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<blockquote data-quote="ptf" data-source="post: 292327" data-attributes="member: 45662">In this video, let&#039;s compare the different kinds of atomic&#039;s memory orders: relaxed, consume, acquire, release, sequentially consistent, and understand their differences and when to use them.[MEDIA=youtube]UzYVDki31Hg[/MEDIA][CODE=cpp]// Video lecture: https://www.youtube.com/watch?v=UzYVDki31Hg#include &lt;iostream&gt;#include &lt;atomic&gt;#include &lt;thread&gt;#include &lt;random&gt;#include &lt;vector&gt;#include &lt;cassert&gt;// #include &quot;jthread.hpp&quot;using namespace std::chrono_literals;void test_atomic_relaxed() {&nbsp; &nbsp; // Atomic operations tagged memory_order_relaxed are not synchronization operations; they do not&nbsp; &nbsp; // impose an order among concurrent memory accesses. They only guarantee atomicity and&nbsp; &nbsp; // modification order consistency.&nbsp; &nbsp; std::atomic&lt;int&gt; x = {0};&nbsp; &nbsp; std::atomic&lt;int&gt; y = {0};&nbsp; &nbsp; std::jthread t1([&amp;]() {&nbsp; &nbsp; &nbsp; &nbsp; auto r1 = y.load(std::memory_order_relaxed);&nbsp; // A&nbsp; &nbsp; &nbsp; &nbsp; x.fetch_add(r1, std::memory_order_relaxed);&nbsp;&nbsp; // B&nbsp; &nbsp; });&nbsp; &nbsp; std::jthread t2([&amp;]() {&nbsp; &nbsp; &nbsp; &nbsp; auto r2 = x.load(std::memory_order_relaxed);&nbsp; // C&nbsp; &nbsp; &nbsp; &nbsp; y.fetch_add(42, std::memory_order_relaxed);&nbsp;&nbsp; // D&nbsp; &nbsp; });&nbsp; &nbsp; // Possible outcomes&nbsp; &nbsp; // CDAB: r1 = 42, r2 = 0&nbsp; &nbsp; // ABCD: r1 = 0, r2 = 42&nbsp; &nbsp; // DABC: r1 = 42, r2 = 42&nbsp; &nbsp; // ...}void test_consume_release() {&nbsp; &nbsp; // If an atomic store in thread A is tagged memory_order_release and an atomic load in thread B&nbsp; &nbsp; // from the same variable that read the stored value is tagged memory_order_consume, all memory&nbsp; &nbsp; // writes (non-atomic and relaxed atomic) that happened-before the atomic store from the point&nbsp; &nbsp; // of view of thread A, become visible side-effects within those operations in thread B into&nbsp; &nbsp; // which the load operation carries dependency, that is, once the atomic load is completed,&nbsp; &nbsp; // those operators and functions in thread B that use the value obtained from the load are&nbsp; &nbsp; // guaranteed to see what thread A wrote to memory.&nbsp; &nbsp; // The synchronization is established only between the threads releasing and consuming the same&nbsp; &nbsp; // atomic variable. Other threads can see different order of memory accesses than either or both&nbsp; &nbsp; // of the synchronized threads.&nbsp; &nbsp; std::atomic&lt;std::string*&gt; ptr{};&nbsp; &nbsp; int data;&nbsp; &nbsp; std::string* p{};&nbsp; &nbsp; std::jthread producer([&amp;]() {&nbsp; &nbsp; &nbsp; &nbsp; p = new std::string(&quot;Hello&quot;);&nbsp; &nbsp; &nbsp; &nbsp; data = 42;&nbsp; &nbsp; &nbsp; &nbsp; ptr.store(p, std::memory_order_release);&nbsp; &nbsp; });&nbsp; &nbsp; std::jthread consumer([&amp;]() {&nbsp; &nbsp; &nbsp; &nbsp; std::string* p2{};&nbsp; &nbsp; &nbsp; &nbsp; while (!(p2 = ptr.load(std::memory_order_consume)))&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; ;&nbsp; &nbsp; &nbsp; &nbsp; assert(*p == &quot;Hello&quot;);&nbsp;&nbsp; // always true&nbsp; &nbsp; &nbsp; &nbsp; assert(*p2 == &quot;Hello&quot;);&nbsp; // always true: *p2 carries dependency from ptr&nbsp; &nbsp; &nbsp; &nbsp; assert(data == 42);&nbsp; &nbsp; &nbsp; // may and may not be true: data does not carry dependency from ptr&nbsp; &nbsp; &nbsp; &nbsp; delete p2;&nbsp; &nbsp; });}void test_acquire_release_1() {&nbsp; &nbsp; // If an atomic store in thread A is tagged memory_order_release and an atomic load in thread B&nbsp; &nbsp; // from the same variable is tagged memory_order_acquire, all memory writes (non-atomic and&nbsp; &nbsp; // relaxed atomic) that happened-before the atomic store from the point of view of thread A,&nbsp; &nbsp; // become visible side-effects in thread B. That is, once the atomic load is completed, thread B&nbsp; &nbsp; // is guaranteed to see everything thread A wrote to memory.&nbsp; &nbsp; std::atomic&lt;std::string*&gt; ptr{};&nbsp; &nbsp; int data;&nbsp; &nbsp; std::string* p{};&nbsp; &nbsp; std::jthread producer([&amp;]() {&nbsp; &nbsp; &nbsp; &nbsp; p = new std::string(&quot;Hello&quot;);&nbsp; &nbsp; &nbsp; &nbsp; data = 42;&nbsp; &nbsp; &nbsp; &nbsp; ptr.store(p, std::memory_order_release);&nbsp; &nbsp; });&nbsp; &nbsp; std::jthread consumer([&amp;]() {&nbsp; &nbsp; &nbsp; &nbsp; std::string* p2{};&nbsp; &nbsp; &nbsp; &nbsp; while (!(p2 = ptr.load(std::memory_order_acquire)))&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; ;&nbsp; &nbsp; &nbsp; &nbsp; assert(*p == &quot;Hello&quot;);&nbsp;&nbsp; // always true&nbsp; &nbsp; &nbsp; &nbsp; assert(*p2 == &quot;Hello&quot;);&nbsp; // always true&nbsp; &nbsp; &nbsp; &nbsp; assert(data == 42);&nbsp; &nbsp; &nbsp; // always true&nbsp; &nbsp; &nbsp; &nbsp; delete p2;&nbsp; &nbsp; });}void test_acquire_release_2() {&nbsp; &nbsp; // If an atomic store in thread A is tagged memory_order_release and an atomic load in thread B&nbsp; &nbsp; // from the same variable is tagged memory_order_acquire, all memory writes (non-atomic and&nbsp; &nbsp; // relaxed atomic) that happened-before the atomic store from the point of view of thread A,&nbsp; &nbsp; // become visible side-effects in thread B. That is, once the atomic load is completed, thread B&nbsp; &nbsp; // is guaranteed to see everything thread A wrote to memory.&nbsp; &nbsp; std::vector&lt;int&gt; data;&nbsp; &nbsp; std::atomic&lt;int&gt; flag = {0};&nbsp; &nbsp; std::jthread producer([&amp;]() {&nbsp; &nbsp; &nbsp; &nbsp; data.push_back(42);&nbsp; &nbsp; &nbsp; &nbsp; flag.store(1, std::memory_order_release);&nbsp; &nbsp; });&nbsp; &nbsp; std::jthread consumer([&amp;]() {&nbsp; &nbsp; &nbsp; &nbsp; int expected = 1;&nbsp; &nbsp; &nbsp; &nbsp; // Compares the contents of the flag with expected:&nbsp; &nbsp; &nbsp; &nbsp; // - if true, it replaces the flag value with 2. (performs read-modify-write operation)&nbsp; &nbsp; &nbsp; &nbsp; // - if false, it replaces expected with the flag. (performs load operation)&nbsp; &nbsp; &nbsp; &nbsp; // returns&nbsp; &nbsp; &nbsp; &nbsp; // - true if expected compares equal to the contained value.&nbsp; &nbsp; &nbsp; &nbsp; // - false otherwise.&nbsp; &nbsp; &nbsp; &nbsp; while (!flag.compare_exchange_weak(expected, 2, std::memory_order_acq_rel)) {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; expected = 1;&nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; &nbsp; &nbsp; assert(data.at(0) == 42);&nbsp; // always true&nbsp; &nbsp; });}void test_seq_cst() {&nbsp; &nbsp; // A load operation with this memory order performs an acquire operation, a store performs a&nbsp; &nbsp; // release operation, and read-modify-write performs both an acquire operation and a release&nbsp; &nbsp; // operation, plus a single total order exists in which all threads observe all modifications in&nbsp; &nbsp; // the same order&nbsp; &nbsp; std::atomic&lt;bool&gt; x = {false};&nbsp; &nbsp; std::atomic&lt;bool&gt; y = {false};&nbsp; &nbsp; std::atomic&lt;int&gt; z = {0};&nbsp; &nbsp; {&nbsp; &nbsp; &nbsp; &nbsp; std::jthread write_x([&amp;]() {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; //&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; x.store(true, std::memory_order_seq_cst);&nbsp; &nbsp; &nbsp; &nbsp; });&nbsp; &nbsp; &nbsp; &nbsp; std::jthread write_y([&amp;]() {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; y.store(true, std::memory_order_seq_cst);&nbsp; &nbsp; &nbsp; &nbsp; });&nbsp; &nbsp; &nbsp; &nbsp; std::jthread read_x_then_y([&amp;]() {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; while (!x.load(std::memory_order_seq_cst))&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; ;&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; if (y.load(std::memory_order_seq_cst)) {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; ++z;&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; &nbsp; &nbsp; });&nbsp; &nbsp; &nbsp; &nbsp; std::jthread read_y_then_x([&amp;]() {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; while (!y.load(std::memory_order_seq_cst))&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; ;&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; if (x.load(std::memory_order_seq_cst)) {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; ++z;&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; &nbsp; &nbsp; });&nbsp; &nbsp; }&nbsp; &nbsp; assert(z.load() != 0);}int main() {&nbsp; &nbsp; test_atomic_relaxed();&nbsp; &nbsp; test_consume_release();&nbsp; &nbsp; test_acquire_release_1();&nbsp; &nbsp; test_acquire_release_2();&nbsp; &nbsp; test_seq_cst();&nbsp; &nbsp; return 0;}[/CODE]</blockquote>

[QUOTE="ptf, post: 292327, member: 45662"] In this video, let's compare the different kinds of atomic's memory orders: relaxed, consume, acquire, release, sequentially consistent, and understand their differences and when to use them. [MEDIA=youtube]UzYVDki31Hg[/MEDIA] [CODE=cpp]// Video lecture: https://www.youtube.com/watch?v=UzYVDki31Hg #include <iostream> #include <atomic> #include <thread> #include <random> #include <vector> #include <cassert> // #include "jthread.hpp" using namespace std::chrono_literals; void test_atomic_relaxed() { // Atomic operations tagged memory_order_relaxed are not synchronization operations; they do not // impose an order among concurrent memory accesses. They only guarantee atomicity and // modification order consistency. std::atomic<int> x = {0}; std::atomic<int> y = {0}; std::jthread t1([&]() { auto r1 = y.load(std::memory_order_relaxed); // A x.fetch_add(r1, std::memory_order_relaxed); // B }); std::jthread t2([&]() { auto r2 = x.load(std::memory_order_relaxed); // C y.fetch_add(42, std::memory_order_relaxed); // D }); // Possible outcomes // CDAB: r1 = 42, r2 = 0 // ABCD: r1 = 0, r2 = 42 // DABC: r1 = 42, r2 = 42 // ... } void test_consume_release() { // If an atomic store in thread A is tagged memory_order_release and an atomic load in thread B // from the same variable that read the stored value is tagged memory_order_consume, all memory // writes (non-atomic and relaxed atomic) that happened-before the atomic store from the point // of view of thread A, become visible side-effects within those operations in thread B into // which the load operation carries dependency, that is, once the atomic load is completed, // those operators and functions in thread B that use the value obtained from the load are // guaranteed to see what thread A wrote to memory. // The synchronization is established only between the threads releasing and consuming the same // atomic variable. Other threads can see different order of memory accesses than either or both // of the synchronized threads. std::atomic<std::string*> ptr{}; int data; std::string* p{}; std::jthread producer([&]() { p = new std::string("Hello"); data = 42; ptr.store(p, std::memory_order_release); }); std::jthread consumer([&]() { std::string* p2{}; while (!(p2 = ptr.load(std::memory_order_consume))) ; assert(*p == "Hello"); // always true assert(*p2 == "Hello"); // always true: *p2 carries dependency from ptr assert(data == 42); // may and may not be true: data does not carry dependency from ptr delete p2; }); } void test_acquire_release_1() { // If an atomic store in thread A is tagged memory_order_release and an atomic load in thread B // from the same variable is tagged memory_order_acquire, all memory writes (non-atomic and // relaxed atomic) that happened-before the atomic store from the point of view of thread A, // become visible side-effects in thread B. That is, once the atomic load is completed, thread B // is guaranteed to see everything thread A wrote to memory. std::atomic<std::string*> ptr{}; int data; std::string* p{}; std::jthread producer([&]() { p = new std::string("Hello"); data = 42; ptr.store(p, std::memory_order_release); }); std::jthread consumer([&]() { std::string* p2{}; while (!(p2 = ptr.load(std::memory_order_acquire))) ; assert(*p == "Hello"); // always true assert(*p2 == "Hello"); // always true assert(data == 42); // always true delete p2; }); } void test_acquire_release_2() { // If an atomic store in thread A is tagged memory_order_release and an atomic load in thread B // from the same variable is tagged memory_order_acquire, all memory writes (non-atomic and // relaxed atomic) that happened-before the atomic store from the point of view of thread A, // become visible side-effects in thread B. That is, once the atomic load is completed, thread B // is guaranteed to see everything thread A wrote to memory. std::vector<int> data; std::atomic<int> flag = {0}; std::jthread producer([&]() { data.push_back(42); flag.store(1, std::memory_order_release); }); std::jthread consumer([&]() { int expected = 1; // Compares the contents of the flag with expected: // - if true, it replaces the flag value with 2. (performs read-modify-write operation) // - if false, it replaces expected with the flag. (performs load operation) // returns // - true if expected compares equal to the contained value. // - false otherwise. while (!flag.compare_exchange_weak(expected, 2, std::memory_order_acq_rel)) { expected = 1; } assert(data.at(0) == 42); // always true }); } void test_seq_cst() { // A load operation with this memory order performs an acquire operation, a store performs a // release operation, and read-modify-write performs both an acquire operation and a release // operation, plus a single total order exists in which all threads observe all modifications in // the same order std::atomic<bool> x = {false}; std::atomic<bool> y = {false}; std::atomic<int> z = {0}; { std::jthread write_x([&]() { // x.store(true, std::memory_order_seq_cst); }); std::jthread write_y([&]() { y.store(true, std::memory_order_seq_cst); }); std::jthread read_x_then_y([&]() { while (!x.load(std::memory_order_seq_cst)) ; if (y.load(std::memory_order_seq_cst)) { ++z; } }); std::jthread read_y_then_x([&]() { while (!y.load(std::memory_order_seq_cst)) ; if (x.load(std::memory_order_seq_cst)) { ++z; } }); } assert(z.load() != 0); } int main() { test_atomic_relaxed(); test_consume_release(); test_acquire_release_1(); test_acquire_release_2(); test_seq_cst(); return 0; }[/CODE] [/QUOTE]

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