Size active queried series worker pool from GOMAXPROCS

CHANGELOG.md

@@ -77,6 +77,7 @@
 * [BUGFIX] Querier: Fix panic due to request tracker truncating multi-byte UTF-8 character #7640
 * [BUGFIX] Ingester: Fix panic (`HistogramProtoToHistogram called with a float histogram`) when ingesting a float native histogram with a zero count (e.g. a staleness marker or empty histogram). The decoder is now selected by histogram type via `IsFloatHistogram()` instead of by count value. #7645
 * [BUGFIX] Querier: Fix parquet queryable fallback returning a nil error instead of the actual query error in `LabelValues` and `LabelNames`. #7638
+* [BUGFIX] Ingester: Size the active queried series worker pool from `GOMAXPROCS` instead of `runtime.NumCPU()`, so it respects the container CPU quota (set via automaxprocs) rather than the host core count. #XXXX
 
 ## 1.21.0 2026-04-24
 

pkg/ingester/active_queried_series.go

@@ -385,8 +385,10 @@ type ActiveQueriedSeriesService struct {
 
 // NewActiveQueriedSeriesService creates a new ActiveQueriedSeriesService service.
 func NewActiveQueriedSeriesService(logger log.Logger, registerer prometheus.Registerer) *ActiveQueriedSeriesService {
-	// Cap at 4 workers to avoid excessive goroutines
-	numWorkers := max(min(runtime.NumCPU()/2, 4), 1)
+	// Cap at 4 workers to avoid excessive goroutines. Use GOMAXPROCS (which
+	// automaxprocs sets from the CPU cgroup quota) rather than NumCPU, so the
+	// pool is sized to the container's CPU budget instead of the host cores.
+	numWorkers := max(min(runtime.GOMAXPROCS(0)/2, 4), 1)
 
 	m := &ActiveQueriedSeriesService{
 		updateChan: make(chan activeQueriedSeriesUpdate, 10000), // Buffered channel to avoid blocking

pkg/ingester/active_queried_series_test.go

@@ -3,6 +3,7 @@ package ingester
 import (
 	"context"
 	"fmt"
+	"runtime"
 	"sync"
 	"testing"
 	"time"
@@ -546,3 +547,30 @@ func TestActiveQueriedSeriesService_NoSendOnClosedChannelOnShutdown(t *testing.T
 		t.Fatalf("producer goroutine panicked during shutdown: %v", panicMsg.Load())
 	}
 }
+
+func TestActiveQueriedSeriesService_NumWorkersFollowsGOMAXPROCS(t *testing.T) {
+	// The worker pool must be sized from GOMAXPROCS (which automaxprocs derives
+	// from the CPU cgroup quota) and not from runtime.NumCPU (host cores, which
+	// ignores cgroup limits). Save and restore GOMAXPROCS around the test.
+	defer runtime.GOMAXPROCS(runtime.GOMAXPROCS(1))
+
+	for _, tc := range []struct {
+		gomaxprocs      int
+		expectedWorkers int
+	}{
+		// GOMAXPROCS(1)/2 == 0, floored to the minimum of 1 worker. Against the
+		// buggy runtime.NumCPU() this yields min(NumCPU/2, 4) on a multi-core
+		// host and fails.
+		{gomaxprocs: 1, expectedWorkers: 1},
+		{gomaxprocs: 2, expectedWorkers: 1},
+		{gomaxprocs: 4, expectedWorkers: 2},
+		// Capped at 4 workers regardless of how many CPUs are available.
+		{gomaxprocs: 16, expectedWorkers: 4},
+	} {
+		t.Run(fmt.Sprintf("gomaxprocs=%d", tc.gomaxprocs), func(t *testing.T) {
+			runtime.GOMAXPROCS(tc.gomaxprocs)
+			svc := NewActiveQueriedSeriesService(log.NewNopLogger(), nil)
+			assert.Equal(t, tc.expectedWorkers, svc.numWorkers)
+		})
+	}
+}