record: allow unbounded queued blocks in LogWriter

Previously, a LogWriter would allocate up to 16 blocks of 32 KiB for buffering
WAL writes. If all 16 blocks had been allocated and no free blocks were
available, a batch writing to the WAL would queue until the flushing goroutine
freed blocks. In testing of write-heavy workloads, especially with larger value
sizes, we've seen queueing at the LogWriter. This queueing blocks the commit
pipeline, preventing any batches from committing regardless of priority and
whether they require waiting for fsync.

This commit modifies LogWriter to allow the queueing of an unbounded number of
blocks. In practice, for the current WAL, the memtable size serves as an upper
bound. With a 64 MiB memtable, at most 64 MiB / 32 KiB = 2,048 blocks may
queue. This is not an unreasonable of additional memory overhead for a
write-heavy workload.

Beyond improving throughput for write-heavy workloads, removing this hard bound
improves tolerance of momentary disk stalls.

Informs cockroachdb/cockroach#88699.

batch.go

@@ -319,7 +319,11 @@ type BatchCommitStats struct {
 	// commitPipeline.Commit.
 	SemaphoreWaitDuration time.Duration
 	// WALQueueWaitDuration is the wait time for allocating memory blocks in the
-	// LogWriter (due to the LogWriter not writing fast enough).
+	// LogWriter (due to the LogWriter not writing fast enough). At the moment
+	// this is duration is always zero because a single WAL will allow
+	// allocating memory blocks up to the entire memtable size. In the future,
+	// we may pipeline WALs and bound the WAL queued blocks separately, so this
+	// field is preserved for that possibility.
 	WALQueueWaitDuration time.Duration
 	// MemTableWriteStallDuration is the wait caused by a write stall due to too
 	// many memtables (due to not flushing fast enough).

batch_test.go

@@ -23,7 +23,6 @@ import (
 	"github.com/cockroachdb/pebble/internal/batchskl"
 	"github.com/cockroachdb/pebble/internal/keyspan"
 	"github.com/cockroachdb/pebble/internal/testkeys"
-	"github.com/cockroachdb/pebble/record"
 	"github.com/cockroachdb/pebble/vfs"
 	"github.com/stretchr/testify/require"
 )
@@ -1233,28 +1232,6 @@ func TestBatchCommitStats(t *testing.T) {
 			}
 		}
 
-		// WAL queue stall funcs.
-		//
-		// The LogWriter gets changed when stalling/unstalling the memtable, so we
-		// need to use a hook to tell us about the latest LogWriter.
-		var unstallWALQueue func()
-		stallWALQueue := func() {
-			var unstallLatestWALQueue func()
-			db.mu.Lock()
-			defer db.mu.Unlock()
-			db.mu.log.registerLogWriterForTesting = func(w *record.LogWriter) {
-				// db.mu will be held when this is called.
-				unstallLatestWALQueue = w.ReserveAllFreeBlocksForTesting()
-			}
-			db.mu.log.registerLogWriterForTesting(db.mu.log.LogWriter)
-			unstallWALQueue = func() {
-				db.mu.Lock()
-				defer db.mu.Unlock()
-				db.mu.log.registerLogWriterForTesting = nil
-				unstallLatestWALQueue()
-			}
-		}
-
 		// Commit wait stall funcs.
 		var unstallCommitWait func()
 		stallCommitWait := func() {
@@ -1268,12 +1245,9 @@ func TestBatchCommitStats(t *testing.T) {
 		stallCommitSemaphore()
 		stallMemtable()
 		stallL0ReadAmp()
-		stallWALQueue()
 		stallCommitWait()
 
 		// Exceed initialMemTableSize -- this is needed to make stallMemtable work.
-		// It also exceeds record.blockSize, requiring a new block to be allocated,
-		// which is what we need for stallWALQueue to work.
 		require.NoError(t, b.Set(make([]byte, initialMemTableSize), nil, nil))
 
 		var commitWG sync.WaitGroup
@@ -1291,8 +1265,6 @@ func TestBatchCommitStats(t *testing.T) {
 		time.Sleep(sleepDuration)
 		unstallL0ReadAmp()
 		time.Sleep(sleepDuration)
-		unstallWALQueue()
-		time.Sleep(sleepDuration)
 		unstallCommitWait()
 
 		// Wait for Apply to return.
@@ -1303,10 +1275,6 @@ func TestBatchCommitStats(t *testing.T) {
 			return errors.Errorf("SemaphoreWaitDuration %s is too low",
 				stats.SemaphoreWaitDuration.String())
 		}
-		if expectedDuration > stats.WALQueueWaitDuration {
-			return errors.Errorf("WALQueueWaitDuration %s is too low",
-				stats.WALQueueWaitDuration.String())
-		}
 		if expectedDuration > stats.MemTableWriteStallDuration {
 			return errors.Errorf("MemTableWriteStallDuration %s is too low",
 				stats.MemTableWriteStallDuration.String())

commit_test.go

@@ -245,7 +245,7 @@ func TestCommitPipelineWALClose(t *testing.T) {
 			return nil
 		},
 		write: func(b *Batch, syncWG *sync.WaitGroup, syncErr *error) (*memTable, error) {
-			_, _, err := wal.SyncRecord(b.data, syncWG, syncErr)
+			_, err := wal.SyncRecord(b.data, syncWG, syncErr)
 			return nil, err
 		},
 	}
@@ -316,7 +316,7 @@ func BenchmarkCommitPipeline(b *testing.B) {
 								break
 							}
 
-							_, _, err := wal.SyncRecord(b.data, syncWG, syncErr)
+							_, err := wal.SyncRecord(b.data, syncWG, syncErr)
 							return mem, err
 						},
 					}

db.go

@@ -909,7 +909,7 @@ func (d *DB) commitWrite(b *Batch, syncWG *sync.WaitGroup, syncErr *error) (*mem
 		b.flushable.setSeqNum(b.SeqNum())
 		if !d.opts.DisableWAL {
 			var err error
-			size, b.commitStats.WALQueueWaitDuration, err = d.mu.log.SyncRecord(repr, syncWG, syncErr)
+			size, err = d.mu.log.SyncRecord(repr, syncWG, syncErr)
 			if err != nil {
 				panic(err)
 			}
@@ -947,7 +947,7 @@ func (d *DB) commitWrite(b *Batch, syncWG *sync.WaitGroup, syncErr *error) (*mem
 	}
 
 	if b.flushable == nil {
-		size, b.commitStats.WALQueueWaitDuration, err = d.mu.log.SyncRecord(repr, syncWG, syncErr)
+		size, err = d.mu.log.SyncRecord(repr, syncWG, syncErr)
 		if err != nil {
 			panic(err)
 		}

record/log_writer.go

@@ -266,10 +266,7 @@ type LogWriter struct {
 	block *block
 	free  struct {
 		sync.Mutex
-		// Condition variable used to signal a block is freed.
-		cond      sync.Cond
-		blocks    []*block
-		allocated int
+		blocks []*block
 	}
 
 	flusher struct {
@@ -313,9 +310,10 @@ type LogWriterConfig struct {
 	QueueSemChan chan struct{}
 }
 
-// CapAllocatedBlocks is the maximum number of blocks allocated by the
-// LogWriter.
-const CapAllocatedBlocks = 16
+// initialAllocatedBlocksCap is the initial capacity of the various slices
+// intended to hold LogWriter blocks. The LogWriter may allocate more blocks
+// than this threshold allows.
+const initialAllocatedBlocksCap = 32
 
 // NewLogWriter returns a new LogWriter.
 func NewLogWriter(w io.Writer, logNum base.FileNum, logWriterConfig LogWriterConfig) *LogWriter {
@@ -335,9 +333,7 @@ func NewLogWriter(w io.Writer, logNum base.FileNum, logWriterConfig LogWriterCon
 		},
 		queueSemChan: logWriterConfig.QueueSemChan,
 	}
-	r.free.cond.L = &r.free.Mutex
-	r.free.blocks = make([]*block, 0, CapAllocatedBlocks)
-	r.free.allocated = 1
+	r.free.blocks = make([]*block, 0, initialAllocatedBlocksCap)
 	r.block = &block{}
 	r.flusher.ready.init(&r.flusher.Mutex, &r.flusher.syncQ)
 	r.flusher.closed = make(chan struct{})
@@ -405,8 +401,12 @@ func (w *LogWriter) flushLoop(context.Context) {
 	//   the flush work (w.block.written.Load()).
 
 	// The list of full blocks that need to be written. This is copied from
-	// f.pending on every loop iteration, though the number of elements is small
-	// (usually 1, max 16).
+	// f.pending on every loop iteration, though the number of elements is
+	// usually small (most frequently 1). In the case of the WAL LogWriter, the
+	// number of blocks is bounded by the size of the WAL's corresponding
+	// memtable (MemtableSize/BlockSize). With the default 64 MiB memtables,
+	// this works out to at most 2048 elements if the entirety of the memtable's
+	// contents are queued.
 	pending := make([]*block, 0, cap(f.pending))
 	for {
 		for {
@@ -432,8 +432,7 @@ func (w *LogWriter) flushLoop(context.Context) {
 		// Found work to do, so no longer idle.
 		workStartTime := time.Now()
 		idleDuration := workStartTime.Sub(idleStartTime)
-		pending = pending[:len(f.pending)]
-		copy(pending, f.pending)
+		pending = append(pending[:0], f.pending...)
 		f.pending = f.pending[:0]
 		f.metrics.PendingBufferLen.AddSample(int64(len(pending)))
 
@@ -556,28 +555,18 @@ func (w *LogWriter) flushBlock(b *block) error {
 	b.flushed = 0
 	w.free.Lock()
 	w.free.blocks = append(w.free.blocks, b)
-	w.free.cond.Signal()
 	w.free.Unlock()
 	return nil
 }
 
 // queueBlock queues the current block for writing to the underlying writer,
 // allocates a new block and reserves space for the next header.
-func (w *LogWriter) queueBlock() (waitDuration time.Duration) {
+func (w *LogWriter) queueBlock() {
 	// Allocate a new block, blocking until one is available. We do this first
 	// because w.block is protected by w.flusher.Mutex.
 	w.free.Lock()
 	if len(w.free.blocks) == 0 {
-		if w.free.allocated < cap(w.free.blocks) {
-			w.free.allocated++
-			w.free.blocks = append(w.free.blocks, &block{})
-		} else {
-			now := time.Now()
-			for len(w.free.blocks) == 0 {
-				w.free.cond.Wait()
-			}
-			waitDuration = time.Since(now)
-		}
+		w.free.blocks = append(w.free.blocks, &block{})
 	}
 	nextBlock := w.free.blocks[len(w.free.blocks)-1]
 	w.free.blocks = w.free.blocks[:len(w.free.blocks)-1]
@@ -592,28 +581,6 @@ func (w *LogWriter) queueBlock() (waitDuration time.Duration) {
 	f.Unlock()
 
 	w.blockNum++
-	return waitDuration
-}
-
-// ReserveAllFreeBlocksForTesting is used to only for testing.
-func (w *LogWriter) ReserveAllFreeBlocksForTesting() (releaseFunc func()) {
-	w.free.Lock()
-	defer w.free.Unlock()
-	free := w.free.blocks
-	w.free.blocks = nil
-	return func() {
-		w.free.Lock()
-		defer w.free.Unlock()
-		// It is possible that someone has pushed a free block and w.free.blocks
-		// is no longer nil. That is harmless. Also, the waiter loops on the
-		// condition len(w.free.blocks) == 0, so to actually unblock it we need to
-		// give it a free block.
-		if len(free) == 0 {
-			free = append(free, &block{})
-		}
-		w.free.blocks = free
-		w.free.cond.Broadcast()
-	}
 }
 
 // Close flushes and syncs any unwritten data and closes the writer.
@@ -665,7 +632,7 @@ func (w *LogWriter) Close() error {
 // of the record.
 // External synchronisation provided by commitPipeline.mu.
 func (w *LogWriter) WriteRecord(p []byte) (int64, error) {
-	logSize, _, err := w.SyncRecord(p, nil, nil)
+	logSize, err := w.SyncRecord(p, nil, nil)
 	return logSize, err
 }
 
@@ -676,19 +643,17 @@ func (w *LogWriter) WriteRecord(p []byte) (int64, error) {
 // External synchronisation provided by commitPipeline.mu.
 func (w *LogWriter) SyncRecord(
 	p []byte, wg *sync.WaitGroup, err *error,
-) (logSize int64, waitDuration time.Duration, err2 error) {
+) (logSize int64, err2 error) {
 	if w.err != nil {
-		return -1, 0, w.err
+		return -1, w.err
 	}
 
 	// The `i == 0` condition ensures we handle empty records. Such records can
 	// possibly be generated for VersionEdits stored in the MANIFEST. While the
 	// MANIFEST is currently written using Writer, it is good to support the same
 	// semantics with LogWriter.
 	for i := 0; i == 0 || len(p) > 0; i++ {
-		var wd time.Duration
-		p, wd = w.emitFragment(i, p)
-		waitDuration += wd
+		p = w.emitFragment(i, p)
 	}
 
 	if wg != nil {
@@ -707,7 +672,7 @@ func (w *LogWriter) SyncRecord(
 	// race with our read. That's ok because the only error we could be seeing is
 	// one to syncing for which the caller can receive notification of by passing
 	// in a non-nil err argument.
-	return offset, waitDuration, nil
+	return offset, nil
 }
 
 // Size returns the current size of the file.
@@ -728,7 +693,7 @@ func (w *LogWriter) emitEOFTrailer() {
 	b.written.Store(i + int32(recyclableHeaderSize))
 }
 
-func (w *LogWriter) emitFragment(n int, p []byte) (remainingP []byte, waitDuration time.Duration) {
+func (w *LogWriter) emitFragment(n int, p []byte) (remainingP []byte) {
 	b := w.block
 	i := b.written.Load()
 	first := n == 0
@@ -762,9 +727,9 @@ func (w *LogWriter) emitFragment(n int, p []byte) (remainingP []byte, waitDurati
 		for i := b.written.Load(); i < blockSize; i++ {
 			b.buf[i] = 0
 		}
-		waitDuration = w.queueBlock()
+		w.queueBlock()
 	}
-	return p[r:], waitDuration
+	return p[r:]
 }
 
 // Metrics must be called after Close. The callee will no longer modify the