ddtrace/tracer: Remove log.Debug from hot path (-2 allocs; -5% CPU)

ddtrace/tracer/tracer.go

@@ -434,7 +434,16 @@ func (t *tracer) StartSpan(operationName string, options ...ddtrace.StartSpanOpt
 	if t.config.profilerHotspots || t.config.profilerEndpoints {
 		t.applyPPROFLabels(pprofContext, span)
 	}
-	log.Debug("Started Span: %v, Operation: %s, Resource: %s, Tags: %v, %v", span, span.Name, span.Resource, span.Meta, span.Metrics)
+	if t.config.serviceMappings != nil {
+		if newSvc, ok := t.config.serviceMappings[span.Service]; ok {
+			span.Service = newSvc
+		}
+	}
+	if log.DebugEnabled() {
+		// avoid allocating the ...interface{} argument if debug logging is disabled
+		log.Debug("Started Span: %v, Operation: %s, Resource: %s, Tags: %v, %v",
+			span, span.Name, span.Resource, span.Meta, span.Metrics)
+	}
 	return span
 }
 

internal/log/log.go

@@ -56,12 +56,18 @@ func SetLevel(lvl Level) {
 	level = lvl
 }
 
-// Debug prints the given message if the level is LevelDebug.
-func Debug(fmt string, a ...interface{}) {
+// DebugEnabled returns true if debug log messages are enabled. This can be used in extremely
+// hot code paths to avoid allocating the ...interface{} argument.
+func DebugEnabled() bool {
 	mu.RLock()
 	lvl := level
 	mu.RUnlock()
-	if lvl != LevelDebug {
+	return lvl == LevelDebug
+}
+
+// Debug prints the given message if the level is LevelDebug.
+func Debug(fmt string, a ...interface{}) {
+	if !DebugEnabled() {
 		return
 	}
 	printMsg("DEBUG", fmt, a...)

internal/log/log_test.go

@@ -60,13 +60,15 @@ func TestLog(t *testing.T) {
 			tp.Reset()
 			defer func(old Level) { level = old }(level)
 			SetLevel(LevelDebug)
+			assert.True(t, DebugEnabled())
 
 			Debug("message %d", 3)
 			assert.Equal(t, msg("DEBUG", "message 3"), tp.Lines()[0])
 		})
 
 		t.Run("off", func(t *testing.T) {
 			tp.Reset()
+			assert.False(t, DebugEnabled())
 			Debug("message %d", 2)
 			assert.Len(t, tp.Lines(), 0)
 		})