diff --git a/.vscode/settings.json b/.vscode/settings.json
index 907e121..58b46dc 100644
--- a/.vscode/settings.json
+++ b/.vscode/settings.json
@@ -1,5 +1,8 @@
 {
     "cSpell.words": [
-        "datashake"
+        "datashake",
+        "rebalances",
+        "rebalancing",
+        "sourcegraph"
     ]
 }
diff --git a/datashake.go b/datashake.go
index 95bc91c..f6f67f1 100644
--- a/datashake.go
+++ b/datashake.go
@@ -1,6 +1,7 @@
 package main
 
 import (
+	"context"
 	"encoding/json"
 	"flag"
 	"fmt"
@@ -9,13 +10,37 @@ import (
 	"os"
 	"os/signal"
 	"path/filepath"
+	"runtime"
+	"sync"
 	"sync/atomic"
 	"syscall"
+
+	"github.com/sourcegraph/conc/pool"
+)
+
+var (
+	sourceDir   = flag.String("source", ".", "source data directory")
+	tempDir     = flag.String("temp", "/tmp", "temporary storage directory")
+	dbPath      = flag.String("db", "datashake.json", "database file path")
+	minimumSize = flag.Int64("min-size", 1024*1024, "minimum size in bytes")
+	concurrency = flag.Int("concurrency", 1, "concurrent processing limit")
 )
 
 func main() {
+	ctx, stop := signal.NotifyContext(
+		context.Background(),
+		syscall.SIGINT, syscall.SIGTERM,
+	)
+	defer stop()
+
 	flag.Parse()
 
+	concurrency := *concurrency
+	if concurrency < 1 {
+		concurrency = runtime.GOMAXPROCS(0)
+	}
+	tasks = pool.New().WithMaxGoroutines(concurrency)
+
 	if err := loadDb(); err != nil {
 		fmt.Println("error", err)
 		os.Exit(1)
@@ -26,12 +51,10 @@ func main() {
 		if err := filepath.WalkDir(*sourceDir, process); err != nil {
 			errors <- err
 		}
+		pending.Wait()
 		close(errors)
 	}()
 
-	signals := make(chan os.Signal, 1)
-	signal.Notify(signals, syscall.SIGINT, syscall.SIGTERM)
-
 Loop:
 	for {
 		select {
@@ -41,7 +64,7 @@ Loop:
 			} else {
 				break Loop
 			}
-		case <-signals:
+		case <-ctx.Done():
 			running.Store(false)
 		}
 	}
@@ -52,69 +75,50 @@ Loop:
 	}
 }
 
-var sourceDir = flag.String("source", "", "source data directory")
-var tempDir = flag.String("temp", "", "temporary storage directory")
-var dbPath = flag.String("db", "datashake.json", "database file path")
-var minimumSize = flag.Int64("min-size", 1024*1024, "minimum size in bytes")
+var (
+	running atomic.Bool
 
-var errors = make(chan error)
-var db = DB{
-	Processed: make(map[string]struct{}),
-}
-var running atomic.Bool
+	tasks   *pool.Pool
+	pending sync.WaitGroup
+	errors  = make(chan error)
 
-func loadDb() error {
-	if *dbPath == "" {
-		return nil
+	db = DB{
+		Processed: make(map[string]struct{}),
 	}
-	dbFile, err := os.Open(*dbPath)
-	if err != nil {
-		return nil
-	}
-	defer func() {
-		_ = dbFile.Close()
-	}()
-	d := json.NewDecoder(dbFile)
-	err = d.Decode(&db)
-	return err
-}
+	dbLock sync.Mutex
+)
 
-func saveDb() error {
-	if *dbPath == "" {
-		return nil
-	}
-	dbFile, err := os.Create(*dbPath)
-	if err != nil {
-		return err
-	}
-	defer func() {
-		_ = dbFile.Close()
-	}()
-	e := json.NewEncoder(dbFile)
-	err = e.Encode(&db)
-	return err
-}
-
-// process is a visitor for `filepath.WalkDir` that implements the rebalancing
-// algorithm.
+// process is a visitor for `filepath.WalkDir` that performs the rebalancing
+// algorithm against regular files.
 //
-// This function never returns an error, since that would stop the directory
+// This function normally never returns an error, since that would stop the
 // directory walk. Instead, any errors are sent to the `errors` channel.
-func process(path string, d fs.DirEntry, err error) (alwaysNil error) {
+func process(path string, d fs.DirEntry, err error) (typicallyNil error) {
 	if !running.Load() {
 		return fs.SkipAll
 	}
+	if err != nil || d.IsDir() || !d.Type().IsRegular() {
+		return
+	}
+	pending.Add(1)
+	tasks.Go(func() {
+		defer pending.Done()
+		if running.Load() {
+			work(path, d)
+		}
+	})
+	return
+}
 
+// work rebalances a single file.
+func work(path string, d fs.DirEntry) {
+	var err error
 	defer func() {
 		if err != nil {
 			errors <- err
 		}
 	}()
 
-	if err != nil || d.IsDir() {
-		return
-	}
-
 	srcFileName := d.Name()
 	srcFilePath, err := filepath.Abs(path)
 	if err != nil {
@@ -168,8 +172,6 @@ func process(path string, d fs.DirEntry, err error) (alwaysNil error) {
 	}
 	safeToRemoveTemp = true
 	db.Remember(srcFilePath)
-
-	return
 }
 
 // copy opens the file from the source path, then creates a copy of it at the
@@ -214,15 +216,57 @@ func copy(srcPath, dstPath string) error {
 	return err
 }
 
+// DB holds a set of files which have been rebalanced.
+//
+// These files are skipped on future runs of the program.
+//
+// The database is loaded from a JSON file when the program starts and saved
+// back to that JSON file as the program finishes.
 type DB struct {
 	Processed map[string]struct{}
 }
 
 func (db *DB) Contains(path string) bool {
+	dbLock.Lock()
+	defer dbLock.Unlock()
 	_, ok := db.Processed[path]
 	return ok
 }
 
 func (db *DB) Remember(path string) {
+	dbLock.Lock()
+	defer dbLock.Unlock()
 	db.Processed[path] = struct{}{}
 }
+
+func loadDb() error {
+	if *dbPath == "" {
+		return nil
+	}
+	dbFile, err := os.Open(*dbPath)
+	if err != nil {
+		return nil
+	}
+	defer func() {
+		_ = dbFile.Close()
+	}()
+	d := json.NewDecoder(dbFile)
+	err = d.Decode(&db)
+	return err
+}
+
+func saveDb() error {
+	if *dbPath == "" {
+		return nil
+	}
+	dbFile, err := os.Create(*dbPath)
+	if err != nil {
+		return err
+	}
+	defer func() {
+		_ = dbFile.Close()
+	}()
+	e := json.NewEncoder(dbFile)
+	err = e.Encode(&db)
+	return err
+}
diff --git a/go.mod b/go.mod
index 333f7ed..daedd34 100644
--- a/go.mod
+++ b/go.mod
@@ -1,3 +1,5 @@
 module gogs.humancabbage.net/datashake
 
 go 1.21
+
+require github.com/sourcegraph/conc v0.3.0 // indirect
diff --git a/go.sum b/go.sum
new file mode 100644
index 0000000..29e2b58
--- /dev/null
+++ b/go.sum
@@ -0,0 +1,2 @@
+github.com/sourcegraph/conc v0.3.0 h1:OQTbbt6P72L20UqAkXXuLOj79LfEanQ+YQFNpLA9ySo=
+github.com/sourcegraph/conc v0.3.0/go.mod h1:Sdozi7LEKbFPqYX2/J+iBAM6HpqSLTASQIKqDmF7Mt0=