Update startup compilation thresholds

This change aims to compile less startup methods to save RAM and
reduce code size. Numbers are from shortly after launching the apps.

Leave cutoff as 1 for high end devices for now. We may want to
revisit that later.

For low ram devices, the cutoff is 256. This regresses startup time
by 1% but saves more RAM. Results:
(odex+vdex) PSS: -25% average
odex size: -42% average
vdex size unchanged
Apps: Gmail, Maps, YouTube, Search

Spreadsheet at go/compilethreshold

Bug: 36457259
Test: test-art-host
Change-Id: I49d157df9379532d8d7a1ea1d844bb4beb195bb2

2 files changed, 13 insertions, 4 deletions

diff --git a/runtime/jit/profile_saver.cc b/runtime/jit/profile_saver.cc
index 68f33bde03..edce9cd96c 100644
--- a/runtime/jit/profile_saver.cc
+++ b/runtime/jit/profile_saver.cc
@@ -251,7 +251,8 @@ void ProfileSaver::FetchAndCacheResolvedClassesAndMethods() {
   MethodReferenceCollection hot_methods(allocator.Adapter(), allocator.Adapter());
   MethodReferenceCollection startup_methods(allocator.Adapter(), allocator.Adapter());
   TypeReferenceCollection resolved_classes(allocator.Adapter(), allocator.Adapter());
-  const size_t hot_threshold = options_.GetHotStartupMethodSamples();
+  const bool is_low_ram = Runtime::Current()->GetHeap()->IsLowMemoryMode();
+  const size_t hot_threshold = options_.GetHotStartupMethodSamples(is_low_ram);
   {
     ScopedObjectAccess soa(self);
     gc::ScopedGCCriticalSection sgcs(self,
diff --git a/runtime/jit/profile_saver_options.h b/runtime/jit/profile_saver_options.h
index 455bc1aae4..44550f4ddb 100644
--- a/runtime/jit/profile_saver_options.h
+++ b/runtime/jit/profile_saver_options.h
@@ -24,16 +24,18 @@ struct ProfileSaverOptions {
   static constexpr uint32_t kSaveResolvedClassesDelayMs = 5 * 1000;  // 5 seconds
   // Minimum number of JIT samples during launch to mark a method as hot in the profile.
   static constexpr uint32_t kHotStartupMethodSamples = 1;
+  static constexpr uint32_t kHotStartupMethodSamplesLowRam = 256;
   static constexpr uint32_t kMinMethodsToSave = 10;
   static constexpr uint32_t kMinClassesToSave = 10;
   static constexpr uint32_t kMinNotificationBeforeWake = 10;
   static constexpr uint32_t kMaxNotificationBeforeWake = 50;
+  static constexpr uint32_t kHotStartupMethodSamplesNotSet = std::numeric_limits<uint32_t>::max();
 
   ProfileSaverOptions() :
     enabled_(false),
     min_save_period_ms_(kMinSavePeriodMs),
     save_resolved_classes_delay_ms_(kSaveResolvedClassesDelayMs),
-    hot_startup_method_samples_(kHotStartupMethodSamples),
+    hot_startup_method_samples_(kHotStartupMethodSamplesNotSet),
     min_methods_to_save_(kMinMethodsToSave),
     min_classes_to_save_(kMinClassesToSave),
     min_notification_before_wake_(kMinNotificationBeforeWake),
@@ -73,8 +75,12 @@ struct ProfileSaverOptions {
   uint32_t GetSaveResolvedClassesDelayMs() const {
     return save_resolved_classes_delay_ms_;
   }
-  uint32_t GetHotStartupMethodSamples() const {
-    return hot_startup_method_samples_;
+  uint32_t GetHotStartupMethodSamples(bool is_low_ram) const {
+    uint32_t ret = hot_startup_method_samples_;
+    if (ret == kHotStartupMethodSamplesNotSet) {
+      ret = is_low_ram ? kHotStartupMethodSamplesLowRam : kHotStartupMethodSamples;
+    }
+    return ret;
   }
   uint32_t GetMinMethodsToSave() const {
     return min_methods_to_save_;
@@ -107,6 +113,8 @@ struct ProfileSaverOptions {
   bool enabled_;
   uint32_t min_save_period_ms_;
   uint32_t save_resolved_classes_delay_ms_;
+  // Do not access hot_startup_method_samples_ directly for reading since it may be set to the
+  // placeholder default.
   uint32_t hot_startup_method_samples_;
   uint32_t min_methods_to_save_;
   uint32_t min_classes_to_save_;