pkg/runtime: optimize FDUsage by removing sort

No need sort when we just want the counts.

Signed-off-by: Gyuho Lee <leegyuho@amazon.com>

pkg/runtime/fds_linux.go

@@ -16,7 +16,7 @@
 package runtime
 
 import (
-	"io/ioutil"
+	"os"
 	"syscall"
 )
 
@@ -29,9 +29,20 @@ func FDLimit() (uint64, error) {
 }
 
 func FDUsage() (uint64, error) {
-	fds, err := ioutil.ReadDir("/proc/self/fd")
+	return countFiles("/proc/self/fd")
+}
+
+// countFiles reads the directory named by dirname and returns the count.
+// This is same as stdlib "io/ioutil.ReadDir" but without sorting.
+func countFiles(dirname string) (uint64, error) {
+	f, err := os.Open(dirname)
 	if err != nil {
 		return 0, err
 	}
-	return uint64(len(fds)), nil
+	list, err := f.Readdir(-1)
+	f.Close()
+	if err != nil {
+		return 0, err
+	}
+	return uint64(len(list)), nil
 }