fix(stress): keep platform burn responsive under load

audit/internal/platform/platform_stress.go

@@ -10,9 +10,11 @@ import (
 	"os"
 	"os/exec"
 	"path/filepath"
+	"runtime"
 	"strconv"
 	"strings"
 	"sync"
+	"syscall"
 	"time"
 )
 
@@ -374,10 +376,17 @@ func buildCPUStressCmd(ctx context.Context) (*exec.Cmd, error) {
 		return nil, fmt.Errorf("stressapptest not found: %w", err)
 	}
 	// Use a very long duration; the context timeout will kill it at the right time.
-	cmd := exec.CommandContext(ctx, path, "-s", "86400", "-W", "--cc_test")
+	cmdArgs := []string{"-s", "86400", "-W", "--cc_test"}
+	if threads := platformStressCPUThreads(); threads > 0 {
+		cmdArgs = append(cmdArgs, "-m", strconv.Itoa(threads))
+	}
+	if mb := platformStressMemoryMB(); mb > 0 {
+		cmdArgs = append(cmdArgs, "-M", strconv.Itoa(mb))
+	}
+	cmd := exec.CommandContext(ctx, path, cmdArgs...)
 	cmd.Stdout = nil
 	cmd.Stderr = nil
-	if err := cmd.Start(); err != nil {
+	if err := startLowPriorityCmd(cmd, 15); err != nil {
 		return nil, fmt.Errorf("stressapptest start: %w", err)
 	}
 	return cmd, nil
@@ -418,7 +427,7 @@ func buildAMDGPUStressCmd(ctx context.Context) *exec.Cmd {
 	cmd := exec.CommandContext(ctx, rvsPath, "-c", cfgFile)
 	cmd.Stdout = nil
 	cmd.Stderr = nil
-	_ = cmd.Start()
+	_ = startLowPriorityCmd(cmd, 10)
 	return cmd
 }
 
@@ -433,10 +442,50 @@ func buildNvidiaGPUStressCmd(ctx context.Context) *exec.Cmd {
 	cmd := exec.CommandContext(ctx, path, "--seconds", "86400", "--size-mb", "64")
 	cmd.Stdout = nil
 	cmd.Stderr = nil
-	_ = cmd.Start()
+	_ = startLowPriorityCmd(cmd, 10)
 	return cmd
 }
 
+func startLowPriorityCmd(cmd *exec.Cmd, nice int) error {
+	if err := cmd.Start(); err != nil {
+		return err
+	}
+	if cmd.Process != nil {
+		_ = syscall.Setpriority(syscall.PRIO_PROCESS, cmd.Process.Pid, nice)
+	}
+	return nil
+}
+
+func platformStressCPUThreads() int {
+	if n := envInt("BEE_PLATFORM_STRESS_THREADS", 0); n > 0 {
+		return n
+	}
+	cpus := runtime.NumCPU()
+	switch {
+	case cpus <= 2:
+		return 1
+	case cpus <= 8:
+		return cpus - 1
+	default:
+		return cpus - 2
+	}
+}
+
+func platformStressMemoryMB() int {
+	if mb := envInt("BEE_PLATFORM_STRESS_MB", 0); mb > 0 {
+		return mb
+	}
+	free := freeMemBytes()
+	if free <= 0 {
+		return 0
+	}
+	mb := int((free * 60) / 100 / (1024 * 1024))
+	if mb < 1024 {
+		return 1024
+	}
+	return mb
+}
+
 func packPlatformDir(dir, dest string) error {
 	f, err := os.Create(dest)
 	if err != nil {

audit/internal/platform/platform_stress_test.go

@@ -0,0 +1,34 @@
+package platform
+
+import (
+	"runtime"
+	"testing"
+)
+
+func TestPlatformStressCPUThreadsOverride(t *testing.T) {
+	t.Setenv("BEE_PLATFORM_STRESS_THREADS", "7")
+	if got := platformStressCPUThreads(); got != 7 {
+		t.Fatalf("platformStressCPUThreads=%d want 7", got)
+	}
+}
+
+func TestPlatformStressCPUThreadsDefaultLeavesHeadroom(t *testing.T) {
+	t.Setenv("BEE_PLATFORM_STRESS_THREADS", "")
+	got := platformStressCPUThreads()
+	if got < 1 {
+		t.Fatalf("platformStressCPUThreads=%d want >= 1", got)
+	}
+	if got > runtime.NumCPU() {
+		t.Fatalf("platformStressCPUThreads=%d want <= NumCPU=%d", got, runtime.NumCPU())
+	}
+	if runtime.NumCPU() > 2 && got >= runtime.NumCPU() {
+		t.Fatalf("platformStressCPUThreads=%d want headroom below NumCPU=%d", got, runtime.NumCPU())
+	}
+}
+
+func TestPlatformStressMemoryMBOverride(t *testing.T) {
+	t.Setenv("BEE_PLATFORM_STRESS_MB", "8192")
+	if got := platformStressMemoryMB(); got != 8192 {
+		t.Fatalf("platformStressMemoryMB=%d want 8192", got)
+	}
+}