commit.go

// Copyright 2018 The LevelDB-Go and Pebble Authors. All rights reserved. Use
// of this source code is governed by a BSD-style license that can be found in
// the LICENSE file.

package pebble

import (
	"runtime"
	"sync"
	"sync/atomic"
	"time"
	"unsafe"

	"github.com/cockroachdb/pebble/record"
)

// commitQueue is a lock-free fixed-size single-producer, multi-consumer
// queue. The single producer can enqueue (push) to the head, and consumers can
// dequeue (pop) from the tail.
//
// It has the added feature that it nils out unused slots to avoid unnecessary
// retention of objects.
type commitQueue struct {
	// headTail packs together a 32-bit head index and a 32-bit tail index. Both
	// are indexes into slots modulo len(slots)-1.
	//
	// tail = index of oldest data in queue
	// head = index of next slot to fill
	//
	// Slots in the range [tail, head) are owned by consumers.  A consumer
	// continues to own a slot outside this range until it nils the slot, at
	// which point ownership passes to the producer.
	//
	// The head index is stored in the most-significant bits so that we can
	// atomically add to it and the overflow is harmless.
	headTail uint64

	// slots is a ring buffer of values stored in this queue. The size must be a
	// power of 2. A slot is in use until *both* the tail index has moved beyond
	// it and the slot value has been set to nil. The slot value is set to nil
	// atomically by the consumer and read atomically by the producer.
	slots [record.SyncConcurrency]unsafe.Pointer
}

const dequeueBits = 32

func (q *commitQueue) unpack(ptrs uint64) (head, tail uint32) {
	const mask = 1<<dequeueBits - 1
	head = uint32((ptrs >> dequeueBits) & mask)
	tail = uint32(ptrs & mask)
	return
}

func (q *commitQueue) pack(head, tail uint32) uint64 {
	const mask = 1<<dequeueBits - 1
	return (uint64(head) << dequeueBits) |
		uint64(tail&mask)
}

func (q *commitQueue) enqueue(b *Batch) {
	ptrs := atomic.LoadUint64(&q.headTail)
	head, tail := q.unpack(ptrs)
	if (tail+uint32(len(q.slots)))&(1<<dequeueBits-1) == head {
		// Queue is full. This should never be reached because commitPipeline.commitQueueSem
		// limits the number of concurrent operations.
		panic("pebble: not reached")
	}
	slot := &q.slots[head&uint32(len(q.slots)-1)]

	// Check if the head slot has been released by dequeue.
	for atomic.LoadPointer(slot) != nil {
		// Another goroutine is still cleaning up the tail, so the queue is
		// actually still full. We spin because this should resolve itself
		// momentarily.
		runtime.Gosched()
	}

	// The head slot is free, so we own it.
	atomic.StorePointer(slot, unsafe.Pointer(b))

	// Increment head. This passes ownership of slot to dequeue and acts as a
	// store barrier for writing the slot.
	atomic.AddUint64(&q.headTail, 1<<dequeueBits)
}

func (q *commitQueue) dequeue() *Batch {
	for {
		ptrs := atomic.LoadUint64(&q.headTail)
		head, tail := q.unpack(ptrs)
		if tail == head {
			// Queue is empty.
			return nil
		}

		slot := &q.slots[tail&uint32(len(q.slots)-1)]
		b := (*Batch)(atomic.LoadPointer(slot))
		if b == nil || atomic.LoadUint32(&b.applied) == 0 {
			// The batch is not ready to be dequeued, or another goroutine has
			// already dequeued it.
			return nil
		}

		// Confirm head and tail (for our speculative check above) and increment
		// tail. If this succeeds, then we own the slot at tail.
		ptrs2 := q.pack(head, tail+1)
		if atomic.CompareAndSwapUint64(&q.headTail, ptrs, ptrs2) {
			// We now own slot.
			//
			// Tell enqueue that we're done with this slot. Zeroing the slot is also
			// important so we don't leave behind references that could keep this object
			// live longer than necessary.
			atomic.StorePointer(slot, nil)
			// At this point enqueue owns the slot.
			return b
		}
	}
}

// commitEnv contains the environment that a commitPipeline interacts
// with. This allows fine-grained testing of commitPipeline behavior without
// construction of an entire DB.
type commitEnv struct {
	// The next sequence number to give to a batch. Protected by
	// commitPipeline.mu.
	logSeqNum *uint64
	// The visible sequence number at which reads should be performed. Ratcheted
	// upwards atomically as batches are applied to the memtable.
	visibleSeqNum *uint64

	// Apply the batch to the specified memtable. Called concurrently.
	apply func(b *Batch, mem *memTable) error
	// Write the batch to the WAL. If wg != nil, the data will be persisted
	// asynchronously and done will be called on wg upon completion. If wg != nil
	// and err != nil, a failure to persist the WAL will populate *err. Returns
	// the memtable the batch should be applied to. Serial execution enforced by
	// commitPipeline.mu.
	write func(b *Batch, wg *sync.WaitGroup, err *error) (*memTable, error)
}

// A commitPipeline manages the stages of committing a set of mutations
// (contained in a single Batch) atomically to the DB. The steps are
// conceptually:
//
//  1. Write the batch to the WAL and optionally sync the WAL
//  2. Apply the mutations in the batch to the memtable
//
// These two simple steps are made complicated by the desire for high
// performance. In the absence of concurrency, performance is limited by how
// fast a batch can be written (and synced) to the WAL and then added to the
// memtable, both of which are outside the purview of the commit
// pipeline. Performance under concurrency is the primary concern of the commit
// pipeline, though it also needs to maintain two invariants:
//
//  1. Batches need to be written to the WAL in sequence number order.
//  2. Batches need to be made visible for reads in sequence number order. This
//     invariant arises from the use of a single sequence number which
//     indicates which mutations are visible.
//
// Taking these invariants into account, let's revisit the work the commit
// pipeline needs to perform. Writing the batch to the WAL is necessarily
// serialized as there is a single WAL object. The order of the entries in the
// WAL defines the sequence number order. Note that writing to the WAL is
// extremely fast, usually just a memory copy. Applying the mutations in a
// batch to the memtable can occur concurrently as the underlying skiplist
// supports concurrent insertions. Publishing the visible sequence number is
// another serialization point, but one with a twist: the visible sequence
// number cannot be bumped until the mutations for earlier batches have
// finished applying to the memtable (the visible sequence number only ratchets
// up). Lastly, if requested, the commit waits for the WAL to sync. Note that
// waiting for the WAL sync after ratcheting the visible sequence number allows
// another goroutine to read committed data before the WAL has synced. This is
// similar behavior to RocksDB's manual WAL flush functionality. Application
// code needs to protect against this if necessary.
//
// The full outline of the commit pipeline operation is as follows:
//
//	with commitPipeline mutex locked:
//	  assign batch sequence number
//	  write batch to WAL
//	(optionally) add batch to WAL sync list
//	apply batch to memtable (concurrently)
//	wait for earlier batches to apply
//	ratchet read sequence number
//	(optionally) wait for the WAL to sync
//
// As soon as a batch has been written to the WAL, the commitPipeline mutex is
// released allowing another batch to write to the WAL. Each commit operation
// individually applies its batch to the memtable providing concurrency. The
// WAL sync happens concurrently with applying to the memtable (see
// commitPipeline.syncLoop).
//
// The "waits for earlier batches to apply" work is more complicated than might
// be expected. The obvious approach would be to keep a queue of pending
// batches and for each batch to wait for the previous batch to finish
// committing. This approach was tried initially and turned out to be too
// slow. The problem is that it causes excessive goroutine activity as each
// committing goroutine needs to wake up in order for the next goroutine to be
// unblocked. The approach taken in the current code is conceptually similar,
// though it avoids waking a goroutine to perform work that another goroutine
// can perform. A commitQueue (a single-producer, multiple-consumer queue)
// holds the ordered list of committing batches. Addition to the queue is done
// while holding commitPipeline.mutex ensuring the same ordering of batches in
// the queue as the ordering in the WAL. When a batch finishes applying to the
// memtable, it atomically updates its Batch.applied field. Ratcheting of the
// visible sequence number is done by commitPipeline.publish which loops
// dequeueing "applied" batches and ratcheting the visible sequence number. If
// we hit an unapplied batch at the head of the queue we can block as we know
// that committing of that unapplied batch will eventually find our (applied)
// batch in the queue. See commitPipeline.publish for additional commentary.
type commitPipeline struct {
	// WARNING: The following struct `commitQueue` contains fields which will
	// be accessed atomically.
	//
	// Go allocations are guaranteed to be 64-bit aligned which we take advantage
	// of by placing the 64-bit fields which we access atomically at the beginning
	// of the commitPipeline struct.
	// For more information, see https://golang.org/pkg/sync/atomic/#pkg-note-BUG.
	// Queue of pending batches to commit.
	pending commitQueue
	env     commitEnv
	// The commit path has two queues:
	// - commitPipeline.pending contains batches whose seqnums have not yet been
	//   published. It is a lock-free single producer multi consumer queue.
	// - LogWriter.flusher.syncQ contains state for batches that have asked for
	//   a sync. It is a lock-free single producer single consumer queue.
	// These lock-free queues have a fixed capacity. And since they are
	// lock-free, we cannot do blocking waits when pushing onto these queues, in
	// case they are full. Additionally, adding to these queues happens while
	// holding commitPipeline.mu, and we don't want to block while holding that
	// mutex since it is also needed by other code.
	//
	// Popping from these queues is independent and for a particular batch can
	// occur in either order, though it is more common that popping from the
	// commitPipeline.pending will happen first.
	//
	// Due to these constraints, we reserve a unit of space in each queue before
	// acquiring commitPipeline.mu, which also ensures that the push operation
	// is guaranteed to have space in the queue. The commitQueueSem and
	// logSyncQSem are used for this reservation.
	commitQueueSem chan struct{}
	logSyncQSem    chan struct{}
	// The mutex to use for synchronizing access to logSeqNum and serializing
	// calls to commitEnv.write().
	mu sync.Mutex
}

func newCommitPipeline(env commitEnv) *commitPipeline {
	p := &commitPipeline{
		env: env,
		// The capacity of both commitQueue.slots and syncQueue.slots is set to
		// record.SyncConcurrency, which also determines the value of these
		// semaphores. We used to have a single semaphore, which required that the
		// capacity of these queues be the same. Now that we have two semaphores,
		// the capacity of these queues could be changed to be different. Say half
		// of the batches asked to be synced, but syncing took 5x the latency of
		// adding to the memtable and publishing. Then syncQueue.slots could be
		// sized as 0.5*5 of the commitQueue.slots. We can explore this if we find
		// that LogWriterMetrics.SyncQueueLen has high utilization under some
		// workloads.
		//
		// NB: the commit concurrency is one less than SyncConcurrency because we
		// have to allow one "slot" for a concurrent WAL rotation which will close
		// and sync the WAL.
		commitQueueSem: make(chan struct{}, record.SyncConcurrency-1),
		logSyncQSem:    make(chan struct{}, record.SyncConcurrency-1),
	}
	return p
}

// directWrite is used to directly write to the WAL. commitPipeline.mu must be
// held while this is called. DB.mu must not be held. directWrite will only
// return once the WAL sync is complete. Note that DirectWrite is a special case
// function which is currently only used when ingesting sstables as a flushable.
// Reason carefully about the correctness argument when calling this function
// from any context.
func (p *commitPipeline) directWrite(b *Batch) error {
	var syncWG sync.WaitGroup
	var syncErr error
	syncWG.Add(1)
	p.logSyncQSem <- struct{}{}
	_, err := p.env.write(b, &syncWG, &syncErr)
	syncWG.Wait()
	err = firstError(err, syncErr)
	return err
}

// Commit the specified batch, writing it to the WAL, optionally syncing the
// WAL, and applying the batch to the memtable. Upon successful return the
// batch's mutations will be visible for reading.
// REQUIRES: noSyncWait => syncWAL
func (p *commitPipeline) Commit(b *Batch, syncWAL bool, noSyncWait bool) error {
	if b.Empty() {
		return nil
	}

	commitStartTime := time.Now()
	// Acquire semaphores.
	p.commitQueueSem <- struct{}{}
	if syncWAL {
		p.logSyncQSem <- struct{}{}
	}
	b.commitStats.SemaphoreWaitDuration = time.Since(commitStartTime)

	// Prepare the batch for committing: enqueuing the batch in the pending
	// queue, determining the batch sequence number and writing the data to the
	// WAL.
	//
	// NB: We set Batch.commitErr on error so that the batch won't be a candidate
	// for reuse. See Batch.release().
	mem, err := p.prepare(b, syncWAL, noSyncWait)
	if err != nil {
		b.db = nil // prevent batch reuse on error
		// NB: we are not doing <-p.commitQueueSem since the batch is still
		// sitting in the pending queue. We should consider fixing this by also
		// removing the batch from the pending queue.
		return err
	}

	// Apply the batch to the memtable.
	if err := p.env.apply(b, mem); err != nil {
		b.db = nil // prevent batch reuse on error
		// NB: we are not doing <-p.commitQueueSem since the batch is still
		// sitting in the pending queue. We should consider fixing this by also
		// removing the batch from the pending queue.
		return err
	}

	// Publish the batch sequence number.
	p.publish(b)

	<-p.commitQueueSem

	if !noSyncWait {
		// Already waited for commit, so look at the error.
		if b.commitErr != nil {
			b.db = nil // prevent batch reuse on error
			err = b.commitErr
		}
	}
	// Else noSyncWait. The LogWriter can be concurrently writing to
	// b.commitErr. We will read b.commitErr in Batch.SyncWait after the
	// LogWriter is done writing.

	b.commitStats.TotalDuration = time.Since(commitStartTime)

	return err
}

// AllocateSeqNum allocates count sequence numbers, invokes the prepare
// callback, then the apply callback, and then publishes the sequence
// numbers. AllocateSeqNum does not write to the WAL or add entries to the
// memtable. AllocateSeqNum can be used to sequence an operation such as
// sstable ingestion within the commit pipeline. The prepare callback is
// invoked with commitPipeline.mu held, but note that DB.mu is not held and
// must be locked if necessary.
func (p *commitPipeline) AllocateSeqNum(
	count int, prepare func(seqNum uint64), apply func(seqNum uint64),
) {
	// This method is similar to Commit and prepare. Be careful about trying to
	// share additional code with those methods because Commit and prepare are
	// performance critical code paths.

	b := newBatch(nil)
	defer b.release()

	// Give the batch a count of 1 so that the log and visible sequence number
	// are incremented correctly.
	b.data = make([]byte, batchHeaderLen)
	b.setCount(uint32(count))
	b.commit.Add(1)

	p.commitQueueSem <- struct{}{}

	p.mu.Lock()

	// Enqueue the batch in the pending queue. Note that while the pending queue
	// is lock-free, we want the order of batches to be the same as the sequence
	// number order.
	p.pending.enqueue(b)

	// Assign the batch a sequence number. Note that we use atomic operations
	// here to handle concurrent reads of logSeqNum. commitPipeline.mu provides
	// mutual exclusion for other goroutines writing to logSeqNum.
	logSeqNum := atomic.AddUint64(p.env.logSeqNum, uint64(count)) - uint64(count)
	seqNum := logSeqNum
	if seqNum == 0 {
		// We can't use the value 0 for the global seqnum during ingestion, because
		// 0 indicates no global seqnum. So allocate one more seqnum.
		atomic.AddUint64(p.env.logSeqNum, 1)
		seqNum++
	}
	b.setSeqNum(seqNum)

	// Wait for any outstanding writes to the memtable to complete. This is
	// necessary for ingestion so that the check for memtable overlap can see any
	// writes that were sequenced before the ingestion. The spin loop is
	// unfortunate, but obviates the need for additional synchronization.
	for {
		visibleSeqNum := atomic.LoadUint64(p.env.visibleSeqNum)
		if visibleSeqNum == logSeqNum {
			break
		}
		runtime.Gosched()
	}

	// Invoke the prepare callback. Note the lack of error reporting. Even if the
	// callback internally fails, the sequence number needs to be published in
	// order to allow the commit pipeline to proceed.
	prepare(b.SeqNum())

	p.mu.Unlock()

	// Invoke the apply callback.
	apply(b.SeqNum())

	// Publish the sequence number.
	p.publish(b)

	<-p.commitQueueSem
}

func (p *commitPipeline) prepare(b *Batch, syncWAL bool, noSyncWait bool) (*memTable, error) {
	n := uint64(b.Count())
	if n == invalidBatchCount {
		return nil, ErrInvalidBatch
	}
	var syncWG *sync.WaitGroup
	var syncErr *error
	switch {
	case !syncWAL:
		// Only need to wait for the publish.
		b.commit.Add(1)
	// Remaining cases represent syncWAL=true.
	case noSyncWait:
		syncErr = &b.commitErr
		syncWG = &b.fsyncWait
		// Only need to wait synchronously for the publish. The user will
		// (asynchronously) wait on the batch's fsyncWait.
		b.commit.Add(1)
		b.fsyncWait.Add(1)
	case !noSyncWait:
		syncErr = &b.commitErr
		syncWG = &b.commit
		// Must wait for both the publish and the WAL fsync.
		b.commit.Add(2)
	}

	p.mu.Lock()

	// Enqueue the batch in the pending queue. Note that while the pending queue
	// is lock-free, we want the order of batches to be the same as the sequence
	// number order.
	p.pending.enqueue(b)

	// Assign the batch a sequence number. Note that we use atomic operations
	// here to handle concurrent reads of logSeqNum. commitPipeline.mu provides
	// mutual exclusion for other goroutines writing to logSeqNum.
	b.setSeqNum(atomic.AddUint64(p.env.logSeqNum, n) - n)

	// Write the data to the WAL.
	mem, err := p.env.write(b, syncWG, syncErr)

	p.mu.Unlock()

	return mem, err
}

func (p *commitPipeline) publish(b *Batch) {
	// Mark the batch as applied.
	atomic.StoreUint32(&b.applied, 1)

	// Loop dequeuing applied batches from the pending queue. If our batch was
	// the head of the pending queue we are guaranteed that either we'll publish
	// it or someone else will dequeue and publish it. If our batch is not the
	// head of the queue then either we'll dequeue applied batches and reach our
	// batch or there is an unapplied batch blocking us. When that unapplied
	// batch applies it will go through the same process and publish our batch
	// for us.
	for {
		t := p.pending.dequeue()
		if t == nil {
			// Wait for another goroutine to publish us. We might also be waiting for
			// the WAL sync to finish.
			now := time.Now()
			b.commit.Wait()
			b.commitStats.CommitWaitDuration += time.Since(now)
			break
		}
		if atomic.LoadUint32(&t.applied) != 1 {
			panic("not reached")
		}

		// We're responsible for publishing the sequence number for batch t, but
		// another concurrent goroutine might sneak in and publish the sequence
		// number for a subsequent batch. That's ok as all we're guaranteeing is
		// that the sequence number ratchets up.
		for {
			curSeqNum := atomic.LoadUint64(p.env.visibleSeqNum)
			newSeqNum := t.SeqNum() + uint64(t.Count())
			if newSeqNum <= curSeqNum {
				// t's sequence number has already been published.
				break
			}
			if atomic.CompareAndSwapUint64(p.env.visibleSeqNum, curSeqNum, newSeqNum) {
				// We successfully published t's sequence number.
				break
			}
		}

		t.commit.Done()
	}
}