From 175240909f94936074ca8bae11337fef6a0b3313 Mon Sep 17 00:00:00 2001
From: Aaron Jacobs <jacobsa@google.com>
Date: Thu, 23 Jul 2015 14:20:35 +1000
Subject: [PATCH] Stop over-allocating receive buffers for large writes.

The maxWrite constant used to size buffers was 16 MiB, even though
InitRequest.Respond always tells the kernel we want writes of 128 KiB.
Caveat: mount_darwin.go implies that on darwin we were still telling
the kernel 16 MiB, but experimentally it was capping this to 1 MiB.

Now use exactly those sizes on those operating systems. On FreeBSD
continue to use 128 KiB, because I'm not sure what it does.

Allocating and zeroing buffers in allocMessage is a large source of CPU
usage in a read-heavy load; I believe this change will help there.
---
 fuse.go          | 5 +----
 mount_darwin.go  | 5 +++++
 mount_freebsd.go | 3 +++
 mount_linux.go   | 4 ++++
 4 files changed, 13 insertions(+), 4 deletions(-)

diff --git a/fuse.go b/fuse.go
index ca988967..8cfe149d 100644
--- a/fuse.go
+++ b/fuse.go
@@ -235,7 +235,7 @@ func initMount(c *Conn, conf *mountConfig) error {
 	s := &InitResponse{
 		Library:      proto,
 		MaxReadahead: conf.maxReadahead,
-		MaxWrite:     128 * 1024,
+		MaxWrite:     maxWrite,
 		Flags:        InitBigWrites | conf.initFlags,
 	}
 	r.Respond(s)
@@ -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()
diff --git a/mount_darwin.go b/mount_darwin.go
index ef09bb78..73513f84 100644
--- a/mount_darwin.go
+++ b/mount_darwin.go
@@ -13,6 +13,11 @@ import (
 	"syscall"
 )
 
+// OS X appears to cap the size of writes to 1 MiB. This constant is also used
+// for sizing receive buffers, so make it as small as it can be without
+// limiting write sizes.
+const maxWrite = 1 << 20
+
 var (
 	errNoAvail         = errors.New("no available fuse devices")
 	errNotLoaded       = errors.New("osxfuse is not loaded")
diff --git a/mount_freebsd.go b/mount_freebsd.go
index 70bb4102..26d884c4 100644
--- a/mount_freebsd.go
+++ b/mount_freebsd.go
@@ -10,6 +10,9 @@ import (
 	"syscall"
 )
 
+// Maximum file write size we are prepared to receive from the kernel.
+const maxWrite = 128 * 1024
+
 func handleMountFusefsStderr(errCh chan<- error) func(line string) (ignore bool) {
 	return func(line string) (ignore bool) {
 		const (
diff --git a/mount_linux.go b/mount_linux.go
index 197d1044..03a50ef4 100644
--- a/mount_linux.go
+++ b/mount_linux.go
@@ -11,6 +11,10 @@ import (
 	"syscall"
 )
 
+// Maximum file write size we are prepared to receive from the kernel. Linux
+// appears to limit writes to 128 KiB.
+const maxWrite = 128 * 1024
+
 func handleFusermountStderr(errCh chan<- error) func(line string) (ignore bool) {
 	return func(line string) (ignore bool) {
 		if line == `fusermount: failed to open /etc/fuse.conf: Permission denied` {