NodeSelector using system load metric (#94)

* use load when selecting nodes

* Apply suggestions from code review

Co-authored-by: David Zhao <david@davidzhao.com>

* sysload selector test

* wire nodeSelectorFromConfig

* fix tests

* Update pkg/routing/selectorsystemload_test.go

Co-authored-by: David Zhao <david@davidzhao.com>

* dz review: nits

Co-authored-by: David Zhao <david@davidzhao.com>

pkg/routing/selectorsystemload.go

@@ -0,0 +1,34 @@
+package routing
+
+import (
+	livekit "github.com/livekit/livekit-server/proto"
+	"github.com/thoas/go-funk"
+)
+
+type SystemLoadSelector struct {
+	SysloadLimit float32
+}
+
+func (s *SystemLoadSelector) SelectNode(nodes []*livekit.Node, room *livekit.Room) (*livekit.Node, error) {
+	nodes = GetAvailableNodes(nodes)
+	if len(nodes) == 0 {
+		return nil, ErrNoAvailableNodes
+	}
+
+	nodesLowLoad := []*livekit.Node{}
+	for _, node := range nodes {
+		numCpus := node.Stats.NumCpus
+		if numCpus == 0 {
+			numCpus = 1
+		}
+		if node.Stats.LoadAvgLast1Min/float32(numCpus) < s.SysloadLimit {
+			nodesLowLoad = append(nodesLowLoad, node)
+		}
+	}
+	if len(nodesLowLoad) > 0 {
+		nodes = nodesLowLoad
+	}
+
+	idx := funk.RandomInt(0, len(nodes))
+	return nodes[idx], nil
+}

pkg/routing/selectorsystemload_test.go

@@ -0,0 +1,54 @@
+package routing_test
+
+import (
+	"testing"
+	"time"
+
+	"github.com/livekit/livekit-server/pkg/routing"
+	livekit "github.com/livekit/livekit-server/proto"
+	"github.com/stretchr/testify/require"
+)
+
+var (
+	nodeLoadLow = &livekit.Node{
+		Stats: &livekit.NodeStats{
+			UpdatedAt:       time.Now().Unix(),
+			NumCpus:         1,
+			LoadAvgLast1Min: 0.0,
+		},
+	}
+
+	nodeLoadHigh = &livekit.Node{
+		Stats: &livekit.NodeStats{
+			UpdatedAt:       time.Now().Unix(),
+			NumCpus:         1,
+			LoadAvgLast1Min: 2.0,
+		},
+	}
+)
+
+func TestSystemLoadSelector_SelectNode(t *testing.T) {
+	selector := routing.SystemLoadSelector{SysloadLimit: 1.0}
+
+	nodes := []*livekit.Node{}
+	_, err := selector.SelectNode(nodes, nil)
+	require.Error(t, err, "should error no available nodes")
+
+	// Select a node with high load when no nodes with low load are available
+	nodes = []*livekit.Node{nodeLoadHigh}
+	if _, err := selector.SelectNode(nodes, nil); err != nil {
+		t.Error(err)
+	}
+
+	// Select a node with low load when available
+	nodes = []*livekit.Node{nodeLoadLow, nodeLoadHigh}
+	for i := 0; i < 5; i++ {
+		node, err := selector.SelectNode(nodes, nil)
+		if err != nil {
+			t.Error(err)
+		}
+		if node != nodeLoadLow {
+			t.Error("selected the wrong node")
+		}
+	}
+}