Enable readahead by default

The Linux kernel and FUSE default to a readahead size of 128 KB, but
because there is no default value for conf.maxReadhead, it gets
completely disabled leading to huge overhead due to many page sized
FUSE read requests with only 4 KB.

fuse.go

@@ -232,9 +232,17 @@ func initMount(c *Conn, conf *mountConfig) error {
 	}
 	c.proto = proto
 
+	maxReadahead := conf.maxReadahead
+	if maxReadahead == 0 {
+		maxReadahead = defaultReadahead
+	}
+	if maxReadahead > r.MaxReadahead {
+		maxReadahead = r.MaxReadahead
+	}
+
 	s := &InitResponse{
 		Library:      proto,
-		MaxReadahead: conf.maxReadahead,
+		MaxReadahead: maxReadahead,
 		MaxWrite:     maxWrite,
 		Flags:        InitBigWrites | conf.initFlags,
 	}

fuse_darwin.go

@@ -7,3 +7,6 @@ package fuse
 // 16MB value. See TestSetxattr16MB and
 // https://github.com/bazil/fuse/issues/42
 const maxWrite = 16 * 1024 * 1024
+
+// Default kernel readahead.
+const defaultReadahead = 128 * 1024

fuse_freebsd.go

@@ -4,3 +4,6 @@ package fuse
 //
 // This number is just a guess.
 const maxWrite = 128 * 1024
+
+// Default kernel readahead.
+const defaultReadahead = 128 * 1024

fuse_linux.go

@@ -5,3 +5,6 @@ package fuse
 // Linux 4.2.0 has been observed to cap this value at 128kB
 // (FUSE_MAX_PAGES_PER_REQ=32, 4kB pages).
 const maxWrite = 128 * 1024
+
+// Default kernel readahead.
+const defaultReadahead = 128 * 1024