asim: don't collect traces when we're not going to record them

This should speed up asim when not using `-rewrite`.

Author: tbg

pkg/kv/kvserver/asim/asim.go

@@ -98,13 +98,18 @@ func NewSimulator(
 	changer := state.NewReplicaChanger()
 	controllers := make(map[state.StoreID]op.Controller)
 
+	var onRecording func(storeID state.StoreID, atDuration time.Duration, rec tracingpb.Recording)
+	if fn := settings.OnRecording; fn != nil {
+		onRecording = func(storeID state.StoreID, atDuration time.Duration, rec tracingpb.Recording) {
+			fn(int64(storeID), atDuration, rec)
+		}
+	}
+
 	s := &Simulator{
 		AmbientContext: log.MakeTestingAmbientCtxWithNewTracer(),
-		onRecording: func(storeID state.StoreID, atDuration time.Duration, rec tracingpb.Recording) {
-			if fn := settings.OnRecording; fn != nil {
-				fn(int64(storeID), atDuration, rec)
-			}
-		},
+		// onRecording is intentionally nil if settings.OnRecording is nil, to
+		// short-circuit trace creation overhead in that case.
+		onRecording: onRecording,
 		curr:        settings.StartTime,
 		end:         settings.StartTime.Add(duration),
 		interval:    settings.TickInterval,

pkg/kv/kvserver/asim/tests/datadriven_simulation_test.go

@@ -577,8 +577,12 @@ func TestDataDriven(t *testing.T) {
 							seedGen := rand.New(rand.NewSource(seed))
 							for sample := 0; sample < samples; sample++ {
 								tr := makeTraceHelper(rewrite, plotDir, testName, sample+1, duration)
-								settingsGen.Settings.OnRecording = func(storeID int64, atDuration time.Duration, rec tracingpb.Recording) {
-									tr.OnRecording(t, storeID, atDuration, rec)
+								if !tr.enabled {
+									// Only populate OnRecording if we're going to save the results.
+									// That way, we avoid creating trace spans during normal test runs.
+									settingsGen.Settings.OnRecording = func(storeID int64, atDuration time.Duration, rec tracingpb.Recording) {
+										tr.OnRecording(t, storeID, atDuration, rec)
+									}
 								}
 
 								assertionFailures := []string{}