export: reduce node buffer to 32

To avoid races with IAVL pruning, the SDK must begin exports of all IAVL stores at the same time. Lowering the IAVL export buffer reduces the memory overhead of doing this, while still keeping it large enough to mitigate context switching penalties.

export.go

@@ -6,6 +6,11 @@ import (
 	"github.com/pkg/errors"
 )
 
+// exportBufferSize is the number of nodes to buffer in the exporter. It improves throughput by
+// processing multiple nodes per context switch, but take care to avoid excessive memory usage,
+// especially since callers may export several IAVL stores in parallel (e.g. the Cosmos SDK).
+const exportBufferSize = 32
+
 // ExportDone is returned by Exporter.Next() when all items have been exported.
 var ExportDone = errors.New("export is complete") // nolint:golint
 
@@ -33,7 +38,7 @@ func newExporter(tree *ImmutableTree) *Exporter {
 	ctx, cancel := context.WithCancel(context.Background())
 	exporter := &Exporter{
 		tree:   tree,
-		ch:     make(chan *ExportNode, 64),
+		ch:     make(chan *ExportNode, exportBufferSize),
 		cancel: cancel,
 	}