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uid_map.go
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/
uid_map.go
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package main
import (
"container/list"
"encoding/binary"
"sync"
"sync/atomic"
"github.com/dgraph-io/badger"
"github.com/dgraph-io/dgraph/x"
farm "github.com/dgryski/go-farm"
)
const (
// Memory used is in the order of numShards * (avgKeySize + constant
// overhead per key) * lruSize.
numShards = 1 << 10
lruSize = 1 << 9
)
type uidMap struct {
lease uint64
shards [numShards]shard
kv *badger.KV
}
type shard struct {
sync.Mutex
lastUsed uint64
lease uint64
elems map[string]*list.Element
queue *list.List
beingEvicted map[string]uint64
kv *badger.KV
}
type mapping struct {
xid string
uid uint64
persisted bool
}
func newUIDMap(kv *badger.KV) *uidMap {
um := &uidMap{
lease: 1,
kv: kv,
}
for i := range um.shards {
um.shards[i].elems = make(map[string]*list.Element)
um.shards[i].queue = list.New()
um.shards[i].kv = kv
}
return um
}
// assignUID creates new or looks up existing XID to UID mappings.
func (m *uidMap) assignUID(xid string) (uid uint64, isNew bool) {
fp := farm.Fingerprint64([]byte(xid))
idx := fp % numShards
sh := &m.shards[idx]
sh.Lock()
defer sh.Unlock()
var ok bool
uid, ok = sh.lookup(xid)
if ok {
return uid, false
}
var item badger.KVItem
x.Check(m.kv.Get([]byte(xid), &item))
x.Check(item.Value(func(uidBuf []byte) error {
if uidBuf == nil {
return nil
}
var n int
uid, n = binary.Uvarint(uidBuf)
x.AssertTrue(n == len(uidBuf))
ok = true
return nil
}))
if ok {
sh.add(xid, uid, true)
return uid, false
}
const leaseChunk = 1e5
if sh.lastUsed == sh.lease {
sh.lease = atomic.AddUint64(&m.lease, leaseChunk)
sh.lastUsed = sh.lease - leaseChunk
}
sh.lastUsed++
sh.add(xid, sh.lastUsed, false)
return sh.lastUsed, true
}
func (s *shard) lookup(xid string) (uint64, bool) {
elem, ok := s.elems[xid]
if ok {
s.queue.MoveToBack(elem)
return elem.Value.(*mapping).uid, true
}
if uid, ok := s.beingEvicted[xid]; ok {
s.add(xid, uid, true)
return uid, true
}
return 0, false
}
func (s *shard) add(xid string, uid uint64, persisted bool) {
if s.queue.Len() >= lruSize && len(s.beingEvicted) == 0 {
s.evict()
}
m := &mapping{
xid: xid,
uid: uid,
persisted: persisted,
}
elem := s.queue.PushBack(m)
s.elems[xid] = elem
}
func (s *shard) evict() {
const evictRatio = 0.5
evict := int(float64(s.queue.Len()) * evictRatio)
s.beingEvicted = make(map[string]uint64)
batch := make([]*badger.Entry, 0, evict)
for i := 0; i < evict; i++ {
m := s.queue.Remove(s.queue.Front()).(*mapping)
delete(s.elems, m.xid)
s.beingEvicted[m.xid] = m.uid
if !m.persisted {
var uidBuf [binary.MaxVarintLen64]byte
n := binary.PutUvarint(uidBuf[:], m.uid)
batch = append(batch, &badger.Entry{
Key: []byte(m.xid),
Value: uidBuf[:n],
})
}
}
s.kv.BatchSetAsync(batch, func(err error) {
x.Check(err)
for _, e := range batch {
x.Check(e.Error)
}
s.Lock()
s.beingEvicted = nil
s.Unlock()
})
}