Game detection: foreign GPU holders take an external lock hold (GAME_PROCS, GPU_FOREIGN_VRAM_MB)

internal/config/config.go

@@ -61,6 +61,17 @@ type Config struct {
 	ComfyIdleTimeout  time.Duration
 	ComfyStartTimeout time.Duration
 
+	// GameProcs (GAME_PROCS) is a watch list of process names; while any of
+	// them runs, the GPU is treated as held by a foreign process. The
+	// nvidia-smi path (GPUForeignVRAMMB, GPU_FOREIGN_VRAM_MB) does the same
+	// when a process not in GPUIgnoreProcs (GPU_IGNORE_PROCS) holds more than
+	// that many MiB of VRAM. GamePollInterval (GAME_POLL_INTERVAL) is how
+	// often both checks run.
+	GameProcs        []string
+	GPUForeignVRAMMB int
+	GPUIgnoreProcs   []string
+	GamePollInterval time.Duration
+
 	WarmModel string
 	LogLevel  slog.Level
 	LogJSON   bool
@@ -102,6 +113,11 @@ func Defaults() Config {
 		ComfyIdleTimeout:  5 * time.Minute,
 		ComfyStartTimeout: 2 * time.Minute,
 
+		// ComfyUI runs under python; excluding it (and Ollama) by name keeps
+		// our own consumers from tripping the foreign-VRAM check.
+		GPUIgnoreProcs:   []string{"ollama", "ollama app", "ollama_llama_server", "python", "pythonw"},
+		GamePollInterval: 5 * time.Second,
+
 		LogLevel: slog.LevelWarn,
 	}
 }
@@ -137,6 +153,17 @@ func ParseEnvFile(r io.Reader) (map[string]string, error) {
 	return values, scanner.Err()
 }
 
+// splitList parses a comma-separated setting into trimmed, non-empty items.
+func splitList(v string) []string {
+	var out []string
+	for _, item := range strings.Split(v, ",") {
+		if item = strings.TrimSpace(item); item != "" {
+			out = append(out, item)
+		}
+	}
+	return out
+}
+
 func envDuration(getenv func(string) string, name string, dst *time.Duration) error {
 	v := getenv(name)
 	if v == "" {
@@ -192,11 +219,25 @@ func Load(getenv func(string) string) (Config, error) {
 		{"COMFY_IDLE_TIMEOUT", &cfg.ComfyIdleTimeout},
 		{"COMFY_START_TIMEOUT", &cfg.ComfyStartTimeout},
 		{"UPDATE_INTERVAL", &cfg.UpdateInterval},
+		{"GAME_POLL_INTERVAL", &cfg.GamePollInterval},
 	} {
 		if err := envDuration(getenv, e.name, e.dst); err != nil {
 			return cfg, err
 		}
 	}
+	if v := getenv("GAME_PROCS"); v != "" {
+		cfg.GameProcs = splitList(v)
+	}
+	if v := getenv("GPU_IGNORE_PROCS"); v != "" {
+		cfg.GPUIgnoreProcs = splitList(v)
+	}
+	if v := getenv("GPU_FOREIGN_VRAM_MB"); v != "" {
+		n, err := strconv.Atoi(v)
+		if err != nil || n < 0 {
+			return cfg, fmt.Errorf("GPU_FOREIGN_VRAM_MB: must be a non-negative integer (MiB, 0 = disabled)")
+		}
+		cfg.GPUForeignVRAMMB = n
+	}
 	if v := getenv("PROMPT_CAPTURE_LIMIT"); v != "" {
 		n, err := strconv.ParseInt(v, 10, 64)
 		if err != nil || n < 0 {

internal/config/config_test.go

@@ -157,6 +157,8 @@ func TestLoadErrors(t *testing.T) {
 		{"LLM_BUSY_MODE", "bogus"},
 		{"LLM_BUSY_STATUS", "200"},
 		{"BUSY_RETRY_AFTER", "0"},
+		{"GPU_FOREIGN_VRAM_MB", "-1"},
+		{"GAME_POLL_INTERVAL", "bogus"},
 	} {
 		_, err := Load(func(k string) string {
 			if k == tc.key {
@@ -172,3 +174,51 @@ func TestLoadErrors(t *testing.T) {
 		}
 	}
 }
+
+func TestGameDetectionSettings(t *testing.T) {
+	cfg, err := Load(func(k string) string {
+		switch k {
+		case "OLLAMA_URL":
+			return "http://127.0.0.1:11435"
+		case "GAME_PROCS":
+			return " cyberpunk2077.exe, hl2.exe ,, "
+		case "GPU_FOREIGN_VRAM_MB":
+			return "1024"
+		case "GPU_IGNORE_PROCS":
+			return "ollama, my-trainer"
+		}
+		return ""
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(cfg.GameProcs) != 2 || cfg.GameProcs[0] != "cyberpunk2077.exe" || cfg.GameProcs[1] != "hl2.exe" {
+		t.Fatalf("GameProcs = %v", cfg.GameProcs)
+	}
+	if cfg.GPUForeignVRAMMB != 1024 {
+		t.Fatalf("GPUForeignVRAMMB = %d", cfg.GPUForeignVRAMMB)
+	}
+	if len(cfg.GPUIgnoreProcs) != 2 || cfg.GPUIgnoreProcs[1] != "my-trainer" {
+		t.Fatalf("GPUIgnoreProcs = %v", cfg.GPUIgnoreProcs)
+	}
+	if cfg.GamePollInterval != 5*time.Second {
+		t.Fatalf("GamePollInterval = %v, want 5s default", cfg.GamePollInterval)
+	}
+
+	// Defaults: both detection paths off, ignore list covers our consumers.
+	cfg, err = Load(func(k string) string {
+		if k == "OLLAMA_URL" {
+			return "http://127.0.0.1:11435"
+		}
+		return ""
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(cfg.GameProcs) != 0 || cfg.GPUForeignVRAMMB != 0 {
+		t.Fatalf("detection must be off by default: %v %d", cfg.GameProcs, cfg.GPUForeignVRAMMB)
+	}
+	if len(cfg.GPUIgnoreProcs) == 0 {
+		t.Fatal("GPUIgnoreProcs default must not be empty")
+	}
+}

internal/config/sample.go

@@ -32,6 +32,10 @@ func sampleEntries(logFile string) []sampleEntry {
 		{"COMFY_DIR", `C:\ComfyUI`, "Working directory for COMFY_CMD (default: empty = inherit)", false},
 		{"COMFY_IDLE_TIMEOUT", "5m", "Stop the managed ComfyUI after this long without requests or jobs (frees VRAM)", false},
 		{"COMFY_START_TIMEOUT", "2m", "How long a request waits for the managed ComfyUI to come up", false},
+		{"GAME_PROCS", "cyberpunk2077.exe,hl2.exe", "While a listed process runs, the GPU counts as held by it: requests wait, Ollama unloads, managed ComfyUI stops (default: empty = disabled)", false},
+		{"GPU_FOREIGN_VRAM_MB", "1024", "Also treat the GPU as held when a process not in GPU_IGNORE_PROCS uses more VRAM than this (needs nvidia-smi; 0/empty = disabled)", false},
+		{"GPU_IGNORE_PROCS", "ollama,ollama app,ollama_llama_server,python,pythonw", "Process names never counted as foreign GPU users (ComfyUI runs under python)", false},
+		{"GAME_POLL_INTERVAL", "5s", "How often game/VRAM detection runs", false},
 		{"UNLOAD_TIMEOUT", "60s", "How long to wait for Ollama to unload a model", false},
 		{"JOB_TIMEOUT", "15m", "Maximum time to wait for a ComfyUI job", false},
 		{"LLM_WAIT_TIMEOUT", "10m", "Max time an LLM request waits for the GPU before being answered 503 (wait mode)", false},

internal/game/game.go

@@ -0,0 +1,159 @@
+// Package game detects processes outside gpu-turnstile's control that hold
+// the GPU — typically a game — so the proxy can block new GPU work and free
+// VRAM while they run. Two detection paths: an explicit process watch list
+// (GAME_PROCS) and a foreign-VRAM threshold via nvidia-smi
+// (GPU_FOREIGN_VRAM_MB) that catches anything not on the ignore list.
+package game
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"log/slog"
+	"os/exec"
+	"strconv"
+	"strings"
+)
+
+// Process is one running OS process.
+type Process struct {
+	PID  int
+	Name string
+}
+
+// computeApp is one process holding GPU memory, as reported by nvidia-smi.
+type computeApp struct {
+	PID    int
+	UsedMB int
+}
+
+// Detector checks whether a foreign process holds the GPU. The zero value
+// (no watch list, no threshold) never detects anything; main only starts the
+// poll loop when at least one path is configured.
+type Detector struct {
+	procs    map[string]bool // normalized names from GAME_PROCS
+	vramMB   int             // foreign VRAM threshold; 0 = disabled
+	ignore   map[string]bool // normalized names never counted as foreign
+	log      *slog.Logger
+	noNvidia bool // nvidia-smi was not found; VRAM path disabled for good
+}
+
+// New builds a Detector from the configured watch list, VRAM threshold in
+// MiB (0 disables the nvidia-smi path) and ignore list. Names are matched
+// case-insensitively, with or without a trailing ".exe".
+func New(procs []string, vramMB int, ignore []string, log *slog.Logger) *Detector {
+	if log == nil {
+		log = slog.Default()
+	}
+	return &Detector{
+		procs:  nameSet(procs),
+		vramMB: vramMB,
+		ignore: nameSet(ignore),
+		log:    log,
+	}
+}
+
+// normName lowercases a process name and strips a trailing ".exe" so the
+// watch and ignore lists match on Windows and Linux spellings alike.
+func normName(s string) string {
+	return strings.TrimSuffix(strings.ToLower(strings.TrimSpace(s)), ".exe")
+}
+
+func nameSet(names []string) map[string]bool {
+	set := make(map[string]bool, len(names))
+	for _, n := range names {
+		if n = normName(n); n != "" {
+			set[n] = true
+		}
+	}
+	return set
+}
+
+// Check looks once for foreign GPU holders and returns a human-readable
+// description of each (empty when the GPU is free for gpu-turnstile's
+// consumers). A failing nvidia-smi call is returned as an error only when
+// the process list found nothing; a missing nvidia-smi binary disables the
+// VRAM path permanently (logged once).
+func (d *Detector) Check(ctx context.Context) ([]string, error) {
+	ps, psErr := processes()
+	if d.vramMB <= 0 || d.noNvidia {
+		return d.detect(ps, nil), psErr
+	}
+	apps, err := queryComputeApps(ctx)
+	if errors.Is(err, exec.ErrNotFound) {
+		d.noNvidia = true
+		d.log.Warn("GPU_FOREIGN_VRAM_MB is set but nvidia-smi was not found; VRAM detection disabled")
+		return d.detect(ps, nil), nil
+	}
+	if err != nil {
+		return d.detect(ps, nil), err
+	}
+	return d.detect(ps, apps), nil
+}
+
+// detect is the pure core of Check: given the process table and (optionally)
+// the nvidia-smi compute-apps list, it returns the foreign holders.
+func (d *Detector) detect(ps []Process, apps []computeApp) []string {
+	var holders []string
+	for _, p := range ps {
+		if d.procs[normName(p.Name)] {
+			holders = append(holders, fmt.Sprintf("%s (pid %d)", p.Name, p.PID))
+		}
+	}
+	if d.vramMB > 0 && apps != nil {
+		names := make(map[int]string, len(ps))
+		for _, p := range ps {
+			names[p.PID] = p.Name
+		}
+		for _, a := range apps {
+			name := names[a.PID]
+			if d.ignore[normName(name)] || a.UsedMB < d.vramMB {
+				continue
+			}
+			if name == "" {
+				name = "unknown process"
+			}
+			holders = append(holders, fmt.Sprintf("%s (pid %d) using %d MiB VRAM", name, a.PID, a.UsedMB))
+		}
+	}
+	return holders
+}
+
+// queryComputeApps runs nvidia-smi and parses the per-process VRAM list.
+// Note: under Windows' WDDM driver, nvidia-smi only sees compute
+// allocations, so graphics-only games may not appear there — GAME_PROCS is
+// the reliable path on Windows; on Linux both work.
+func queryComputeApps(ctx context.Context) ([]computeApp, error) {
+	out, err := exec.CommandContext(ctx, "nvidia-smi",
+		"--query-compute-apps=pid,used_memory", "--format=csv,noheader,nounits").Output()
+	if err != nil {
+		return nil, err
+	}
+	return parseComputeApps(string(out))
+}
+
+// parseComputeApps parses "pid, used_memory" CSV lines (no header, MiB
+// units). Unsupported rows ("N/A" on WDDM) are skipped.
+func parseComputeApps(out string) ([]computeApp, error) {
+	var apps []computeApp
+	for _, line := range strings.Split(out, "\n") {
+		line = strings.TrimSpace(line)
+		if line == "" {
+			continue
+		}
+		pidStr, memStr, ok := strings.Cut(line, ",")
+		if !ok {
+			return nil, fmt.Errorf("nvidia-smi: unexpected line %q", line)
+		}
+		pid, err := strconv.Atoi(strings.TrimSpace(pidStr))
+		if err != nil {
+			return nil, fmt.Errorf("nvidia-smi: unexpected pid in %q", line)
+		}
+		mem, err := strconv.Atoi(strings.TrimSpace(memStr))
+		if err != nil {
+			continue // "N/A" and friends: unsupported under WDDM
+		}
+		apps = append(apps, computeApp{PID: pid, UsedMB: mem})
+	}
+	return apps, nil
+}

internal/game/game_test.go

@@ -0,0 +1,96 @@
+package game
+
+import (
+	"runtime"
+	"slices"
+	"testing"
+)
+
+func TestNormName(t *testing.T) {
+	for in, want := range map[string]string{
+		"Cyberpunk2077.exe": "cyberpunk2077",
+		"ollama":            "ollama",
+		"OLLAMA APP.EXE":    "ollama app",
+		" python.exe ":      "python",
+		"hl2":               "hl2",
+	} {
+		if got := normName(in); got != want {
+			t.Errorf("normName(%q) = %q, want %q", in, got, want)
+		}
+	}
+}
+
+func TestParseComputeApps(t *testing.T) {
+	apps, err := parseComputeApps("1234, 512\n  42 , 8192 \n\n")
+	if err != nil {
+		t.Fatal(err)
+	}
+	want := []computeApp{{PID: 1234, UsedMB: 512}, {PID: 42, UsedMB: 8192}}
+	if !slices.Equal(apps, want) {
+		t.Errorf("got %+v, want %+v", apps, want)
+	}
+
+	// WDDM reports "N/A" for memory; those rows are skipped, not fatal.
+	apps, err = parseComputeApps("1234, N/A\n42, 1024\n")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !slices.Equal(apps, []computeApp{{PID: 42, UsedMB: 1024}}) {
+		t.Errorf("got %+v", apps)
+	}
+
+	if _, err := parseComputeApps("garbage\n"); err == nil {
+		t.Error("expected an error for a malformed line")
+	}
+}
+
+func TestDetect(t *testing.T) {
+	d := New([]string{"Cyberpunk2077.exe", "hl2"}, 1024,
+		[]string{"ollama", "python", "pythonw"}, nil)
+	ps := []Process{
+		{PID: 10, Name: "ollama.exe"},
+		{PID: 20, Name: "python.exe"},
+		{PID: 30, Name: "cyberpunk2077.exe"},
+	}
+	apps := []computeApp{
+		{PID: 10, UsedMB: 8192}, // ignored: ollama
+		{PID: 20, UsedMB: 4096}, // ignored: python (ComfyUI)
+		{PID: 40, UsedMB: 2048}, // foreign, above threshold
+		{PID: 50, UsedMB: 100},  // foreign but below threshold
+	}
+	holders := d.detect(ps, apps)
+	if len(holders) != 2 {
+		t.Fatalf("got %v, want 2 holders", holders)
+	}
+	if holders[0] != "cyberpunk2077.exe (pid 30)" {
+		t.Errorf("holders[0] = %q", holders[0])
+	}
+	if holders[1] != "unknown process (pid 40) using 2048 MiB VRAM" {
+		t.Errorf("holders[1] = %q", holders[1])
+	}
+}
+
+func TestDetectNothingConfigured(t *testing.T) {
+	d := New(nil, 0, nil, nil)
+	if got := d.detect([]Process{{PID: 1, Name: "game.exe"}}, nil); len(got) != 0 {
+		t.Errorf("got %v, want none", got)
+	}
+}
+
+func TestProcessesLive(t *testing.T) {
+	if runtime.GOOS != "windows" && runtime.GOOS != "linux" {
+		t.Skip("no process listing on this platform")
+	}
+	ps, err := processes()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(ps) == 0 {
+		t.Fatal("no processes listed")
+	}
+	for _, p := range ps {
+		if p.Name == "" {
+			t.Errorf("pid %d has an empty name", p.PID)
+		}
+	}
+}

internal/game/procs_linux.go

@@ -0,0 +1,30 @@
+//go:build linux
+
+package game
+
+import (
+	"os"
+	"strconv"
+	"strings"
+)
+
+// processes lists the running processes from /proc/<pid>/comm.
+func processes() ([]Process, error) {
+	entries, err := os.ReadDir("/proc")
+	if err != nil {
+		return nil, err
+	}
+	var ps []Process
+	for _, e := range entries {
+		pid, err := strconv.Atoi(e.Name())
+		if err != nil {
+			continue
+		}
+		comm, err := os.ReadFile("/proc/" + e.Name() + "/comm")
+		if err != nil {
+			continue // process vanished mid-walk
+		}
+		ps = append(ps, Process{PID: pid, Name: strings.TrimSpace(string(comm))})
+	}
+	return ps, nil
+}

internal/game/procs_other.go

@@ -0,0 +1,7 @@
+//go:build !windows && !linux
+
+package game
+
+// processes is unsupported on this platform; the process watch list never
+// matches and the nvidia-smi path reports PIDs without names.
+func processes() ([]Process, error) { return nil, nil }

internal/game/procs_windows.go

@@ -0,0 +1,35 @@
+//go:build windows
+
+package game
+
+import (
+	"unsafe"
+
+	"golang.org/x/sys/windows"
+)
+
+// processes lists the running processes via the Toolhelp32 snapshot API.
+func processes() ([]Process, error) {
+	h, err := windows.CreateToolhelp32Snapshot(windows.TH32CS_SNAPPROCESS, 0)
+	if err != nil {
+		return nil, err
+	}
+	defer windows.CloseHandle(h) //nolint:errcheck // best effort
+
+	var entry windows.ProcessEntry32
+	entry.Size = uint32(unsafe.Sizeof(entry))
+	if err := windows.Process32First(h, &entry); err != nil {
+		return nil, err
+	}
+	var ps []Process
+	for {
+		ps = append(ps, Process{
+			PID:  int(entry.ProcessID),
+			Name: windows.UTF16ToString(entry.ExeFile[:]),
+		})
+		if err := windows.Process32Next(h, &entry); err != nil {
+			break // ERROR_NO_MORE_FILES ends the walk
+		}
+	}
+	return ps, nil
+}

internal/lock/lock.go

@@ -1,6 +1,8 @@
 // Package lock implements the two-mode GPU arbitration lock: any number of
 // concurrent LLM requests ("readers") or exactly one image job ("writer"),
-// with image jobs taking priority over newly arriving LLM requests.
+// with image jobs taking priority over newly arriving LLM requests. An
+// external hold (SetExternal) blocks new grants of both kinds while a
+// foreign process — e.g. a game — holds the GPU; in-flight work drains.
 package lock
 
 import (
@@ -13,9 +15,10 @@ import (
 type State string
 
 const (
-	StateIdle  State = "idle"
-	StateLLM   State = "llm"
-	StateImage State = "image"
+	StateIdle     State = "idle"
+	StateLLM      State = "llm"
+	StateImage    State = "image"
+	StateExternal State = "external"
 )
 
 type imageWaiter struct{ id uint64 }
@@ -30,6 +33,7 @@ type Lock struct {
 	imageActive bool // an image job holds the GPU
 	imageQ      []imageWaiter
 	nextID      uint64
+	external    string // non-empty: a foreign process (e.g. a game) holds the GPU
 
 	log *slog.Logger
 }
@@ -52,12 +56,41 @@ func (l *Lock) logTransition(msg string, args ...any) {
 	}
 }
 
+// SetExternal records that a process outside gpu-turnstile's control (a
+// game, another ML job) holds the GPU: new LLM and image grants block until
+// ClearExternal. In-flight work is not preempted. holder describes the
+// process for logs and busy responses.
+func (l *Lock) SetExternal(holder string) {
+	l.mu.Lock()
+	l.external = holder
+	l.broadcast()
+	l.mu.Unlock()
+	l.logTransition("lock transition", "state", StateExternal, "holder", holder)
+}
+
+// ClearExternal lifts the external hold; waiting LLM and image requests
+// proceed.
+func (l *Lock) ClearExternal() {
+	l.mu.Lock()
+	l.external = ""
+	l.broadcast()
+	l.mu.Unlock()
+	l.logTransition("lock transition", "state", StateIdle)
+}
+
+// External returns the current external holder, or "" when none.
+func (l *Lock) External() string {
+	l.mu.Lock()
+	defer l.mu.Unlock()
+	return l.external
+}
+
 // AcquireLLM blocks until no image job is active or pending, then registers
 // one in-flight LLM request. Returns ctx.Err() if the context is cancelled
 // while waiting; no state is changed in that case.
 func (l *Lock) AcquireLLM(ctx context.Context) error {
 	l.mu.Lock()
-	for l.imageActive || len(l.imageQ) > 0 {
+	for l.imageActive || len(l.imageQ) > 0 || l.external != "" {
 		ch := l.change
 		l.mu.Unlock()
 		select {
@@ -76,10 +109,10 @@ func (l *Lock) AcquireLLM(ctx context.Context) error {
 
 // TryAcquireLLM acquires one in-flight LLM slot without waiting and
 // reports whether it succeeded. It fails when an image job is active or
-// pending.
+// pending or an external hold is set.
 func (l *Lock) TryAcquireLLM() bool {
 	l.mu.Lock()
-	if l.imageActive || len(l.imageQ) > 0 {
+	if l.imageActive || len(l.imageQ) > 0 || l.external != "" {
 		l.mu.Unlock()
 		return false
 	}
@@ -120,7 +153,7 @@ func (l *Lock) AcquireImage(ctx context.Context) error {
 
 	for {
 		l.mu.Lock()
-		if l.imageQ[0].id == w.id && l.n == 0 && !l.imageActive {
+		if l.imageQ[0].id == w.id && l.n == 0 && !l.imageActive && l.external == "" {
 			l.imageQ = l.imageQ[1:]
 			l.imageActive = true
 			l.mu.Unlock()
@@ -166,6 +199,8 @@ func (l *Lock) Snapshot() (state State, llmInflight int, imagePending bool) {
 		state = StateImage
 	case l.n > 0:
 		state = StateLLM
+	case l.external != "":
+		state = StateExternal
 	default:
 		state = StateIdle
 	}

internal/lock/lock_test.go

@@ -226,3 +226,70 @@ func TestRace(t *testing.T) {
 		t.Fatalf("leaked lock state: state=%s n=%d pending=%v", state, n, pending)
 	}
 }
+
+func TestExternalHoldBlocksBoth(t *testing.T) {
+	lk := New(nil)
+	ctx := context.Background()
+
+	lk.SetExternal("game.exe (pid 42)")
+	if lk.TryAcquireLLM() {
+		t.Fatal("TryAcquireLLM succeeded during external hold")
+	}
+	if got := lk.External(); got != "game.exe (pid 42)" {
+		t.Fatalf("External() = %q", got)
+	}
+	if state, _, _ := lk.Snapshot(); state != StateExternal {
+		t.Fatalf("state=%s, want external", state)
+	}
+
+	llmAcquired := make(chan struct{})
+	go func() {
+		if err := lk.AcquireLLM(ctx); err != nil {
+			t.Error(err)
+		}
+		close(llmAcquired)
+	}()
+	assertBlocked(t, llmAcquired, "LLM acquire during external hold")
+
+	imageAcquired := make(chan struct{})
+	go func() {
+		if err := lk.AcquireImage(ctx); err != nil {
+			t.Error(err)
+		}
+		close(imageAcquired)
+	}()
+	assertBlocked(t, imageAcquired, "image acquire during external hold")
+
+	lk.ClearExternal()
+	// The queued image job wins over the LLM waiter (image priority).
+	waitFor(t, imageAcquired, "image acquire after ClearExternal")
+	assertBlocked(t, llmAcquired, "LLM acquire while image active")
+	lk.ReleaseImage()
+	waitFor(t, llmAcquired, "LLM acquire after image release")
+	lk.ReleaseLLM()
+}
+
+func TestExternalHoldDoesNotPreempt(t *testing.T) {
+	lk := New(nil)
+	ctx := context.Background()
+	if err := lk.AcquireLLM(ctx); err != nil {
+		t.Fatal(err)
+	}
+
+	lk.SetExternal("game.exe")
+	// In-flight LLM work keeps the llm state; the hold blocks new grants.
+	if state, _, _ := lk.Snapshot(); state != StateLLM {
+		t.Fatalf("state=%s, want llm while work in flight", state)
+	}
+	if lk.TryAcquireLLM() {
+		t.Fatal("TryAcquireLLM succeeded during external hold")
+	}
+	lk.ReleaseLLM()
+	if state, _, _ := lk.Snapshot(); state != StateExternal {
+		t.Fatalf("state=%s, want external after drain", state)
+	}
+	lk.ClearExternal()
+	if state, _, _ := lk.Snapshot(); state != StateIdle {
+		t.Fatalf("state=%s, want idle", state)
+	}
+}

internal/metrics/metrics.go

@@ -89,7 +89,7 @@ func (m *Metrics) Render(w io.Writer, state string, llmInflight int, imagePendin
 	fmt.Fprint(w, `# HELP gpu_turnstile_state Current GPU state (1 for the active state).
 # TYPE gpu_turnstile_state gauge
 `)
-	for _, s := range []string{"idle", "llm", "image"} {
+	for _, s := range []string{"idle", "llm", "image", "external"} {
 		v := 0
 		if s == state {
 			v = 1

internal/proxy/proxy.go

@@ -464,10 +464,14 @@ func (s *Server) OllamaHandler() http.Handler {
 		if s.busyMode == "reject" {
 			if !s.cfg.Lock.TryAcquireLLM() {
 				s.cfg.Metrics.ObserveLockWait("llm", time.Since(start).Seconds())
+				msg := "GPU busy: image job active or queued"
+				if holder := s.cfg.Lock.External(); holder != "" {
+					msg = "GPU busy: " + holder
+				}
 				s.log.Info("llm request rejected; GPU busy",
 					"path", r.URL.Path, "status", s.busyStatus)
 				w.Header().Set("Retry-After", strconv.Itoa(s.busyRetryAfter))
-				http.Error(w, "GPU busy: image job active or queued", s.busyStatus)
+				http.Error(w, msg, s.busyStatus)
 				return
 			}
 			s.cfg.Metrics.ObserveLockWait("llm", time.Since(start).Seconds())
@@ -508,7 +512,14 @@ func (s *Server) ComfyHandler() http.Handler {
 		}
 		if s.cfg.ComfySup != nil {
 			// Any other ComfyUI request also wakes the managed server; the
-			// retry backoff bridges the time it needs to come up.
+			// retry backoff bridges the time it needs to come up. While a
+			// foreign process holds the GPU we refuse to spawn it — the
+			// request gets the busy answer instead of fighting for VRAM.
+			if holder := s.cfg.Lock.External(); holder != "" && !s.cfg.ComfySup.Running() {
+				w.Header().Set("Retry-After", strconv.Itoa(s.busyRetryAfter))
+				http.Error(w, "GPU busy: "+holder, http.StatusServiceUnavailable)
+				return
+			}
 			if err := s.cfg.ComfySup.EnsureRunning(); err != nil {
 				http.Error(w, fmt.Sprintf("cannot start ComfyUI: %v", err), http.StatusBadGateway)
 				return
@@ -553,10 +564,13 @@ func (w *captureWriter) Unwrap() http.ResponseWriter { return w.ResponseWriter }
 func (s *Server) handlePrompt(w http.ResponseWriter, r *http.Request) {
 	log := s.log.With("op", "image")
 
-	if s.cfg.ComfySup != nil {
-		// Bring the managed server up *before* taking the GPU lock: torch
-		// can take a minute to load, and LLM traffic should keep flowing
-		// in the meantime.
+	// Normally the managed server is brought up *before* taking the GPU
+	// lock: torch can take a minute to load, and LLM traffic should keep
+	// flowing in the meantime. While a foreign process holds the GPU, LLM
+	// traffic is blocked anyway and a fresh ComfyUI would fight it for
+	// VRAM — so the lock comes first in that case.
+	comfyFirst := s.cfg.ComfySup != nil && s.cfg.Lock.External() == ""
+	if comfyFirst {
 		if err := s.cfg.ComfySup.EnsureRunning(); err != nil {
 			http.Error(w, fmt.Sprintf("cannot start ComfyUI: %v", err), http.StatusBadGateway)
 			return
@@ -577,6 +591,19 @@ func (s *Server) handlePrompt(w http.ResponseWriter, r *http.Request) {
 	s.cfg.Metrics.ObserveLockWait("image", time.Since(start).Seconds())
 	log.Info("image lock acquired")
 
+	if s.cfg.ComfySup != nil && !comfyFirst {
+		if err := s.cfg.ComfySup.EnsureRunning(); err != nil {
+			s.cfg.Lock.ReleaseImage()
+			http.Error(w, fmt.Sprintf("cannot start ComfyUI: %v", err), http.StatusBadGateway)
+			return
+		}
+		if err := s.cfg.ComfySup.WaitReady(r.Context()); err != nil {
+			s.cfg.Lock.ReleaseImage()
+			http.Error(w, fmt.Sprintf("ComfyUI did not become ready: %v", err), http.StatusBadGateway)
+			return
+		}
+	}
+
 	if s.cfg.Ollama != nil {
 		uctx, ucancel := context.WithTimeout(r.Context(), s.cfg.UnloadTimeout)
 		elapsed, uerr := s.cfg.Ollama.UnloadAll(uctx)