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2023-09-17 12:18:09 +03:00
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vendor/github.com/cockroachdb/pebble/internal/batchskl/README.md generated vendored Normal file
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# batchskl
Fast, non-concurrent skiplist implementation in Go that supports
forward and backward iteration.
## Limitations
* The interface is tailored for use in indexing pebble batches. Keys
and values are stored outside of the skiplist making the skiplist
awkward for general purpose use.
* Deletion is not supported. Instead, higher-level code is expected to
add deletion tombstones and needs to process those tombstones
appropriately.
## Pedigree
This code is based on Andy Kimball's arenaskl code.
The arenaskl code is based on the skiplist found in Badger, a Go-based
KV store:
https://github.com/dgraph-io/badger/tree/master/skl
The skiplist in Badger is itself based on a C++ skiplist built for
Facebook's RocksDB:
https://github.com/facebook/rocksdb/tree/master/memtable
## Benchmarks
The benchmarks consist of a mix of reads and writes executed in parallel. The
fraction of reads is indicated in the run name: "frac_X" indicates a run where
X percent of the operations are reads.
```
name time/op
ReadWrite/frac_0 1.03µs ± 2%
ReadWrite/frac_10 1.32µs ± 1%
ReadWrite/frac_20 1.26µs ± 1%
ReadWrite/frac_30 1.18µs ± 1%
ReadWrite/frac_40 1.09µs ± 1%
ReadWrite/frac_50 987ns ± 2%
ReadWrite/frac_60 1.07µs ± 1%
ReadWrite/frac_70 909ns ± 1%
ReadWrite/frac_80 693ns ± 2%
ReadWrite/frac_90 599ns ± 2%
ReadWrite/frac_100 45.3ns ± 3%
```
Forward and backward iteration are also fast:
```
name time/op
IterNext 4.49ns ± 3%
IterPrev 4.48ns ± 3%
```

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vendor/github.com/cockroachdb/pebble/internal/batchskl/iterator.go generated vendored Normal file
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/*
* Copyright 2017 Dgraph Labs, Inc. and Contributors
* Modifications copyright (C) 2017 Andy Kimball and Contributors
*
* 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 batchskl
import "github.com/cockroachdb/pebble/internal/base"
type splice struct {
prev uint32
next uint32
}
// Iterator is an iterator over the skiplist object. Use Skiplist.NewIter
// to construct an iterator. The current state of the iterator can be cloned
// by simply value copying the struct.
type Iterator struct {
list *Skiplist
nd uint32
key base.InternalKey
lower []byte
upper []byte
}
// Close resets the iterator.
func (it *Iterator) Close() error {
it.list = nil
it.nd = 0
return nil
}
// SeekGE moves the iterator to the first entry whose key is greater than or
// equal to the given key. Returns true if the iterator is pointing at a valid
// entry and false otherwise. Note that SeekGE only checks the upper bound. It
// is up to the caller to ensure that key is greater than or equal to the lower
// bound.
func (it *Iterator) SeekGE(key []byte, flags base.SeekGEFlags) *base.InternalKey {
if flags.TrySeekUsingNext() {
if it.nd == it.list.tail {
// Iterator is done.
return nil
}
less := it.list.cmp(it.key.UserKey, key) < 0
// Arbitrary constant. By measuring the seek cost as a function of the
// number of elements in the skip list, and fitting to a model, we
// could adjust the number of nexts based on the current size of the
// skip list.
const numNexts = 5
for i := 0; less && i < numNexts; i++ {
k := it.Next()
if k == nil {
// Iterator is done.
return nil
}
less = it.list.cmp(k.UserKey, key) < 0
}
if !less {
return &it.key
}
}
_, it.nd = it.seekForBaseSplice(key, it.list.abbreviatedKey(key))
if it.nd == it.list.tail {
return nil
}
nodeKey := it.list.getKey(it.nd)
if it.upper != nil && it.list.cmp(it.upper, nodeKey.UserKey) <= 0 {
it.nd = it.list.tail
return nil
}
it.key = nodeKey
return &it.key
}
// SeekLT moves the iterator to the last entry whose key is less the given
// key. Returns true if the iterator is pointing at a valid entry and false
// otherwise. Note that SeekLT only checks the lower bound. It is up to the
// caller to ensure that key is less than the upper bound.
func (it *Iterator) SeekLT(key []byte) *base.InternalKey {
it.nd, _ = it.seekForBaseSplice(key, it.list.abbreviatedKey(key))
if it.nd == it.list.head {
return nil
}
nodeKey := it.list.getKey(it.nd)
if it.lower != nil && it.list.cmp(it.lower, nodeKey.UserKey) > 0 {
it.nd = it.list.head
return nil
}
it.key = nodeKey
return &it.key
}
// First seeks position at the first entry in list. Final state of iterator is
// Valid() iff list is not empty. Note that First only checks the upper
// bound. It is up to the caller to ensure that key is greater than or equal to
// the lower bound (e.g. via a call to SeekGE(lower)).
func (it *Iterator) First() *base.InternalKey {
it.nd = it.list.getNext(it.list.head, 0)
if it.nd == it.list.tail {
return nil
}
nodeKey := it.list.getKey(it.nd)
if it.upper != nil && it.list.cmp(it.upper, nodeKey.UserKey) <= 0 {
it.nd = it.list.tail
return nil
}
it.key = nodeKey
return &it.key
}
// Last seeks position at the last entry in list. Final state of iterator is
// Valid() iff list is not empty. Note that Last only checks the lower
// bound. It is up to the caller to ensure that key is less than the upper
// bound (e.g. via a call to SeekLT(upper)).
func (it *Iterator) Last() *base.InternalKey {
it.nd = it.list.getPrev(it.list.tail, 0)
if it.nd == it.list.head {
return nil
}
nodeKey := it.list.getKey(it.nd)
if it.lower != nil && it.list.cmp(it.lower, nodeKey.UserKey) > 0 {
it.nd = it.list.head
return nil
}
it.key = nodeKey
return &it.key
}
// Next advances to the next position. If there are no following nodes, then
// Valid() will be false after this call.
func (it *Iterator) Next() *base.InternalKey {
it.nd = it.list.getNext(it.nd, 0)
if it.nd == it.list.tail {
return nil
}
nodeKey := it.list.getKey(it.nd)
if it.upper != nil && it.list.cmp(it.upper, nodeKey.UserKey) <= 0 {
it.nd = it.list.tail
return nil
}
it.key = nodeKey
return &it.key
}
// Prev moves to the previous position. If there are no previous nodes, then
// Valid() will be false after this call.
func (it *Iterator) Prev() *base.InternalKey {
it.nd = it.list.getPrev(it.nd, 0)
if it.nd == it.list.head {
return nil
}
nodeKey := it.list.getKey(it.nd)
if it.lower != nil && it.list.cmp(it.lower, nodeKey.UserKey) > 0 {
it.nd = it.list.head
return nil
}
it.key = nodeKey
return &it.key
}
// Key returns the key at the current position.
func (it *Iterator) Key() *base.InternalKey {
return &it.key
}
// KeyInfo returns the offset of the start of the record, the start of the key,
// and the end of the key.
func (it *Iterator) KeyInfo() (offset, keyStart, keyEnd uint32) {
n := it.list.node(it.nd)
return n.offset, n.keyStart, n.keyEnd
}
// Head true iff the iterator is positioned at the sentinel head node.
func (it *Iterator) Head() bool {
return it.nd == it.list.head
}
// Tail true iff the iterator is positioned at the sentinel tail node.
func (it *Iterator) Tail() bool {
return it.nd == it.list.tail
}
// Valid returns nil iff the iterator is positioned at a valid node.
func (it *Iterator) Valid() bool {
return it.list != nil && it.nd != it.list.head && it.nd != it.list.tail
}
func (it *Iterator) String() string {
return "batch"
}
// SetBounds sets the lower and upper bounds for the iterator. Note that the
// result of Next and Prev will be undefined until the iterator has been
// repositioned with SeekGE, SeekLT, First, or Last.
func (it *Iterator) SetBounds(lower, upper []byte) {
it.lower = lower
it.upper = upper
}
func (it *Iterator) seekForBaseSplice(key []byte, abbreviatedKey uint64) (prev, next uint32) {
prev = it.list.head
for level := it.list.height - 1; ; level-- {
prev, next = it.list.findSpliceForLevel(key, abbreviatedKey, level, prev)
if level == 0 {
break
}
}
return
}

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vendor/github.com/cockroachdb/pebble/internal/batchskl/skl.go generated vendored Normal file
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/*
* Copyright 2017 Dgraph Labs, Inc. and Contributors
* Modifications copyright (C) 2017 Andy Kimball and Contributors
*
* 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.
*/
/*
Adapted from RocksDB inline skiplist.
Key differences:
- No optimization for sequential inserts (no "prev").
- No custom comparator.
- Support overwrites. This requires care when we see the same key when inserting.
For RocksDB or LevelDB, overwrites are implemented as a newer sequence number in the key, so
there is no need for values. We don't intend to support versioning. In-place updates of values
would be more efficient.
- We discard all non-concurrent code.
- We do not support Splices. This simplifies the code a lot.
- No AllocateNode or other pointer arithmetic.
- We combine the findLessThan, findGreaterOrEqual, etc into one function.
*/
/*
Further adapted from Badger: https://github.com/dgraph-io/badger.
Key differences:
- Support for previous pointers - doubly linked lists. Note that it's up to higher
level code to deal with the intermediate state that occurs during insertion,
where node A is linked to node B, but node B is not yet linked back to node A.
- Iterator includes mutator functions.
*/
/*
Further adapted from arenaskl: https://github.com/andy-kimball/arenaskl
Key differences:
- Removed support for deletion.
- Removed support for concurrency.
- External storage of keys.
- Node storage grows to an arbitrary size.
*/
package batchskl // import "github.com/cockroachdb/pebble/internal/batchskl"
import (
"bytes"
"encoding/binary"
"fmt"
"math"
"time"
"unsafe"
"github.com/cockroachdb/errors"
"github.com/cockroachdb/pebble/internal/base"
"github.com/cockroachdb/pebble/internal/constants"
"golang.org/x/exp/rand"
)
const (
maxHeight = 20
maxNodeSize = uint64(unsafe.Sizeof(node{}))
linksSize = uint64(unsafe.Sizeof(links{}))
maxNodesSize = constants.MaxUint32OrInt
)
var (
// ErrExists indicates that a duplicate record was inserted. This should never
// happen for normal usage of batchskl as every key should have a unique
// sequence number.
ErrExists = errors.New("record with this key already exists")
// ErrTooManyRecords is a sentinel error returned when the size of the raw
// nodes slice exceeds the maximum allowed size (currently 1 << 32 - 1). This
// corresponds to ~117 M skiplist entries.
ErrTooManyRecords = errors.New("too many records")
)
type links struct {
next uint32
prev uint32
}
type node struct {
// The offset of the start of the record in the storage.
offset uint32
// The offset of the start and end of the key in storage.
keyStart uint32
keyEnd uint32
// A fixed 8-byte abbreviation of the key, used to avoid retrieval of the key
// during seek operations. The key retrieval can be expensive purely due to
// cache misses while the abbreviatedKey stored here will be in the same
// cache line as the key and the links making accessing and comparing against
// it almost free.
abbreviatedKey uint64
// Most nodes do not need to use the full height of the link tower, since the
// probability of each successive level decreases exponentially. Because
// these elements are never accessed, they do not need to be allocated.
// Therefore, when a node is allocated, its memory footprint is deliberately
// truncated to not include unneeded link elements.
links [maxHeight]links
}
// Skiplist is a fast, non-cocnurrent skiplist implementation that supports
// forward and backward iteration. See arenaskl.Skiplist for a concurrent
// skiplist. Keys and values are stored externally from the skiplist via the
// Storage interface. Deletion is not supported. Instead, higher-level code is
// expected to perform deletion via tombstones and needs to process those
// tombstones appropriately during retrieval operations.
type Skiplist struct {
storage *[]byte
cmp base.Compare
abbreviatedKey base.AbbreviatedKey
nodes []byte
head uint32
tail uint32
height uint32 // Current height: 1 <= height <= maxHeight
rand rand.PCGSource
}
var (
probabilities [maxHeight]uint32
)
func init() {
const pValue = 1 / math.E
// Precompute the skiplist probabilities so that only a single random number
// needs to be generated and so that the optimal pvalue can be used (inverse
// of Euler's number).
p := float64(1.0)
for i := 0; i < maxHeight; i++ {
probabilities[i] = uint32(float64(math.MaxUint32) * p)
p *= pValue
}
}
// NewSkiplist constructs and initializes a new, empty skiplist.
func NewSkiplist(storage *[]byte, cmp base.Compare, abbreviatedKey base.AbbreviatedKey) *Skiplist {
s := &Skiplist{}
s.Init(storage, cmp, abbreviatedKey)
return s
}
// Reset the fields in the skiplist for reuse.
func (s *Skiplist) Reset() {
*s = Skiplist{
nodes: s.nodes[:0],
height: 1,
}
const batchMaxRetainedSize = 1 << 20 // 1 MB
if cap(s.nodes) > batchMaxRetainedSize {
s.nodes = nil
}
}
// Init the skiplist to empty and re-initialize.
func (s *Skiplist) Init(storage *[]byte, cmp base.Compare, abbreviatedKey base.AbbreviatedKey) {
*s = Skiplist{
storage: storage,
cmp: cmp,
abbreviatedKey: abbreviatedKey,
nodes: s.nodes[:0],
height: 1,
}
s.rand.Seed(uint64(time.Now().UnixNano()))
const initBufSize = 256
if cap(s.nodes) < initBufSize {
s.nodes = make([]byte, 0, initBufSize)
}
// Allocate head and tail nodes. While allocating a new node can fail, in the
// context of initializing the skiplist we consider it unrecoverable.
var err error
s.head, err = s.newNode(maxHeight, 0, 0, 0, 0)
if err != nil {
panic(err)
}
s.tail, err = s.newNode(maxHeight, 0, 0, 0, 0)
if err != nil {
panic(err)
}
// Link all head/tail levels together.
headNode := s.node(s.head)
tailNode := s.node(s.tail)
for i := uint32(0); i < maxHeight; i++ {
headNode.links[i].next = s.tail
tailNode.links[i].prev = s.head
}
}
// Add adds a new key to the skiplist if it does not yet exist. If the record
// already exists, then Add returns ErrRecordExists.
func (s *Skiplist) Add(keyOffset uint32) error {
data := (*s.storage)[keyOffset+1:]
v, n := binary.Uvarint(data)
if n <= 0 {
return errors.Errorf("corrupted batch entry: %d", errors.Safe(keyOffset))
}
data = data[n:]
if v > uint64(len(data)) {
return errors.Errorf("corrupted batch entry: %d", errors.Safe(keyOffset))
}
keyStart := 1 + keyOffset + uint32(n)
keyEnd := keyStart + uint32(v)
key := data[:v]
abbreviatedKey := s.abbreviatedKey(key)
// spl holds the list of next and previous links for each level in the
// skiplist indicating where the new node will be inserted.
var spl [maxHeight]splice
// Fast-path for in-order insertion of keys: compare the new key against the
// last key.
prev := s.getPrev(s.tail, 0)
if prevNode := s.node(prev); prev == s.head ||
abbreviatedKey > prevNode.abbreviatedKey ||
(abbreviatedKey == prevNode.abbreviatedKey &&
s.cmp(key, (*s.storage)[prevNode.keyStart:prevNode.keyEnd]) > 0) {
for level := uint32(0); level < s.height; level++ {
spl[level].prev = s.getPrev(s.tail, level)
spl[level].next = s.tail
}
} else {
s.findSplice(key, abbreviatedKey, &spl)
}
height := s.randomHeight()
// Increase s.height as necessary.
for ; s.height < height; s.height++ {
spl[s.height].next = s.tail
spl[s.height].prev = s.head
}
// We always insert from the base level and up. After you add a node in base
// level, we cannot create a node in the level above because it would have
// discovered the node in the base level.
nd, err := s.newNode(height, keyOffset, keyStart, keyEnd, abbreviatedKey)
if err != nil {
return err
}
newNode := s.node(nd)
for level := uint32(0); level < height; level++ {
next := spl[level].next
prev := spl[level].prev
newNode.links[level].next = next
newNode.links[level].prev = prev
s.node(next).links[level].prev = nd
s.node(prev).links[level].next = nd
}
return nil
}
// NewIter returns a new Iterator object. The lower and upper bound parameters
// control the range of keys the iterator will return. Specifying for nil for
// lower or upper bound disables the check for that boundary. Note that lower
// bound is not checked on {SeekGE,First} and upper bound is not check on
// {SeekLT,Last}. The user is expected to perform that check. Note that it is
// safe for an iterator to be copied by value.
func (s *Skiplist) NewIter(lower, upper []byte) Iterator {
return Iterator{list: s, lower: lower, upper: upper}
}
func (s *Skiplist) newNode(
height,
offset, keyStart, keyEnd uint32, abbreviatedKey uint64,
) (uint32, error) {
if height < 1 || height > maxHeight {
panic("height cannot be less than one or greater than the max height")
}
unusedSize := uint64(maxHeight-int(height)) * linksSize
nodeOffset, err := s.alloc(uint32(maxNodeSize - unusedSize))
if err != nil {
return 0, err
}
nd := s.node(nodeOffset)
nd.offset = offset
nd.keyStart = keyStart
nd.keyEnd = keyEnd
nd.abbreviatedKey = abbreviatedKey
return nodeOffset, nil
}
func (s *Skiplist) alloc(size uint32) (uint32, error) {
offset := uint64(len(s.nodes))
// We only have a need for memory up to offset + size, but we never want
// to allocate a node whose tail points into unallocated memory.
minAllocSize := offset + maxNodeSize
if uint64(cap(s.nodes)) < minAllocSize {
allocSize := uint64(cap(s.nodes)) * 2
if allocSize < minAllocSize {
allocSize = minAllocSize
}
// Cap the allocation at the max allowed size to avoid wasted capacity.
if allocSize > maxNodesSize {
// The new record may still not fit within the allocation, in which case
// we return early with an error. This avoids the panic below when we
// resize the slice. It also avoids the allocation and copy.
if uint64(offset)+uint64(size) > maxNodesSize {
return 0, errors.Wrapf(ErrTooManyRecords,
"alloc of new record (size=%d) would overflow uint32 (current size=%d)",
uint64(offset)+uint64(size), offset,
)
}
allocSize = maxNodesSize
}
tmp := make([]byte, len(s.nodes), allocSize)
copy(tmp, s.nodes)
s.nodes = tmp
}
newSize := uint32(offset) + size
s.nodes = s.nodes[:newSize]
return uint32(offset), nil
}
func (s *Skiplist) node(offset uint32) *node {
return (*node)(unsafe.Pointer(&s.nodes[offset]))
}
func (s *Skiplist) randomHeight() uint32 {
rnd := uint32(s.rand.Uint64())
h := uint32(1)
for h < maxHeight && rnd <= probabilities[h] {
h++
}
return h
}
func (s *Skiplist) findSplice(key []byte, abbreviatedKey uint64, spl *[maxHeight]splice) {
prev := s.head
for level := s.height - 1; ; level-- {
// The code in this loop is the same as findSpliceForLevel(). For some
// reason, calling findSpliceForLevel() here is much much slower than the
// inlined code below. The excess time is also caught up in the final
// return statement which makes little sense. Revisit when in go1.14 or
// later if inlining improves.
next := s.getNext(prev, level)
for next != s.tail {
// Assume prev.key < key.
nextNode := s.node(next)
nextAbbreviatedKey := nextNode.abbreviatedKey
if abbreviatedKey < nextAbbreviatedKey {
// We are done for this level, since prev.key < key < next.key.
break
}
if abbreviatedKey == nextAbbreviatedKey {
if s.cmp(key, (*s.storage)[nextNode.keyStart:nextNode.keyEnd]) <= 0 {
// We are done for this level, since prev.key < key <= next.key.
break
}
}
// Keep moving right on this level.
prev = next
next = nextNode.links[level].next
}
spl[level].prev = prev
spl[level].next = next
if level == 0 {
break
}
}
}
func (s *Skiplist) findSpliceForLevel(
key []byte, abbreviatedKey uint64, level, start uint32,
) (prev, next uint32) {
prev = start
next = s.getNext(prev, level)
for next != s.tail {
// Assume prev.key < key.
nextNode := s.node(next)
nextAbbreviatedKey := nextNode.abbreviatedKey
if abbreviatedKey < nextAbbreviatedKey {
// We are done for this level, since prev.key < key < next.key.
break
}
if abbreviatedKey == nextAbbreviatedKey {
if s.cmp(key, (*s.storage)[nextNode.keyStart:nextNode.keyEnd]) <= 0 {
// We are done for this level, since prev.key < key < next.key.
break
}
}
// Keep moving right on this level.
prev = next
next = nextNode.links[level].next
}
return
}
func (s *Skiplist) getKey(nd uint32) base.InternalKey {
n := s.node(nd)
kind := base.InternalKeyKind((*s.storage)[n.offset])
key := (*s.storage)[n.keyStart:n.keyEnd]
return base.MakeInternalKey(key, uint64(n.offset)|base.InternalKeySeqNumBatch, kind)
}
func (s *Skiplist) getNext(nd, h uint32) uint32 {
return s.node(nd).links[h].next
}
func (s *Skiplist) getPrev(nd, h uint32) uint32 {
return s.node(nd).links[h].prev
}
func (s *Skiplist) debug() string {
var buf bytes.Buffer
for level := uint32(0); level < s.height; level++ {
var count int
for nd := s.head; nd != s.tail; nd = s.getNext(nd, level) {
count++
}
fmt.Fprintf(&buf, "%d: %d\n", level, count)
}
return buf.String()
}
// Silence unused warning.
var _ = (*Skiplist).debug