Limit maximum number of concurrent queries.

A high number of concurrent queries can slow each other down
so that none of them is reasonbly responsive. This commit limits
the number of queries being concurrently executed.

promql/engine.go

@@ -33,6 +33,7 @@ import (
 var (
 	stalenessDelta       = flag.Duration("query.staleness-delta", 300*time.Second, "Staleness delta allowance during expression evaluations.")
 	defaultQueryTimeout  = flag.Duration("query.timeout", 2*time.Minute, "Maximum time a query may take before being aborted.")
+	maxConcurrentQueries = flag.Int("query.max-concurrency", 20, "Maximum number of queries executed concurrently.")
 )
 
 // SampleStream is a stream of Values belonging to an attached COWMetric.
@@ -215,10 +216,7 @@ func (q *query) Cancel() {
 
 // Exec implements the Query interface.
 func (q *query) Exec() *Result {
-	ctx, cancel := context.WithTimeout(q.ng.baseCtx, *defaultQueryTimeout)
-	q.cancel = cancel
-
-	res, err := q.ng.exec(ctx, q)
+	res, err := q.ng.exec(q)
 	return &Result{Err: err, Value: res}
 }
 
@@ -249,6 +247,8 @@ type Engine struct {
 	// The base context for all queries and its cancellation function.
 	baseCtx       context.Context
 	cancelQueries func()
+	// The gate limiting the maximum number of concurrent and waiting queries.
+	gate *queryGate
 }
 
 // NewEngine returns a new engine.
@@ -258,6 +258,7 @@ func NewEngine(storage local.Storage) *Engine {
 		storage:       storage,
 		baseCtx:       ctx,
 		cancelQueries: cancel,
+		gate:          newQueryGate(*maxConcurrentQueries),
 	}
 }
 
@@ -316,9 +317,21 @@ func (ng *Engine) newTestQuery(stmts ...Statement) Query {
 //
 // At this point per query only one EvalStmt is evaluated. Alert and record
 // statements are not handled by the Engine.
-func (ng *Engine) exec(ctx context.Context, q *query) (Value, error) {
+func (ng *Engine) exec(q *query) (Value, error) {
 	const env = "query execution"
 
+	ctx, cancel := context.WithTimeout(q.ng.baseCtx, *defaultQueryTimeout)
+	q.cancel = cancel
+
+	queueTimer := q.stats.GetTimer(stats.ExecQueueTime).Start()
+
+	if err := ng.gate.Start(ctx); err != nil {
+		return nil, err
+	}
+	defer ng.gate.Done()
+
+	queueTimer.Stop()
+
 	// Cancel when execution is done or an error was raised.
 	defer q.cancel()
 
@@ -1125,3 +1138,35 @@ func interpolateSamples(first, second *metric.SamplePair, timestamp clientmodel.
 		Timestamp: timestamp,
 	}
 }
+
+// A queryGate controls the maximum number of concurrently running and waiting queries.
+type queryGate struct {
+	ch chan struct{}
+}
+
+// newQueryGate returns a query gate that limits the number of queries
+// being concurrently executed.
+func newQueryGate(length int) *queryGate {
+	return &queryGate{
+		ch: make(chan struct{}, length),
+	}
+}
+
+// Start blocks until the gate has a free spot or the context is done.
+func (g *queryGate) Start(ctx context.Context) error {
+	select {
+	case <-ctx.Done():
+		return contextDone(ctx, "query queue")
+	case g.ch <- struct{}{}:
+		return nil
+	}
+}
+
+// Done releases a single spot in the gate.
+func (g *queryGate) Done() {
+	select {
+	case <-g.ch:
+	default:
+		panic("engine.queryGate.Done: more operations done than started")
+	}
+}

promql/engine_test.go

@@ -6,14 +6,61 @@ import (
 	"time"
 
 	"golang.org/x/net/context"
-
-	"github.com/prometheus/prometheus/storage/local"
 )
 
 var noop = testStmt(func(context.Context) error {
 	return nil
 })
 
+func TestQueryConcurreny(t *testing.T) {
+	engine := NewEngine(nil)
+	defer engine.Stop()
+
+	block := make(chan struct{})
+	processing := make(chan struct{})
+	f1 := testStmt(func(context.Context) error {
+		processing <- struct{}{}
+		<-block
+		return nil
+	})
+
+	for i := 0; i < *maxConcurrentQueries; i++ {
+		q := engine.newTestQuery(f1)
+		go q.Exec()
+		select {
+		case <-processing:
+			// Expected.
+		case <-time.After(5 * time.Millisecond):
+			t.Fatalf("Query within concurrency threshold not being executed")
+		}
+	}
+
+	q := engine.newTestQuery(f1)
+	go q.Exec()
+
+	select {
+	case <-processing:
+		t.Fatalf("Query above concurrency threhosld being executed")
+	case <-time.After(5 * time.Millisecond):
+		// Expected.
+	}
+
+	// Terminate a running query.
+	block <- struct{}{}
+
+	select {
+	case <-processing:
+		// Expected.
+	case <-time.After(5 * time.Millisecond):
+		t.Fatalf("Query within concurrency threshold not being executed")
+	}
+
+	// Terminate remaining queries.
+	for i := 0; i < *maxConcurrentQueries; i++ {
+		block <- struct{}{}
+	}
+}
+
 func TestQueryTimeout(t *testing.T) {
 	*defaultQueryTimeout = 5 * time.Millisecond
 	defer func() {
@@ -21,10 +68,7 @@ func TestQueryTimeout(t *testing.T) {
 		*defaultQueryTimeout = 2 * time.Minute
 	}()
 
-	storage, closer := local.NewTestStorage(t, 1)
-	defer closer.Close()
-
-	engine := NewEngine(storage)
+	engine := NewEngine(nil)
 	defer engine.Stop()
 
 	f1 := testStmt(func(context.Context) error {
@@ -46,10 +90,7 @@ func TestQueryTimeout(t *testing.T) {
 }
 
 func TestQueryCancel(t *testing.T) {
-	storage, closer := local.NewTestStorage(t, 1)
-	defer closer.Close()
-
-	engine := NewEngine(storage)
+	engine := NewEngine(nil)
 	defer engine.Stop()
 
 	// As for timeouts, cancellation is only checked at designated points. We ensure
@@ -91,10 +132,7 @@ func TestQueryCancel(t *testing.T) {
 }
 
 func TestEngineShutdown(t *testing.T) {
-	storage, closer := local.NewTestStorage(t, 1)
-	defer closer.Close()
-
-	engine := NewEngine(storage)
+	engine := NewEngine(nil)
 
 	handlerExecutions := 0
 	// Shutdown engine on first handler execution. Should handler execution ever become

stats/query_stats.go

@@ -31,7 +31,7 @@ const (
 	GetValueAtTimeTime
 	GetBoundaryValuesTime
 	GetRangeValuesTime
-	ViewQueueTime
+	ExecQueueTime
 	ViewDiskPreparationTime
 	ViewDataExtractionTime
 	ViewDiskExtractionTime
@@ -64,8 +64,8 @@ func (s QueryTiming) String() string {
 		return "GetBoundaryValues() time"
 	case GetRangeValuesTime:
 		return "GetRangeValues() time"
-	case ViewQueueTime:
-		return "View queue wait time"
+	case ExecQueueTime:
+		return "Exec queue wait time"
 	case ViewDiskPreparationTime:
 		return "View building disk preparation time"
 	case ViewDataExtractionTime: