Make memory stress size follow available RAM

audit/internal/platform/sat.go

@@ -21,10 +21,11 @@ import (
 )
 
 var (
-	satExecCommand = exec.Command
-	satLookPath    = exec.LookPath
-	satGlob        = filepath.Glob
-	satStat        = os.Stat
+	satExecCommand  = exec.Command
+	satLookPath     = exec.LookPath
+	satGlob         = filepath.Glob
+	satStat         = os.Stat
+	satFreeMemBytes = freeMemBytes
 
 	rocmSMIExecutableGlobs = []string{
 		"/opt/rocm/bin/rocm-smi",
@@ -407,6 +408,25 @@ func resolveDCGMGPUIndices(gpuIndices []int) ([]int, error) {
 	return all, nil
 }
 
+func memoryStressSizeArg() string {
+	if mb := envInt("BEE_VM_STRESS_SIZE_MB", 0); mb > 0 {
+		return fmt.Sprintf("%dM", mb)
+	}
+	availBytes := satFreeMemBytes()
+	if availBytes <= 0 {
+		return "80%"
+	}
+	availMB := availBytes / (1024 * 1024)
+	targetMB := (availMB * 2) / 3
+	if targetMB >= 256 {
+		targetMB = (targetMB / 256) * 256
+	}
+	if targetMB <= 0 {
+		return "80%"
+	}
+	return fmt.Sprintf("%dM", targetMB)
+}
+
 func (s *System) RunMemoryAcceptancePack(ctx context.Context, baseDir string, logFunc func(string)) (string, error) {
 	sizeMB := envInt("BEE_MEMTESTER_SIZE_MB", 128)
 	passes := envInt("BEE_MEMTESTER_PASSES", 1)
@@ -422,11 +442,9 @@ func (s *System) RunMemoryStressPack(ctx context.Context, baseDir string, durati
 	if seconds <= 0 {
 		seconds = envInt("BEE_VM_STRESS_SECONDS", 300)
 	}
-	// Use 80% of RAM by default; override with BEE_VM_STRESS_SIZE_MB.
-	sizeArg := "80%"
-	if mb := envInt("BEE_VM_STRESS_SIZE_MB", 0); mb > 0 {
-		sizeArg = fmt.Sprintf("%dM", mb)
-	}
+	// Base the default on current MemAvailable and keep headroom for the OS and
+	// concurrent stressors so mixed burn runs do not trip the OOM killer.
+	sizeArg := memoryStressSizeArg()
 	return runAcceptancePackCtx(ctx, baseDir, "memory-stress", []satJob{
 		{name: "01-free-before.log", cmd: []string{"free", "-h"}},
 		{name: "02-stress-ng-vm.log", cmd: []string{