents time when value was last accessed.
}
// Cache holds the required variables to compose an in memory cache system
// which also provides expiring key mechanism and also maxSize.
type Cache struct {
// Mutex is used for handling the concurrent
// read/write requests for cache
mutex sync.Mutex
// Once is used for resetting GC once after
// peak cache usage.
onceGC sync.Once
// maxSize is a total size for overall cache
maxSize uint64
// maxCacheEntrySize is a total size per key buffer.
maxCacheEntrySize uint64
// currentSize is a current size in memory
currentSize uint64
// OnEviction - callback function for eviction
OnEviction func(key string)
// totalEvicted counter to keep track of total expirys
totalEvicted int
// map of objectName and its contents
entries map[string]*buffer
// Expiry in time duration.
expiry time.Duration
// Stop garbage collection routine, stops any running GC routine.
stopGC chan struct{}
}
// New - Return a new cache with a given default expiry
// duration. If the expiry duration is less than one
// (or NoExpiry), the items in the cache never expire
// (by default), and must be deleted manually.
func New(maxSize uint64, expiry time.Duration) (c *Cache, err error) {
if maxSize == 0 {
err = errors.New("invalid maximum cache size")
return c, err
}
// A garbage collection is triggered when the ratio
// of freshly allocated data to live data remaining
// after the previous collection reaches this percentage.
//
// - https://golang.org/pkg/runtime/debug/#SetGCPercent
//
// This means that by default GC is triggered after
// we've allocated an extra amount of memory proportional
// to the amount already in use.
//
// If gcpercent=100 and we're using 4M, we'll gc again
// when we get to 8M.
//
// Set this value to 40% if caching is enabled.
debug.SetGCPercent(defaultGCPercent)
// Max cache entry size - indicates the
// maximum buffer per key that can be held in
// memory. Currently this value is 1/10th
// the size of requested cache size.
maxCacheEntrySize := func() uint64 {
i := maxSize / defaultBufferRatio
if i == 0 {
i = maxSize
}
return i
}()
c = &Cache{
onceGC: sync.Once{},
maxSize: maxSize,
maxCacheEntrySize: maxCacheEntrySize,
entries: make(map[string]*buffer),
expiry: expiry,
}
// We have expiry start the janitor routine.
if expiry > 0 {
// Initialize a new stop GC channel.
c.stopGC = make(chan struct{})
// Start garbage collection routine to expire objects.
c.StartGC()
}
return c, nil
}
// Create - validates if object size fits with in cache size limit and returns a io.WriteCloser
// to which object contents can be written and finally Close()'d. During Close() we
// checks if the amount of data written is equal to the size of the object, in which
// case it saves the contents to object cache.
func (c *Cache) Create(key string, size int64) (w io.WriteCloser, err error) {
// Recovers any panic generated and return errors appropriately.
defer func() {
if r := recover(); r != nil {
// Recover any panic and return ErrCacheFull.
err = ErrCacheFull
}
}() // Do not crash the server.
valueLen := uint64(size)
// Check if the size of the object is > 1/10th the size
// of the cache, if yes then we ignore it.
if valueLen > c.maxCacheEntrySize {
return nil, ErrCacheFull
}
c.mutex.Lock()
// Check if the incoming size is going to exceed the
// effective cache size, if yes return error instead.
if c.currentSize+valueLen > c.maxSize {
c.mutex.Unlock()
return nil, ErrCacheFull
}
// Change GC percent if the current cache usage might
// become 75% of the maximum allowed usage, change
// the GC percent.
if c.currentSize+valueLen > (75 * c.maxSize / 100) {
c.onceGC.Do(func() { debug.SetGCPercent(defaultGCPercent - 25) })
}
c.mutex.Unlock()
cbuf := &cappedWriter{
offset: 0,
cap: size,
buffer: make([]byte, size),
}
// Function called on close which saves the object contents
// to the object cache.
onClose := func() error {
c.mutex.Lock()
defer c.mutex.Unlock()
if size != cbuf.offset {
cbuf.Reset() // Reset resets the buffer to be empty.
// Full object not available hence do not save buf to object cache.
return io.ErrShortBuffer
}
// Full object available in buf, save it to cache.
c.entries[key] = &buffer{
value: cbuf.buffer,
lastAccessed: time.Now().UTC(), // Save last accessed time.
}
// Account for the memory allocated above.
c.currentSize += uint64(size)
return nil
}
// Object contents that is written - cappedWriter.Write(data)
// will be accumulated in buf which implements io.Writer.
cbuf.>
// Capped writer.
return cbuf, nil
}
// Open - open the in-memory file, returns an in memory read seeker.
// returns an error ErrNotFoundInCache, if the key does not exist.
// Returns ErrKeyNotFoundInCache if entry's lastAccessedTime is older
// than objModTime.
func (c *Cache) Open(key string, objModTime time.Time) (io.ReaderAt, error) {
// Entry exists, return the readable buffer.
c.mutex.Lock()
defer c.mutex.Unlock()
buf, ok := c.entries[key]
if !ok {
return nil, ErrKeyNotFoundInCache
}
// Check if buf is recent copy of the object on disk.
if buf.lastAccessed.Before(objModTime) {
c.delete(key)
return nil, ErrKeyNotFoundInCache
}
buf.lastAccessed = time.Now().UTC()
return bytes.NewReader(buf.value), nil
}
// Delete - delete deletes an entry from the cache.
func (c *Cache) Delete(key string) {
c.mutex.Lock()
c.delete(key)
c.mutex.Unlock()
if c.OnEviction != nil {
c.OnEviction(key)
}
}
// gc - garbage collect all the expired entries from the cache.
func (c *Cache) gc() {
var evictedEntries []string
c.mutex.Lock()
for k, v := range c.entries {
if c.expiry > 0 && time.Now().UTC().Sub(v.lastAccessed) > c.expiry {
c.delete(k)
evictedEntries = append(evictedEntries, k)
}
}
c.mutex.Unlock()
for _, k := range evictedEntries {
if c.OnEviction != nil {
c.OnEviction(k)
}
}
}
// StopGC sends a message to the expiry routine to stop
// expiring cached entries. NOTE: once this is called, cached
// entries will not be expired, be careful if you are using this.
func (c *Cache) StopGC() {
if c.stopGC != nil {
c.stopGC <- struct{}{}
}
}
// StartGC starts running a routine ticking at expiry interval,
// on each interval this routine does a sweep across the cache
// entries and garbage collects all the expired entries.
func (c *Cache) StartGC() {
go func() {
for {
select {
// Wait till cleanup interval and initiate delete expired entries.
case <-time.After(c.expiry / 4):
c.gc()
// Stop the routine, usually called by the user of object cache during cleanup.
case <-c.stopGC:
return
}
}
}()
}
// Deletes a requested entry from the cache.
func (c *Cache) delete(key string) {
if _, ok := c.entries[key]; ok {
deletedSize := uint64(len(c.entries[key].value))
delete(c.entries, key)
c.currentSize -= deletedSize
c.totalEvicted++
}
}