Optimize for _count=3

Author: Dan Lambright

fdbrpc/ReplicationPolicy.cpp

@@ -177,7 +177,6 @@ bool PolicyAcross::validate(std::vector<LocalityEntry> const& solutionSet,
 // Choose new servers from "least utilized" alsoServers and append the new servers to results
 // fromserverse are the servers that have already been chosen and
 // that should be excluded from being selected as replicas.
-// fromServers are the servers that must have;
 // alsoServers are the servers you can choose.
 bool PolicyAcross::selectReplicas(Reference<LocalitySet>& fromServers,
                                   std::vector<LocalityEntry> const& alsoServers,
@@ -189,14 +188,14 @@ bool PolicyAcross::selectReplicas(Reference<LocalitySet>& fromServers,
 	int resultsInit = results.size();
 
 	// Clear the member variables
+	AttribValue firstUsedValue, secondUsedValue;
 	_usedValues.clear();
 	_newResults.clear();
 	_addedResults.resize(_arena, 0);
-
 	for (auto& alsoServer : alsoServers) {
 		auto value = fromServers->getValueViaGroupKey(alsoServer, groupIndexKey);
 		if (value.present()) {
-			if (!_usedValues.contains(value.get())) {
+			if (firstUsedValue != value.get() && secondUsedValue != value.get() && !_usedValues.contains(value.get())) {
 				//_selected is a set of processes that have the same indexKey and indexValue (value)
 				_selected = fromServers->restrict(indexKey, value.get());
 				if (_selected->size()) {
@@ -211,7 +210,13 @@ bool PolicyAcross::selectReplicas(Reference<LocalitySet>& fromServers,
 						}
 						if (count >= _count)
 							break;
-						_usedValues.insert(value.get());
+						if (firstUsedValue._id == -1) {
+							firstUsedValue = AttribValue(value.get());
+						} else if (secondUsedValue._id == -1) {
+							secondUsedValue = AttribValue(value.get());
+						} else {
+							_usedValues.insert(value.get());
+						}
 					}
 				}
 			}
@@ -256,14 +261,20 @@ bool PolicyAcross::selectReplicas(Reference<LocalitySet>& fromServers,
 			auto& entry = mutableArray[recordIndex];
 			auto value = fromServers->getValueViaGroupKey(entry, groupIndexKey);
 			if (value.present()) {
-				if (!_usedValues.contains(value.get())) {
+				if (firstUsedValue != value.get() && secondUsedValue != value.get() && !_usedValues.contains(value.get())) {
 					_selected = fromServers->restrict(indexKey, value.get());
 					if (_selected->size()) {
 						if (_policy->selectReplicas(_selected, emptyEntryArray, results)) {
 							count++;
 							if (count >= _count)
 								break;
-							_usedValues.insert(value.get());
+							if (firstUsedValue._id == -1) {
+								firstUsedValue = AttribValue(value.get());
+							} else if (secondUsedValue._id == -1) {
+								secondUsedValue = AttribValue(value.get());
+							} else {
+								_usedValues.insert(value.get());
+							}
 						}
 					}
 				}

fdbrpc/include/fdbrpc/ReplicationTypes.h

@@ -57,7 +57,8 @@ struct AttribValue {
 namespace std {
 template <>
 struct hash<AttribValue> {
-	std::size_t operator()(const AttribValue& v) const noexcept { return std::hash<int>{}(v._id); }
+	std::size_t operator()(const AttribValue& v) const noexcept { return static_cast<std::size_t>(v._id); }
+	// return std::hash<int>{}(v._id);
 };
 } // namespace std
 struct LocalityEntry {