Bug 1864828 - pt 2. Add a pref for PHC's minimum RAM size r=glandium

Differential Revision: https://phabricator.services.mozilla.com/D194363

modules/libpref/init/StaticPrefList.yaml

@@ -11126,6 +11126,11 @@
   value: @IS_EARLY_BETA_OR_EARLIER@
   mirror: always
 
+- name: memory.phc.min_ram_mb
+  type: uint32_t
+  value: 8000
+  mirror: always
+
 #---------------------------------------------------------------------------
 # Prefs starting with "midi."
 #---------------------------------------------------------------------------

toolkit/components/nimbus/FeatureManifest.yaml

@@ -2015,6 +2015,10 @@ phc:
       description: Whether to enable PHC
       type: boolean
       setPref: memory.phc.enabled
+    phcMinRamMB:
+      description: The minimum amount of RAM required to enable PHC
+      type: int
+      setPref: memory.phc.min_ram_mb
 
 mailto:
   description: Prefs to control aspects of the mailto handler

xpcom/base/PHCManager.cpp

@@ -7,6 +7,7 @@
 #include "PHCManager.h"
 
 #include "PHC.h"
+#include "mozilla/Literals.h"
 #include "mozilla/Preferences.h"
 #include "mozilla/StaticPrefs_memory.h"
 #include "mozilla/Telemetry.h"
@@ -16,31 +17,37 @@ namespace mozilla {
 
 using namespace phc;
 
-static const char kPHCPref[] = "memory.phc.enabled";
+static const char kPHCEnabledPref[] = "memory.phc.enabled";
+static const char kPHCMinRamMBPref[] = "memory.phc.min_ram_mb";
 
-static PHCState GetPHCStateFromPref() {
-  return StaticPrefs::memory_phc_enabled() ? Enabled : OnlyFree;
-}
+static void UpdatePHCState() {
+  size_t mem_size = PR_GetPhysicalMemorySize() / (1_MiB);
+  size_t min_mem_size = StaticPrefs::memory_phc_min_ram_mb();
 
-static void PrefChangeCallback(const char* aPrefName, void* aNull) {
-  MOZ_ASSERT(0 == strcmp(aPrefName, kPHCPref));
-
-  SetPHCState(GetPHCStateFromPref());
-}
-
-void InitPHCState() {
-  size_t memSize = PR_GetPhysicalMemorySize();
   // Only enable PHC if there are at least 8GB of ram.  Note that we use
   // 1000 bytes per kilobyte rather than 1024.  Some 8GB machines will have
   // slightly lower actual RAM available after some hardware devices
   // reserve some.
-  if (memSize >= size_t(8'000'000'000llu)) {
-    SetPHCState(GetPHCStateFromPref());
-
-    Preferences::RegisterCallback(PrefChangeCallback, kPHCPref);
+  if (StaticPrefs::memory_phc_enabled() && mem_size >= min_mem_size) {
+    SetPHCState(Enabled);
+  } else {
+    SetPHCState(OnlyFree);
   }
 }
 
+static void PrefChangeCallback(const char* aPrefName, void* aNull) {
+  MOZ_ASSERT((0 == strcmp(aPrefName, kPHCEnabledPref)) ||
+             (0 == strcmp(aPrefName, kPHCMinRamMBPref)));
+
+  UpdatePHCState();
+}
+
+void InitPHCState() {
+  Preferences::RegisterCallback(PrefChangeCallback, kPHCEnabledPref);
+  Preferences::RegisterCallback(PrefChangeCallback, kPHCMinRamMBPref);
+  UpdatePHCState();
+}
+
 void ReportPHCTelemetry() {
   MemoryUsage usage;
   PHCMemoryUsage(usage);