fix(compaction): Use separate compactors for L0, L1 (#1466)

Related to #1459

This PR contains the following changes to compactions
- Use a separate thread for compacting Level 0 and 1 and a separate one for other levels
- Pick levels to compact based on score.
- Stall Level 0 if compactions cannot keep up (we had added this in #1186)
- Limit the number of open table builders to 5 in compactions.

Author: Ibrahim Jarif

badger/main.go

@@ -29,7 +29,7 @@ func main() {
 	go func() {
 		for i := 8080; i < 9080; i++ {
 			fmt.Printf("Listening for /debug HTTP requests at port: %d\n", i)
-			if err := http.ListenAndServe(fmt.Sprintf("localhost:%d", i), nil); err != nil {
+			if err := http.ListenAndServe(fmt.Sprintf("0.0.0.0:%d", i), nil); err != nil {
 				fmt.Println("Port busy. Trying another one...")
 				continue
 

db.go

@@ -192,6 +192,12 @@ func (db *DB) replayFunction() func(Entry, valuePointer) error {
 
 // Open returns a new DB object.
 func Open(opt Options) (db *DB, err error) {
+	// It's okay to have zero compactors which will disable all compactions but
+	// we cannot have just one compactor otherwise we will end up with all data
+	// one level 2.
+	if opt.NumCompactors == 1 {
+		return nil, errors.New("Cannot have 1 compactor. Need at least 2")
+	}
 	if opt.InMemory && (opt.Dir != "" || opt.ValueDir != "") {
 		return nil, errors.New("Cannot use badger in Disk-less mode with Dir or ValueDir set")
 	}
@@ -534,7 +540,7 @@ func (db *DB) close() (err error) {
 	// Force Compact L0
 	// We don't need to care about cstatus since no parallel compaction is running.
 	if db.opt.CompactL0OnClose {
-		err := db.lc.doCompact(compactionPriority{level: 0, score: 1.73})
+		err := db.lc.doCompact(173, compactionPriority{level: 0, score: 1.73})
 		switch err {
 		case errFillTables:
 			// This error only means that there might be enough tables to do a compaction. So, we
@@ -1461,7 +1467,7 @@ func (db *DB) Flatten(workers int) error {
 		errCh := make(chan error, 1)
 		for i := 0; i < workers; i++ {
 			go func() {
-				errCh <- db.lc.doCompact(cp)
+				errCh <- db.lc.doCompact(175, cp)
 			}()
 		}
 		var success int

level_handler.go

@@ -188,9 +188,8 @@ func (s *levelHandler) tryAddLevel0Table(t *table.Table) bool {
 	// Need lock as we may be deleting the first table during a level 0 compaction.
 	s.Lock()
 	defer s.Unlock()
-	// Return false only if L0 is in memory and number of tables is more than number of
-	// ZeroTableStall. For on disk L0, we should just add the tables to the level.
-	if s.db.opt.KeepL0InMemory && len(s.tables) >= s.db.opt.NumLevelZeroTablesStall {
+	// Stall (by returning false) if we are above the specified stall setting for L0.
+	if len(s.tables) >= s.db.opt.NumLevelZeroTablesStall {
 		return false
 	}
 

levels.go

@@ -306,7 +306,7 @@ func (s *levelsController) dropPrefixes(prefixes [][]byte) error {
 					// function in logs, and forces a compaction.
 					dropPrefixes: prefixes,
 				}
-				if err := s.doCompact(cp); err != nil {
+				if err := s.doCompact(174, cp); err != nil {
 					opt.Warningf("While compacting level 0: %v", err)
 					return nil
 				}
@@ -366,11 +366,13 @@ func (s *levelsController) startCompact(lc *y.Closer) {
 	n := s.kv.opt.NumCompactors
 	lc.AddRunning(n - 1)
 	for i := 0; i < n; i++ {
-		go s.runWorker(lc)
+		// The worker with id=0 is dedicated to L0 and L1. This is not counted
+		// towards the user specified NumCompactors.
+		go s.runCompactor(i, lc)
 	}
 }
 
-func (s *levelsController) runWorker(lc *y.Closer) {
+func (s *levelsController) runCompactor(id int, lc *y.Closer) {
 	defer lc.Done()
 
 	randomDelay := time.NewTimer(time.Duration(rand.Int31n(1000)) * time.Millisecond)
@@ -381,7 +383,7 @@ func (s *levelsController) runWorker(lc *y.Closer) {
 		return
 	}
 
-	ticker := time.NewTicker(time.Second)
+	ticker := time.NewTicker(100 * time.Millisecond)
 	defer ticker.Stop()
 
 	for {
@@ -391,7 +393,15 @@ func (s *levelsController) runWorker(lc *y.Closer) {
 			prios := s.pickCompactLevels()
 		loop:
 			for _, p := range prios {
-				err := s.doCompact(p)
+				if id == 0 && p.level > 1 {
+					// If I'm ID zero, I only compact L0 and L1.
+					continue
+				}
+				if id != 0 && p.level <= 1 {
+					// If I'm ID non-zero, I do NOT compact L0 and L1.
+					continue
+				}
+				err := s.doCompact(id, p)
 				switch err {
 				case nil:
 					break loop
@@ -453,10 +463,11 @@ func (s *levelsController) pickCompactLevels() (prios []compactionPriority) {
 			prios = append(prios, pri)
 		}
 	}
-	// We used to sort compaction priorities based on the score. But, we
-	// decided to compact based on the level, not the priority. So, upper
-	// levels (level 0, level 1, etc) always get compacted first, before the
-	// lower levels -- this allows us to avoid stalls.
+	// We should continue to sort the compaction priorities by score. Now that we have a dedicated
+	// compactor for L0 and L1, we don't need to sort by level here.
+	sort.Slice(prios, func(i, j int) bool {
+		return prios[i].score > prios[j].score
+	})
 	return prios
 }
 
@@ -541,15 +552,13 @@ nextTable:
 	// that would affect the snapshot view guarantee provided by transactions.
 	discardTs := s.kv.orc.discardAtOrBelow()
 
-	// Start generating new tables.
-	type newTableResult struct {
-		table *table.Table
-		err   error
-	}
-	resultCh := make(chan newTableResult)
 	var numBuilds, numVersions int
 	var lastKey, skipKey []byte
 	var vp valuePointer
+	var newTables []*table.Table
+	mu := new(sync.Mutex) // Guards newTables
+
+	inflightBuilders := y.NewThrottle(5)
 	for it.Valid() {
 		timeStart := time.Now()
 		dk, err := s.kv.registry.latestDataKey()
@@ -646,19 +655,6 @@ nextTable:
 		// called Add() at least once, and builder is not Empty().
 		s.kv.opt.Debugf("LOG Compact. Added %d keys. Skipped %d keys. Iteration took: %v",
 			numKeys, numSkips, time.Since(timeStart))
-		build := func(fileID uint64) (*table.Table, error) {
-			fd, err := y.CreateSyncedFile(table.NewFilename(fileID, s.kv.opt.Dir), true)
-			if err != nil {
-				return nil, errors.Wrapf(err, "While opening new table: %d", fileID)
-			}
-
-			if _, err := fd.Write(builder.Finish()); err != nil {
-				return nil, errors.Wrapf(err, "Unable to write to file: %d", fileID)
-			}
-			tbl, err := table.OpenTable(fd, bopts)
-			// decrRef is added below.
-			return tbl, errors.Wrapf(err, "Unable to open table: %q", fd.Name())
-		}
 		if builder.Empty() {
 			// Cleanup builder resources:
 			builder.Finish()
@@ -667,49 +663,61 @@ nextTable:
 		}
 		numBuilds++
 		fileID := s.reserveFileID()
+		if err := inflightBuilders.Do(); err != nil {
+			// Can't return from here, until I decrRef all the tables that I built so far.
+			break
+		}
 		go func(builder *table.Builder) {
 			defer builder.Close()
-			var (
-				tbl *table.Table
-				err error
-			)
+			defer inflightBuilders.Done(err)
+
+			build := func(fileID uint64) (*table.Table, error) {
+				fd, err := y.CreateSyncedFile(table.NewFilename(fileID, s.kv.opt.Dir), true)
+				if err != nil {
+					return nil, errors.Wrapf(err, "While opening new table: %d", fileID)
+				}
+
+				if _, err := fd.Write(builder.Finish()); err != nil {
+					return nil, errors.Wrapf(err, "Unable to write to file: %d", fileID)
+				}
+				tbl, err := table.OpenTable(fd, bopts)
+				// decrRef is added below.
+				return tbl, errors.Wrapf(err, "Unable to open table: %q", fd.Name())
+			}
+
+			var tbl *table.Table
+			var err error
 			if s.kv.opt.InMemory {
 				tbl, err = table.OpenInMemoryTable(builder.Finish(), fileID, &bopts)
 			} else {
 				tbl, err = build(fileID)
 			}
-			resultCh <- newTableResult{tbl, err}
-		}(builder)
-	}
 
-	newTables := make([]*table.Table, 0, 20)
-	// Wait for all table builders to finish.
-	var firstErr error
-	for x := 0; x < numBuilds; x++ {
-		res := <-resultCh
-		newTables = append(newTables, res.table)
-		if firstErr == nil {
-			firstErr = res.err
-		}
+			// If we couldn't build the table, return fast.
+			if err != nil {
+				return
+			}
+
+			mu.Lock()
+			newTables = append(newTables, tbl)
+			mu.Unlock()
+		}(builder)
 	}
 
-	if firstErr == nil {
+	// Wait for all table builders to finish and also for newTables accumulator to finish.
+	err := inflightBuilders.Finish()
+	if err == nil {
 		// Ensure created files' directory entries are visible.  We don't mind the extra latency
 		// from not doing this ASAP after all file creation has finished because this is a
 		// background operation.
-		firstErr = s.kv.syncDir(s.kv.opt.Dir)
+		err = s.kv.syncDir(s.kv.opt.Dir)
 	}
 
-	if firstErr != nil {
+	if err != nil {
 		// An error happened.  Delete all the newly created table files (by calling DecrRef
 		// -- we're the only holders of a ref).
-		for j := 0; j < numBuilds; j++ {
-			if newTables[j] != nil {
-				_ = newTables[j].DecrRef()
-			}
-		}
-		errorReturn := errors.Wrapf(firstErr, "While running compaction for: %+v", cd)
-		return nil, nil, errorReturn
+		_ = decrRefs(newTables)
+		return nil, nil, errors.Wrapf(err, "while running compactions for: %+v", cd)
 	}
 
 	sort.Slice(newTables, func(i, j int) bool {
@@ -963,7 +971,7 @@ func (s *levelsController) runCompactDef(l int, cd compactDef) (err error) {
 var errFillTables = errors.New("Unable to fill tables")
 
 // doCompact picks some table on level l and compacts it away to the next level.
-func (s *levelsController) doCompact(p compactionPriority) error {
+func (s *levelsController) doCompact(id int, p compactionPriority) error {
 	l := p.level
 	y.AssertTrue(l+1 < s.kv.opt.MaxLevels) // Sanity check.
 
@@ -976,7 +984,7 @@ func (s *levelsController) doCompact(p compactionPriority) error {
 	cd.elog.SetMaxEvents(100)
 	defer cd.elog.Finish()
 
-	s.kv.opt.Infof("Got compaction priority: %+v", p)
+	s.kv.opt.Debugf("[Compactor: %d] Attempting to run compaction: %+v", id, p)
 
 	// While picking tables to be compacted, both levels' tables are expected to
 	// remain unchanged.
@@ -992,16 +1000,17 @@ func (s *levelsController) doCompact(p compactionPriority) error {
 	}
 	defer s.cstatus.delete(cd) // Remove the ranges from compaction status.
 
-	s.kv.opt.Infof("Running for level: %d\n", cd.thisLevel.level)
+	s.kv.opt.Infof("[Compactor: %d] Running compaction: %+v for level: %d\n",
+		id, p, cd.thisLevel.level)
 	s.cstatus.toLog(cd.elog)
 	if err := s.runCompactDef(l, cd); err != nil {
 		// This compaction couldn't be done successfully.
-		s.kv.opt.Warningf("LOG Compact FAILED with error: %+v: %+v", err, cd)
+		s.kv.opt.Warningf("[Compactor: %d] LOG Compact FAILED with error: %+v: %+v", id, err, cd)
 		return err
 	}
 
 	s.cstatus.toLog(cd.elog)
-	s.kv.opt.Infof("Compaction for level: %d DONE", cd.thisLevel.level)
+	s.kv.opt.Infof("[Compactor: %d] Compaction for level: %d DONE", id, cd.thisLevel.level)
 	return nil
 }
 
@@ -1025,7 +1034,7 @@ func (s *levelsController) addLevel0Table(t *table.Table) error {
 		// Stall. Make sure all levels are healthy before we unstall.
 		var timeStart time.Time
 		{
-			s.kv.opt.Debugf("STALLED STALLED STALLED: %v\n", time.Since(s.lastUnstalled))
+			s.kv.opt.Infof("STALLED STALLED STALLED: %v\n", time.Since(s.lastUnstalled))
 			s.cstatus.RLock()
 			for i := 0; i < s.kv.opt.MaxLevels; i++ {
 				s.kv.opt.Debugf("level=%d. Status=%s Size=%d\n",