add read-only api to query hotness of hash

Reviewed By: stuclar, therealgymmy

Differential Revision: D38539036

fbshipit-source-id: 1bb42e3604c41f629c7c2adb5f7e7ffe208546db

cachelib/common/hothash/HotHashDetector.cpp

@@ -81,6 +81,27 @@ uint8_t HotHashDetector::bumpHash(uint64_t hash) {
   return result;
 }
 
+bool HotHashDetector::isHotHash(uint64_t hash) const {
+  auto idx = l1HashFunction(hash);
+  auto l1count = l1Vector_[idx];
+  // Checking against threshold/2 since hot index should pass after one decay
+  if (LIKELY(l1count < (l1Threshold_ / 2))) {
+    return false;
+  }
+  auto idx2 = l2HashFunction(hash);
+  uint32_t l2count = l2Vector_[idx2].count;
+  if (l2count == 0) {
+    return false;
+  }
+  for (unsigned i = 0; i < kScanLen; ++i) {
+    auto& cell = l2Vector_[(idx2 + i) & bucketsMask_];
+    if (cell.hash == hash) {
+      return true;
+    }
+  }
+  return false;
+}
+
 void HotHashDetector::doMaintenance() {
   bumpsSinceMaintenance_ = 0;
 

cachelib/common/hothash/HotHashDetector.h

@@ -106,6 +106,14 @@ class HotHashDetector {
   //                hotter, but any non-zero result here means it's very hot.
   uint8_t bumpHash(uint64_t hash);
 
+  // Read-only method to check the hotness of a key-hash.
+  //
+  // @param hash    hash value of an item. Zero is a valid value for hash, but
+  //                may cause false positives.
+  //
+  // @return        True if the hash is hot.
+  bool isHotHash(uint64_t hash) const;
+
   // Manually trigger the data structure maintenance procedure.
   void doMaintenance();
 

cachelib/common/hothash/HotHashDetectorTest.cpp

@@ -58,7 +58,11 @@ static void testHotHashes(uint32_t seed,
       }
       hotCount = 0;
       for (size_t i = 0; i < numHashes; ++i) {
-        hotCount += detector.bumpHash(hashes[i]) ? 1 : 0;
+        bool isHot = detector.bumpHash(hashes[i]) > 0;
+        hotCount += isHot ? 1 : 0;
+        if (isHot) {
+          EXPECT_TRUE(detector.isHotHash(hashes[i]));
+        }
       }
       ++iteration;
     }