1 // Copyright 2021 The Abseil Authors 2 // 3 // Licensed under the Apache License, Version 2.0 (the "License"); 4 // you may not use this file except in compliance with the License. 5 // You may obtain a copy of the License at 6 // 7 // https://www.apache.org/licenses/LICENSE-2.0 8 // 9 // Unless required by applicable law or agreed to in writing, software 10 // distributed under the License is distributed on an "AS IS" BASIS, 11 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 12 // See the License for the specific language governing permissions and 13 // limitations under the License. 14 15 #ifndef ABSL_STRINGS_INTERNAL_CORDZ_UPDATE_TRACKER_H_ 16 #define ABSL_STRINGS_INTERNAL_CORDZ_UPDATE_TRACKER_H_ 17 18 #include <atomic> 19 #include <cstdint> 20 21 #include "absl/base/config.h" 22 23 namespace absl { 24 ABSL_NAMESPACE_BEGIN 25 namespace cord_internal { 26 27 // CordzUpdateTracker tracks counters for Cord update methods. 28 // 29 // The purpose of CordzUpdateTracker is to track the number of calls to methods 30 // updating Cord data for sampled cords. The class internally uses 'lossy' 31 // atomic operations: Cord is thread-compatible, so there is no need to 32 // synchronize updates. However, Cordz collection threads may call 'Value()' at 33 // any point, so the class needs to provide thread safe access. 34 // 35 // This class is thread-safe. But as per above comments, all non-const methods 36 // should be used single-threaded only: updates are thread-safe but lossy. 37 class CordzUpdateTracker { 38 public: 39 // Tracked update methods. 40 enum MethodIdentifier { 41 kUnknown, 42 kAppendCord, 43 kAppendExternalMemory, 44 kAppendString, 45 kAssignCord, 46 kAssignString, 47 kClear, 48 kConstructorCord, 49 kConstructorString, 50 kCordReader, 51 kFlatten, 52 kGetAppendRegion, 53 kMakeCordFromExternal, 54 kMoveAppendCord, 55 kMoveAssignCord, 56 kMovePrependCord, 57 kPrependCord, 58 kPrependString, 59 kRemovePrefix, 60 kRemoveSuffix, 61 kSubCord, 62 63 // kNumMethods defines the number of entries: must be the last entry. 64 kNumMethods, 65 }; 66 67 // Constructs a new instance. All counters are zero-initialized. CordzUpdateTracker()68 constexpr CordzUpdateTracker() noexcept : values_{} {} 69 70 // Copy constructs a new instance. CordzUpdateTracker(const CordzUpdateTracker & rhs)71 CordzUpdateTracker(const CordzUpdateTracker& rhs) noexcept { *this = rhs; } 72 73 // Assigns the provided value to this instance. 74 CordzUpdateTracker& operator=(const CordzUpdateTracker& rhs) noexcept { 75 for (int i = 0; i < kNumMethods; ++i) { 76 values_[i].store(rhs.values_[i].load(std::memory_order_relaxed), 77 std::memory_order_relaxed); 78 } 79 return *this; 80 } 81 82 // Returns the value for the specified method. Value(MethodIdentifier method)83 int64_t Value(MethodIdentifier method) const { 84 return values_[method].load(std::memory_order_relaxed); 85 } 86 87 // Increases the value for the specified method by `n` 88 void LossyAdd(MethodIdentifier method, int64_t n = 1) { 89 auto& value = values_[method]; 90 value.store(value.load(std::memory_order_relaxed) + n, 91 std::memory_order_relaxed); 92 } 93 94 // Adds all the values from `src` to this instance LossyAdd(const CordzUpdateTracker & src)95 void LossyAdd(const CordzUpdateTracker& src) { 96 for (int i = 0; i < kNumMethods; ++i) { 97 MethodIdentifier method = static_cast<MethodIdentifier>(i); 98 if (int64_t value = src.Value(method)) { 99 LossyAdd(method, value); 100 } 101 } 102 } 103 104 private: 105 // Until C++20 std::atomic is not constexpr default-constructible, so we need 106 // a wrapper for this class to be constexpr constructible. 107 class Counter : public std::atomic<int64_t> { 108 public: Counter()109 constexpr Counter() noexcept : std::atomic<int64_t>(0) {} 110 }; 111 112 Counter values_[kNumMethods]; 113 }; 114 115 } // namespace cord_internal 116 ABSL_NAMESPACE_END 117 } // namespace absl 118 119 #endif // ABSL_STRINGS_INTERNAL_CORDZ_UPDATE_TRACKER_H_ 120