parallelize batch flushing

1. Modern storage devices (i.e., SSDs) tend to be highly parallel.
2. Allows us to read and write at the same time (avoids pausing while flushing).

fixes #898 (comment)

License: MIT
Signed-off-by: Steven Allen <steven@stebalien.com>

merkledag/batch.go

@@ -0,0 +1,98 @@
+package merkledag
+
+import (
+	"runtime"
+
+	cid "gx/ipfs/QmNp85zy9RLrQ5oQD4hPyS39ezrrXpcaa7R4Y9kxdWQLLQ/go-cid"
+	node "gx/ipfs/QmPN7cwmpcc4DWXb4KTB9dNAJgjuPY69h3npsMfhRrQL9c/go-ipld-format"
+	blocks "gx/ipfs/QmSn9Td7xgxm9EV7iEjTckpUWmWApggzPxu7eFGWkkpwin/go-block-format"
+)
+
+// ParallelBatchCommits is the number of batch commits that can be in-flight before blocking.
+// TODO: Experiment with multiple datastores, storage devices, and CPUs to find
+// the right value/formula.
+var ParallelBatchCommits = runtime.NumCPU() * 2
+
+// Batch is a buffer for batching adds to a dag.
+type Batch struct {
+	ds *dagService
+
+	activeCommits int
+	commitError   error
+	commitResults chan error
+
+	blocks []blocks.Block
+	size   int
+
+	MaxSize   int
+	MaxBlocks int
+}
+
+func (t *Batch) processResults() {
+	for t.activeCommits > 0 && t.commitError == nil {
+		select {
+		case err := <-t.commitResults:
+			t.activeCommits--
+			if err != nil {
+				t.commitError = err
+			}
+		default:
+			return
+		}
+	}
+}
+
+func (t *Batch) asyncCommit() {
+	if len(t.blocks) == 0 || t.commitError != nil {
+		return
+	}
+	if t.activeCommits >= ParallelBatchCommits {
+		err := <-t.commitResults
+		t.activeCommits--
+
+		if err != nil {
+			t.commitError = err
+			return
+		}
+	}
+	go func(b []blocks.Block) {
+		_, err := t.ds.Blocks.AddBlocks(b)
+		t.commitResults <- err
+	}(t.blocks)
+
+	t.activeCommits++
+	t.blocks = nil
+	t.size = 0
+
+	return
+}
+
+// Add adds a node to the batch and commits the batch if necessary.
+func (t *Batch) Add(nd node.Node) (*cid.Cid, error) {
+	// Not strictly necessary but allows us to catch errors early.
+	t.processResults()
+	if t.commitError != nil {
+		return nil, t.commitError
+	}
+
+	t.blocks = append(t.blocks, nd)
+	t.size += len(nd.RawData())
+	if t.size > t.MaxSize || len(t.blocks) > t.MaxBlocks {
+		t.asyncCommit()
+	}
+	return nd.Cid(), t.commitError
+}
+
+// Commit commits batched nodes.
+func (t *Batch) Commit() error {
+	t.asyncCommit()
+	for t.activeCommits > 0 && t.commitError == nil {
+		err := <-t.commitResults
+		t.activeCommits--
+		if err != nil {
+			t.commitError = err
+		}
+	}
+
+	return t.commitError
+}

merkledag/merkledag.go

@@ -11,7 +11,6 @@ import (
 
 	cid "gx/ipfs/QmNp85zy9RLrQ5oQD4hPyS39ezrrXpcaa7R4Y9kxdWQLLQ/go-cid"
 	node "gx/ipfs/QmPN7cwmpcc4DWXb4KTB9dNAJgjuPY69h3npsMfhRrQL9c/go-ipld-format"
-	blocks "gx/ipfs/QmSn9Td7xgxm9EV7iEjTckpUWmWApggzPxu7eFGWkkpwin/go-block-format"
 	ipldcbor "gx/ipfs/QmWCs8kMecJwCPK8JThue8TjgM2ieJ2HjTLDu7Cv2NEmZi/go-ipld-cbor"
 )
 
@@ -75,8 +74,9 @@ func (n *dagService) Add(nd node.Node) (*cid.Cid, error) {
 
 func (n *dagService) Batch() *Batch {
 	return &Batch{
-		ds:      n,
-		MaxSize: 8 << 20,
+		ds:            n,
+		commitResults: make(chan error, ParallelBatchCommits),
+		MaxSize:       8 << 20,
 
 		// By default, only batch up to 128 nodes at a time.
 		// The current implementation of flatfs opens this many file
@@ -389,31 +389,6 @@ func (np *nodePromise) Get(ctx context.Context) (node.Node, error) {
 	}
 }
 
-type Batch struct {
-	ds *dagService
-
-	blocks    []blocks.Block
-	size      int
-	MaxSize   int
-	MaxBlocks int
-}
-
-func (t *Batch) Add(nd node.Node) (*cid.Cid, error) {
-	t.blocks = append(t.blocks, nd)
-	t.size += len(nd.RawData())
-	if t.size > t.MaxSize || len(t.blocks) > t.MaxBlocks {
-		return nd.Cid(), t.Commit()
-	}
-	return nd.Cid(), nil
-}
-
-func (t *Batch) Commit() error {
-	_, err := t.ds.Blocks.AddBlocks(t.blocks)
-	t.blocks = nil
-	t.size = 0
-	return err
-}
-
 type GetLinks func(context.Context, *cid.Cid) ([]*node.Link, error)
 
 // EnumerateChildren will walk the dag below the given root node and add all