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This article introduces the knowledge about "how to use go cache freecache". In the actual case operation process, many people will encounter such difficulties. Next, let Xiaobian lead you to learn how to deal with these situations! I hope you can read carefully and learn something!

Go development caching scenarios generally use map or caching frameworks, and sync.Map or thread-safe caching frameworks are used for thread safety.

In cache scenarios, if the amount of data is larger than one million, you need to consider the impact of data type on gc (note that the bottom layer of string type is pointer +Len+Cap, so it is also a pointer type). If the cache key and value are both non-pointer types, there is no need to worry. However, in actual application scenarios, it is very common for key and value to be (contain) pointer type data, so the use of cache frameworks requires special attention to its impact on GC. From the perspective of whether it affects GC, cache frameworks are roughly divided into two categories:

Zero GC overhead: such as freecache or bigcache, the bottom layer is based on ringbuf, reducing the number of pointers;

GC overhead: Cache framework implemented directly from Map.

For map, gc scans all key/value pairs, and if they are primitive types, gc does not scan them again. The following is an example of freecache analysis of its implementation principle, the code example is as follows:

func main() { cacheSize := 100 * 1024 * 1024 cache := freecache.NewCache(cacheSize) for i := 0; i

< N; i++ { str := strconv.Itoa(i) _ = cache.Set([]byte(str), []byte(str), 1) } now := time.Now() runtime.GC() fmt.Printf("freecache, GC took: %s\n", time.Since(now)) _, _ = cache.Get([]byte("aa")) now = time.Now() for i := 0; i < N; i++ { str := strconv.Itoa(i) _, _ = cache.Get([]byte(str)) } fmt.Printf("freecache, Get took: %s\n\n", time.Since(now))}1 初始化 freecache.NewCache会初始化本地缓存，size表示存储空间大小，freecache会初始化256个segment，每个segment是独立的存储单元，freecache加锁维度也是基于segment的，每个segment有一个ringbuf，初始大小为size/256。freecache号称零GC的来源就是其指针是固定的，只有512个，每个segment有2个，分别是rb和slotData（注意切片为指针类型）。 type segment struct { rb RingBuf // ring buffer that stores data segId int _ uint32 // 占位 missCount int64 hitCount int64 entryCount int64 totalCount int64 // number of entries in ring buffer, including deleted entries. totalTime int64 // used to calculate least recent used entry. timer Timer // Timer giving current time totalEvacuate int64 // used for debug totalExpired int64 // used for debug overwrites int64 // used for debug touched int64 // used for debug vacuumLen int64 // up to vacuumLen, new data can be written without overwriting old data. slotLens [256]int32 // The actual length for every slot. slotCap int32 // max number of entry pointers a slot can hold. slotsData []entryPtr // 索引指针}func NewCacheCustomTimer(size int, timer Timer) (cache *Cache) { cache = new(Cache) for i := 0; i < segmentCount; i++ { cache.segments[i] = newSegment(size/segmentCount, i, timer) }}func newSegment(bufSize int, segId int, timer Timer) (seg segment) { seg.rb = NewRingBuf(bufSize, 0) seg.segId = segId seg.timer = timer seg.vacuumLen = int64(bufSize) seg.slotCap = 1 seg.slotsData = make([]entryPtr, 256*seg.slotCap) // 每个slotData初始化256个单位大小}2 读写流程 freecache的key和value都是[]byte数组，使用时需要自行序列化和反序列化，如果缓存复杂对象不可忽略其序列化和反序列化带来的影响，首先看下Set流程： _ = cache.Set([]byte(str), []byte(str), 1) Set流程首先对key进行hash，hashVal类型uint64，其低8位segID对应segment数组，低8-15位表示slotId对应slotsData下标，高16位表示slotsData下标对应的[]entryPtr某个数据，这里需要查找操作。注意[]entryPtr数组大小为slotCap(初始为1)，当扩容时会slotCap倍增。 每个segment对应一个lock（sync.Mutex），因此其能够支持较大并发量，而不像sync.Map只有一个锁。 func (cache *Cache) Set(key, value []byte, expireSeconds int) (err error) { hashVal := hashFunc(key) segID := hashVal & segmentAndOpVal // 低8位 cache.locks[segID].Lock() // 加锁 err = cache.segments[segID].set(key, value, hashVal, expireSeconds) cache.locks[segID].Unlock()}func (seg *segment) set(key, value []byte, hashVal uint64, expireSeconds int) (err error) { slotId := uint8(hashVal >

> 8) hash26 := uint16(hashVal >> 16) slot := seg.getSlot(slotId) idx, match := seg.lookup(slot, hash26, key) var hdrBuf [ENTRY_HDR_SIZE]byte hdr := (*entryHdr)(unsafe.Pointer(&hdrBuf[0])) if match { //data update operation matchedPtr := &slot[idx] seg.rb.ReadAt(hdrBuf[:], matchedPtr.offset) hdr.slotId = slotId hdr.hash26 = hash26 hdr.keyLen = uint16(len(key)) originAccessTime := hdr.accessTime hdr.accessTime = now hdr.expireAt = expireAt hdr.valLen = uint32(len(value)) if hdr.valCap >= hdr.valLen { //already exists data value space can store this value size atomic.AddInt64(&seg.totalTime, int64(hdr.accessTime)-int64(originAccessTime)) seg.rb.WriteAt(hdrBuf[:], matchedPtr.offset) seg.rb.WriteAt(value, matchedPtr.offset+ENTRY_HDR_SIZE+int64(hdr.keyLen)) atomic.AddInt64(&seg.overwrites, 1) return } //delete corresponding entryPtr, involving slotsData memory copy, ringbug only mark delete seg.delEntryPtr(slotId, slot, idx) match = false // increase capacity and limit entry len. for hdr.valCap

< hdr.valLen { hdr.valCap *= 2 } if hdr.valCap >

uint32(maxKeyValLen-len(key)) { hdr.valCap = uint32(maxKeyValLen - len(key)) } } else { //No data hdr.slotId = slotId hdr.hash26 = hash26 hdr.keyLen = uint16(len(key)) hdr.accessTime = now hdr.expireAt = expireAt hdr.valLen = uint32(len(value)) hdr.valCap = uint32(len(value)) if hdr.valCap == 0 { // avoid infinite loop when increasing capacity. hdr.valCap = 1 } } //Actual length of data is ENTRY_HDR_SIZE=24 + length of key and value entryLen := ENTRY_HDR_SIZE + int64(len(key)) + int64(hdr.valCap) slotModified := seg.evacuate(entryLen, slotId, now) if slotModified { // the slot has been modified during evacuation, we need to looked up for the 'idx' again. // otherwise there would be index out of bound error. slot = seg.getSlot(slotId) idx, match = seg.lookup(slot, hash26, key) // assert(match == false) } newOff := seg.rb.End() seg.insertEntryPtr(slotId, hash26, newOff, idx, hdr.keyLen) seg.rb.Write(hdrBuf[:]) seg.rb.Write(key) seg.rb.Write(value) seg.rb.Skip(int64(hdr.valCap - hdr.valLen)) atomic.AddInt64(&seg.totalTime, int64(now)) atomic.AddInt64(&seg.totalCount, 1) seg.vacuumLen -= entryLen return}

seg.evacuate will evaluate whether ringbuf has enough space to store key/value. If there is not enough space, it will start scanning from the end of free space (that is, the beginning position of the data to be eliminated)(oldOff := seg.rb.End() + seg.vacuumLen - seg.rb.Size()). If the corresponding data has been logically deleted or expired, then the memory of this block can be directly reclaimed. If the reclamation conditions are not met, the entry will be transposed from the ring head to the ring tail, and then the index of entry will be updated. If this loop fails for 5 times, the current oldHdrBuf needs to be reclaimed to meet the memory needs.

The space required to execute seg.evacuate must be satisfied, and then the index and data are written. insertEntryPtr is the index writing operation. When the number of elements in []entryPtr is greater than seg.slotCap(initial 1), the expansion operation is required. For the corresponding method, see seg.expand, which will not be described here.

Write ringbuf is to execute rb.Write.

func (seg *segment) evacuate(entryLen int64, slotId uint8, now uint32) (slotModified bool) { var oldHdrBuf [ENTRY_HDR_SIZE]byte consecutiveEvacuate := 0 for seg.vacuumLen

< entryLen { oldOff := seg.rb.End() + seg.vacuumLen - seg.rb.Size() seg.rb.ReadAt(oldHdrBuf[:], oldOff) oldHdr := (*entryHdr)(unsafe.Pointer(&oldHdrBuf[0])) oldEntryLen := ENTRY_HDR_SIZE + int64(oldHdr.keyLen) + int64(oldHdr.valCap) if oldHdr.deleted { // 已删除 consecutiveEvacuate = 0 atomic.AddInt64(&seg.totalTime, -int64(oldHdr.accessTime)) atomic.AddInt64(&seg.totalCount, -1) seg.vacuumLen += oldEntryLen continue } expired := oldHdr.expireAt != 0 && oldHdr.expireAt < now leastRecentUsed := int64(oldHdr.accessTime)*atomic.LoadInt64(&seg.totalCount) 5 { // 可以回收 seg.delEntryPtrByOffset(oldHdr.slotId, oldHdr.hash26, oldOff) if oldHdr.slotId == slotId { slotModified = true } consecutiveEvacuate = 0 atomic.AddInt64(&seg.totalTime, -int64(oldHdr.accessTime)) atomic.AddInt64(&seg.totalCount, -1) seg.vacuumLen += oldEntryLen if expired { atomic.AddInt64(&seg.totalExpired, 1) } else { atomic.AddInt64(&seg.totalEvacuate, 1) } } else { // evacuate an old entry that has been accessed recently for better cache hit rate. newOff := seg.rb.Evacuate(oldOff, int(oldEntryLen)) seg.updateEntryPtr(oldHdr.slotId, oldHdr.hash26, oldOff, newOff) consecutiveEvacuate++ atomic.AddInt64(&seg.totalEvacuate, 1) } }} freecache的Get流程相对来说简单点，通过hash找到对应segment，通过slotId找到对应索引slot，然后通过二分+遍历寻找数据，如果找不到直接返回ErrNotFound，否则更新一些time指标。Get流程还会更新缓存命中率相关指标。 func (cache *Cache) Get(key []byte) (value []byte, err error) { hashVal := hashFunc(key) segID := hashVal & segmentAndOpVal cache.locks[segID].Lock() value, _, err = cache.segments[segID].get(key, nil, hashVal, false) cache.locks[segID].Unlock() return}func (seg *segment) get(key, buf []byte, hashVal uint64, peek bool) (value []byte, expireAt uint32, err error) { hdr, ptr, err := seg.locate(key, hashVal, peek) // hash+定位查找 if err != nil { return } expireAt = hdr.expireAt if cap(buf) >

= int(hdr.valLen) { value = buf[:hdr.valLen] } else { value = make([]byte, hdr.valLen) } seg.rb.ReadAt(value, ptr.offset+ENTRY_HDR_SIZE+int64(hdr.keyLen))}

After locating the data, read ringbuf, note that generally the value read is the newly created memory space, so it involves the copy operation of []byte data.

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