autopilot: parallelize betweenness centrality

This commit parallelizes betweenness centrality calculation, by
distributing the algo to N workers and creating partial results.

autopilot/betweenness_centrality.go

@@ -1,5 +1,10 @@
 package autopilot
 
+import (
+	"fmt"
+	"sync"
+)
+
 // stack is a simple int stack to help with readability of Brandes'
 // betweenness centrality implementation below.
 type stack struct {
@@ -50,6 +55,10 @@ func (q *queue) empty() bool {
 // shortest paths starting or ending at that node. This is a useful metric
 // to measure control of individual nodes over the whole network.
 type BetweennessCentrality struct {
+	// workers number of goroutines are used to parallelize
+	// centrality calculation.
+	workers int
+
 	// centrality stores original (not normalized) centrality values for
 	// each node in the graph.
 	centrality map[NodeID]float64
@@ -62,8 +71,15 @@ type BetweennessCentrality struct {
 }
 
 // NewBetweennessCentralityMetric creates a new BetweennessCentrality instance.
-func NewBetweennessCentralityMetric() *BetweennessCentrality {
-	return &BetweennessCentrality{}
+// Users can specify the number of workers to use for calculating centrality.
+func NewBetweennessCentralityMetric(workers int) (*BetweennessCentrality, error) {
+	// There should be at least one worker.
+	if workers < 1 {
+		return nil, fmt.Errorf("workers must be positive")
+	}
+	return &BetweennessCentrality{
+		workers: workers,
+	}, nil
 }
 
 // Name returns the name of the metric.
@@ -158,10 +174,47 @@ func (bc *BetweennessCentrality) Refresh(graph ChannelGraph) error {
 		return err
 	}
 
-	// TODO: parallelize updates to centrality.
-	centrality := make([]float64, len(cache.Nodes))
+	var wg sync.WaitGroup
+	work := make(chan int)
+	partials := make(chan []float64, bc.workers)
+
+	// Each worker will compute a partial result. This
+	// partial result is a sum of centrality updates on
+	// roughly N / workers nodes.
+	worker := func() {
+		defer wg.Done()
+		partial := make([]float64, len(cache.Nodes))
+
+		// Consume the next node, update centrality
+		// parital to avoid unnecessary synchronizaton.
+		for node := range work {
+			betweennessCentrality(cache, node, partial)
+		}
+		partials <- partial
+	}
+
+	// Now start the N workers.
+	wg.Add(bc.workers)
+	for i := 0; i < bc.workers; i++ {
+		go worker()
+	}
+
+	// Distribute work amongst workers Should be
+	// fair when graph is sufficiently large.
 	for node := range cache.Nodes {
-		betweennessCentrality(cache, node, centrality)
+		work <- node
+	}
+
+	close(work)
+	wg.Wait()
+	close(partials)
+
+	// Collect and sum partials for final result.
+	centrality := make([]float64, len(cache.Nodes))
+	for partial := range partials {
+		for i := 0; i < len(partial); i++ {
+			centrality[i] += partial[i]
+		}
 	}
 
 	// Get min/max to be able to normalize
@@ -169,11 +222,11 @@ func (bc *BetweennessCentrality) Refresh(graph ChannelGraph) error {
 	bc.min = 0
 	bc.max = 0
 	if len(centrality) > 0 {
-		for i := 1; i < len(centrality); i++ {
-			if centrality[i] < bc.min {
-				bc.min = centrality[i]
-			} else if centrality[i] > bc.max {
-				bc.max = centrality[i]
+		for _, v := range centrality {
+			if v < bc.min {
+				bc.min = v
+			} else if v > bc.max {
+				bc.max = v
 			}
 		}
 	}

autopilot/betweenness_centrality_test.go

@@ -1,15 +1,38 @@
 package autopilot
 
 import (
+	"fmt"
 	"testing"
 
 	"github.com/btcsuite/btcd/btcec"
 	"github.com/btcsuite/btcutil"
 )
 
+func TestBetweennessCentralityMetricConstruction(t *testing.T) {
+	failing := []int{-1, 0}
+	ok := []int{1, 10}
+
+	for _, workers := range failing {
+		m, err := NewBetweennessCentralityMetric(workers)
+		if m != nil || err == nil {
+			t.Fatalf("construction must fail with <= 0 workers")
+		}
+	}
+
+	for _, workers := range ok {
+		m, err := NewBetweennessCentralityMetric(workers)
+		if m == nil || err != nil {
+			t.Fatalf("construction must succeed with >= 1 workers")
+		}
+	}
+}
+
 // Tests that empty graph results in empty centrality result.
 func TestBetweennessCentralityEmptyGraph(t *testing.T) {
-	centralityMetric := NewBetweennessCentralityMetric()
+	centralityMetric, err := NewBetweennessCentralityMetric(1)
+	if err != nil {
+		t.Fatalf("construction must succeed with positive number of workers")
+	}
 
 	for _, chanGraph := range chanGraphs {
 		graph, cleanup, err := chanGraph.genFunc()
@@ -91,8 +114,9 @@ func TestBetweennessCentralityWithNonEmptyGraph(t *testing.T) {
 		},
 	}
 
-	tests := []struct {
-		name       string
+	workers := []int{1, 3, 9, 100}
+
+	results := []struct {
 		normalize  bool
 		centrality []float64
 	}{
@@ -110,7 +134,9 @@ func TestBetweennessCentralityWithNonEmptyGraph(t *testing.T) {
 		},
 	}
 
+	for _, numWorkers := range workers {
 		for _, chanGraph := range chanGraphs {
+			numWorkers := numWorkers
 			graph, cleanup, err := chanGraph.genFunc()
 			if err != nil {
 				t.Fatalf("unable to create graph: %v", err)
@@ -119,23 +145,27 @@ func TestBetweennessCentralityWithNonEmptyGraph(t *testing.T) {
 				defer cleanup()
 			}
 
-		success := t.Run(chanGraph.name, func(t1 *testing.T) {
-			centralityMetric := NewBetweennessCentralityMetric()
-			graphNodes := buildTestGraph(t1, graph, graphDesc)
+			testName := fmt.Sprintf("%v %d workers", chanGraph.name, numWorkers)
+			success := t.Run(testName, func(t1 *testing.T) {
+				centralityMetric, err := NewBetweennessCentralityMetric(numWorkers)
+				if err != nil {
+					t.Fatalf("construction must succeed with positive number of workers")
+				}
 
+				graphNodes := buildTestGraph(t1, graph, graphDesc)
 				if err := centralityMetric.Refresh(graph); err != nil {
 					t1.Fatalf("error while calculating betweeness centrality")
 				}
-			for _, test := range tests {
-				test := test
-				centrality := centralityMetric.GetMetric(test.normalize)
+				for _, expected := range results {
+					expected := expected
+					centrality := centralityMetric.GetMetric(expected.normalize)
 
 					if len(centrality) != graphDesc.nodes {
 						t.Fatalf("expected %v values, got: %v",
 							graphDesc.nodes, len(centrality))
 					}
 
-				for node, nodeCentrality := range test.centrality {
+					for node, nodeCentrality := range expected.centrality {
 						nodeID := NewNodeID(graphNodes[node])
 						calculatedCentrality, ok := centrality[nodeID]
 						if !ok {
@@ -144,7 +174,7 @@ func TestBetweennessCentralityWithNonEmptyGraph(t *testing.T) {
 
 						if nodeCentrality != calculatedCentrality {
 							t1.Errorf("centrality for node: %v should be %v, got: %v",
-							node, test.centrality[node], calculatedCentrality)
+								node, nodeCentrality, calculatedCentrality)
 						}
 					}
 				}
@@ -153,4 +183,5 @@ func TestBetweennessCentralityWithNonEmptyGraph(t *testing.T) {
 				break
 			}
 		}
+	}
 }

rpcserver.go

@@ -10,6 +10,7 @@ import (
 	"io"
 	"math"
 	"net/http"
+	"runtime"
 	"sort"
 	"strings"
 	"sync"
@@ -4693,7 +4694,12 @@ func (r *rpcServer) GetNodeMetrics(ctx context.Context,
 	// Calculate betweenness centrality if requested. Note that depending on the
 	// graph size, this may take up to a few minutes.
 	channelGraph := autopilot.ChannelGraphFromDatabase(graph)
-	centralityMetric := autopilot.NewBetweennessCentralityMetric()
+	centralityMetric, err := autopilot.NewBetweennessCentralityMetric(
+		runtime.NumCPU(),
+	)
+	if err != nil {
+		return nil, err
+	}
 	if err := centralityMetric.Refresh(channelGraph); err != nil {
 		return nil, err
 	}