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mergeplan: a placeholder planner that merges all segments

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A stepping stone to fleshing out the API contract.
This commit is contained in:
Steve Yen 2017-12-11 10:40:35 -08:00
parent 58ef21a88a
commit 3461fb741f
3 changed files with 394 additions and 0 deletions
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275
index/scorch/mergeplan/merge_plan.go Normal file
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// Copyright (c) 2017 Couchbase, 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 mergeplan
import (
"math"
"sort"
)
// A Segment represents the information that the planner needs to
// calculate segment merging.
type Segment interface {
// Unique id of the segment -- used for sorting.
Id() uint64
// Full segment size (the size before any logical deletions).
FullSize() int64
// Size of the live data of the segment; i.e., FullSize() minus
// any logical deletions.
LiveSize() int64
}
// Plan() will functionally compute a merge plan. A segment will be
// assigned to at most a single MergeTask in the output MergePlan. A
// segment not assigned to any MergeTask means the segment should
// remain unmerged.
func Plan(segments []Segment, o *MergePlanOptions) (
result *MergePlan, err error) {
if len(segments) <= 1 {
return nil, nil
}
// TODO: PLACEHOLDER implementation for now, that always merges
// all the candidates.
return &MergePlan{
Tasks: []*MergeTask{
&MergeTask{
Segments: segments,
},
},
}, nil
}
// A MergePlan is the result of the Plan() API.
//
// The planner doesn’t know how or whether these tasks are executed --
// that’s up to a separate merge execution system, which might execute
// these tasks concurrently or not, and which might execute all the
// tasks or not.
type MergePlan struct {
Tasks []*MergeTask
}
// A MergeTask represents several segments that should be merged
// together into a single segment.
type MergeTask struct {
Segments []Segment
}
// The MergePlanOptions is designed to be reusable between planning calls.
type MergePlanOptions struct {
// Max # segments per logarithmic tier, or max width of any
// logarithmic “step”. Smaller values mean more merging but fewer
// segments. Should be >= SegmentsPerMergeTask, else you'll have
// too much merging.
MaxSegmentsPerTier int
// Max size of any segment produced after merging. Actual
// merging, however, may produce segment sizes different than the
// planner’s predicted sizes.
MaxSegmentSize int64
// The number of segments in any resulting MergeTask. e.g.,
// len(result.Tasks[ * ].Segments) == SegmentsPerMergeTask.
SegmentsPerMergeTask int
// Small segments are rounded up to this size, i.e., treated as
// equal (floor) size for consideration. This is to prevent lots
// of tiny segments from resulting in a long tail in the index.
FloorSegmentSize int64
// Controls how aggressively merges that reclaim more deletions
// are favored. Higher values will more aggressively target
// merges that reclaim deletions, but be careful not to go so high
// that way too much merging takes place; a value of 3.0 is
// probably nearly too high. A value of 0.0 means deletions don't
// impact merge selection.
ReclaimDeletesWeight float64
// Only consider a segment for merging if its delete percentage is
// over this threshold.
MinDeletesPct float64
// Optional, defaults to mergeplan.CalcBudget().
CalcBudget func(totalSize int64, firstTierSize int64,
o *MergePlanOptions) (budgetNumSegments int)
// Optional, defaults to mergeplan.ScoreSegments().
ScoreSegments func(segments []Segment, o *MergePlanOptions) float64
// Optional.
Logger func(string)
}
// Returns the higher of the input or FloorSegmentSize.
func (o *MergePlanOptions) RaiseToFloorSegmentSize(s int64) int64 {
if s > o.FloorSegmentSize {
return s
}
return o.FloorSegmentSize
}
// Suggested default options.
var DefaultMergePlanOptions = MergePlanOptions{
MaxSegmentsPerTier: 10,
MaxSegmentSize: 5000000,
SegmentsPerMergeTask: 10,
FloorSegmentSize: 2000,
ReclaimDeletesWeight: 2.0,
MinDeletesPct: 10.0,
}
// -------------------------------------------
func plan(segmentsIn []Segment, o *MergePlanOptions) (
result *MergePlan, err error) {
if len(segmentsIn) <= 1 {
return nil, nil
}
if o == nil {
o = &DefaultMergePlanOptions
}
segments := append([]Segment(nil), segmentsIn...) // Copy.
sort.Sort(byLiveSizeDescending(segments))
var segmentsLiveSize int64
var minLiveSize int64 = math.MaxInt64
var eligible []Segment
var eligibleLiveSize int64
for _, segment := range segments {
segmentsLiveSize += segment.LiveSize()
if minLiveSize > segment.LiveSize() {
minLiveSize = segment.LiveSize()
}
// Only small-enough segments are eligible.
if segment.LiveSize() < o.MaxSegmentSize/2 {
eligible = append(eligible, segment)
eligibleLiveSize += segment.LiveSize()
}
}
minLiveSize = o.RaiseToFloorSegmentSize(minLiveSize)
calcBudget := o.CalcBudget
if calcBudget == nil {
calcBudget = CalcBudget
}
budgetNumSegments := CalcBudget(eligibleLiveSize, minLiveSize, o)
scoreSegments := o.ScoreSegments
if scoreSegments == nil {
scoreSegments = ScoreSegments
}
rv := &MergePlan{}
// While we’re over budget, keep looping, which might produce
// another MergeTask.
for len(eligible) > budgetNumSegments {
// Track a current best roster as we examine and score
// potential rosters of merges.
var bestRoster []Segment
var bestRosterScore float64 // Lower score is better.
for startIdx := 0; startIdx < len(eligible)-o.SegmentsPerMergeTask; startIdx++ {
var roster []Segment
var rosterLiveSize int64
for idx := startIdx; idx < len(eligible) && len(roster) < o.SegmentsPerMergeTask; idx++ {
rosterCandidate := eligible[idx]
if rosterLiveSize+rosterCandidate.LiveSize() > o.MaxSegmentSize {
// NOTE: We continue the loop, to try to “pack”
// the roster with smaller segments to get closer
// to the max size; but, we aren't doing full,
// comprehensive "bin-packing" permutations.
continue
}
roster = append(roster, rosterCandidate)
rosterLiveSize += rosterCandidate.LiveSize()
}
rosterScore := scoreSegments(roster, o)
if len(bestRoster) <= 0 || rosterScore < bestRosterScore {
bestRoster = roster
bestRosterScore = rosterScore
}
}
if len(bestRoster) <= 0 {
return rv, nil
}
rv.Tasks = append(rv.Tasks, &MergeTask{
Segments: bestRoster,
})
eligible = removeSegments(eligible, bestRoster)
}
return rv, nil
}
// Compute the number of segments that would be needed to cover the
// totalSize, by climbing up a logarithmic staircase of segment tiers.
func CalcBudget(totalSize int64, firstTierSize int64, o *MergePlanOptions) (
budgetNumSegments int) {
tierSize := firstTierSize
for totalSize > 0 {
segmentsInTier := float64(totalSize) / float64(tierSize)
if segmentsInTier < float64(o.MaxSegmentsPerTier) {
budgetNumSegments += int(math.Ceil(segmentsInTier))
break
}
budgetNumSegments += o.MaxSegmentsPerTier
totalSize -= int64(o.MaxSegmentsPerTier) * tierSize
tierSize *= int64(o.SegmentsPerMergeTask)
}
return budgetNumSegments
}
// removeSegments() keeps the ordering of the result segments stable.
func removeSegments(segments []Segment, toRemove []Segment) (rv []Segment) {
OUTER:
for _, segment := range segments {
for _, r := range toRemove {
if segment == r {
continue OUTER
}
}
rv = append(rv, segment)
}
return rv
}
func ScoreSegments(segments []Segment, o *MergePlanOptions) float64 {
return 0 // TODO. Bogus score.
}

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index/scorch/mergeplan/merge_plan_test.go Normal file
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// Copyright (c) 2017 Couchbase, 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 mergeplan
import (
"reflect"
"testing"
)
// Implements the Segment interface for testing,
type segment struct {
id uint64
fullSize int64
liveSize int64
}
func (s *segment) Id() uint64 { return s.id }
func (s *segment) FullSize() int64 { return s.fullSize }
func (s *segment) LiveSize() int64 { return s.liveSize }
func makeLinearSegments(n int) (rv []Segment) {
for i := 0; i < n; i++ {
rv = append(rv, &segment{
id: uint64(i),
fullSize: int64(i),
liveSize: int64(i),
})
}
return rv
}
func TestSimplePlan(t *testing.T) {
segs := makeLinearSegments(10)
tests := []struct {
desc string
segments []Segment
expectPlan *MergePlan
expectErr error
}{
{"nil candidates",
nil, nil, nil},
{"empty candidates",
[]Segment{}, nil, nil},
{"1 candidate",
[]Segment{segs[0]},
nil,
nil,
},
{"2 candidates",
[]Segment{
segs[0],
segs[1],
},
&MergePlan{
[]*MergeTask{
&MergeTask{
Segments: []Segment{
segs[0],
segs[1],
},
},
},
},
nil,
},
}
for testi, test := range tests {
plan, err := Plan(test.segments, &DefaultMergePlanOptions)
if err != test.expectErr {
t.Errorf("testi: %d, test: %v, got err: %v",
testi, test, err)
}
if !reflect.DeepEqual(plan, test.expectPlan) {
t.Errorf("testi: %d, test: %v, got plan: %v",
testi, test, plan)
}
}
}

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index/scorch/mergeplan/sort.go Normal file
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// Copyright (c) 2017 Couchbase, 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 mergeplan
type byLiveSizeDescending []Segment
func (a byLiveSizeDescending) Len() int { return len(a) }
func (a byLiveSizeDescending) Swap(i, j int) { a[i], a[j] = a[j], a[i] }
func (a byLiveSizeDescending) Less(i, j int) bool {
if a[i].LiveSize() != a[j].LiveSize() {
return a[i].LiveSize() < a[j].LiveSize()
}
return a[i].Id() < a[j].Id()
}