Initial QSfera import
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Курнат Андрей
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/*
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* Copyright 2020 Dgraph Labs, Inc. and Contributors
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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package ristretto
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import (
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"math"
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"sync"
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"sync/atomic"
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"github.com/dgraph-io/ristretto/z"
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)
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const (
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// lfuSample is the number of items to sample when looking at eviction
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// candidates. 5 seems to be the most optimal number [citation needed].
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lfuSample = 5
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)
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// policy is the interface encapsulating eviction/admission behavior.
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// TODO: remove this interface and just rename defaultPolicy to policy, as we
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// are probably only going to use/implement/maintain one policy.
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type policy interface {
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ringConsumer
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// Add attempts to Add the key-cost pair to the Policy. It returns a slice
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// of evicted keys and a bool denoting whether or not the key-cost pair
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// was added. If it returns true, the key should be stored in cache.
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Add(uint64, int64) ([]*Item, bool)
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// Has returns true if the key exists in the Policy.
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Has(uint64) bool
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// Del deletes the key from the Policy.
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Del(uint64)
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// Cap returns the available capacity.
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Cap() int64
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// Close stops all goroutines and closes all channels.
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Close()
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// Update updates the cost value for the key.
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Update(uint64, int64)
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// Cost returns the cost value of a key or -1 if missing.
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Cost(uint64) int64
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// Optionally, set stats object to track how policy is performing.
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CollectMetrics(*Metrics)
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// Clear zeroes out all counters and clears hashmaps.
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Clear()
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// MaxCost returns the current max cost of the cache policy.
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MaxCost() int64
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// UpdateMaxCost updates the max cost of the cache policy.
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UpdateMaxCost(int64)
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}
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func newPolicy(numCounters, maxCost int64) policy {
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return newDefaultPolicy(numCounters, maxCost)
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}
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type defaultPolicy struct {
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sync.Mutex
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admit *tinyLFU
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evict *sampledLFU
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itemsCh chan []uint64
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stop chan struct{}
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isClosed bool
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metrics *Metrics
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}
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func newDefaultPolicy(numCounters, maxCost int64) *defaultPolicy {
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p := &defaultPolicy{
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admit: newTinyLFU(numCounters),
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evict: newSampledLFU(maxCost),
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itemsCh: make(chan []uint64, 3),
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stop: make(chan struct{}),
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}
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go p.processItems()
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return p
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}
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func (p *defaultPolicy) CollectMetrics(metrics *Metrics) {
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p.metrics = metrics
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p.evict.metrics = metrics
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}
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type policyPair struct {
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key uint64
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cost int64
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}
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func (p *defaultPolicy) processItems() {
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for {
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select {
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case items := <-p.itemsCh:
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p.Lock()
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p.admit.Push(items)
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p.Unlock()
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case <-p.stop:
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return
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}
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}
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}
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func (p *defaultPolicy) Push(keys []uint64) bool {
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if p.isClosed {
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return false
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}
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if len(keys) == 0 {
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return true
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}
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select {
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case p.itemsCh <- keys:
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p.metrics.add(keepGets, keys[0], uint64(len(keys)))
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return true
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default:
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p.metrics.add(dropGets, keys[0], uint64(len(keys)))
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return false
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}
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}
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// Add decides whether the item with the given key and cost should be accepted by
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// the policy. It returns the list of victims that have been evicted and a boolean
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// indicating whether the incoming item should be accepted.
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func (p *defaultPolicy) Add(key uint64, cost int64) ([]*Item, bool) {
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p.Lock()
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defer p.Unlock()
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// Cannot add an item bigger than entire cache.
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if cost > p.evict.getMaxCost() {
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return nil, false
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}
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// No need to go any further if the item is already in the cache.
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if has := p.evict.updateIfHas(key, cost); has {
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// An update does not count as an addition, so return false.
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return nil, false
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}
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// If the execution reaches this point, the key doesn't exist in the cache.
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// Calculate the remaining room in the cache (usually bytes).
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room := p.evict.roomLeft(cost)
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if room >= 0 {
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// There's enough room in the cache to store the new item without
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// overflowing. Do that now and stop here.
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p.evict.add(key, cost)
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p.metrics.add(costAdd, key, uint64(cost))
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return nil, true
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}
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// incHits is the hit count for the incoming item.
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incHits := p.admit.Estimate(key)
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// sample is the eviction candidate pool to be filled via random sampling.
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// TODO: perhaps we should use a min heap here. Right now our time
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// complexity is N for finding the min. Min heap should bring it down to
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// O(lg N).
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sample := make([]*policyPair, 0, lfuSample)
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// As items are evicted they will be appended to victims.
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victims := make([]*Item, 0)
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// Delete victims until there's enough space or a minKey is found that has
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// more hits than incoming item.
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for ; room < 0; room = p.evict.roomLeft(cost) {
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// Fill up empty slots in sample.
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sample = p.evict.fillSample(sample)
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// Find minimally used item in sample.
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minKey, minHits, minId, minCost := uint64(0), int64(math.MaxInt64), 0, int64(0)
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for i, pair := range sample {
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// Look up hit count for sample key.
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if hits := p.admit.Estimate(pair.key); hits < minHits {
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minKey, minHits, minId, minCost = pair.key, hits, i, pair.cost
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}
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}
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// If the incoming item isn't worth keeping in the policy, reject.
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if incHits < minHits {
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p.metrics.add(rejectSets, key, 1)
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return victims, false
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}
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// Delete the victim from metadata.
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p.evict.del(minKey)
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// Delete the victim from sample.
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sample[minId] = sample[len(sample)-1]
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sample = sample[:len(sample)-1]
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// Store victim in evicted victims slice.
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victims = append(victims, &Item{
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Key: minKey,
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Conflict: 0,
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Cost: minCost,
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})
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}
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p.evict.add(key, cost)
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p.metrics.add(costAdd, key, uint64(cost))
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return victims, true
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}
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func (p *defaultPolicy) Has(key uint64) bool {
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p.Lock()
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_, exists := p.evict.keyCosts[key]
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p.Unlock()
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return exists
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}
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func (p *defaultPolicy) Del(key uint64) {
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p.Lock()
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p.evict.del(key)
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p.Unlock()
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}
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func (p *defaultPolicy) Cap() int64 {
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p.Lock()
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capacity := p.evict.getMaxCost() - p.evict.used
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p.Unlock()
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return capacity
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}
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func (p *defaultPolicy) Update(key uint64, cost int64) {
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p.Lock()
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p.evict.updateIfHas(key, cost)
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p.Unlock()
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}
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func (p *defaultPolicy) Cost(key uint64) int64 {
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p.Lock()
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if cost, found := p.evict.keyCosts[key]; found {
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p.Unlock()
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return cost
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}
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p.Unlock()
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return -1
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}
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func (p *defaultPolicy) Clear() {
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p.Lock()
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p.admit.clear()
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p.evict.clear()
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p.Unlock()
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}
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func (p *defaultPolicy) Close() {
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if p.isClosed {
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return
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}
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// Block until the p.processItems goroutine returns.
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p.stop <- struct{}{}
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close(p.stop)
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close(p.itemsCh)
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p.isClosed = true
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}
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func (p *defaultPolicy) MaxCost() int64 {
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if p == nil || p.evict == nil {
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return 0
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}
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return p.evict.getMaxCost()
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}
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func (p *defaultPolicy) UpdateMaxCost(maxCost int64) {
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if p == nil || p.evict == nil {
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return
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}
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p.evict.updateMaxCost(maxCost)
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}
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// sampledLFU is an eviction helper storing key-cost pairs.
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type sampledLFU struct {
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// NOTE: align maxCost to 64-bit boundary for use with atomic.
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// As per https://golang.org/pkg/sync/atomic/: "On ARM, x86-32,
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// and 32-bit MIPS, it is the caller’s responsibility to arrange
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// for 64-bit alignment of 64-bit words accessed atomically.
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// The first word in a variable or in an allocated struct, array,
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// or slice can be relied upon to be 64-bit aligned."
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maxCost int64
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used int64
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metrics *Metrics
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keyCosts map[uint64]int64
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}
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func newSampledLFU(maxCost int64) *sampledLFU {
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return &sampledLFU{
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keyCosts: make(map[uint64]int64),
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maxCost: maxCost,
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}
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}
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func (p *sampledLFU) getMaxCost() int64 {
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return atomic.LoadInt64(&p.maxCost)
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}
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func (p *sampledLFU) updateMaxCost(maxCost int64) {
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atomic.StoreInt64(&p.maxCost, maxCost)
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}
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func (p *sampledLFU) roomLeft(cost int64) int64 {
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return p.getMaxCost() - (p.used + cost)
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}
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func (p *sampledLFU) fillSample(in []*policyPair) []*policyPair {
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if len(in) >= lfuSample {
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return in
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}
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for key, cost := range p.keyCosts {
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in = append(in, &policyPair{key, cost})
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if len(in) >= lfuSample {
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return in
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}
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}
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return in
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}
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func (p *sampledLFU) del(key uint64) {
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cost, ok := p.keyCosts[key]
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if !ok {
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return
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}
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p.used -= cost
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delete(p.keyCosts, key)
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p.metrics.add(costEvict, key, uint64(cost))
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p.metrics.add(keyEvict, key, 1)
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}
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func (p *sampledLFU) add(key uint64, cost int64) {
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p.keyCosts[key] = cost
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p.used += cost
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}
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func (p *sampledLFU) updateIfHas(key uint64, cost int64) bool {
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if prev, found := p.keyCosts[key]; found {
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// Update the cost of an existing key, but don't worry about evicting.
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// Evictions will be handled the next time a new item is added.
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p.metrics.add(keyUpdate, key, 1)
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if prev > cost {
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diff := prev - cost
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p.metrics.add(costAdd, key, ^(uint64(diff) - 1))
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} else if cost > prev {
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diff := cost - prev
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p.metrics.add(costAdd, key, uint64(diff))
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}
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p.used += cost - prev
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p.keyCosts[key] = cost
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return true
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}
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return false
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}
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func (p *sampledLFU) clear() {
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p.used = 0
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p.keyCosts = make(map[uint64]int64)
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}
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// tinyLFU is an admission helper that keeps track of access frequency using
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// tiny (4-bit) counters in the form of a count-min sketch.
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// tinyLFU is NOT thread safe.
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type tinyLFU struct {
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freq *cmSketch
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door *z.Bloom
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incrs int64
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resetAt int64
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}
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func newTinyLFU(numCounters int64) *tinyLFU {
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return &tinyLFU{
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freq: newCmSketch(numCounters),
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door: z.NewBloomFilter(float64(numCounters), 0.01),
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resetAt: numCounters,
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}
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}
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func (p *tinyLFU) Push(keys []uint64) {
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for _, key := range keys {
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p.Increment(key)
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}
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}
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func (p *tinyLFU) Estimate(key uint64) int64 {
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hits := p.freq.Estimate(key)
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if p.door.Has(key) {
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hits++
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}
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return hits
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}
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func (p *tinyLFU) Increment(key uint64) {
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// Flip doorkeeper bit if not already done.
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if added := p.door.AddIfNotHas(key); !added {
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// Increment count-min counter if doorkeeper bit is already set.
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p.freq.Increment(key)
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}
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p.incrs++
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if p.incrs >= p.resetAt {
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p.reset()
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}
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}
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func (p *tinyLFU) reset() {
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// Zero out incrs.
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p.incrs = 0
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// clears doorkeeper bits
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p.door.Clear()
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// halves count-min counters
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p.freq.Reset()
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}
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func (p *tinyLFU) clear() {
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p.incrs = 0
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p.door.Clear()
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p.freq.Clear()
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}
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