Parallel encoding reworked

- Encode in parallel only branches, up to N goroutines
- Set the max N to the number of CPUs
- Only write to buffer if no error was encountered

lib/trie/branch/encode.go

@@ -8,6 +8,7 @@ import (
 	"fmt"
 	"hash"
 	"io"
+	"runtime"
 
 	"github.com/ChainSafe/gossamer/lib/common"
 	"github.com/ChainSafe/gossamer/lib/trie/encode"
@@ -120,43 +121,72 @@ func (b *Branch) Encode(buffer encode.Buffer) (err error) {
 		}
 	}
 
-	const parallel = false // TODO
-	if parallel {
-		err = encodeChildrenInParallel(b.Children, buffer)
-	} else {
-		err = encodeChildrenSequentially(b.Children, buffer)
-	}
+	err = encodeChildrenOpportunisticParallel(b.Children, buffer)
 	if err != nil {
 		return fmt.Errorf("cannot encode children of branch: %w", err)
 	}
 
 	return nil
 }
 
-func encodeChildrenInParallel(children [16]node.Node, buffer io.Writer) (err error) {
-	type result struct {
-		index  int
-		buffer *bytes.Buffer
-		err    error
+type encodingAsyncResult struct {
+	index  int
+	buffer *bytes.Buffer
+	err    error
+}
+
+func runEncodeChild(child node.Node, index int,
+	results chan<- encodingAsyncResult, rateLimit <-chan struct{}) {
+	buffer := pools.EncodingBuffers.Get().(*bytes.Buffer)
+	buffer.Reset()
+	// buffer is put back in the pool after processing its
+	// data in the select block below.
+
+	err := encodeChild(child, buffer)
+
+	results <- encodingAsyncResult{
+		index:  index,
+		buffer: buffer,
+		err:    err,
+	}
+	if rateLimit != nil {
+		// Only run if runEncodeChild is launched
+		// in its own goroutine.
+		<-rateLimit
 	}
+}
+
+var parallelLimit = runtime.NumCPU()
+
+var parallelEncodingRateLimit = make(chan struct{}, parallelLimit)
 
-	resultsCh := make(chan result)
+// encodeChildrenOpportunisticParallel encodes children in parallel eventually.
+// Leaves are encoded in a blocking way, and branches are encoded in separate
+// goroutines IF they are less than the parallelLimit number of goroutines already
+// running. This is designed to limit the total number of goroutines in order to
+// avoid using too much memory on the stack.
+func encodeChildrenOpportunisticParallel(children [16]node.Node, buffer io.Writer) (err error) {
+	// Buffered channels since children might be encoded in this
+	// goroutine or another one.
+	resultsCh := make(chan encodingAsyncResult, len(children))
 
 	for i, child := range children {
-		go func(index int, child node.Node) {
-			buffer := pools.EncodingBuffers.Get().(*bytes.Buffer)
-			buffer.Reset()
-			// buffer is put back in the pool after processing its
-			// data in the select block below.
-
-			err := encodeChild(child, buffer)
-
-			resultsCh <- result{
-				index:  index,
-				buffer: buffer,
-				err:    err,
-			}
-		}(i, child)
+		if isNodeNil(child) || child.Type() == node.LeafType {
+			runEncodeChild(child, i, resultsCh, nil)
+			continue
+		}
+
+		// Branch child
+		select {
+		case parallelEncodingRateLimit <- struct{}{}:
+			// We have a goroutine available to encode
+			// the branch in parallel.
+			go runEncodeChild(child, i, resultsCh, parallelEncodingRateLimit)
+		default:
+			// we reached the maximum parallel goroutines
+			// so encode this branch in this goroutine
+			runEncodeChild(child, i, resultsCh, nil)
+		}
 	}
 
 	currentIndex := 0
@@ -173,7 +203,7 @@ func encodeChildrenInParallel(children [16]node.Node, buffer io.Writer) (err err
 		for currentIndex < len(children) &&
 			resultBuffers[currentIndex] != nil {
 			bufferSlice := resultBuffers[currentIndex].Bytes()
-			if len(bufferSlice) > 0 {
+			if err == nil && len(bufferSlice) > 0 {
 				// note buffer.Write copies the byte slice given as argument
 				_, writeErr := buffer.Write(bufferSlice)
 				if writeErr != nil && err == nil {
@@ -210,17 +240,20 @@ func encodeChildrenSequentially(children [16]node.Node, buffer io.Writer) (err e
 	return nil
 }
 
-func encodeChild(child node.Node, buffer io.Writer) (err error) {
-	var isNil bool
-	switch impl := child.(type) {
+func isNodeNil(n node.Node) (isNil bool) {
+	switch impl := n.(type) {
 	case *Branch:
 		isNil = impl == nil
 	case *leaf.Leaf:
 		isNil = impl == nil
 	default:
-		isNil = child == nil
+		isNil = n == nil
 	}
-	if isNil {
+	return isNil
+}
+
+func encodeChild(child node.Node, buffer io.Writer) (err error) {
+	if isNodeNil(child) {
 		return nil
 	}
 

lib/trie/branch/encode_test.go

@@ -146,7 +146,7 @@ func Test_Branch_Encode(t *testing.T) {
 			wrappedErr: errTest,
 			errMessage: "cannot write encoded value to buffer: test error",
 		},
-		"buffer write error for children encoded sequentially": {
+		"buffer write error for children encoding": {
 			branch: &Branch{
 				Key:   []byte{1, 2, 3},
 				Value: []byte{100},
@@ -175,10 +175,10 @@ func Test_Branch_Encode(t *testing.T) {
 			},
 			wrappedErr: errTest,
 			errMessage: "cannot encode children of branch: " +
-				"cannot encode child at index 3: " +
-				"failed to write child to buffer: test error",
+				"cannot write encoding of child at index 3: " +
+				"test error",
 		},
-		"success with sequential children encoding": {
+		"success with children encoding": {
 			branch: &Branch{
 				Key:   []byte{1, 2, 3},
 				Value: []byte{100},
@@ -241,7 +241,7 @@ func Test_Branch_Encode(t *testing.T) {
 	}
 }
 
-func Test_encodeChildrenInParallel(t *testing.T) {
+func Test_encodeChildrenOpportunisticParallel(t *testing.T) {
 	t.Parallel()
 
 	testCases := map[string]struct {
@@ -329,7 +329,7 @@ func Test_encodeChildrenInParallel(t *testing.T) {
 				previousCall = call
 			}
 
-			err := encodeChildrenInParallel(testCase.children, buffer)
+			err := encodeChildrenOpportunisticParallel(testCase.children, buffer)
 
 			if testCase.wrappedErr != nil {
 				assert.ErrorIs(t, err, testCase.wrappedErr)