etcd: fix dereferencing issue in commit queue causing contention and change design to be more scalable

docs/release-notes/release-notes-0.14.0.md

@@ -107,6 +107,10 @@ you.
 * [Optimized payment sequence generation](https://github.com/lightningnetwork/lnd/pull/5514/)
   to make LNDs payment throughput (and latency) with better when using etcd.
 
+* [More robust commit queue design](https://github.com/lightningnetwork/lnd/pull/5513)
+  to make it less likely that we retry etcd transactions and make the commit
+  queue more scalable.
+
 ## Performance improvements
 
 * [Update MC store in blocks](https://github.com/lightningnetwork/lnd/pull/5515)
@@ -119,6 +123,7 @@ currnet DNS seeds when in SigNet
 mode](https://github.com/lightningnetwork/lnd/pull/5564).
 
 # Contributors (Alphabetical Order)
+* Andras Banki-Horvath
 * ErikEk
 * Martin Habovstiak
 * Zero-1729

kvdb/etcd/commit_queue.go

@@ -3,14 +3,11 @@
 package etcd
 
 import (
+	"container/list"
 	"context"
 	"sync"
 )
 
-// commitQueueSize is the maximum number of commits we let to queue up. All
-// remaining commits will block on commitQueue.Add().
-const commitQueueSize = 100
-
 // commitQueue is a simple execution queue to manage conflicts for transactions
 // and thereby reduce the number of times conflicting transactions need to be
 // retried. When a new transaction is added to the queue, we first upgrade the
@@ -25,9 +22,18 @@ type commitQueue struct {
 	readerMap map[string]int
 	writerMap map[string]int
 
-	commitMutex sync.RWMutex
-	queue       chan (func())
-	wg          sync.WaitGroup
+	queue     *list.List
+	queueMx   sync.Mutex
+	queueCond *sync.Cond
+
+	shutdown chan struct{}
+}
+
+type commitQueueTxn struct {
+	commitLoop func()
+	blocked    bool
+	rset       []string
+	wset       []string
 }
 
 // NewCommitQueue creates a new commit queue, with the passed abort context.
@@ -36,32 +42,37 @@ func NewCommitQueue(ctx context.Context) *commitQueue {
 		ctx:       ctx,
 		readerMap: make(map[string]int),
 		writerMap: make(map[string]int),
-		queue:     make(chan func(), commitQueueSize),
+		queue:     list.New(),
+		shutdown:  make(chan struct{}),
 	}
+	q.queueCond = sync.NewCond(&q.queueMx)
 
 	// Start the queue consumer loop.
-	q.wg.Add(1)
 	go q.mainLoop()
 
 	return q
 }
 
-// Wait waits for the queue to stop (after the queue context has been canceled).
-func (c *commitQueue) Wait() {
-	c.wg.Wait()
+// Stop signals the queue to stop after the queue context has been canceled and
+// waits until the has stopped.
+func (c *commitQueue) Stop() {
+	// Signal the queue's condition variable to ensure the mainLoop reliably
+	// unblocks to check for the exit condition.
+	c.queueCond.Signal()
+	<-c.shutdown
 }
 
 // Add increases lock counts and queues up tx commit closure for execution.
 // Transactions that don't have any conflicts are executed immediately by
 // "downgrading" the count mutex to allow concurrency.
-func (c *commitQueue) Add(commitLoop func(), rset readSet, wset writeSet) {
+func (c *commitQueue) Add(commitLoop func(), rset []string, wset []string) {
 	c.mx.Lock()
 	blocked := false
 
 	// Mark as blocked if there's any writer changing any of the keys in
 	// the read set. Do not increment the reader counts yet as we'll need to
 	// use the original reader counts when scanning through the write set.
-	for key := range rset {
+	for _, key := range rset {
 		if c.writerMap[key] > 0 {
 			blocked = true
 			break
@@ -70,56 +81,45 @@ func (c *commitQueue) Add(commitLoop func(), rset readSet, wset writeSet) {
 
 	// Mark as blocked if there's any writer or reader for any of the keys
 	// in the write set.
-	for key := range wset {
+	for _, key := range wset {
 		blocked = blocked || c.readerMap[key] > 0 || c.writerMap[key] > 0
 
 		// Increment the writer count.
 		c.writerMap[key] += 1
 	}
 
 	// Finally we can increment the reader counts for keys in the read set.
-	for key := range rset {
+	for _, key := range rset {
 		c.readerMap[key] += 1
 	}
 
-	if blocked {
-		// Add the transaction to the queue if conflicts with an already
-		// queued one.
-		c.mx.Unlock()
+	c.queueCond.L.Lock()
+	c.queue.PushBack(&commitQueueTxn{
+		commitLoop: commitLoop,
+		blocked:    blocked,
+		rset:       rset,
+		wset:       wset,
+	})
+	c.queueCond.L.Unlock()
 
-		select {
-		case c.queue <- commitLoop:
-		case <-c.ctx.Done():
-		}
-	} else {
-		// To make sure we don't add a new tx to the queue that depends
-		// on this "unblocked" tx, grab the commitMutex before lifting
-		// the mutex guarding the lock maps.
-		c.commitMutex.RLock()
-		c.mx.Unlock()
-
-		// At this point we're safe to execute the "unblocked" tx, as
-		// we cannot execute blocked tx that may have been read from the
-		// queue until the commitMutex is held.
-		commitLoop()
-
-		c.commitMutex.RUnlock()
-	}
+	c.mx.Unlock()
+
+	c.queueCond.Signal()
 }
 
-// Done decreases lock counts of the keys in the read/write sets.
-func (c *commitQueue) Done(rset readSet, wset writeSet) {
+// done decreases lock counts of the keys in the read/write sets.
+func (c *commitQueue) done(rset []string, wset []string) {
 	c.mx.Lock()
 	defer c.mx.Unlock()
 
-	for key := range rset {
+	for _, key := range rset {
 		c.readerMap[key] -= 1
 		if c.readerMap[key] == 0 {
 			delete(c.readerMap, key)
 		}
 	}
 
-	for key := range wset {
+	for _, key := range wset {
 		c.writerMap[key] -= 1
 		if c.writerMap[key] == 0 {
 			delete(c.writerMap, key)
@@ -131,20 +131,82 @@ func (c *commitQueue) Done(rset readSet, wset writeSet) {
 // dependencies. The queue ensures that the top element doesn't conflict with
 // any other transactions and therefore can be executed freely.
 func (c *commitQueue) mainLoop() {
-	defer c.wg.Done()
+	defer close(c.shutdown)
 
 	for {
+		// Wait until there are no unblocked transactions being
+		// executed, and for there to be at least one blocked
+		// transaction in our queue.
+		c.queueCond.L.Lock()
+		for c.queue.Front() == nil {
+			c.queueCond.Wait()
+
+			// Check the exit condition before looping again.
+			select {
+			case <-c.ctx.Done():
+				c.queueCond.L.Unlock()
+				return
+			default:
+			}
+		}
+
+		// Now collect all txns until we find the next blocking one.
+		// These shouldn't conflict (if the precollected read/write
+		// keys sets don't grow), meaning we can safely commit them
+		// in parallel.
+		work := make([]*commitQueueTxn, 1)
+		e := c.queue.Front()
+		work[0] = c.queue.Remove(e).(*commitQueueTxn)
+
+		for {
+			e := c.queue.Front()
+			if e == nil {
+				break
+			}
+
+			next := e.Value.(*commitQueueTxn)
+			if !next.blocked {
+				work = append(work, next)
+				c.queue.Remove(e)
+			} else {
+				// We found the next blocking txn which means
+				// the block of work needs to be cut here.
+				break
+			}
+		}
+
+		c.queueCond.L.Unlock()
+
+		// Check if we need to exit before continuing.
 		select {
-		case top := <-c.queue:
-			// Execute the next blocked transaction. As it is
-			// the top element in the queue it means that it doesn't
-			// depend on any other transactions anymore.
-			c.commitMutex.Lock()
-			top()
-			c.commitMutex.Unlock()
+		case <-c.ctx.Done():
+			return
+		default:
+		}
+
+		var wg sync.WaitGroup
+		wg.Add(len(work))
 
+		// Fire up N goroutines where each will run its commit loop
+		// and then clean up the reader/writer maps.
+		for _, txn := range work {
+			go func(txn *commitQueueTxn) {
+				defer wg.Done()
+				txn.commitLoop()
+
+				// We can safely cleanup here as done only
+				// holds the main mutex.
+				c.done(txn.rset, txn.wset)
+			}(txn)
+		}
+
+		wg.Wait()
+
+		// Check if we need to exit before continuing.
+		select {
 		case <-c.ctx.Done():
 			return
+		default:
 		}
 	}
 }

kvdb/etcd/commit_queue_test.go

@@ -17,7 +17,7 @@ import (
 func TestCommitQueue(t *testing.T) {
 	// The duration of each commit.
 	const commitDuration = time.Millisecond * 500
-	const numCommits = 4
+	const numCommits = 5
 
 	var wg sync.WaitGroup
 	commits := make([]string, numCommits)
@@ -30,69 +30,53 @@ func TestCommitQueue(t *testing.T) {
 			// Update our log of commit order. Avoid blocking
 			// by preallocating the commit log and increasing
 			// the log index atomically.
-			i := atomic.AddInt32(&idx, 1)
-			commits[i] = tag
-
 			if sleep {
 				time.Sleep(commitDuration)
 			}
-		}
-	}
 
-	// Helper function to create a read set from the passed keys.
-	makeReadSet := func(keys []string) readSet {
-		rs := make(map[string]stmGet)
-
-		for _, key := range keys {
-			rs[key] = stmGet{}
-		}
-
-		return rs
-	}
-
-	// Helper function to create a write set from the passed keys.
-	makeWriteSet := func(keys []string) writeSet {
-		ws := make(map[string]stmPut)
-
-		for _, key := range keys {
-			ws[key] = stmPut{}
+			i := atomic.AddInt32(&idx, 1)
+			commits[i] = tag
 		}
-
-		return ws
 	}
 
 	ctx := context.Background()
 	ctx, cancel := context.WithCancel(ctx)
 	q := NewCommitQueue(ctx)
-	defer q.Wait()
+	defer q.Stop()
 	defer cancel()
 
 	wg.Add(numCommits)
 	t1 := time.Now()
 
-	// Tx1: reads: key1, key2, writes: key3, conflict: none
+	// Tx1 (long): reads: key1, key2, writes: key3, conflict: none
 	q.Add(
 		commit("free", true),
-		makeReadSet([]string{"key1", "key2"}),
-		makeWriteSet([]string{"key3"}),
+		[]string{"key1", "key2"},
+		[]string{"key3"},
 	)
 	// Tx2: reads: key1, key2, writes: key3, conflict: Tx1
 	q.Add(
 		commit("blocked1", false),
-		makeReadSet([]string{"key1", "key2"}),
-		makeWriteSet([]string{"key3"}),
+		[]string{"key1", "key2"},
+		[]string{"key3"},
+	)
+	// Tx3 (long): reads: key1, writes: key4, conflict: none
+	q.Add(
+		commit("free", true),
+		[]string{"key1", "key2"},
+		[]string{"key4"},
 	)
-	// Tx3: reads: key1, writes: key4, conflict: none
+	// Tx4 (long): reads: key1, writes: none, conflict: none
 	q.Add(
 		commit("free", true),
-		makeReadSet([]string{"key1", "key2"}),
-		makeWriteSet([]string{"key4"}),
+		[]string{"key1", "key2"},
+		[]string{},
 	)
 	// Tx4: reads: key2, writes: key4 conflict: Tx3
 	q.Add(
 		commit("blocked2", false),
-		makeReadSet([]string{"key2"}),
-		makeWriteSet([]string{"key4"}),
+		[]string{"key2"},
+		[]string{"key4"},
 	)
 
 	// Wait for all commits.
@@ -109,7 +93,7 @@ func TestCommitQueue(t *testing.T) {
 	// before the blocking ones, and the blocking ones are executed in
 	// the order of addition.
 	require.Equal(t,
-		[]string{"free", "free", "blocked1", "blocked2"},
+		[]string{"free", "blocked1", "free", "free", "blocked2"},
 		commits,
 	)
 }