From 8cb1c36cfcbce5b625d606ee281fcc3edf322460 Mon Sep 17 00:00:00 2001
From: Luke Lombardi <luke@beam.cloud>
Date: Sun, 24 May 2026 16:24:17 -0400
Subject: [PATCH] Tune FUSE init limits for geesefs

---
 connection.go                       | 16 ++++++++++++++++
 conversions.go                      |  5 ++---
 internal/buffer/in_message_linux.go |  4 ++--
 mount_config.go                     | 10 ++++++++++
 ops.go                              | 11 ++++++-----
 5 files changed, 36 insertions(+), 10 deletions(-)

diff --git a/connection.go b/connection.go
index 0d99f0e..073eb4d 100644
--- a/connection.go
+++ b/connection.go
@@ -56,6 +56,11 @@ var contextKey interface{} = contextKeyType(0)
 // Reading a page at a time is a drag. Ask for a larger size.
 const maxReadahead = 1 << 20
 
+const (
+	defaultMaxBackground       = 128
+	defaultCongestionThreshold = 96
+)
+
 // Connection represents a connection to the fuse kernel process. It is used to
 // receive and reply to requests from the kernel.
 type Connection struct {
@@ -159,6 +164,17 @@ func (c *Connection) Init() error {
 	initOp.Library = c.protocol
 	initOp.MaxReadahead = maxReadahead
 	initOp.MaxWrite = buffer.MaxWriteSize
+	initOp.MaxBackground = c.cfg.MaxBackground
+	if initOp.MaxBackground == 0 {
+		initOp.MaxBackground = defaultMaxBackground
+	}
+	initOp.CongestionThreshold = c.cfg.CongestionThreshold
+	if initOp.CongestionThreshold == 0 {
+		initOp.CongestionThreshold = defaultCongestionThreshold
+		if initOp.CongestionThreshold >= initOp.MaxBackground {
+			initOp.CongestionThreshold = initOp.MaxBackground * 3 / 4
+		}
+	}
 
 	initOp.Flags = 0
 
diff --git a/conversions.go b/conversions.go
index 4d76f36..91d5ade 100644
--- a/conversions.go
+++ b/conversions.go
@@ -1061,9 +1061,8 @@ func (c *Connection) kernelResponseForOp(
 		out.Minor = o.Library.Minor
 		out.MaxReadahead = o.MaxReadahead
 		out.Flags = uint32(o.Flags)
-		// Default values
-		out.MaxBackground = 12
-		out.CongestionThreshold = 9
+		out.MaxBackground = o.MaxBackground
+		out.CongestionThreshold = o.CongestionThreshold
 		out.MaxWrite = o.MaxWrite
 		out.TimeGran = 1
 		out.MaxPages = o.MaxPages
diff --git a/internal/buffer/in_message_linux.go b/internal/buffer/in_message_linux.go
index 6273733..d7410ab 100644
--- a/internal/buffer/in_message_linux.go
+++ b/internal/buffer/in_message_linux.go
@@ -16,5 +16,5 @@ package buffer
 
 // The maximum fuse write request size that InMessage can acommodate.
 //
-// As of kernel 4.20 Linux accepts writes up to 256 pages or 1MiB
-const MaxWriteSize = 1 << 17
+// As of kernel 4.20 Linux accepts writes up to 256 pages or 1MiB.
+const MaxWriteSize = 1 << 20
diff --git a/mount_config.go b/mount_config.go
index b21d5ff..192d910 100644
--- a/mount_config.go
+++ b/mount_config.go
@@ -169,6 +169,16 @@ type MountConfig struct {
 	// being read from the file as a list of slices in ReadFileOp.Data.
 	UseVectoredRead bool
 
+	// Linux only. Controls the maximum number of outstanding FUSE requests the
+	// kernel may queue before applying backpressure. If zero, the library uses a
+	// high-throughput default.
+	MaxBackground uint16
+
+	// Linux only. Controls the number of outstanding FUSE requests at which the
+	// kernel considers the connection congested. If zero, the library derives a
+	// value from MaxBackground.
+	CongestionThreshold uint16
+
 	// OS X only.
 	//
 	// The name of the mounted volume, as displayed in the Finder. If empty, a
diff --git a/ops.go b/ops.go
index fe21a64..03008dc 100644
--- a/ops.go
+++ b/ops.go
@@ -40,9 +40,10 @@ type initOp struct {
 	Flags fusekernel.InitFlags
 
 	// Out
-	Library       fusekernel.Protocol
-	MaxReadahead  uint32
-	MaxBackground uint16
-	MaxWrite      uint32
-	MaxPages      uint16
+	Library             fusekernel.Protocol
+	MaxReadahead        uint32
+	MaxBackground       uint16
+	CongestionThreshold uint16
+	MaxWrite            uint32
+	MaxPages            uint16
 }