dircache: increase cache size & lifetime

Looking at the dircache debug output, we see
that a "git status" workload has a very bad
cache hit rate because the entries expire or
get evicted before they can be reused.

Increase both cache size and lifetime for
a 4x speedup:

Before: 75s
After:  17s

https://github.com/rfjakob/gocryptfs/issues/410

internal/fusefrontend/dircache.go

@@ -12,11 +12,11 @@ import (
 )
 
 const (
-	// Number of entries in the dirCache. Three entries work well for two
-	// (probably also three) parallel tar extracts (hit rate around 92%).
+	// Number of entries in the dirCache.
+	// 20 entries work well for "git stat" on a small git repo on sshfs.
 	// Keep in sync with test_helpers.maxCacheFds !
 	// TODO: How to share this constant without causing an import cycle?
-	dirCacheSize = 3
+	dirCacheSize = 20
 	// Enable Lookup/Store/Clear debug messages
 	enableDebugMessages = false
 	// Enable hit rate statistics printing
@@ -151,7 +151,7 @@ func (d *dirCacheStruct) Lookup(dirRelPath string) (fd int, iv []byte) {
 // expireThread is started on the first Lookup()
 func (d *dirCacheStruct) expireThread() {
 	for {
-		time.Sleep(1 * time.Second)
+		time.Sleep(60 * time.Second)
 		d.Clear()
 		if enableStats {
 			d.Lock()

tests/test_helpers/mount_unmount.go

@@ -14,6 +14,11 @@ import (
 	"time"
 )
 
+// gocryptfs may hold up to maxCacheFds open for caching
+// Keep in sync with fusefrontend.dirCacheSize
+// TODO: How to share this constant without causing an import cycle?!
+const maxCacheFds = 20
+
 // Indexed by mountpoint. Initialized in doInit().
 var MountInfo map[string]mountInfo
 
@@ -137,11 +142,6 @@ func UnmountPanic(dir string) {
 	}
 }
 
-// gocryptfs may hold up to maxCacheFds open for caching
-// Keep in sync with fusefrontend.dirCacheSize
-// TODO: How to share this constant without causing an import cycle?!
-const maxCacheFds = 3
-
 // UnmountErr tries to unmount "dir", retrying 10 times, and returns the
 // resulting error.
 func UnmountErr(dir string) (err error) {