Implementation

NOTE:
Rebased from main after refactor in #13014

Signed-off-by: Danny Kopping <danny.kopping@grafana.com>

cmd/prometheus/main.go

@@ -137,6 +137,7 @@ type flagConfig struct {
 	forGracePeriod      model.Duration
 	outageTolerance     model.Duration
 	resendDelay         model.Duration
+	maxConcurrentEvals  int64
 	web                 web.Options
 	scrape              scrape.Options
 	tsdb                tsdbOptions
@@ -411,6 +412,9 @@ func main() {
 	serverOnlyFlag(a, "rules.alert.resend-delay", "Minimum amount of time to wait before resending an alert to Alertmanager.").
 		Default("1m").SetValue(&cfg.resendDelay)
 
+	serverOnlyFlag(a, "rules.max-concurrent-rule-evals", "Global concurrency limit for independent rules which can run concurrently.").
+		Default("4").Int64Var(&cfg.maxConcurrentEvals)
+
 	a.Flag("scrape.adjust-timestamps", "Adjust scrape timestamps by up to `scrape.timestamp-tolerance` to align them to the intended schedule. See https://github.com/prometheus/prometheus/issues/7846 for more context. Experimental. This flag will be removed in a future release.").
 		Hidden().Default("true").BoolVar(&scrape.AlignScrapeTimestamps)
 
@@ -749,17 +753,18 @@ func main() {
 		queryEngine = promql.NewEngine(opts)
 
 		ruleManager = rules.NewManager(&rules.ManagerOptions{
-			Appendable:      fanoutStorage,
-			Queryable:       localStorage,
-			QueryFunc:       rules.EngineQueryFunc(queryEngine, fanoutStorage),
-			NotifyFunc:      rules.SendAlerts(notifierManager, cfg.web.ExternalURL.String()),
-			Context:         ctxRule,
-			ExternalURL:     cfg.web.ExternalURL,
-			Registerer:      prometheus.DefaultRegisterer,
-			Logger:          log.With(logger, "component", "rule manager"),
-			OutageTolerance: time.Duration(cfg.outageTolerance),
-			ForGracePeriod:  time.Duration(cfg.forGracePeriod),
-			ResendDelay:     time.Duration(cfg.resendDelay),
+			Appendable:         fanoutStorage,
+			Queryable:          localStorage,
+			QueryFunc:          rules.EngineQueryFunc(queryEngine, fanoutStorage),
+			NotifyFunc:         rules.SendAlerts(notifierManager, cfg.web.ExternalURL.String()),
+			Context:            ctxRule,
+			ExternalURL:        cfg.web.ExternalURL,
+			Registerer:         prometheus.DefaultRegisterer,
+			Logger:             log.With(logger, "component", "rule manager"),
+			OutageTolerance:    time.Duration(cfg.outageTolerance),
+			ForGracePeriod:     time.Duration(cfg.forGracePeriod),
+			ResendDelay:        time.Duration(cfg.resendDelay),
+			MaxConcurrentEvals: cfg.maxConcurrentEvals,
 		})
 	}
 

docs/command-line/prometheus.md

@@ -48,6 +48,7 @@ The Prometheus monitoring server
 | <code class="text-nowrap">--rules.alert.for-outage-tolerance</code> | Max time to tolerate prometheus outage for restoring "for" state of alert. Use with server mode only. | `1h` |
 | <code class="text-nowrap">--rules.alert.for-grace-period</code> | Minimum duration between alert and restored "for" state. This is maintained only for alerts with configured "for" time greater than grace period. Use with server mode only. | `10m` |
 | <code class="text-nowrap">--rules.alert.resend-delay</code> | Minimum amount of time to wait before resending an alert to Alertmanager. Use with server mode only. | `1m` |
+| <code class="text-nowrap">--rules.max-concurrent-rule-evals</code> | Global concurrency limit for independent rules which can run concurrently. Use with server mode only. | `4` |
 | <code class="text-nowrap">--alertmanager.notification-queue-capacity</code> | The capacity of the queue for pending Alertmanager notifications. Use with server mode only. | `10000` |
 | <code class="text-nowrap">--query.lookback-delta</code> | The maximum lookback duration for retrieving metrics during expression evaluations and federation. Use with server mode only. | `5m` |
 | <code class="text-nowrap">--query.timeout</code> | Maximum time a query may take before being aborted. Use with server mode only. | `2m` |

rules/fixtures/rules_dependencies.yaml

@@ -0,0 +1,7 @@
+groups:
+  - name: test
+    rules:
+      - record: job:http_requests:rate5m
+        expr: sum by (job)(rate(http_requests_total[5m]))
+      - record: HighRequestRate
+        expr: job:http_requests:rate5m > 100

rules/fixtures/rules_multiple.yaml

@@ -0,0 +1,14 @@
+groups:
+  - name: test
+    rules:
+      # independents
+      - record: job:http_requests:rate1m
+        expr: sum by (job)(rate(http_requests_total[1m]))
+      - record: job:http_requests:rate5m
+        expr: sum by (job)(rate(http_requests_total[5m]))
+
+      # dependents
+      - record: job:http_requests:rate15m
+        expr: sum by (job)(rate(http_requests_total[15m]))
+      - record: TooManyRequests
+        expr: job:http_requests:rate15m > 100

rules/fixtures/rules_multiple_groups.yaml

@@ -0,0 +1,28 @@
+groups:
+  - name: http
+    rules:
+      # independents
+      - record: job:http_requests:rate1m
+        expr: sum by (job)(rate(http_requests_total[1m]))
+      - record: job:http_requests:rate5m
+        expr: sum by (job)(rate(http_requests_total[5m]))
+
+      # dependents
+      - record: job:http_requests:rate15m
+        expr: sum by (job)(rate(http_requests_total[15m]))
+      - record: TooManyHTTPRequests
+        expr: job:http_requests:rate15m > 100
+
+  - name: grpc
+    rules:
+      # independents
+      - record: job:grpc_requests:rate1m
+        expr: sum by (job)(rate(grpc_requests_total[1m]))
+      - record: job:grpc_requests:rate5m
+        expr: sum by (job)(rate(grpc_requests_total[5m]))
+
+      # dependents
+      - record: job:grpc_requests:rate15m
+        expr: sum by (job)(rate(grpc_requests_total[15m]))
+      - record: TooManyGRPCRequests
+        expr: job:grpc_requests:rate15m > 100

rules/group.go

@@ -21,8 +21,11 @@ import (
 	"sync"
 	"time"
 
+	"go.uber.org/atomic"
 	"golang.org/x/exp/slices"
 
+	"github.com/prometheus/prometheus/promql/parser"
+
 	"github.com/go-kit/log"
 	"github.com/go-kit/log/level"
 	"github.com/prometheus/client_golang/prometheus"
@@ -68,6 +71,7 @@ type Group struct {
 	// Rule group evaluation iteration function,
 	// defaults to DefaultEvalIterationFunc.
 	evalIterationFunc GroupEvalIterationFunc
+	dependencyMap     dependencyMap
 }
 
 // GroupEvalIterationFunc is used to implement and extend rule group
@@ -126,6 +130,7 @@ func NewGroup(o GroupOptions) *Group {
 		logger:               log.With(o.Opts.Logger, "file", o.File, "group", o.Name),
 		metrics:              metrics,
 		evalIterationFunc:    evalIterationFunc,
+		dependencyMap:        buildDependencyMap(o.Rules),
 	}
 }
 
@@ -421,7 +426,7 @@ func (g *Group) CopyState(from *Group) {
 
 // Eval runs a single evaluation cycle in which all rules are evaluated sequentially.
 func (g *Group) Eval(ctx context.Context, ts time.Time) {
-	var samplesTotal float64
+	var samplesTotal atomic.Float64
 	for i, rule := range g.rules {
 		select {
 		case <-g.done:
@@ -429,7 +434,12 @@ func (g *Group) Eval(ctx context.Context, ts time.Time) {
 		default:
 		}
 
-		func(i int, rule Rule) {
+		eval := func(i int, rule Rule, async bool) {
+			if async {
+				defer func() {
+					g.opts.ConcurrentEvalSema.Release(1)
+				}()
+			}
 			logger := log.WithPrefix(g.logger, "name", rule.Name(), "index", i)
 			ctx, sp := otel.Tracer("").Start(ctx, "rule")
 			sp.SetAttributes(attribute.String("name", rule.Name()))
@@ -465,7 +475,7 @@ func (g *Group) Eval(ctx context.Context, ts time.Time) {
 			}
 			rule.SetHealth(HealthGood)
 			rule.SetLastError(nil)
-			samplesTotal += float64(len(vector))
+			samplesTotal.Add(float64(len(vector)))
 
 			if ar, ok := rule.(*AlertingRule); ok {
 				ar.sendAlerts(ctx, ts, g.opts.ResendDelay, g.interval, g.opts.NotifyFunc)
@@ -554,10 +564,19 @@ func (g *Group) Eval(ctx context.Context, ts time.Time) {
 					}
 				}
 			}
-		}(i, rule)
+		}
+
+		// If the rule has no dependencies, it can run concurrently because no other rules in this group depend on its output.
+		// Try run concurrently if there are slots available.
+		if g.dependencyMap.isIndependent(rule) && g.opts.ConcurrentEvalSema != nil && g.opts.ConcurrentEvalSema.TryAcquire(1) {
+			go eval(i, rule, true)
+		} else {
+			eval(i, rule, false)
+		}
 	}
+
 	if g.metrics != nil {
-		g.metrics.GroupSamples.WithLabelValues(GroupKey(g.File(), g.Name())).Set(samplesTotal)
+		g.metrics.GroupSamples.WithLabelValues(GroupKey(g.File(), g.Name())).Set(samplesTotal.Load())
 	}
 	g.cleanupStaleSeries(ctx, ts)
 }
@@ -866,3 +885,109 @@ func NewGroupMetrics(reg prometheus.Registerer) *Metrics {
 
 	return m
 }
+
+// dependencyMap is a data-structure which contains the relationships between rules within a group.
+// It is used to describe the dependency associations between recording rules in a group whereby one rule uses the
+// output metric produced by another recording rule in its expression (i.e. as its "input").
+type dependencyMap map[Rule][]Rule
+
+// dependents returns all rules which use the output of the given rule as one of their inputs.
+func (m dependencyMap) dependents(r Rule) []Rule {
+	if len(m) == 0 {
+		return nil
+	}
+
+	return m[r]
+}
+
+// dependencies returns all the rules on which the given rule is dependent for input.
+func (m dependencyMap) dependencies(r Rule) []Rule {
+	if len(m) == 0 {
+		return nil
+	}
+
+	var parents []Rule
+	for parent, children := range m {
+		if len(children) == 0 {
+			continue
+		}
+
+		for _, child := range children {
+			if child == r {
+				parents = append(parents, parent)
+			}
+		}
+	}
+
+	return parents
+}
+
+func (m dependencyMap) isIndependent(r Rule) bool {
+	if m == nil {
+		return false
+	}
+
+	return len(m.dependents(r)) == 0 && len(m.dependencies(r)) == 0
+}
+
+// buildDependencyMap builds a data-structure which contains the relationships between rules within a group.
+func buildDependencyMap(rules []Rule) dependencyMap {
+	dependencies := make(dependencyMap)
+
+	if len(rules) <= 1 {
+		// No relationships if group has 1 or fewer rules.
+		return nil
+	}
+
+	inputs := make(map[string][]Rule, len(rules))
+	outputs := make(map[string][]Rule, len(rules))
+
+	var indeterminate bool
+
+	for _, rule := range rules {
+		rule := rule
+
+		name := rule.Name()
+		outputs[name] = append(outputs[name], rule)
+
+		parser.Inspect(rule.Query(), func(node parser.Node, path []parser.Node) error {
+			if n, ok := node.(*parser.VectorSelector); ok {
+				// A wildcard metric expression means we cannot reliably determine if this rule depends on any other,
+				// which means we cannot safely run any rules concurrently.
+				if n.Name == "" && len(n.LabelMatchers) > 0 {
+					indeterminate = true
+					return nil
+				}
+
+				// Rules which depend on "meta-metrics" like ALERTS and ALERTS_FOR_STATE will have undefined behaviour
+				// if they run concurrently.
+				if n.Name == alertMetricName || n.Name == alertForStateMetricName {
+					indeterminate = true
+					return nil
+				}
+
+				inputs[n.Name] = append(inputs[n.Name], rule)
+			}
+			return nil
+		})
+	}
+
+	if indeterminate {
+		return nil
+	}
+
+	if len(inputs) == 0 || len(outputs) == 0 {
+		// No relationships can be inferred.
+		return nil
+	}
+
+	for output, outRules := range outputs {
+		for _, outRule := range outRules {
+			if rs, found := inputs[output]; found && len(rs) > 0 {
+				dependencies[outRule] = append(dependencies[outRule], rs...)
+			}
+		}
+	}
+
+	return dependencies
+}

rules/manager.go

@@ -26,6 +26,7 @@ import (
 	"github.com/go-kit/log/level"
 	"github.com/prometheus/client_golang/prometheus"
 	"golang.org/x/exp/slices"
+	"golang.org/x/sync/semaphore"
 
 	"github.com/prometheus/prometheus/model/labels"
 	"github.com/prometheus/prometheus/model/rulefmt"
@@ -103,18 +104,20 @@ type NotifyFunc func(ctx context.Context, expr string, alerts ...*Alert)
 
 // ManagerOptions bundles options for the Manager.
 type ManagerOptions struct {
-	ExternalURL     *url.URL
-	QueryFunc       QueryFunc
-	NotifyFunc      NotifyFunc
-	Context         context.Context
-	Appendable      storage.Appendable
-	Queryable       storage.Queryable
-	Logger          log.Logger
-	Registerer      prometheus.Registerer
-	OutageTolerance time.Duration
-	ForGracePeriod  time.Duration
-	ResendDelay     time.Duration
-	GroupLoader     GroupLoader
+	ExternalURL        *url.URL
+	QueryFunc          QueryFunc
+	NotifyFunc         NotifyFunc
+	Context            context.Context
+	Appendable         storage.Appendable
+	Queryable          storage.Queryable
+	Logger             log.Logger
+	Registerer         prometheus.Registerer
+	OutageTolerance    time.Duration
+	ForGracePeriod     time.Duration
+	ResendDelay        time.Duration
+	MaxConcurrentEvals int64
+	ConcurrentEvalSema *semaphore.Weighted
+	GroupLoader        GroupLoader
 
 	Metrics *Metrics
 }
@@ -130,6 +133,8 @@ func NewManager(o *ManagerOptions) *Manager {
 		o.GroupLoader = FileLoader{}
 	}
 
+	o.ConcurrentEvalSema = semaphore.NewWeighted(o.MaxConcurrentEvals)
+
 	m := &Manager{
 		groups: map[string]*Group{},
 		opts:   o,

rules/manager_test.go

@@ -19,15 +19,18 @@ import (
 	"math"
 	"os"
 	"sort"
+	"sync"
 	"testing"
 	"time"
 
 	"github.com/go-kit/log"
 	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/testutil"
 	"github.com/prometheus/common/model"
 	"github.com/stretchr/testify/require"
 	"go.uber.org/atomic"
 	"go.uber.org/goleak"
+	"golang.org/x/sync/semaphore"
 	"gopkg.in/yaml.v2"
 
 	"github.com/prometheus/prometheus/model/labels"
@@ -1402,3 +1405,382 @@ func TestNativeHistogramsInRecordingRules(t *testing.T) {
 	require.Equal(t, expHist, fh)
 	require.Equal(t, chunkenc.ValNone, it.Next())
 }
+
+func TestDependencyMap(t *testing.T) {
+	ctx := context.Background()
+	opts := &ManagerOptions{
+		Context: ctx,
+		Logger:  log.NewNopLogger(),
+	}
+
+	expr, err := parser.ParseExpr("sum by (user) (rate(requests[1m]))")
+	require.NoError(t, err)
+	rule := NewRecordingRule("user:requests:rate1m", expr, labels.Labels{})
+
+	expr, err = parser.ParseExpr("user:requests:rate1m <= 0")
+	require.NoError(t, err)
+	rule2 := NewAlertingRule("ZeroRequests", expr, 0, 0, labels.Labels{}, labels.Labels{}, labels.EmptyLabels(), "", true, log.NewNopLogger())
+	group := NewGroup(GroupOptions{
+		Name:     "rule_group",
+		Interval: time.Second,
+		Rules:    []Rule{rule, rule2},
+		Opts:     opts,
+	})
+
+	require.Equal(t, []Rule{rule2}, group.dependencyMap.dependents(rule))
+	require.Len(t, group.dependencyMap.dependencies(rule), 0)
+	require.False(t, group.dependencyMap.isIndependent(rule))
+
+	require.Len(t, group.dependencyMap.dependents(rule2), 0)
+	require.Equal(t, []Rule{rule}, group.dependencyMap.dependencies(rule2))
+	require.False(t, group.dependencyMap.isIndependent(rule2))
+}
+
+func TestNoDependency(t *testing.T) {
+	ctx := context.Background()
+	opts := &ManagerOptions{
+		Context: ctx,
+		Logger:  log.NewNopLogger(),
+	}
+
+	expr, err := parser.ParseExpr("sum by (user) (rate(requests[1m]))")
+	require.NoError(t, err)
+	rule := NewRecordingRule("user:requests:rate1m", expr, labels.Labels{})
+
+	group := NewGroup(GroupOptions{
+		Name:     "rule_group",
+		Interval: time.Second,
+		Rules:    []Rule{rule},
+		Opts:     opts,
+	})
+
+	// A group with only one rule cannot have dependencies.
+	require.False(t, group.dependencyMap.isIndependent(rule))
+}
+
+func TestNoMetricSelector(t *testing.T) {
+	ctx := context.Background()
+	opts := &ManagerOptions{
+		Context: ctx,
+		Logger:  log.NewNopLogger(),
+	}
+
+	expr, err := parser.ParseExpr("sum by (user) (rate(requests[1m]))")
+	require.NoError(t, err)
+	rule := NewRecordingRule("user:requests:rate1m", expr, labels.Labels{})
+
+	expr, err = parser.ParseExpr(`count({user="bob"})`)
+	require.NoError(t, err)
+	rule2 := NewRecordingRule("user:requests:rate1m", expr, labels.Labels{})
+
+	group := NewGroup(GroupOptions{
+		Name:     "rule_group",
+		Interval: time.Second,
+		Rules:    []Rule{rule, rule2},
+		Opts:     opts,
+	})
+
+	// A rule with no metric selector cannot be reliably determined to have no dependencies on other rules, and therefore
+	// all rules are not considered independent.
+	require.False(t, group.dependencyMap.isIndependent(rule))
+	require.False(t, group.dependencyMap.isIndependent(rule2))
+}
+
+func TestDependentRulesWithNonMetricExpression(t *testing.T) {
+	ctx := context.Background()
+	opts := &ManagerOptions{
+		Context: ctx,
+		Logger:  log.NewNopLogger(),
+	}
+
+	expr, err := parser.ParseExpr("sum by (user) (rate(requests[1m]))")
+	require.NoError(t, err)
+	rule := NewRecordingRule("user:requests:rate1m", expr, labels.Labels{})
+
+	expr, err = parser.ParseExpr("user:requests:rate1m <= 0")
+	require.NoError(t, err)
+	rule2 := NewAlertingRule("ZeroRequests", expr, 0, 0, labels.Labels{}, labels.Labels{}, labels.EmptyLabels(), "", true, log.NewNopLogger())
+
+	expr, err = parser.ParseExpr("3")
+	require.NoError(t, err)
+	rule3 := NewRecordingRule("three", expr, labels.Labels{})
+
+	group := NewGroup(GroupOptions{
+		Name:     "rule_group",
+		Interval: time.Second,
+		Rules:    []Rule{rule, rule2, rule3},
+		Opts:     opts,
+	})
+
+	require.False(t, group.dependencyMap.isIndependent(rule))
+	require.False(t, group.dependencyMap.isIndependent(rule2))
+	require.True(t, group.dependencyMap.isIndependent(rule3))
+}
+
+func TestRulesDependentOnMetaMetrics(t *testing.T) {
+	ctx := context.Background()
+	opts := &ManagerOptions{
+		Context: ctx,
+		Logger:  log.NewNopLogger(),
+	}
+
+	// This rule is not dependent on any other rules in its group but it does depend on `ALERTS`, which is produced by
+	// the rule engine, and is therefore not independent.
+	expr, err := parser.ParseExpr("count(ALERTS)")
+	require.NoError(t, err)
+	rule := NewRecordingRule("alert_count", expr, labels.Labels{})
+
+	// Create another rule so a dependency map is built (no map is built if a group contains one or fewer rules).
+	expr, err = parser.ParseExpr("1")
+	require.NoError(t, err)
+	rule2 := NewRecordingRule("one", expr, labels.Labels{})
+
+	group := NewGroup(GroupOptions{
+		Name:     "rule_group",
+		Interval: time.Second,
+		Rules:    []Rule{rule, rule2},
+		Opts:     opts,
+	})
+
+	require.False(t, group.dependencyMap.isIndependent(rule))
+}
+
+func TestDependencyMapUpdatesOnGroupUpdate(t *testing.T) {
+	files := []string{"fixtures/rules.yaml"}
+	ruleManager := NewManager(&ManagerOptions{
+		Context: context.Background(),
+		Logger:  log.NewNopLogger(),
+	})
+
+	ruleManager.start()
+	defer ruleManager.Stop()
+
+	err := ruleManager.Update(10*time.Second, files, labels.EmptyLabels(), "", nil)
+	require.NoError(t, err)
+	require.Greater(t, len(ruleManager.groups), 0, "expected non-empty rule groups")
+
+	orig := make(map[string]dependencyMap, len(ruleManager.groups))
+	for _, g := range ruleManager.groups {
+		// No dependency map is expected because there is only one rule in the group.
+		require.Empty(t, g.dependencyMap)
+		orig[g.Name()] = g.dependencyMap
+	}
+
+	// Update once without changing groups.
+	err = ruleManager.Update(10*time.Second, files, labels.EmptyLabels(), "", nil)
+	require.NoError(t, err)
+	for h, g := range ruleManager.groups {
+		// Dependency maps are the same because of no updates.
+		require.Equal(t, orig[h], g.dependencyMap)
+	}
+
+	// Groups will be recreated when updated.
+	files[0] = "fixtures/rules_dependencies.yaml"
+	err = ruleManager.Update(10*time.Second, files, labels.EmptyLabels(), "", nil)
+	require.NoError(t, err)
+
+	for h, g := range ruleManager.groups {
+		// Dependency maps must change because the groups would've been updated.
+		require.NotEqual(t, orig[h], g.dependencyMap)
+		// We expect there to be some dependencies since the new rule group contains a dependency.
+		require.Greater(t, len(g.dependencyMap), 0)
+	}
+}
+
+func TestAsyncRuleEvaluation(t *testing.T) {
+	storage := teststorage.New(t)
+	t.Cleanup(func() { storage.Close() })
+
+	const artificialDelay = time.Second
+
+	var (
+		inflightQueries atomic.Int32
+		maxInflight     atomic.Int32
+	)
+
+	files := []string{"fixtures/rules_multiple.yaml"}
+	ruleManager := NewManager(&ManagerOptions{
+		Context:    context.Background(),
+		Logger:     log.NewNopLogger(),
+		Appendable: storage,
+		QueryFunc: func(ctx context.Context, q string, ts time.Time) (promql.Vector, error) {
+			inflightQueries.Add(1)
+			defer func() {
+				inflightQueries.Add(-1)
+			}()
+
+			// Artificially delay all query executions to highly concurrent execution improvement.
+			time.Sleep(artificialDelay)
+
+			// return a stub sample
+			return promql.Vector{
+				promql.Sample{Metric: labels.FromStrings("__name__", "test"), T: ts.UnixMilli(), F: 12345},
+			}, nil
+		},
+	})
+
+	// Evaluate groups manually to show the impact of async rule evaluations.
+	groups, errs := ruleManager.LoadGroups(time.Second, labels.EmptyLabels(), "", nil, files...)
+	require.Empty(t, errs)
+	require.Len(t, groups, 1)
+
+	expectedRules := 4
+
+	t.Run("synchronous evaluation with independent rules", func(t *testing.T) {
+		ctx, cancel := context.WithCancel(context.Background())
+
+		for _, group := range groups {
+			require.Len(t, group.rules, expectedRules)
+
+			start := time.Now()
+
+			// Never expect more than 1 inflight query at a time.
+			go func() {
+				for {
+					select {
+					case <-ctx.Done():
+						return
+					default:
+						highWatermark := maxInflight.Load()
+						current := inflightQueries.Load()
+						if current > highWatermark {
+							maxInflight.Store(current)
+						}
+
+						time.Sleep(time.Millisecond)
+					}
+				}
+			}()
+
+			group.Eval(ctx, start)
+
+			require.EqualValues(t, 1, maxInflight.Load())
+			// Each rule should take at least 1 second to execute sequentially.
+			require.GreaterOrEqual(t, time.Since(start).Seconds(), (time.Duration(expectedRules) * artificialDelay).Seconds())
+			// Each rule produces one vector.
+			require.EqualValues(t, expectedRules, testutil.ToFloat64(group.metrics.GroupSamples))
+		}
+
+		cancel()
+	})
+
+	t.Run("asynchronous evaluation with independent rules", func(t *testing.T) {
+		// Reset.
+		inflightQueries.Store(0)
+		maxInflight.Store(0)
+		ctx, cancel := context.WithCancel(context.Background())
+
+		for _, group := range groups {
+			// Allow up to 2 concurrent rule evaluations.
+			group.opts.ConcurrentEvalSema = semaphore.NewWeighted(2)
+			require.Len(t, group.rules, expectedRules)
+
+			start := time.Now()
+
+			go func() {
+				for {
+					select {
+					case <-ctx.Done():
+						return
+					default:
+						highWatermark := maxInflight.Load()
+						current := inflightQueries.Load()
+						if current > highWatermark {
+							maxInflight.Store(current)
+						}
+
+						time.Sleep(time.Millisecond)
+					}
+				}
+			}()
+
+			group.Eval(ctx, start)
+
+			require.EqualValues(t, 3, maxInflight.Load())
+			// Some rules should execute concurrently so should complete quicker.
+			require.Less(t, time.Since(start).Seconds(), (time.Duration(expectedRules) * artificialDelay).Seconds())
+			// Each rule produces one vector.
+			require.EqualValues(t, expectedRules, testutil.ToFloat64(group.metrics.GroupSamples))
+		}
+
+		cancel()
+	})
+}
+
+func TestBoundedRuleEvalConcurrency(t *testing.T) {
+	storage := teststorage.New(t)
+	t.Cleanup(func() { storage.Close() })
+
+	const artificialDelay = time.Millisecond * 100
+
+	var (
+		inflightQueries atomic.Int32
+		maxInflight     atomic.Int32
+		maxConcurrency  int64 = 3
+		groupCount            = 2
+	)
+
+	files := []string{"fixtures/rules_multiple_groups.yaml"}
+	ruleManager := NewManager(&ManagerOptions{
+		Context:            context.Background(),
+		Logger:             log.NewNopLogger(),
+		Appendable:         storage,
+		MaxConcurrentEvals: maxConcurrency,
+		QueryFunc: func(ctx context.Context, q string, ts time.Time) (promql.Vector, error) {
+			inflightQueries.Add(1)
+			defer func() {
+				inflightQueries.Add(-1)
+			}()
+
+			// Artificially delay all query executions to highly concurrent execution improvement.
+			time.Sleep(artificialDelay)
+
+			// return a stub sample
+			return promql.Vector{
+				promql.Sample{Metric: labels.FromStrings("__name__", "test"), T: ts.UnixMilli(), F: 12345},
+			}, nil
+		},
+	})
+
+	// Evaluate groups manually to show the impact of async rule evaluations.
+	groups, errs := ruleManager.LoadGroups(time.Second, labels.EmptyLabels(), "", nil, files...)
+	require.Empty(t, errs)
+	require.Len(t, groups, groupCount)
+
+	ctx, cancel := context.WithCancel(context.Background())
+
+	go func() {
+		for {
+			select {
+			case <-ctx.Done():
+				return
+			default:
+				highWatermark := maxInflight.Load()
+				current := inflightQueries.Load()
+				if current > highWatermark {
+					maxInflight.Store(current)
+				}
+
+				time.Sleep(time.Millisecond)
+			}
+		}
+	}()
+
+	// Evaluate groups concurrently (like they normally do).
+	var wg sync.WaitGroup
+	for _, group := range groups {
+		group := group
+
+		wg.Add(1)
+		go func() {
+			group.Eval(ctx, time.Now())
+			wg.Done()
+		}()
+	}
+
+	wg.Wait()
+	cancel()
+
+	// Synchronous queries also count towards inflight, so at most we can have maxConcurrency+$groupCount inflight evaluations.
+	require.EqualValues(t, maxInflight.Load(), int32(maxConcurrency)+int32(groupCount))
+}