Add EventLogging option (dgraph-io#1035)

This introduces an option to disable or enable trace.EventLog logging of
events.  For backward compatibility this defaults to true.

The reason for making this configurable is that under some workloads the
cost of event logging is significant and so can cause performance
issues.

Fixes dgraph-io#938

db.go

@@ -271,13 +271,18 @@ func Open(opt Options) (db *DB, err error) {
 		}
 	}()
 
+	elog := y.NoEventLog
+	if opt.EventLogging {
+		elog = trace.NewEventLog("Badger", "DB")
+	}
+
 	db = &DB{
 		imm:           make([]*skl.Skiplist, 0, opt.NumMemtables),
 		flushChan:     make(chan flushTask, opt.NumMemtables),
 		writeCh:       make(chan *request, kvWriteChCapacity),
 		opt:           opt,
 		manifest:      manifestFile,
-		elog:          trace.NewEventLog("Badger", "DB"),
+		elog:          elog,
 		dirLockGuard:  dirLockGuard,
 		valueDirGuard: valueDirLockGuard,
 		orc:           newOracle(opt),

options.go

@@ -44,6 +44,7 @@ type Options struct {
 	ReadOnly            bool
 	Truncate            bool
 	Logger              Logger
+	EventLogging        bool
 
 	// Fine tuning options.
 
@@ -116,6 +117,7 @@ func DefaultOptions(path string) Options {
 		ValueThreshold:     32,
 		Truncate:           false,
 		Logger:             defaultLogger,
+		EventLogging:       true,
 		LogRotatesToFlush:  2,
 	}
 }
@@ -239,6 +241,16 @@ func (opt Options) WithLogger(val Logger) Options {
 	return opt
 }
 
+// WithEventLogging returns a new Options value with EventLogging set to the given value.
+//
+// EventLogging provides a way to enable or disable trace.EventLog logging.
+//
+// The default value of EventLogging is true.
+func (opt Options) WithEventLogging(enabled bool) Options {
+	opt.EventLogging = enabled
+	return opt
+}
+
 // WithMaxTableSize returns a new Options value with MaxTableSize set to the given value.
 //
 // MaxTableSize sets the maximum size in bytes for each LSM table or file.

txn.go

@@ -70,8 +70,8 @@ func newOracle(opt Options) *oracle {
 		txnMark:  &y.WaterMark{Name: "badger.TxnTimestamp"},
 		closer:   y.NewCloser(2),
 	}
-	orc.readMark.Init(orc.closer)
-	orc.txnMark.Init(orc.closer)
+	orc.readMark.Init(orc.closer, opt.EventLogging)
+	orc.txnMark.Init(orc.closer, opt.EventLogging)
 	return orc
 }
 

value.go

@@ -806,7 +806,10 @@ func (vlog *valueLog) open(db *DB, ptr valuePointer, replayFn logEntry) error {
 	vlog.opt = opt
 	vlog.dirPath = opt.ValueDir
 	vlog.db = db
-	vlog.elog = trace.NewEventLog("Badger", "Valuelog")
+	vlog.elog = y.NoEventLog
+	if opt.EventLogging {
+		vlog.elog = trace.NewEventLog("Badger", "Valuelog")
+	}
 	vlog.garbageCh = make(chan struct{}, 1) // Only allow one GC at a time.
 	vlog.lfDiscardStats = &lfDiscardStats{m: make(map[uint32]int64)}
 	if err := vlog.populateFilesMap(); err != nil {

value_test.go

@@ -406,7 +406,7 @@ func TestValueGC4(t *testing.T) {
 	err = kv.vlog.Close()
 	require.NoError(t, err)
 
-	err = kv.vlog.open(kv, valuePointer{Fid: 2}, kv.replayFunction())
+	err = kv.vlog.open(kv, valuePointer{Fid: 2}, kv.replayFunction(), true)
 	require.NoError(t, err)
 
 	for i := 0; i < 8; i++ {
@@ -629,7 +629,7 @@ func TestPartialAppendToValueLog(t *testing.T) {
 	// Replay value log from beginning, badger head is past k2.
 	require.NoError(t, kv.vlog.Close())
 	require.NoError(t,
-		kv.vlog.open(kv, valuePointer{Fid: 0}, kv.replayFunction()))
+		kv.vlog.open(kv, valuePointer{Fid: 0}, kv.replayFunction(), true))
 	require.NoError(t, kv.Close())
 }
 

y/event_log.go

@@ -0,0 +1,31 @@
+/*
+ * Copyright 2019 Dgraph Labs, Inc. and Contributors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package y
+
+import "golang.org/x/net/trace"
+
+var (
+	NoEventLog trace.EventLog = nilEventLog{}
+)
+
+type nilEventLog struct{}
+
+func (nel nilEventLog) Printf(format string, a ...interface{}) {}
+
+func (nel nilEventLog) Errorf(format string, a ...interface{}) {}
+
+func (nel nilEventLog) Finish() {}

y/watermark.go

@@ -68,9 +68,13 @@ type WaterMark struct {
 }
 
 // Init initializes a WaterMark struct. MUST be called before using it.
-func (w *WaterMark) Init(closer *Closer) {
+func (w *WaterMark) Init(closer *Closer, eventLogging bool) {
 	w.markCh = make(chan mark, 100)
-	w.elog = trace.NewEventLog("Watermark", w.Name)
+	if eventLogging {
+		w.elog = trace.NewEventLog("Watermark", w.Name)
+	} else {
+		w.elog = NoEventLog
+	}
 	go w.process(closer)
 }