Decrease Linux and FreeBSD max request payload size from 16MB to 128kB

OSX is kept at 16MB to avoid triggering a bug with large Setxattr
calls: #42

While this was already using sync.Pool and in theory reusing the
buffers, real world isn't as merciful. Apparently 2*16MB plus some
waste from other parts of the system was enough to keep triggering GC
runs all too often. Decreasing the buffer size helps keep us below
that threshold.

New buffer size is the largest Write payload size observed on Linux,
and the most common size for write-heavy workloads.

Improves performance and decreases memory use on Linux.

Thanks to Aaron Jacobs <[email protected]>
and Damien Tournoud <[email protected]>.

fuse.go

@@ -403,9 +403,6 @@ func (h *Header) RespondError(err error) {
 	h.respond(buf)
 }
 
-// Maximum file write size we are prepared to receive from the kernel.
-const maxWrite = 16 * 1024 * 1024
-
 // All requests read from the kernel, without data, are shorter than
 // this.
 var maxRequestSize = syscall.Getpagesize()

fuse_darwin.go

@@ -0,0 +1,9 @@
+package fuse
+
+// Maximum file write size we are prepared to receive from the kernel.
+//
+// This value has to be >=16MB or OSXFUSE (3.4.0 observed) will
+// forcibly close the /dev/fuse file descriptor on a Setxattr with a
+// 16MB value. See TestSetxattr16MB and
+// https://github.com/bazil/fuse/issues/42
+const maxWrite = 16 * 1024 * 1024

fuse_freebsd.go

@@ -0,0 +1,6 @@
+package fuse
+
+// Maximum file write size we are prepared to receive from the kernel.
+//
+// This number is just a guess.
+const maxWrite = 128 * 1024

fuse_linux.go

@@ -0,0 +1,7 @@
+package fuse
+
+// Maximum file write size we are prepared to receive from the kernel.
+//
+// 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