ThrottledLifoSem::excessValueGuess()

Summary: See comment

Reviewed By: uvdn7

Differential Revision: D79346374

fbshipit-source-id: d19588b71adc0a648cf4d8b1804d92c2e78c4fc8

Author: ot

folly/synchronization/ThrottledLifoSem.h

@@ -139,6 +139,15 @@ class ThrottledLifoSem {
     return static_cast<uint32_t>(state_.load() & kValueMask);
   }
 
+  // Returns the number of posts that are not assigned yet to existing
+  // waiters. This can be negative if there are more waiters than posts.
+  int64_t excessValueGuess() const {
+    uint64_t state = state_.load(std::memory_order_relaxed);
+    uint64_t value = state & kValueMask;
+    uint64_t numWaiters = state >> kNumWaitersShift;
+    return static_cast<int64_t>(value) - static_cast<int64_t>(numWaiters);
+  }
+
  private:
   friend class ThrottledLifoSemTestHelper;
 

folly/synchronization/test/ThrottledLifoSemTest.cpp

@@ -183,6 +183,30 @@ TEST(ThrottledLifoSem, TryPost) {
   EXPECT_EQ(sem.valueGuess(), 0);
 }
 
+TEST(ThrottledLifoSem, ExcessValueGuess) {
+  constexpr size_t kNumWaiters = 64;
+  // Use a large wakeUpInterval so that waiters do not wake up immediately.
+  folly::ThrottledLifoSem sem({.wakeUpInterval = std::chrono::milliseconds(1)});
+
+  std::vector<std::thread> threads;
+  for (size_t i = 0; i < kNumWaiters; ++i) {
+    threads.emplace_back([&] { sem.wait(); });
+  }
+
+  folly::ThrottledLifoSemTestHelper::spinUntilWaiters(
+      sem, kNumWaiters, /* assertExact */ true);
+
+  EXPECT_EQ(sem.excessValueGuess(), -static_cast<int64_t>(kNumWaiters));
+  sem.post(kNumWaiters + 1);
+  EXPECT_EQ(sem.excessValueGuess(), 1);
+  // This should be > 1 but it is time-dependent so we cannot assert it.
+  LOG(INFO) << "Value: " << sem.valueGuess();
+
+  for (auto& t : threads) {
+    t.join();
+  }
+}
+
 TEST(ThrottledLifoSem, MPMCStress) {
   // Same number of producers and consumers.
   constexpr size_t kNumThreads = 16;