util/sysresources, magicsock: scale DERP buffer based on system memory

This adds the util/sysresources package, which currently only contains a
function to return the total memory size of the current system.

Then, we modify magicsock to scale the number of buffered DERP messages
based on the system's available memory, ensuring that we never use a
value lower than the previous constant of 32.

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: Ib763c877de4d0d4ee88869078e7d512f6a3a148d

cmd/tailscaled/depaware.txt

@@ -312,6 +312,7 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
         tailscale.com/util/set                                       from tailscale.com/health+
         tailscale.com/util/singleflight                              from tailscale.com/control/controlclient+
         tailscale.com/util/slicesx                                   from tailscale.com/net/dnscache+
+        tailscale.com/util/sysresources                              from tailscale.com/wgengine/magicsock
         tailscale.com/util/systemd                                   from tailscale.com/control/controlclient+
         tailscale.com/util/uniq                                      from tailscale.com/wgengine/magicsock+
         tailscale.com/util/vizerror                                  from tailscale.com/tsweb

util/sysresources/memory.go

@@ -0,0 +1,10 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package sysresources
+
+// TotalMemory returns the total accessible system memory, in bytes. If the
+// value cannot be determined, then 0 will be returned.
+func TotalMemory() uint64 {
+	return totalMemoryImpl()
+}

util/sysresources/memory_bsd.go

@@ -0,0 +1,16 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build freebsd || openbsd || dragonfly || netbsd
+
+package sysresources
+
+import "golang.org/x/sys/unix"
+
+func totalMemoryImpl() uint64 {
+	val, err := unix.SysctlUint64("hw.physmem")
+	if err != nil {
+		return 0
+	}
+	return val
+}

util/sysresources/memory_darwin.go

@@ -0,0 +1,16 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build darwin
+
+package sysresources
+
+import "golang.org/x/sys/unix"
+
+func totalMemoryImpl() uint64 {
+	val, err := unix.SysctlUint64("hw.memsize")
+	if err != nil {
+		return 0
+	}
+	return val
+}

util/sysresources/memory_linux.go

@@ -0,0 +1,19 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build linux
+
+package sysresources
+
+import "golang.org/x/sys/unix"
+
+func totalMemoryImpl() uint64 {
+	var info unix.Sysinfo_t
+
+	if err := unix.Sysinfo(&info); err != nil {
+		return 0
+	}
+
+	// uint64 casts are required since these might be uint32s
+	return uint64(info.Totalram) * uint64(info.Unit)
+}

util/sysresources/memory_unsupported.go

@@ -0,0 +1,8 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !(linux || darwin || freebsd || openbsd || dragonfly || netbsd)
+
+package sysresources
+
+func totalMemoryImpl() uint64 { return 0 }

util/sysresources/sysresources.go

@@ -0,0 +1,6 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package sysresources provides OS-independent methods of determining the
+// resources available to the current system.
+package sysresources

util/sysresources/sysresources_test.go

@@ -0,0 +1,25 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package sysresources
+
+import (
+	"runtime"
+	"testing"
+)
+
+func TestTotalMemory(t *testing.T) {
+	switch runtime.GOOS {
+	case "linux":
+	case "freebsd", "openbsd", "dragonfly", "netbsd":
+	case "darwin":
+	default:
+		t.Skipf("not supported on runtime.GOOS=%q yet", runtime.GOOS)
+	}
+
+	mem := TotalMemory()
+	if mem == 0 {
+		t.Fatal("wanted TotalMemory > 0")
+	}
+	t.Logf("total memory: %v bytes", mem)
+}

wgengine/magicsock/magicsock.go

@@ -63,6 +63,7 @@ import (
 	"tailscale.com/util/clientmetric"
 	"tailscale.com/util/mak"
 	"tailscale.com/util/ringbuffer"
+	"tailscale.com/util/sysresources"
 	"tailscale.com/util/uniq"
 	"tailscale.com/version"
 	"tailscale.com/wgengine/capture"
@@ -1418,12 +1419,59 @@ func (c *Conn) sendAddr(addr netip.AddrPort, pubKey key.NodePublic, b []byte) (s
 	}
 }
 
-// bufferedDerpWritesBeforeDrop is how many packets writes can be
-// queued up the DERP client to write on the wire before we start
-// dropping.
-//
-// TODO: this is currently arbitrary. Figure out something better?
-const bufferedDerpWritesBeforeDrop = 32
+var (
+	bufferedDerpWrites     int
+	bufferedDerpWritesOnce sync.Once
+)
+
+// bufferedDerpWritesBeforeDrop returns how many packets writes can be queued
+// up the DERP client to write on the wire before we start dropping.
+func bufferedDerpWritesBeforeDrop() int {
+	// For mobile devices, always return the previous minimum value of 32;
+	// we can do this outside the sync.Once to avoid that overhead.
+	if runtime.GOOS == "ios" || runtime.GOOS == "android" {
+		return 32
+	}
+
+	bufferedDerpWritesOnce.Do(func() {
+		// Some rough sizing: for the previous fixed value of 32, the
+		// total consumed memory can be:
+		// = numDerpRegions * messages/region * sizeof(message)
+		//
+		// For sake of this calculation, assume 100 DERP regions; at
+		// time of writing (2023-04-03), we have 24.
+		//
+		// A reasonable upper bound for the worst-case average size of
+		// a message is a *disco.CallMeMaybe message with 16 endpoints;
+		// since sizeof(netip.AddrPort) = 32, that's 512 bytes. Thus:
+		// = 100 * 32 * 512
+		// = 1638400 (1.6MiB)
+		//
+		// On a reasonably-small node with 4GiB of memory that's
+		// connected to each region and handling a lot of load, 1.6MiB
+		// is about 0.04% of the total system memory.
+		//
+		// For sake of this calculation, then, let's double that memory
+		// usage to 0.08% and scale based on total system memory.
+		//
+		// For a 16GiB Linux box, this should buffer just over 256
+		// messages.
+		systemMemory := sysresources.TotalMemory()
+		memoryUsable := float64(systemMemory) * 0.0008
+
+		const (
+			theoreticalDERPRegions  = 100
+			messageMaximumSizeBytes = 512
+		)
+		bufferedDerpWrites = int(memoryUsable / (theoreticalDERPRegions * messageMaximumSizeBytes))
+
+		// Never drop below the previous minimum value.
+		if bufferedDerpWrites < 32 {
+			bufferedDerpWrites = 32
+		}
+	})
+	return bufferedDerpWrites
+}
 
 // derpWriteChanOfAddr returns a DERP client for fake UDP addresses that
 // represent DERP servers, creating them as necessary. For real UDP
@@ -1520,7 +1568,7 @@ func (c *Conn) derpWriteChanOfAddr(addr netip.AddrPort, peer key.NodePublic) cha
 	dc.DNSCache = dnscache.Get()
 
 	ctx, cancel := context.WithCancel(c.connCtx)
-	ch := make(chan derpWriteRequest, bufferedDerpWritesBeforeDrop)
+	ch := make(chan derpWriteRequest, bufferedDerpWritesBeforeDrop())
 
 	ad.c = dc
 	ad.writeCh = ch

wgengine/magicsock/magicsock_test.go

@@ -1853,3 +1853,11 @@ func TestSetNetworkMapWithNoPeers(t *testing.T) {
 		}
 	}
 }
+
+func TestBufferedDerpWritesBeforeDrop(t *testing.T) {
+	vv := bufferedDerpWritesBeforeDrop()
+	if vv < 32 {
+		t.Fatalf("got bufferedDerpWritesBeforeDrop=%d, which is < 32", vv)
+	}
+	t.Logf("bufferedDerpWritesBeforeDrop = %d", vv)
+}