Use a sync.Map for sampling decision to establish baseline perf

x-pack/apm-server/sampling/eventstorage/storage.go

@@ -8,6 +8,7 @@ import (
 	"bytes"
 	"errors"
 	"fmt"
+	"sync"
 	"sync/atomic"
 	"time"
 
@@ -48,6 +49,8 @@ type Storage struct {
 	// pendingSize tracks the total size of pending writes across ReadWriters
 	pendingSize *atomic.Int64
 	codec       Codec
+
+	sampled sync.Map
 }
 
 // Codec provides methods for encoding and decoding events.
@@ -153,32 +156,39 @@ func (rw *ReadWriter) Flush() error {
 
 // WriteTraceSampled records the tail-sampling decision for the given trace ID.
 func (rw *ReadWriter) WriteTraceSampled(traceID string, sampled bool, opts WriterOpts) error {
-	rw.lazyInit()
-
-	key := []byte(prefixSamplingDecision + traceID)
-	meta := entryMetaTraceUnsampled
-	if sampled {
-		meta = entryMetaTraceSampled
-	}
-	return rw.batch.Set(key, []byte{meta}, pebble.NoSync)
+	rw.s.sampled.Store(traceID, sampled)
+	return nil
+	//rw.lazyInit()
+	//
+	//key := []byte(prefixSamplingDecision + traceID)
+	//meta := entryMetaTraceUnsampled
+	//if sampled {
+	//	meta = entryMetaTraceSampled
+	//}
+	//return rw.batch.Set(key, []byte{meta}, pebble.NoSync)
 }
 
 // IsTraceSampled reports whether traceID belongs to a trace that is sampled
 // or unsampled. If no sampling decision has been recorded, IsTraceSampled
 // returns ErrNotFound.
 func (rw *ReadWriter) IsTraceSampled(traceID string) (bool, error) {
-	rw.lazyInit()
-
-	// FIXME: this needs to be fast, as it is in the hot path
-	// It should minimize disk IO on miss due to
-	// 1. (pubsub) remote sampling decision
-	// 2. (hot path) sampling decision not made yet
-	item, closer, err := rw.batch.Get([]byte(prefixSamplingDecision + traceID))
-	if err == pebble.ErrNotFound {
+	if sampled, ok := rw.s.sampled.Load(traceID); !ok {
 		return false, ErrNotFound
+	} else {
+		return sampled.(bool), nil
 	}
-	defer closer.Close()
-	return item[0] == entryMetaTraceSampled, nil
+	//rw.lazyInit()
+	//
+	//// FIXME: this needs to be fast, as it is in the hot path
+	//// It should minimize disk IO on miss due to
+	//// 1. (pubsub) remote sampling decision
+	//// 2. (hot path) sampling decision not made yet
+	//item, closer, err := rw.batch.Get([]byte(prefixSamplingDecision + traceID))
+	//if err == pebble.ErrNotFound {
+	//	return false, ErrNotFound
+	//}
+	//defer closer.Close()
+	//return item[0] == entryMetaTraceSampled, nil
 }
 
 // WriteTraceEvent writes a trace event to storage.