Replace mutexes in observers with SharexMutex to reduce memory usage

Summary: sizeof(SharedMutex) = 4, sizeof(std::mutex) = 40. This diff makes sure that we use smaller mutexes both in Observer Core and in Observables given that we may end up with many instances of these objects, yet there mutexes are accessed only on the update path.

Reviewed By: praihan

Differential Revision: D32271615

fbshipit-source-id: ba7d04632a677a9eb4ae398d681dadac3b06f033

folly/experimental/observer/Observable-inl.h

@@ -43,7 +43,7 @@ class ObserverCreatorContext {
   }
 
   std::shared_ptr<const T> get() {
-    auto state = state_.lock();
+    auto state = state_.wlock();
     state->updateRequested = false;
     return state->value;
   }
@@ -62,10 +62,10 @@ class ObserverCreatorContext {
       // important to not hold state_ lock while running it to avoid possible
       // lock inversion with another code path that needs state_ lock (e.g.
       // get()).
-      std::lock_guard<std::mutex> updateLockGuard(updateLock_);
+      std::lock_guard<SharedMutex> updateLockGuard(updateLock_);
       auto newValue = Traits::get(observable_);
 
-      auto state = state_.lock();
+      auto state = state_.wlock();
       if (!state->updateValue(std::move(newValue))) {
         // Value didn't change, so we can skip the version update.
         return nullptr;
@@ -88,7 +88,7 @@ class ObserverCreatorContext {
   }
 
  private:
-  std::mutex updateLock_;
+  SharedMutex updateLock_;
   struct State {
     bool updateValue(std::shared_ptr<const T> newValue) {
       auto newValuePtr = newValue.get();
@@ -102,7 +102,7 @@ class ObserverCreatorContext {
     std::shared_ptr<const T> value;
     bool updateRequested{false};
   };
-  folly::Synchronized<State, std::mutex> state_;
+  folly::Synchronized<State> state_;
 
   observer_detail::Core::WeakPtr coreWeak_;
 

folly/experimental/observer/detail/Core.cpp

@@ -54,7 +54,7 @@ size_t Core::refresh(size_t version) {
   }
 
   {
-    std::lock_guard<std::mutex> lgRefresh(refreshMutex_);
+    std::lock_guard<SharedMutex> lgRefresh(refreshMutex_);
 
     // Recheck in case this code was already refreshed
     if (version_ >= version) {

folly/experimental/observer/detail/Core.h

@@ -111,7 +111,7 @@ class Core : public std::enable_shared_from_this<Core> {
 
   folly::Function<std::shared_ptr<const void>()> creator_;
 
-  std::mutex refreshMutex_;
+  SharedMutex refreshMutex_;
 
   bool forceRefresh_{false};
 };