balancer/weightedroundrobin: add load balancing policy

balancer/weightedroundrobin/config.go

@@ -0,0 +1,40 @@
+/*
+ *
+ * Copyright 2023 gRPC authors.
+ *
+ * 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 weightedroundrobin
+
+import (
+	"time"
+
+	"google.golang.org/grpc/serviceconfig"
+)
+
+type lbConfig struct {
+	serviceconfig.LoadBalancingConfig `json:"-"`
+
+	EnableOOBLoadReport     bool          `json:"enableOobLoadReport,omitempty"`
+	OOBReportingPeriod      time.Duration `json:"oobReportingPeriod,omitempty"`
+	BlackoutPeriod          time.Duration `json:"blackoutPeriod,omitempty"`
+	WeightExpirationPeriod  time.Duration `json:"weightExpirationPeriod,omitempty"`
+	WeightUpdatePeriod      time.Duration `json:"weightUpdatePeriod,omitempty"`
+	ErrorUtilizationPenalty float64       `json:"errorUtilizationPenalty,omitempty"`
+}
+
+// LBConfigForTesting allows tests to produce a JSON form of the config from
+// the struct instead of using a string.
+type LBConfigForTesting = lbConfig

balancer/weightedroundrobin/internal/internal.go

@@ -0,0 +1,23 @@
+/*
+ *
+ * Copyright 2023 gRPC authors.
+ *
+ * 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 internal
+
+// AllowAnyWeightUpdatePeriod permits any setting of WeightUpdatePeriod for
+// testing.  Normally a minimum of 100ms is applied.
+var AllowAnyWeightUpdatePeriod bool

balancer/weightedroundrobin/logging.go

@@ -0,0 +1,34 @@
+/*
+ *
+ * Copyright 2023 gRPC authors.
+ *
+ * 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 weightedroundrobin
+
+import (
+	"fmt"
+
+	"google.golang.org/grpc/grpclog"
+	internalgrpclog "google.golang.org/grpc/internal/grpclog"
+)
+
+const prefix = "[weighted-round-robin-lb %p] "
+
+var logger = grpclog.Component("xds")
+
+func prefixLogger(p *wrrBalancer) *internalgrpclog.PrefixLogger {
+	return internalgrpclog.NewPrefixLogger(logger, fmt.Sprintf(prefix, p))
+}

balancer/weightedroundrobin/scheduler.go

@@ -0,0 +1,119 @@
+/*
+ *
+ * Copyright 2023 gRPC authors.
+ *
+ * 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 weightedroundrobin
+
+import (
+	"math"
+)
+
+type scheduler interface {
+	nextIndex() int
+}
+
+func newScheduler(scWeights []float64, inc func() uint32) scheduler {
+	n := len(scWeights)
+	if n == 0 {
+		return nil
+	}
+	if n == 1 {
+		return &rrScheduler{numSCs: 1, inc: inc}
+	}
+	sum := float64(0)
+	numZero := 0
+	max := float64(0)
+	for _, w := range scWeights {
+		sum += w
+		if w > max {
+			max = w
+		}
+		if w == 0 {
+			numZero++
+		}
+	}
+	if numZero == n {
+		return &rrScheduler{numSCs: n, inc: inc}
+	}
+	unscaledMean := sum / float64(n-numZero)
+	scalingFactor := maxWeight / max
+	mean := uint16(math.Round(scalingFactor * unscaledMean))
+
+	weights := make([]uint16, n, n)
+	for i, w := range scWeights {
+		if w == 0 {
+			weights[i] = mean
+		} else {
+			weights[i] = uint16(math.Round(scalingFactor * w))
+		}
+	}
+
+	logger.Infof("using edf scheduler with weights: %v", weights)
+	return &edfScheduler{weights: weights, inc: inc}
+}
+
+const maxWeight = math.MaxUint16
+
+type edfScheduler struct {
+	inc     func() uint32
+	weights []uint16
+}
+
+// Returns the index in weights to choose.
+func (s *edfScheduler) nextIndex() int {
+	const offset = maxWeight / 2
+
+	for {
+		idx := uint64(s.inc())
+
+		// The sequence number (idx) is split in two: the lower %n gives the
+		// index of the backend, and the rest gives the number of times we've
+		// iterated through all backends. `generation` is used to
+		// deterministically decide whether we pick or skip the backend on this
+		// iteration, in proportion to the backend's weight.
+
+		backendIndex := idx % uint64(len(s.weights))
+		generation := idx / uint64(len(s.weights))
+		weight := uint64(s.weights[backendIndex])
+
+		// We pick a backend `weight` times per `maxWeight` generations. The
+		// multiply and modulus ~evenly spread out the picks for a given
+		// backend between different generations. The offset by `backendIndex`
+		// helps to reduce the chance of multiple consecutive non-picks: if we
+		// have two consecutive backends with an equal, say, 80% weight of the
+		// max, with no offset we would see 1/5 generations that skipped both.
+		// TODO(b/190488683): add test for offset efficacy.
+		mod := uint64(weight*generation+backendIndex*offset) % maxWeight
+
+		if mod < maxWeight-weight {
+			continue
+		}
+		return int(backendIndex)
+	}
+}
+
+// A simple RR scheduler to use for fallback when all weights are zero or only
+// one subconn exists.
+type rrScheduler struct {
+	inc    func() uint32
+	numSCs int
+}
+
+func (s *rrScheduler) nextIndex() int {
+	idx := int(s.inc())
+	return idx % s.numSCs
+}

balancer/weightedroundrobin/weightedroundrobin.go

@@ -16,16 +16,20 @@
  *
  */
 
-// Package weightedroundrobin defines a weighted roundrobin balancer.
+// Package weightedroundrobin provides an implementation of the weighted round
+// robin LB policy, as defined in gRFC A58:
+// https://github.com/grpc/proposal/blob/master/A58-client-side-weighted-round-robin-lb-policy.md
+//
+// # Experimental
+//
+// Notice: This package is EXPERIMENTAL and may be changed or removed in a
+// later release.
 package weightedroundrobin
 
 import (
 	"google.golang.org/grpc/resolver"
 )
 
-// Name is the name of weighted_round_robin balancer.
-const Name = "weighted_round_robin"
-
 // attributeKey is the type used as the key to store AddrInfo in the
 // BalancerAttributes field of resolver.Address.
 type attributeKey struct{}
@@ -44,23 +48,13 @@ func (a AddrInfo) Equal(o interface{}) bool {
 
 // SetAddrInfo returns a copy of addr in which the BalancerAttributes field is
 // updated with addrInfo.
-//
-// # Experimental
-//
-// Notice: This API is EXPERIMENTAL and may be changed or removed in a
-// later release.
 func SetAddrInfo(addr resolver.Address, addrInfo AddrInfo) resolver.Address {
 	addr.BalancerAttributes = addr.BalancerAttributes.WithValue(attributeKey{}, addrInfo)
 	return addr
 }
 
 // GetAddrInfo returns the AddrInfo stored in the BalancerAttributes field of
 // addr.
-//
-// # Experimental
-//
-// Notice: This API is EXPERIMENTAL and may be changed or removed in a
-// later release.
 func GetAddrInfo(addr resolver.Address) AddrInfo {
 	v := addr.BalancerAttributes.Value(attributeKey{})
 	ai, _ := v.(AddrInfo)

xds/internal/balancer/clusterimpl/picker.go

@@ -160,7 +160,7 @@ func (d *picker) Pick(info balancer.PickInfo) (balancer.PickResult, error) {
 			d.loadStore.CallFinished(lIDStr, info.Err)
 
 			load, ok := info.ServerLoad.(*v3orcapb.OrcaLoadReport)
-			if !ok {
+			if !ok || load == nil {
 				return
 			}
 			d.loadStore.CallServerLoad(lIDStr, serverLoadCPUName, load.CpuUtilization)