├── .clang-format
├── spsc.odg
├── .gitignore
├── src
    ├── SPSCQueueExample.cpp
    ├── SPSCQueueExampleHugepages.cpp
    ├── SPSCQueueTest.cpp
    └── SPSCQueueBenchmark.cpp
├── LICENSE
├── .github
    └── workflows
    │   └── ccpp.yml
├── CMakeLists.txt
├── README.md
├── include
    └── rigtorp
    │   └── SPSCQueue.h
└── spsc.svg


/.clang-format:
--------------------------------------------------------------------------------
1 | BasedOnStyle: LLVM


--------------------------------------------------------------------------------
/spsc.odg:
--------------------------------------------------------------------------------
https://raw.githubusercontent.com/rigtorp/SPSCQueue/HEAD/spsc.odg


--------------------------------------------------------------------------------
/.gitignore:
--------------------------------------------------------------------------------
1 | build
2 | cmake-build-*/
3 | *~
4 | .idea/
5 | .vscode/
6 | .cache/


--------------------------------------------------------------------------------
/src/SPSCQueueExample.cpp:
--------------------------------------------------------------------------------
 1 | #include <iostream>
 2 | #include <rigtorp/SPSCQueue.h>
 3 | #include <thread>
 4 | 
 5 | int main(int argc, char *argv[]) {
 6 |   (void)argc, (void)argv;
 7 | 
 8 |   using namespace rigtorp;
 9 | 
10 |   SPSCQueue<int> q(1);
11 |   auto t = std::thread([&] {
12 |     while (!q.front())
13 |       ;
14 |     std::cout << *q.front() << std::endl;
15 |     q.pop();
16 |   });
17 |   q.push(1);
18 |   t.join();
19 | 
20 |   return 0;
21 | }
22 | 


--------------------------------------------------------------------------------
/LICENSE:
--------------------------------------------------------------------------------
 1 | The MIT License (MIT)
 2 | 
 3 | Copyright (c) 2018 Erik Rigtorp <erik@rigtorp.se>
 4 | 
 5 | Permission is hereby granted, free of charge, to any person obtaining a copy
 6 | of this software and associated documentation files (the "Software"), to deal
 7 | in the Software without restriction, including without limitation the rights
 8 | to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 9 | copies of the Software, and to permit persons to whom the Software is
10 | furnished to do so, subject to the following conditions:
11 | 
12 | The above copyright notice and this permission notice shall be included in all
13 | copies or substantial portions of the Software.
14 | 
15 | THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
16 | IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
17 | FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
18 | AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
19 | LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
20 | OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
21 | SOFTWARE.
22 | 
23 | 


--------------------------------------------------------------------------------
/.github/workflows/ccpp.yml:
--------------------------------------------------------------------------------
 1 | name: C/C++ CI
 2 | 
 3 | on: [push]
 4 | 
 5 | jobs:
 6 |   build-ubuntu:
 7 | 
 8 |     runs-on: ubuntu-latest
 9 |     
10 |     steps:
11 |     - uses: actions/checkout@v1
12 |     - name: Build & Test
13 |       run: |
14 |         cmake -E remove_directory build
15 |         cmake -B build -S . -DCMAKE_BUILD_TYPE=Debug -DCMAKE_CXX_FLAGS="-Werror -O2 -fsanitize=address,undefined"
16 |         cmake --build build
17 |         cd build
18 |         ctest --output-on-failure
19 |         
20 |   build-windows:
21 | 
22 |     runs-on: windows-latest
23 |     
24 |     steps:
25 |     - uses: actions/checkout@v1
26 |     - name: Build & Test
27 |       run: | 
28 |         cmake -E remove_directory build
29 |         cmake -B build -S .
30 |         cmake --build build --config Debug
31 |         cd build
32 |         ctest --output-on-failure
33 | 
34 |   build-macos:
35 | 
36 |     runs-on: macOS-latest
37 | 
38 |     steps:
39 |     - uses: actions/checkout@v1
40 |     - name: Build & Test
41 |       run: |
42 |         cmake -E remove_directory build
43 |         cmake -B build -S . -DCMAKE_BUILD_TYPE=Debug -DCMAKE_CXX_FLAGS="-Werror -O2 -fsanitize=address,undefined"
44 |         cmake --build build
45 |         cd build
46 |         ctest --output-on-failure


--------------------------------------------------------------------------------
/src/SPSCQueueExampleHugepages.cpp:
--------------------------------------------------------------------------------
 1 | #include <iostream>
 2 | #include <rigtorp/SPSCQueue.h>
 3 | #include <sys/mman.h>
 4 | #include <thread>
 5 | 
 6 | template <typename T> struct Allocator {
 7 |   using value_type = T;
 8 | 
 9 |   struct AllocationResult {
10 |     T *ptr;
11 |     size_t count;
12 |   };
13 | 
14 |   size_t roundup(size_t n) { return (((n - 1) >> 21) + 1) << 21; }
15 | 
16 |   AllocationResult allocate_at_least(size_t n) {
17 |     size_t count = roundup(sizeof(T) * n);
18 |     auto p = static_cast<T *>(mmap(nullptr, count, PROT_READ | PROT_WRITE,
19 |                                    MAP_PRIVATE | MAP_ANONYMOUS | MAP_HUGETLB,
20 |                                    -1, 0));
21 |     if (p == MAP_FAILED) {
22 |       throw std::bad_alloc();
23 |     }
24 |     return {p, count / sizeof(T)};
25 |   }
26 | 
27 |   void deallocate(T *p, size_t n) { munmap(p, roundup(sizeof(T) * n)); }
28 | };
29 | 
30 | int main(int argc, char *argv[]) {
31 |   (void)argc, (void)argv;
32 | 
33 |   using namespace rigtorp;
34 | 
35 |   SPSCQueue<int, Allocator<int>> q(2);
36 |   std::cout << q.capacity() << std::endl;
37 |   auto t = std::thread([&] {
38 |     while (!q.front())
39 |       ;
40 |     std::cout << *q.front() << std::endl;
41 |     q.pop();
42 |   });
43 |   q.push(1);
44 |   t.join();
45 | 
46 |   return 0;
47 | }
48 | 


--------------------------------------------------------------------------------
/CMakeLists.txt:
--------------------------------------------------------------------------------
 1 | cmake_minimum_required(VERSION 3.6)
 2 | 
 3 | project(SPSCQueue VERSION 1.1 LANGUAGES CXX)
 4 | 
 5 | add_library(${PROJECT_NAME} INTERFACE)
 6 | add_library(${PROJECT_NAME}::${PROJECT_NAME} ALIAS ${PROJECT_NAME})
 7 | 
 8 | target_compile_features(${PROJECT_NAME} INTERFACE cxx_std_11)
 9 | 
10 | target_include_directories(${PROJECT_NAME} INTERFACE
11 |         $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
12 |         $<INSTALL_INTERFACE:include>)
13 | 
14 | # Tests and examples
15 | if(CMAKE_CURRENT_SOURCE_DIR STREQUAL CMAKE_SOURCE_DIR)
16 |     if(MSVC)
17 |         add_compile_options(/permissive- /W4)
18 |     else()
19 |         add_compile_options(-Wall -Wextra -Wpedantic -Wno-c++17-extensions)
20 |     endif()
21 | 
22 |     find_package(Threads REQUIRED)
23 | 
24 |     if (UNIX AND NOT APPLE)
25 |         add_executable(SPSCQueueBenchmark src/SPSCQueueBenchmark.cpp)
26 |         target_link_libraries(SPSCQueueBenchmark SPSCQueue Threads::Threads)
27 |     endif()
28 | 
29 |     if (((CMAKE_CXX_COMPILER_ID MATCHES "GNU" AND CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 7.0)
30 |         OR (CMAKE_CXX_COMPILER_ID MATCHES "Clang" AND CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 9.0))
31 |         AND UNIX AND NOT APPLE)
32 |         add_executable(SPSCQueueExampleHugepages src/SPSCQueueExampleHugepages.cpp)
33 |         target_link_libraries(SPSCQueueExampleHugepages SPSCQueue Threads::Threads)
34 |         target_compile_features(SPSCQueueExampleHugepages PRIVATE cxx_std_17)
35 |     endif()
36 | 
37 |     add_executable(SPSCQueueExample src/SPSCQueueExample.cpp)
38 |     target_link_libraries(SPSCQueueExample SPSCQueue Threads::Threads)
39 | 
40 |     add_executable(SPSCQueueTest src/SPSCQueueTest.cpp)
41 |     target_link_libraries(SPSCQueueTest SPSCQueue Threads::Threads)
42 | 
43 |     enable_testing()
44 |     add_test(SPSCQueueTest SPSCQueueTest)
45 | endif()
46 | 
47 | # Install
48 | include(GNUInstallDirs)
49 | include(CMakePackageConfigHelpers)
50 | 
51 | write_basic_package_version_file(
52 |   "${CMAKE_CURRENT_BINARY_DIR}/${PROJECT_NAME}ConfigVersion.cmake"
53 |   COMPATIBILITY SameMajorVersion
54 | )
55 | 
56 | export(
57 |     TARGETS ${PROJECT_NAME}
58 |     NAMESPACE ${PROJECT_NAME}::
59 |     FILE "${PROJECT_NAME}Config.cmake"
60 | )
61 | 
62 | if(CMAKE_CURRENT_SOURCE_DIR STREQUAL CMAKE_SOURCE_DIR)
63 |     install(
64 |         DIRECTORY "include/"
65 |         DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}
66 |     )
67 | 
68 |     install(
69 |         TARGETS ${PROJECT_NAME}
70 |         EXPORT "${PROJECT_NAME}Config"
71 |         INCLUDES DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}
72 |     )
73 | 
74 |     install(
75 |         EXPORT "${PROJECT_NAME}Config"
76 |         NAMESPACE ${PROJECT_NAME}::
77 |         DESTINATION "${CMAKE_INSTALL_LIBDIR}/cmake/${PROJECT_NAME}"
78 |     )
79 | 
80 |     install(
81 |         FILES "${CMAKE_CURRENT_BINARY_DIR}/${PROJECT_NAME}ConfigVersion.cmake"
82 |         DESTINATION "${CMAKE_INSTALL_LIBDIR}/cmake/${PROJECT_NAME}"
83 |     )
84 | endif()


--------------------------------------------------------------------------------
/src/SPSCQueueTest.cpp:
--------------------------------------------------------------------------------
  1 | /*
  2 | Copyright (c) 2018 Erik Rigtorp <erik@rigtorp.se>
  3 | 
  4 | Permission is hereby granted, free of charge, to any person obtaining a copy
  5 | of this software and associated documentation files (the "Software"), to deal
  6 | in the Software without restriction, including without limitation the rights
  7 | to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
  8 | copies of the Software, and to permit persons to whom the Software is
  9 | furnished to do so, subject to the following conditions:
 10 | 
 11 | The above copyright notice and this permission notice shall be included in all
 12 | copies or substantial portions of the Software.
 13 | 
 14 | THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 15 | IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 16 | FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 17 | AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 18 | LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 19 | OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 20 | SOFTWARE.
 21 |  */
 22 | 
 23 | #undef NDEBUG
 24 | 
 25 | #include <cassert>
 26 | #include <chrono>
 27 | #include <iostream>
 28 | #include <rigtorp/SPSCQueue.h>
 29 | #include <set>
 30 | #include <thread>
 31 | 
 32 | // TestType tracks correct usage of constructors and destructors
 33 | struct TestType {
 34 |   static std::set<const TestType *> constructed;
 35 |   TestType() noexcept {
 36 |     assert(constructed.count(this) == 0);
 37 |     constructed.insert(this);
 38 |   };
 39 |   TestType(const TestType &other) noexcept {
 40 |     assert(constructed.count(this) == 0);
 41 |     assert(constructed.count(&other) == 1);
 42 |     constructed.insert(this);
 43 |   };
 44 |   TestType(TestType &&other) noexcept {
 45 |     assert(constructed.count(this) == 0);
 46 |     assert(constructed.count(&other) == 1);
 47 |     constructed.insert(this);
 48 |   };
 49 |   TestType &operator=(const TestType &other) noexcept {
 50 |     assert(constructed.count(this) == 1);
 51 |     assert(constructed.count(&other) == 1);
 52 |     return *this;
 53 |   };
 54 |   TestType &operator=(TestType &&other) noexcept {
 55 |     assert(constructed.count(this) == 1);
 56 |     assert(constructed.count(&other) == 1);
 57 |     return *this;
 58 |   }
 59 |   ~TestType() noexcept {
 60 |     assert(constructed.count(this) == 1);
 61 |     constructed.erase(this);
 62 |   };
 63 | };
 64 | 
 65 | std::set<const TestType *> TestType::constructed;
 66 | 
 67 | int main(int argc, char *argv[]) {
 68 |   (void)argc, (void)argv;
 69 | 
 70 |   using namespace rigtorp;
 71 | 
 72 |   // Functionality test
 73 |   {
 74 |     SPSCQueue<TestType> q(10);
 75 |     assert(q.front() == nullptr);
 76 |     assert(q.size() == 0);
 77 |     assert(q.empty() == true);
 78 |     assert(q.capacity() == 10);
 79 |     for (int i = 0; i < 10; i++) {
 80 |       q.emplace();
 81 |     }
 82 |     assert(q.front() != nullptr);
 83 |     assert(q.size() == 10);
 84 |     assert(q.empty() == false);
 85 |     assert(TestType::constructed.size() == 10);
 86 |     assert(q.try_emplace() == false);
 87 |     q.pop();
 88 |     assert(q.size() == 9);
 89 |     assert(TestType::constructed.size() == 9);
 90 |     q.pop();
 91 |     assert(q.try_emplace() == true);
 92 |     assert(TestType::constructed.size() == 9);
 93 |   }
 94 |   assert(TestType::constructed.size() == 0);
 95 | 
 96 |   // Copyable only type
 97 |   {
 98 |     struct Test {
 99 |       Test() {}
100 |       Test(const Test &) {}
101 |       Test(Test &&) = delete;
102 |     };
103 |     SPSCQueue<Test> q(16);
104 |     // lvalue
105 |     Test v;
106 |     q.emplace(v);
107 |     (void)q.try_emplace(v);
108 |     q.push(v);
109 |     (void)q.try_push(v);
110 |     static_assert(noexcept(q.emplace(v)) == false, "");
111 |     static_assert(noexcept(q.try_emplace(v)) == false, "");
112 |     static_assert(noexcept(q.push(v)) == false, "");
113 |     static_assert(noexcept(q.try_push(v)) == false, "");
114 |     // xvalue
115 |     q.push(Test());
116 |     (void)q.try_push(Test());
117 |     static_assert(noexcept(q.push(Test())) == false, "");
118 |     static_assert(noexcept(q.try_push(Test())) == false, "");
119 |   }
120 | 
121 |   // Copyable only type (noexcept)
122 |   {
123 |     struct Test {
124 |       Test() noexcept {}
125 |       Test(const Test &) noexcept {}
126 |       Test(Test &&) = delete;
127 |     };
128 |     SPSCQueue<Test> q(16);
129 |     // lvalue
130 |     Test v;
131 |     q.emplace(v);
132 |     (void)q.try_emplace(v);
133 |     q.push(v);
134 |     (void)q.try_push(v);
135 |     static_assert(noexcept(q.emplace(v)) == true, "");
136 |     static_assert(noexcept(q.try_emplace(v)) == true, "");
137 |     static_assert(noexcept(q.push(v)) == true, "");
138 |     static_assert(noexcept(q.try_push(v)) == true, "");
139 |     // xvalue
140 |     q.push(Test());
141 |     (void)q.try_push(Test());
142 |     static_assert(noexcept(q.push(Test())) == true, "");
143 |     static_assert(noexcept(q.try_push(Test())) == true, "");
144 |   }
145 | 
146 |   // Movable only type
147 |   {
148 |     SPSCQueue<std::unique_ptr<int>> q(16);
149 |     // lvalue
150 |     // auto v = std::unique_ptr<int>(new int(1));
151 |     // q.emplace(v);
152 |     // q.try_emplace(v);
153 |     // q.push(v);
154 |     // q.try_push(v);
155 |     // xvalue
156 |     q.emplace(std::unique_ptr<int>(new int(1)));
157 |     (void)q.try_emplace(std::unique_ptr<int>(new int(1)));
158 |     q.push(std::unique_ptr<int>(new int(1)));
159 |     (void)q.try_push(std::unique_ptr<int>(new int(1)));
160 |     auto v = std::unique_ptr<int>(new int(1));
161 |     static_assert(noexcept(q.emplace(std::move(v))) == true, "");
162 |     static_assert(noexcept(q.try_emplace(std::move(v))) == true, "");
163 |     static_assert(noexcept(q.push(std::move(v))) == true, "");
164 |     static_assert(noexcept(q.try_push(std::move(v))) == true, "");
165 |   }
166 | 
167 |   // capacity < 1
168 |   {
169 |     SPSCQueue<int> q(0);
170 |     assert(q.capacity() == 1);
171 |   }
172 | 
173 |   // Check that padding doesn't overflow capacity
174 |   {
175 |     bool throws = false;
176 |     try {
177 |       SPSCQueue<int> q(SIZE_MAX - 1);
178 |     } catch (...) {
179 |       throws = true;
180 |     }
181 |     assert(throws);
182 |   }
183 | 
184 |   // Fuzz and performance test
185 |   {
186 |     const size_t iter = 100000;
187 |     SPSCQueue<size_t> q(iter / 1000 + 1);
188 |     std::atomic<bool> flag(false);
189 |     std::thread producer([&] {
190 |       while (!flag)
191 |         ;
192 |       for (size_t i = 0; i < iter; ++i) {
193 |         q.emplace(i);
194 |       }
195 |     });
196 | 
197 |     size_t sum = 0;
198 |     auto start = std::chrono::system_clock::now();
199 |     flag = true;
200 |     for (size_t i = 0; i < iter; ++i) {
201 |       while (!q.front())
202 |         ;
203 |       sum += *q.front();
204 |       q.pop();
205 |     }
206 |     auto end = std::chrono::system_clock::now();
207 |     auto duration =
208 |         std::chrono::duration_cast<std::chrono::nanoseconds>(end - start);
209 | 
210 |     assert(q.front() == nullptr);
211 |     assert(sum == iter * (iter - 1) / 2);
212 | 
213 |     producer.join();
214 | 
215 |     std::cout << duration.count() / iter << " ns/iter" << std::endl;
216 |   }
217 | 
218 |   return 0;
219 | }
220 | 


--------------------------------------------------------------------------------
/src/SPSCQueueBenchmark.cpp:
--------------------------------------------------------------------------------
  1 | /*
  2 | Copyright (c) 2018 Erik Rigtorp <erik@rigtorp.se>
  3 | 
  4 | Permission is hereby granted, free of charge, to any person obtaining a copy
  5 | of this software and associated documentation files (the "Software"), to deal
  6 | in the Software without restriction, including without limitation the rights
  7 | to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
  8 | copies of the Software, and to permit persons to whom the Software is
  9 | furnished to do so, subject to the following conditions:
 10 | 
 11 | The above copyright notice and this permission notice shall be included in all
 12 | copies or substantial portions of the Software.
 13 | 
 14 | THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 15 | IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 16 | FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 17 | AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 18 | LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 19 | OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 20 | SOFTWARE.
 21 |  */
 22 | 
 23 | #include <chrono>
 24 | #include <iostream>
 25 | #include <rigtorp/SPSCQueue.h>
 26 | #include <thread>
 27 | 
 28 | #if __has_include(<boost/lockfree/spsc_queue.hpp> )
 29 | #include <boost/lockfree/spsc_queue.hpp>
 30 | #endif
 31 | 
 32 | #if __has_include(<folly/ProducerConsumerQueue.h>)
 33 | #include <folly/ProducerConsumerQueue.h>
 34 | #endif
 35 | 
 36 | void pinThread(int cpu) {
 37 |   if (cpu < 0) {
 38 |     return;
 39 |   }
 40 |   cpu_set_t cpuset;
 41 |   CPU_ZERO(&cpuset);
 42 |   CPU_SET(cpu, &cpuset);
 43 |   if (pthread_setaffinity_np(pthread_self(), sizeof(cpu_set_t), &cpuset) ==
 44 |       -1) {
 45 |     perror("pthread_setaffinity_no");
 46 |     exit(1);
 47 |   }
 48 | }
 49 | 
 50 | int main(int argc, char *argv[]) {
 51 |   (void)argc, (void)argv;
 52 | 
 53 |   using namespace rigtorp;
 54 | 
 55 |   int cpu1 = -1;
 56 |   int cpu2 = -1;
 57 | 
 58 |   if (argc == 3) {
 59 |     cpu1 = std::stoi(argv[1]);
 60 |     cpu2 = std::stoi(argv[2]);
 61 |   }
 62 | 
 63 |   const size_t queueSize = 10000000;
 64 |   const int64_t iters = 10000000;
 65 | 
 66 |   std::cout << "SPSCQueue:" << std::endl;
 67 | 
 68 |   {
 69 |     SPSCQueue<int> q(queueSize);
 70 |     auto t = std::thread([&] {
 71 |       pinThread(cpu1);
 72 |       for (int i = 0; i < iters; ++i) {
 73 |         while (!q.front())
 74 |           ;
 75 |         if (*q.front() != i) {
 76 |           throw std::runtime_error("");
 77 |         }
 78 |         q.pop();
 79 |       }
 80 |     });
 81 | 
 82 |     pinThread(cpu2);
 83 | 
 84 |     auto start = std::chrono::steady_clock::now();
 85 |     for (int i = 0; i < iters; ++i) {
 86 |       q.emplace(i);
 87 |     }
 88 |     t.join();
 89 |     auto stop = std::chrono::steady_clock::now();
 90 |     std::cout << iters * 1000000 /
 91 |                      std::chrono::duration_cast<std::chrono::nanoseconds>(stop -
 92 |                                                                           start)
 93 |                          .count()
 94 |               << " ops/ms" << std::endl;
 95 |   }
 96 | 
 97 |   {
 98 |     SPSCQueue<int> q1(queueSize), q2(queueSize);
 99 |     auto t = std::thread([&] {
100 |       pinThread(cpu1);
101 |       for (int i = 0; i < iters; ++i) {
102 |         while (!q1.front())
103 |           ;
104 |         q2.emplace(*q1.front());
105 |         q1.pop();
106 |       }
107 |     });
108 | 
109 |     pinThread(cpu2);
110 | 
111 |     auto start = std::chrono::steady_clock::now();
112 |     for (int i = 0; i < iters; ++i) {
113 |       q1.emplace(i);
114 |       while (!q2.front())
115 |         ;
116 |       q2.pop();
117 |     }
118 |     auto stop = std::chrono::steady_clock::now();
119 |     t.join();
120 |     std::cout << std::chrono::duration_cast<std::chrono::nanoseconds>(stop -
121 |                                                                       start)
122 |                          .count() /
123 |                      iters
124 |               << " ns RTT" << std::endl;
125 |   }
126 | 
127 | #if __has_include(<boost/lockfree/spsc_queue.hpp> )
128 |   std::cout << "boost::lockfree::spsc:" << std::endl;
129 |   {
130 |     boost::lockfree::spsc_queue<int> q(queueSize);
131 |     auto t = std::thread([&] {
132 |       pinThread(cpu1);
133 |       for (int i = 0; i < iters; ++i) {
134 |         int val;
135 |         while (q.pop(&val, 1) != 1)
136 |           ;
137 |         if (val != i) {
138 |           throw std::runtime_error("");
139 |         }
140 |       }
141 |     });
142 | 
143 |     pinThread(cpu2);
144 | 
145 |     auto start = std::chrono::steady_clock::now();
146 |     for (int i = 0; i < iters; ++i) {
147 |       while (!q.push(i))
148 |         ;
149 |     }
150 |     t.join();
151 |     auto stop = std::chrono::steady_clock::now();
152 |     std::cout << iters * 1000000 /
153 |                      std::chrono::duration_cast<std::chrono::nanoseconds>(stop -
154 |                                                                           start)
155 |                          .count()
156 |               << " ops/ms" << std::endl;
157 |   }
158 | 
159 |   {
160 |     boost::lockfree::spsc_queue<int> q1(queueSize), q2(queueSize);
161 |     auto t = std::thread([&] {
162 |       pinThread(cpu1);
163 |       for (int i = 0; i < iters; ++i) {
164 |         int val;
165 |         while (q1.pop(&val, 1) != 1)
166 |           ;
167 |         while (!q2.push(val))
168 |           ;
169 |       }
170 |     });
171 | 
172 |     pinThread(cpu2);
173 | 
174 |     auto start = std::chrono::steady_clock::now();
175 |     for (int i = 0; i < iters; ++i) {
176 |       while (!q1.push(i))
177 |         ;
178 |       int val;
179 |       while (q2.pop(&val, 1) != 1)
180 |         ;
181 |     }
182 |     auto stop = std::chrono::steady_clock::now();
183 |     t.join();
184 |     std::cout << std::chrono::duration_cast<std::chrono::nanoseconds>(stop -
185 |                                                                       start)
186 |                          .count() /
187 |                      iters
188 |               << " ns RTT" << std::endl;
189 |   }
190 | #endif
191 | 
192 | #if __has_include(<folly/ProducerConsumerQueue.h>)
193 |   std::cout << "folly::ProducerConsumerQueue:" << std::endl;
194 | 
195 |   {
196 |     folly::ProducerConsumerQueue<int> q(queueSize);
197 |     auto t = std::thread([&] {
198 |       pinThread(cpu1);
199 |       for (int i = 0; i < iters; ++i) {
200 |         int val;
201 |         while (!q.read(val))
202 |           ;
203 |         if (val != i) {
204 |           throw std::runtime_error("");
205 |         }
206 |       }
207 |     });
208 | 
209 |     pinThread(cpu2);
210 | 
211 |     auto start = std::chrono::steady_clock::now();
212 |     for (int i = 0; i < iters; ++i) {
213 |       while (!q.write(i))
214 |         ;
215 |     }
216 |     t.join();
217 |     auto stop = std::chrono::steady_clock::now();
218 |     std::cout << iters * 1000000 /
219 |                      std::chrono::duration_cast<std::chrono::nanoseconds>(stop -
220 |                                                                           start)
221 |                          .count()
222 |               << " ops/ms" << std::endl;
223 |   }
224 | 
225 |   {
226 |     folly::ProducerConsumerQueue<int> q1(queueSize), q2(queueSize);
227 |     auto t = std::thread([&] {
228 |       pinThread(cpu1);
229 |       for (int i = 0; i < iters; ++i) {
230 |         int val;
231 |         while (!q1.read(val))
232 |           ;
233 |         q2.write(val);
234 |       }
235 |     });
236 | 
237 |     pinThread(cpu2);
238 | 
239 |     auto start = std::chrono::steady_clock::now();
240 |     for (int i = 0; i < iters; ++i) {
241 |       while (!q1.write(i))
242 |         ;
243 |       int val;
244 |       while (!q2.read(val))
245 |         ;
246 |     }
247 |     auto stop = std::chrono::steady_clock::now();
248 |     t.join();
249 |     std::cout << std::chrono::duration_cast<std::chrono::nanoseconds>(stop -
250 |                                                                       start)
251 |                          .count() /
252 |                      iters
253 |               << " ns RTT" << std::endl;
254 |   }
255 | #endif
256 | 
257 |   return 0;
258 | }
259 | 


--------------------------------------------------------------------------------
/README.md:
--------------------------------------------------------------------------------
  1 | # SPSCQueue.h
  2 | 
  3 | [![C/C++ CI](https://github.com/rigtorp/SPSCQueue/workflows/C/C++%20CI/badge.svg)](https://github.com/rigtorp/SPSCQueue/actions)
  4 | [![License](https://img.shields.io/badge/license-MIT-blue.svg)](https://raw.githubusercontent.com/rigtorp/SPSCQueue/master/LICENSE)
  5 | 
  6 | A single producer single consumer wait-free and lock-free fixed size queue
  7 | written in C++11. This implementation is faster than both
  8 | [*boost::lockfree::spsc*](https://www.boost.org/doc/libs/1_76_0/doc/html/boost/lockfree/spsc_queue.html)
  9 | and [*folly::ProducerConsumerQueue*](https://github.com/facebook/folly/blob/master/folly/docs/ProducerConsumerQueue.md).
 10 | 
 11 | ## Example
 12 | 
 13 | ```cpp
 14 | SPSCQueue<int> q(1);
 15 | auto t = std::thread([&] {
 16 |   while (!q.front());
 17 |   std::cout << *q.front() << std::endl;
 18 |   q.pop();
 19 | });
 20 | q.push(1);
 21 | t.join();
 22 | ```
 23 | 
 24 | See `src/SPSCQueueExample.cpp` for the full example.
 25 | 
 26 | ## Usage
 27 | 
 28 | - `SPSCQueue<T>(size_t capacity);`
 29 | 
 30 |   Create a `SPSCqueue` holding items of type `T` with capacity
 31 |   `capacity`. Capacity needs to be at least 1.
 32 | 
 33 | - `void emplace(Args &&... args);`
 34 | 
 35 |   Enqueue an item using inplace construction. Blocks if queue is full.
 36 | 
 37 | - `bool try_emplace(Args &&... args);`
 38 | 
 39 |   Try to enqueue an item using inplace construction. Returns `true` on
 40 |   success and `false` if queue is full.
 41 | 
 42 | - `void push(const T &v);`
 43 | 
 44 |   Enqueue an item using copy construction. Blocks if queue is full.
 45 | 
 46 | - `template <typename P> void push(P &&v);`
 47 | 
 48 |   Enqueue an item using move construction. Participates in overload
 49 |   resolution only if `std::is_constructible<T, P&&>::value == true`.
 50 |   Blocks if queue is full.
 51 | 
 52 | - `bool try_push(const T &v);`
 53 | 
 54 |   Try to enqueue an item using copy construction. Returns `true` on
 55 |   success and `false` if queue is full.
 56 | 
 57 | - `template <typename P> bool try_push(P &&v);`
 58 | 
 59 |   Try to enqueue an item using move construction. Returns `true` on
 60 |   success and `false` if queue is full. Participates in overload
 61 |   resolution only if `std::is_constructible<T, P&&>::value == true`.
 62 | 
 63 | - `T *front();`
 64 | 
 65 |   Return pointer to front of queue. Returns `nullptr` if queue is
 66 |   empty.
 67 | 
 68 | - `void pop();`
 69 | 
 70 |   Dequeue first item of queue. You must ensure that the queue is non-empty
 71 |   before calling pop. This means that `front()` must have returned a
 72 |   non-`nullptr` before each call to `pop()`. Requires
 73 |   `std::is_nothrow_destructible<T>::value == true`.
 74 | 
 75 | - `size_t size();`
 76 | 
 77 |   Return the number of items available in the queue.
 78 | 
 79 | - `bool empty();`
 80 | 
 81 |   Return true if queue is currently empty.
 82 | 
 83 | Only a single writer thread can perform enqueue operations and only a
 84 | single reader thread can perform dequeue operations. Any other usage
 85 | is invalid.
 86 | 
 87 | ## Huge page support
 88 | 
 89 | In addition to supporting custom allocation through the [standard custom
 90 | allocator interface](https://en.cppreference.com/w/cpp/named_req/Allocator) this
 91 | library also supports standard proposal [P0401R3 Providing size feedback in the
 92 | Allocator
 93 | interface](http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p0401r3.html).
 94 | This allows convenient use of [huge
 95 | pages](https://www.kernel.org/doc/html/latest/admin-guide/mm/hugetlbpage.html)
 96 | without wasting any allocated space. Using size feedback is only supported when
 97 | C++17 is enabled.
 98 | 
 99 | The library currently doesn't include a huge page allocator since the APIs for
100 | allocating huge pages are platform dependent and handling of huge page size and
101 | NUMA awareness is application specific.
102 | 
103 | Below is an example huge page allocator for Linux:
104 | 
105 | ```cpp
106 | #include <sys/mman.h>
107 | 
108 | template <typename T> struct Allocator {
109 |   using value_type = T;
110 | 
111 |   struct AllocationResult {
112 |     T *ptr;
113 |     size_t count;
114 |   };
115 | 
116 |   size_t roundup(size_t n) { return (((n - 1) >> 21) + 1) << 21; }
117 | 
118 |   AllocationResult allocate_at_least(size_t n) {
119 |     size_t count = roundup(sizeof(T) * n);
120 |     auto p = static_cast<T *>(mmap(nullptr, count, PROT_READ | PROT_WRITE,
121 |                                    MAP_PRIVATE | MAP_ANONYMOUS | MAP_HUGETLB,
122 |                                    -1, 0));
123 |     if (p == MAP_FAILED) {
124 |       throw std::bad_alloc();
125 |     }
126 |     return {p, count / sizeof(T)};
127 |   }
128 | 
129 |   void deallocate(T *p, size_t n) { munmap(p, roundup(sizeof(T) * n)); }
130 | };
131 | ```
132 | 
133 | See `src/SPSCQueueExampleHugepages.cpp` for the full example on how to use huge
134 | pages on Linux.
135 | 
136 | ## Implementation
137 | 
138 | ![Memory layout](https://github.com/rigtorp/SPSCQueue/blob/master/spsc.svg)
139 | 
140 | The underlying implementation is based on a [ring
141 | buffer](https://en.wikipedia.org/wiki/Circular_buffer).
142 | 
143 | Care has been taken to make sure to avoid any issues with [false
144 | sharing](https://en.wikipedia.org/wiki/False_sharing). The head and tail indices
145 | are aligned and padded to the false sharing range (cache line size).
146 | Additionally the slots buffer is padded with the false sharing range at the
147 | beginning and end, this prevents false sharing with any adjacent allocations.
148 | 
149 | This implementation has higher throughput than a typical concurrent ring buffer
150 | by locally caching the head and tail indices in the writer and reader
151 | respectively. The caching increases throughput by reducing the amount of cache
152 | coherency traffic.
153 | 
154 | To understand how that works first consider a read operation in absence of
155 | caching: the head index (read index) needs to be updated and thus that cache
156 | line is loaded into the L1 cache in exclusive state. The tail (write index)
157 | needs to be read in order to check that the queue is not empty and is thus
158 | loaded into the L1 cache in shared state. Since a queue write operation needs to
159 | read the head index it's likely that a write operation requires some cache
160 | coherency traffic to bring the head index cache line back into exclusive state.
161 | In the worst case there will be one cache line transition from shared to
162 | exclusive for every read and write operation.
163 | 
164 | Next consider a queue reader that caches the tail index: if the cached tail
165 | index indicates that the queue is empty, then load the tail index into the
166 | cached tail index. If the queue was non-empty multiple read operations up until
167 | the cached tail index can complete without stealing the writer's tail index
168 | cache line's exclusive state. Cache coherency traffic is therefore reduced. An
169 | analogous argument can be made for the queue write operation.
170 | 
171 | This implementation allows for arbitrary non-power of two capacities, instead
172 | allocating a extra queue slot to indicate full queue. If you don't want to waste
173 | storage for a extra queue slot you should use a different implementation.
174 | 
175 | References:
176 | 
177 | - *Intel*. [Avoiding and Identifying False Sharing Among Threads](https://software.intel.com/en-us/articles/avoiding-and-identifying-false-sharing-among-threads).
178 | - *Wikipedia*. [Ring buffer](https://en.wikipedia.org/wiki/Circular_buffer).
179 | - *Wikipedia*. [False sharing](https://en.wikipedia.org/wiki/False_sharing).
180 | 
181 | ## Testing
182 | 
183 | Testing lock-free algorithms is hard. I'm using two approaches to test
184 | the implementation:
185 | 
186 | - A single threaded test that the functionality works as intended,
187 |   including that the item constructor and destructor is invoked
188 |   correctly.
189 | - A multi-threaded fuzz test verifies that all items are enqueued and dequeued
190 |   correctly under heavy contention.
191 | 
192 | ## Benchmarks
193 | 
194 | Throughput benchmark measures throughput between 2 threads for a queue of `int`
195 | items.
196 | 
197 | Latency benchmark measures round trip time between 2 threads communicating using
198 | 2 queues of `int` items.
199 | 
200 | Benchmark results for a AMD Ryzen 9 3900X 12-Core Processor, the 2 threads are
201 | running on different cores on the same chiplet:
202 | 
203 | | Queue                        | Throughput (ops/ms) | Latency RTT (ns) |
204 | | ---------------------------- | ------------------: | ---------------: |
205 | | SPSCQueue                    |              362723 |              133 |
206 | | boost::lockfree::spsc        |              209877 |              222 |
207 | | folly::ProducerConsumerQueue |              148818 |              147 |
208 | 
209 | ## Cited by
210 | 
211 | SPSCQueue have been cited by the following papers:
212 | 
213 | - Peizhao Ou and Brian Demsky. 2018. Towards understanding the costs of avoiding
214 |   out-of-thin-air results. Proc. ACM Program. Lang. 2, OOPSLA, Article 136
215 |   (October 2018), 29 pages. DOI: <https://doi.org/10.1145/3276506>
216 | 
217 | ## About
218 | 
219 | This project was created by [Erik Rigtorp](http://rigtorp.se)
220 | <[erik@rigtorp.se](mailto:erik@rigtorp.se)>.
221 | 


--------------------------------------------------------------------------------
/include/rigtorp/SPSCQueue.h:
--------------------------------------------------------------------------------
  1 | /*
  2 | Copyright (c) 2020 Erik Rigtorp <erik@rigtorp.se>
  3 | 
  4 | Permission is hereby granted, free of charge, to any person obtaining a copy
  5 | of this software and associated documentation files (the "Software"), to deal
  6 | in the Software without restriction, including without limitation the rights
  7 | to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
  8 | copies of the Software, and to permit persons to whom the Software is
  9 | furnished to do so, subject to the following conditions:
 10 | 
 11 | The above copyright notice and this permission notice shall be included in all
 12 | copies or substantial portions of the Software.
 13 | 
 14 | THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 15 | IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 16 | FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 17 | AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 18 | LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 19 | OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 20 | SOFTWARE.
 21 |  */
 22 | 
 23 | #pragma once
 24 | 
 25 | #include <atomic>
 26 | #include <cassert>
 27 | #include <cstddef>
 28 | #include <memory> // std::allocator
 29 | #include <new>    // std::hardware_destructive_interference_size
 30 | #include <stdexcept>
 31 | #include <type_traits> // std::enable_if, std::is_*_constructible
 32 | 
 33 | #ifdef __has_cpp_attribute
 34 | #if __has_cpp_attribute(nodiscard)
 35 | #define RIGTORP_NODISCARD [[nodiscard]]
 36 | #endif
 37 | #endif
 38 | #ifndef RIGTORP_NODISCARD
 39 | #define RIGTORP_NODISCARD
 40 | #endif
 41 | 
 42 | namespace rigtorp {
 43 | 
 44 | template <typename T, typename Allocator = std::allocator<T>> class SPSCQueue {
 45 | 
 46 | #if defined(__cpp_if_constexpr) && defined(__cpp_lib_void_t)
 47 |   template <typename Alloc2, typename = void>
 48 |   struct has_allocate_at_least : std::false_type {};
 49 | 
 50 |   template <typename Alloc2>
 51 |   struct has_allocate_at_least<
 52 |       Alloc2, std::void_t<typename Alloc2::value_type,
 53 |                           decltype(std::declval<Alloc2 &>().allocate_at_least(
 54 |                               size_t{}))>> : std::true_type {};
 55 | #endif
 56 | 
 57 | public:
 58 |   explicit SPSCQueue(const size_t capacity,
 59 |                      const Allocator &allocator = Allocator())
 60 |       : capacity_(capacity), allocator_(allocator) {
 61 |     // The queue needs at least one element
 62 |     if (capacity_ < 1) {
 63 |       capacity_ = 1;
 64 |     }
 65 |     capacity_++; // Needs one slack element
 66 |     // Prevent overflowing size_t
 67 |     if (capacity_ > SIZE_MAX - 2 * kPadding) {
 68 |       capacity_ = SIZE_MAX - 2 * kPadding;
 69 |     }
 70 | 
 71 | #if defined(__cpp_if_constexpr) && defined(__cpp_lib_void_t)
 72 |     if constexpr (has_allocate_at_least<Allocator>::value) {
 73 |       auto res = allocator_.allocate_at_least(capacity_ + 2 * kPadding);
 74 |       slots_ = res.ptr;
 75 |       capacity_ = res.count - 2 * kPadding;
 76 |     } else {
 77 |       slots_ = std::allocator_traits<Allocator>::allocate(
 78 |           allocator_, capacity_ + 2 * kPadding);
 79 |     }
 80 | #else
 81 |     slots_ = std::allocator_traits<Allocator>::allocate(
 82 |         allocator_, capacity_ + 2 * kPadding);
 83 | #endif
 84 | 
 85 |     static_assert(alignof(SPSCQueue<T>) == kCacheLineSize, "");
 86 |     static_assert(sizeof(SPSCQueue<T>) >= 3 * kCacheLineSize, "");
 87 |     assert(reinterpret_cast<char *>(&readIdx_) -
 88 |                reinterpret_cast<char *>(&writeIdx_) >=
 89 |            static_cast<std::ptrdiff_t>(kCacheLineSize));
 90 |   }
 91 | 
 92 |   ~SPSCQueue() {
 93 |     while (front()) {
 94 |       pop();
 95 |     }
 96 |     std::allocator_traits<Allocator>::deallocate(allocator_, slots_,
 97 |                                                  capacity_ + 2 * kPadding);
 98 |   }
 99 | 
100 |   // non-copyable and non-movable
101 |   SPSCQueue(const SPSCQueue &) = delete;
102 |   SPSCQueue &operator=(const SPSCQueue &) = delete;
103 | 
104 |   template <typename... Args>
105 |   void emplace(Args &&...args) noexcept(
106 |       std::is_nothrow_constructible<T, Args &&...>::value) {
107 |     static_assert(std::is_constructible<T, Args &&...>::value,
108 |                   "T must be constructible with Args&&...");
109 |     auto const writeIdx = writeIdx_.load(std::memory_order_relaxed);
110 |     auto nextWriteIdx = writeIdx + 1;
111 |     if (nextWriteIdx == capacity_) {
112 |       nextWriteIdx = 0;
113 |     }
114 |     while (nextWriteIdx == readIdxCache_) {
115 |       readIdxCache_ = readIdx_.load(std::memory_order_acquire);
116 |     }
117 |     new (&slots_[writeIdx + kPadding]) T(std::forward<Args>(args)...);
118 |     writeIdx_.store(nextWriteIdx, std::memory_order_release);
119 |   }
120 | 
121 |   template <typename... Args>
122 |   RIGTORP_NODISCARD bool try_emplace(Args &&...args) noexcept(
123 |       std::is_nothrow_constructible<T, Args &&...>::value) {
124 |     static_assert(std::is_constructible<T, Args &&...>::value,
125 |                   "T must be constructible with Args&&...");
126 |     auto const writeIdx = writeIdx_.load(std::memory_order_relaxed);
127 |     auto nextWriteIdx = writeIdx + 1;
128 |     if (nextWriteIdx == capacity_) {
129 |       nextWriteIdx = 0;
130 |     }
131 |     if (nextWriteIdx == readIdxCache_) {
132 |       readIdxCache_ = readIdx_.load(std::memory_order_acquire);
133 |       if (nextWriteIdx == readIdxCache_) {
134 |         return false;
135 |       }
136 |     }
137 |     new (&slots_[writeIdx + kPadding]) T(std::forward<Args>(args)...);
138 |     writeIdx_.store(nextWriteIdx, std::memory_order_release);
139 |     return true;
140 |   }
141 | 
142 |   void push(const T &v) noexcept(std::is_nothrow_copy_constructible<T>::value) {
143 |     static_assert(std::is_copy_constructible<T>::value,
144 |                   "T must be copy constructible");
145 |     emplace(v);
146 |   }
147 | 
148 |   template <typename P, typename = typename std::enable_if<
149 |                             std::is_constructible<T, P &&>::value>::type>
150 |   void push(P &&v) noexcept(std::is_nothrow_constructible<T, P &&>::value) {
151 |     emplace(std::forward<P>(v));
152 |   }
153 | 
154 |   RIGTORP_NODISCARD bool
155 |   try_push(const T &v) noexcept(std::is_nothrow_copy_constructible<T>::value) {
156 |     static_assert(std::is_copy_constructible<T>::value,
157 |                   "T must be copy constructible");
158 |     return try_emplace(v);
159 |   }
160 | 
161 |   template <typename P, typename = typename std::enable_if<
162 |                             std::is_constructible<T, P &&>::value>::type>
163 |   RIGTORP_NODISCARD bool
164 |   try_push(P &&v) noexcept(std::is_nothrow_constructible<T, P &&>::value) {
165 |     return try_emplace(std::forward<P>(v));
166 |   }
167 | 
168 |   RIGTORP_NODISCARD T *front() noexcept {
169 |     auto const readIdx = readIdx_.load(std::memory_order_relaxed);
170 |     if (readIdx == writeIdxCache_) {
171 |       writeIdxCache_ = writeIdx_.load(std::memory_order_acquire);
172 |       if (writeIdxCache_ == readIdx) {
173 |         return nullptr;
174 |       }
175 |     }
176 |     return &slots_[readIdx + kPadding];
177 |   }
178 | 
179 |   void pop() noexcept {
180 |     static_assert(std::is_nothrow_destructible<T>::value,
181 |                   "T must be nothrow destructible");
182 |     auto const readIdx = readIdx_.load(std::memory_order_relaxed);
183 |     assert(writeIdx_.load(std::memory_order_acquire) != readIdx &&
184 |            "Can only call pop() after front() has returned a non-nullptr");
185 |     slots_[readIdx + kPadding].~T();
186 |     auto nextReadIdx = readIdx + 1;
187 |     if (nextReadIdx == capacity_) {
188 |       nextReadIdx = 0;
189 |     }
190 |     readIdx_.store(nextReadIdx, std::memory_order_release);
191 |   }
192 | 
193 |   RIGTORP_NODISCARD size_t size() const noexcept {
194 |     std::ptrdiff_t diff = writeIdx_.load(std::memory_order_acquire) -
195 |                           readIdx_.load(std::memory_order_acquire);
196 |     if (diff < 0) {
197 |       diff += capacity_;
198 |     }
199 |     return static_cast<size_t>(diff);
200 |   }
201 | 
202 |   RIGTORP_NODISCARD bool empty() const noexcept {
203 |     return writeIdx_.load(std::memory_order_acquire) ==
204 |            readIdx_.load(std::memory_order_acquire);
205 |   }
206 | 
207 |   RIGTORP_NODISCARD size_t capacity() const noexcept { return capacity_ - 1; }
208 | 
209 | private:
210 | #ifdef __cpp_lib_hardware_interference_size
211 |   static constexpr size_t kCacheLineSize =
212 |       std::hardware_destructive_interference_size;
213 | #else
214 |   static constexpr size_t kCacheLineSize = 64;
215 | #endif
216 | 
217 |   // Padding to avoid false sharing between slots_ and adjacent allocations
218 |   static constexpr size_t kPadding = (kCacheLineSize - 1) / sizeof(T) + 1;
219 | 
220 | private:
221 |   size_t capacity_;
222 |   T *slots_;
223 | #if defined(__has_cpp_attribute) && __has_cpp_attribute(no_unique_address)
224 |   Allocator allocator_ [[no_unique_address]];
225 | #else
226 |   Allocator allocator_;
227 | #endif
228 | 
229 |   // Align to cache line size in order to avoid false sharing
230 |   // readIdxCache_ and writeIdxCache_ is used to reduce the amount of cache
231 |   // coherency traffic
232 |   alignas(kCacheLineSize) std::atomic<size_t> writeIdx_ = {0};
233 |   alignas(kCacheLineSize) size_t readIdxCache_ = 0;
234 |   alignas(kCacheLineSize) std::atomic<size_t> readIdx_ = {0};
235 |   alignas(kCacheLineSize) size_t writeIdxCache_ = 0;
236 | };
237 | } // namespace rigtorp
238 | 


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