Merge pull request #1418 from weaveworks/log-n-report-merger

A caching, log(n) complexity report merger

app/collector.go

@@ -1,11 +1,9 @@
 package app
 
 import (
-	"fmt"
 	"sync"
 	"time"
 
-	"github.com/spaolacci/murmur3"
 	"golang.org/x/net/context"
 
 	"github.com/weaveworks/scope/common/mtime"
@@ -35,10 +33,12 @@ type Collector interface {
 // Collector receives published reports from multiple producers. It yields a
 // single merged report, representing all collected reports.
 type collector struct {
-	mtx     sync.Mutex
-	reports []timestampReport
-	window  time.Duration
-	cached  *report.Report
+	mtx        sync.Mutex
+	reports    []report.Report
+	timestamps []time.Time
+	window     time.Duration
+	cached     *report.Report
+	merger     Merger
 	waitableCondition
 }
 
@@ -78,6 +78,7 @@ func NewCollector(window time.Duration) Collector {
 		waitableCondition: waitableCondition{
 			waiters: map[chan struct{}]struct{}{},
 		},
+		merger: NewSmartMerger(),
 	}
 }
 
@@ -85,8 +86,10 @@ func NewCollector(window time.Duration) Collector {
 func (c *collector) Add(_ context.Context, rpt report.Report) error {
 	c.mtx.Lock()
 	defer c.mtx.Unlock()
-	c.reports = append(c.reports, timestampReport{mtime.Now(), rpt})
-	c.reports = clean(c.reports, c.window)
+	c.reports = append(c.reports, rpt)
+	c.timestamps = append(c.timestamps, mtime.Now())
+
+	c.clean()
 	c.cached = nil
 	if rpt.Shortcut {
 		c.Broadcast()
@@ -104,37 +107,27 @@ func (c *collector) Report(_ context.Context) (report.Report, error) {
 	// and there is a cached report, return that.
 	if c.cached != nil && len(c.reports) > 0 {
 		oldest := mtime.Now().Add(-c.window)
-		if c.reports[0].timestamp.After(oldest) {
+		if c.timestamps[0].After(oldest) {
 			return *c.cached, nil
 		}
 	}
-	c.reports = clean(c.reports, c.window)
 
-	rpt := report.MakeReport()
-	id := murmur3.New64()
-	for _, tr := range c.reports {
-		rpt = rpt.Merge(tr.report)
-		id.Write([]byte(tr.report.ID))
-	}
-	rpt.ID = fmt.Sprintf("%x", id.Sum64())
-	c.cached = &rpt
-	return rpt, nil
+	c.clean()
+	return c.merger.Merge(c.reports), nil
 }
 
-type timestampReport struct {
-	timestamp time.Time
-	report    report.Report
-}
-
-func clean(reports []timestampReport, window time.Duration) []timestampReport {
+func (c *collector) clean() {
 	var (
-		cleaned = make([]timestampReport, 0, len(reports))
-		oldest  = mtime.Now().Add(-window)
+		cleanedReports    = make([]report.Report, 0, len(c.reports))
+		cleanedTimestamps = make([]time.Time, 0, len(c.timestamps))
+		oldest            = mtime.Now().Add(-c.window)
 	)
-	for _, tr := range reports {
-		if tr.timestamp.After(oldest) {
-			cleaned = append(cleaned, tr)
+	for i, r := range c.reports {
+		if c.timestamps[i].After(oldest) {
+			cleanedReports = append(cleanedReports, r)
+			cleanedTimestamps = append(cleanedTimestamps, c.timestamps[i])
 		}
 	}
-	return cleaned
+	c.reports = cleanedReports
+	c.timestamps = cleanedTimestamps
 }

app/merger.go

@@ -0,0 +1,138 @@
+package app
+
+import (
+	"fmt"
+	"math"
+	"sort"
+
+	"github.com/bluele/gcache"
+	"github.com/spaolacci/murmur3"
+
+	"github.com/weaveworks/scope/report"
+)
+
+// Merger is the type for a thing that can merge reports.
+type Merger interface {
+	Merge([]report.Report) report.Report
+}
+
+type dumbMerger struct{}
+
+// MakeDumbMerger makes a Merger which merges together reports in the simplest possible way.
+func MakeDumbMerger() Merger {
+	return dumbMerger{}
+}
+
+func (dumbMerger) Merge(reports []report.Report) report.Report {
+	rpt := report.MakeReport()
+	id := murmur3.New64()
+	for _, r := range reports {
+		rpt = rpt.Merge(r)
+		id.Write([]byte(r.ID))
+	}
+	rpt.ID = fmt.Sprintf("%x", id.Sum64())
+	return rpt
+}
+
+type smartMerger struct {
+	cache gcache.Cache
+}
+
+// NewSmartMerger makes a Merger which merges together reports, caching intermediate merges
+// to accelerate future merges. Idea is to cache pair-wise merged reports, forming a merge
+// tree.  Merging a new report into this tree should be log(N).
+func NewSmartMerger() Merger {
+	return &smartMerger{
+		cache: gcache.New(1000).LRU().Build(),
+	}
+}
+
+type node struct {
+	id  uint64
+	rpt report.Report
+}
+
+type byID []*node
+
+func (ns byID) Len() int           { return len(ns) }
+func (ns byID) Swap(i, j int)      { ns[i], ns[j] = ns[j], ns[i] }
+func (ns byID) Less(i, j int) bool { return ns[i].id < ns[j].id }
+
+func hash(ids ...string) uint64 {
+	id := murmur3.New64()
+	for _, i := range ids {
+		id.Write([]byte(i))
+	}
+	return id.Sum64()
+}
+
+func (s *smartMerger) ClearCache() {
+	s.cache.Purge()
+}
+
+func (s *smartMerger) Merge(reports []report.Report) report.Report {
+	// Start with a sorted list of leaves.
+	// Note we must dedupe reports with the same ID to ensure the
+	// algorithm below doesn't go into an infinite loop.  This is
+	// fine as reports with the same ID are assumed to be the same.
+	nodes := []*node{}
+	seen := map[uint64]struct{}{}
+	for _, r := range reports {
+		id := hash(r.ID)
+		if _, ok := seen[id]; ok {
+			continue
+		}
+		seen[id] = struct{}{}
+		nodes = append(nodes, &node{
+			id:  id,
+			rpt: r,
+		})
+	}
+	sort.Sort(byID(nodes))
+
+	// Define how to merge two nodes together.  The result of merging
+	// two reports is cached.
+	merge := func(left, right *node) *node {
+		id := hash(left.rpt.ID, right.rpt.ID)
+
+		if result, err := s.cache.Get(id); err == nil {
+			return result.(*node)
+		}
+
+		n := &node{
+			id:  id,
+			rpt: report.MakeReport().Merge(left.rpt).Merge(right.rpt),
+		}
+		s.cache.Set(id, n)
+		return n
+	}
+
+	// Define how to reduce n nodes to 1.
+	// Min and max are both inclusive!
+	var reduce func(min, max uint64, nodes []*node) *node
+	reduce = func(min, max uint64, nodes []*node) *node {
+		switch len(nodes) {
+		case 0:
+			return &node{rpt: report.MakeReport()}
+		case 1:
+			return nodes[0]
+		case 2:
+			return merge(nodes[0], nodes[1])
+		}
+
+		partition := min + ((max - min) / 2)
+		index := sort.Search(len(nodes), func(i int) bool {
+			return nodes[i].id > partition
+		})
+		if index == len(nodes) {
+			return reduce(min, partition, nodes)
+		} else if index == 0 {
+			return reduce(partition+1, max, nodes)
+		}
+		left := reduce(min, partition, nodes[:index])
+		right := reduce(partition+1, max, nodes[index:])
+		return merge(left, right)
+	}
+
+	return reduce(0, math.MaxUint64, nodes).rpt
+}

app/merger_test.go

@@ -0,0 +1,122 @@
+package app_test
+
+import (
+	"fmt"
+	"math/rand"
+	"testing"
+
+	"github.com/weaveworks/scope/app"
+	"github.com/weaveworks/scope/report"
+	"github.com/weaveworks/scope/test"
+	"github.com/weaveworks/scope/test/reflect"
+)
+
+func TestMerger(t *testing.T) {
+	// Use 3 reports to check the pair-wise merging in SmartMerger
+	report1 := report.MakeReport()
+	report1.Endpoint.AddNode(report.MakeNode("foo"))
+	report2 := report.MakeReport()
+	report2.Endpoint.AddNode(report.MakeNode("bar"))
+	report3 := report.MakeReport()
+	report3.Endpoint.AddNode(report.MakeNode("baz"))
+	reports := []report.Report{
+		report1, report2, report3,
+	}
+	want := report.MakeReport()
+	want.Endpoint.
+		AddNode(report.MakeNode("foo")).
+		AddNode(report.MakeNode("bar")).
+		AddNode(report.MakeNode("baz"))
+
+	for _, merger := range []app.Merger{app.MakeDumbMerger(), app.NewSmartMerger()} {
+		// Test the empty list case
+		if have := merger.Merge([]report.Report{}); !reflect.DeepEqual(have, report.MakeReport()) {
+			t.Errorf("Bad merge: %s", test.Diff(have, want))
+		}
+
+		if have := merger.Merge(reports); !reflect.DeepEqual(have, want) {
+			t.Errorf("Bad merge: %s", test.Diff(have, want))
+		}
+
+		// Repeat the above test to ensure caching works
+		if have := merger.Merge(reports); !reflect.DeepEqual(have, want) {
+			t.Errorf("Bad merge: %s", test.Diff(have, want))
+		}
+	}
+}
+
+func TestSmartMerger(t *testing.T) {
+	// Use 3 reports _WITH SAME ID_
+	report1 := report.MakeReport()
+	report1.Endpoint.AddNode(report.MakeNode("foo"))
+	report1.ID = "foo"
+	report2 := report.MakeReport()
+	report2.Endpoint.AddNode(report.MakeNode("bar"))
+	report2.ID = "foo"
+	report3 := report.MakeReport()
+	report3.Endpoint.AddNode(report.MakeNode("baz"))
+	report3.ID = "foo"
+	reports := []report.Report{
+		report1, report2, report3,
+	}
+	want := report.MakeReport()
+	want.Endpoint.AddNode(report.MakeNode("foo"))
+
+	merger := app.NewSmartMerger()
+	if have := merger.Merge(reports); !reflect.DeepEqual(have, want) {
+		t.Errorf("Bad merge: %s", test.Diff(have, want))
+	}
+}
+
+func BenchmarkSmartMerger(b *testing.B) {
+	benchmarkMerger(b, app.NewSmartMerger(), false)
+}
+
+func BenchmarkSmartMergerWithoutCaching(b *testing.B) {
+	benchmarkMerger(b, app.NewSmartMerger(), true)
+}
+
+func BenchmarkDumbMerger(b *testing.B) {
+	benchmarkMerger(b, app.MakeDumbMerger(), false)
+}
+
+const numHosts = 15
+
+func benchmarkMerger(b *testing.B, merger app.Merger, clearCache bool) {
+	makeReport := func() report.Report {
+		rpt := report.MakeReport()
+		for i := 0; i < 100; i++ {
+			rpt.Endpoint.AddNode(report.MakeNode(fmt.Sprintf("%x", rand.Int63())))
+		}
+		return rpt
+	}
+
+	reports := []report.Report{}
+	for i := 0; i < numHosts*5; i++ {
+		reports = append(reports, makeReport())
+	}
+	merger.Merge(reports) // prime the cache
+	if clearable, ok := merger.(interface {
+		ClearCache()
+	}); ok && clearCache {
+		clearable.ClearCache()
+	}
+
+	b.ReportAllocs()
+	b.ResetTimer()
+
+	for i := 0; i < b.N; i++ {
+		// replace 1/3 of hosts work of reports & merge them all
+		for i := 0; i < numHosts/3; i++ {
+			reports[rand.Intn(len(reports))] = makeReport()
+		}
+
+		merger.Merge(reports)
+
+		if clearable, ok := merger.(interface {
+			ClearCache()
+		}); ok && clearCache {
+			clearable.ClearCache()
+		}
+	}
+}