TechTown - Most Malleable Memory Management Method

Examples for wanting to manage the memory usage of your program can be to reduce the cost of heap allocations, improve locality of reference, or maybe reduce heap fragmentation.

Regardless of reason, PMR, Polymorphic Memory Resource, is available since C++17, and makes your life much easier.

I will show you...

* Tools and techniques for analysing the memory usage of your program.

* How PMR makes memory management easier.

* How to use PMR with the standard library types.

* How to make your own types use PMR.

* Advice, and pitfalls to avoid, on your quest to improving the memory usage of your program.

1. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 1/110 Most Malleable Memory Management Method Björn Fahller
2. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 2/110 Most Malleable Memory Management Method
3. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 3/110 Most Malleable Memory Management Method
4. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 4/110 Most Malleable Memory Management Method
5. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 5/110 Most Malleable Memory Management Method
6. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 6/110 The heap Allows you to... – Allocate objects of any size and alignment – Allocate and deallocate from any thread • Even allocate in one thread and deallocate in another – Let objects lifetimes vary wildly
7. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 7/110 The heap Allows you to... – Allocate objects of any size and alignment – Allocate and deallocate from any thread • Even allocate in one thread and deallocate in another – Let objects lifetimes vary wildly Complex structure with lookup time and space overhead
8. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 8/110 The heap Allows you to... – Allocate objects of any size and alignment – Allocate and deallocate from any thread • Even allocate in one thread and deallocate in another – Let objects lifetimes vary wildly Poor locality of reference
9. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 9/110 The heap Allows you to... – Allocate objects of any size and alignment – Allocate and deallocate from any thread • Even allocate in one thread and deallocate in another – Let objects lifetimes vary wildly Fragmentation may make everything worse over time
10. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 10/110 The heap Allows you to... – Allocate objects of any size and alignment – Allocate and deallocate from any thread • Even allocate in one thread and deallocate in another – Let objects lifetimes vary wildly Synchronisation overhead
11. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 11/110 So you turn to allocators...
12. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 12/110 So you turn to allocators... https://www.edvardmunch.org/the-scream.jsp
13. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 13/110 So you turn to allocators... https://www.edvardmunch.org/the-scream.jsp But PMR allocators suck less
14. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 14/110 Polymorphic Memory Resource “container” PMR allocator <<memory resource>>
15. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 15/110 Polymorphic Memory Resource “container” PMR allocator <<memory resource>> A “container” has a PMR allocator
16. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 16/110 Polymorphic Memory Resource “container” PMR allocator <<memory resource>> A “container” has a PMR allocator A PMR allocator references a memory resource
17. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 17/110 Polymorphic Memory Resource “container” PMR allocator <<memory resource>> A “container” has a PMR allocator A PMR allocator references a memory resource Standard containers are conveniently available under namespace std::pmr, e.g. std::pmr::vector<int>
18. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 18/110 memory_resource class std::pmr::memory_resource { public:         void* allocate(size_t bytes, size_t alignment);         void deallocate(void* addr, size_t bytes, size_t alignment);         bool is_equal(const memory_resource&) const noexcept; private:         virtual void* do_allocate(size_t bytes, size_t align) = 0;         virtual void do_deallocate(void* addr, size_t bytes, size_t align) = 0;         virtual bool do_is_equal(const memory_resource&) const noexcept = 0; }; <memory_resource>
19. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 19/110 memory_resource class std::pmr::memory_resource { public:         void* allocate(size_t bytes, size_t alignment);         void deallocate(void* addr, size_t bytes, size_t alignment);         bool is_equal(const memory_resource&) const noexcept; private:         virtual void* do_allocate(size_t bytes, size_t align) = 0;         virtual void do_deallocate(void* addr, size_t bytes, size_t align) = 0;         virtual bool do_is_equal(const memory_resource&) const noexcept = 0; }; <memory_resource> Inherit and implement these
20. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 20/110 memory_resource class std::pmr::memory_resource { public:         void* allocate(size_t bytes, size_t alignment);         void deallocate(void* addr, size_t bytes, size_t alignment);         bool is_equal(const memory_resource&) const noexcept; private:         virtual void* do_allocate(size_t bytes, size_t align) = 0;         virtual void do_deallocate(void* addr, size_t bytes, size_t align) = 0;         virtual bool do_is_equal(const memory_resource&) const noexcept = 0; }; <memory_resource> Mildly annoying that alignment uses size_t and not align_val_t Inherit and implement these
21. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 21/110 memory_resource class tracing_resource final : public std::pmr::memory_resource { public:     tracing_resource(std::ostream& os) : os_(os) {} private:     void* do_allocate(size_t bytes, size_t align) override    {         auto addr = operator new (bytes, std::align_val_t(align));         os_ << "allocate(" << bytes << ", " << align << ") -> " << addr << '\n';         return addr;     }     void do_deallocate(void* addr, size_t bytes, size_t align) override {         os_ << "deallocate(" << addr << ", " << bytes << ", " << align << ")\n";         operator delete(addr, bytes, std::align_val_t(align));     }     bool do_is_equal(const std::pmr::memory_resource& other) const noexcept override {         return &other == this;     }     std::ostream& os_; };
22. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 22/110 memory_resource class tracing_resource final : public std::pmr::memory_resource { public:     tracing_resource(std::ostream& os) : os_(os) {} private:     void* do_allocate(size_t bytes, size_t align) override    {         auto addr = operator new (bytes, std::align_val_t(align));         os_ << "allocate(" << bytes << ", " << align << ") -> " << addr << '\n';         return addr;     }     void do_deallocate(void* addr, size_t bytes, size_t align) override {         os_ << "deallocate(" << addr << ", " << bytes << ", " << align << ")\n";         operator delete(addr, bytes, std::align_val_t(align));     }     bool do_is_equal(const std::pmr::memory_resource& other) const noexcept override {         return &other == this;     }     std::ostream& os_; };
23. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 23/110 memory_resource class tracing_resource final : public std::pmr::memory_resource { public:     tracing_resource(std::ostream& os) : os_(os) {} private:     void* do_allocate(size_t bytes, size_t align) override    {         auto addr = operator new (bytes, std::align_val_t(align));         os_ << "allocate(" << bytes << ", " << align << ") -> " << addr << '\n';         return addr;     }     void do_deallocate(void* addr, size_t bytes, size_t align) override {         os_ << "deallocate(" << addr << ", " << bytes << ", " << align << ")\n";         operator delete(addr, bytes, std::align_val_t(align));     }     bool do_is_equal(const std::pmr::memory_resource& other) const noexcept override {         return &other == this;     }     std::ostream& os_; };
24. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 24/110 memory_resource class tracing_resource final : public std::pmr::memory_resource { public:     tracing_resource(std::ostream& os) : os_(os) {} private:     void* do_allocate(size_t bytes, size_t align) override    {         auto addr = operator new (bytes, std::align_val_t(align));         os_ << "allocate(" << bytes << ", " << align << ") -> " << addr << '\n';         return addr;     }     void do_deallocate(void* addr, size_t bytes, size_t align) override {         os_ << "deallocate(" << addr << ", " << bytes << ", " << align << ")\n";         operator delete(addr, bytes, std::align_val_t(align));     }     bool do_is_equal(const std::pmr::memory_resource& other) const noexcept override {         return &other == this;     }     std::ostream& os_; };
25. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 25/110 memory_resource class tracing_resource final : public std::pmr::memory_resource { public:     tracing_resource(std::ostream& os) : os_(os) {} private:     void* do_allocate(size_t bytes, size_t align) override    {         auto addr = operator new (bytes, std::align_val_t(align));         os_ << "allocate(" << bytes << ", " << align << ") -> " << addr << '\n';         return addr;     }     void do_deallocate(void* addr, size_t bytes, size_t align) override {         os_ << "deallocate(" << addr << ", " << bytes << ", " << align << ")\n";         operator delete(addr, bytes, std::align_val_t(align));     }     bool do_is_equal(const std::pmr::memory_resource& other) const noexcept override {         return &other == this;     }     std::ostream& os_; };
26. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 26/110 https://godbolt.org/z/P7jojKdTf
27. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 27/110 Most Malleable Memory Management Method It is very important that your memory resource instance outlives all containers that use it.
28. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 28/110 Most Malleable Memory Management Method It is very important that your memory resource instance outlives all containers that use it. It’s very easy to miss using a PMR type, and not so easy to detect the mistake
29. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 29/110 Most Malleable Memory Management Method It is very important that your memory resource instance outlives all containers that use it. It’s very easy to miss using a PMR type, and not so easy to detect the mistake There are tools, though
30. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 30/110 string frequency example class histogram { public:     void add(const std::string& word) {         ++words_[word];     }     void print_top(size_t n, std::ostream& os) const {         using count = std::pair<std::string, size_t>;         std::vector<count> popular(words_.begin(), words_.end());         std::ranges::partial_sort(popular,                                                   popular.begin() + n, std::greater{}, &count::second);         for (const auto& stat : popular | std::ranges::views::take(n)) {             os << stat.first << '\t' << stat.second << '\n';         }     } private:     std::unordered_map<std::string, size_t> words_; };
31. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 31/110 string frequency example class histogram { public:     void add(const std::string& word) {         ++words_[word];     }     void print_top(size_t n, std::ostream& os) const {         using count = std::pair<std::string, size_t>;         std::vector<count> popular(words_.begin(), words_.end());         std::ranges::partial_sort(popular,                                                   popular.begin() + n, std::greater{}, &count::second);         for (const auto& stat : popular | std::ranges::views::take(n)) {             os << stat.first << '\t' << stat.second << '\n';         }     } private:     std::unordered_map<std::string, size_t> words_; };
32. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 32/110 string frequency example class histogram { public:     void add(const std::string& word) {         ++words_[word];     }     void print_top(size_t n, std::ostream& os) const {         using count = std::pair<std::string, size_t>;         std::vector<count> popular(words_.begin(), words_.end());         std::ranges::partial_sort(popular,                                                   popular.begin() + n, std::greater{}, &count::second);         for (const auto& stat : popular | std::ranges::views::take(n)) {             os << stat.first << '\t' << stat.second << '\n';         }     } private:     std::unordered_map<std::string, size_t> words_; };
33. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 33/110 string frequency example class histogram { public:     void add(const std::string& word) {         ++words_[word];     }     void print_top(size_t n, std::ostream& os) const {         using count = std::pair<std::string, size_t>;         std::vector<count> popular(words_.begin(), words_.end());         std::ranges::partial_sort(popular,                                                   popular.begin() + n, std::greater{}, &count::second);         for (const auto& stat : popular | std::ranges::views::take(n)) {             os << stat.first << '\t' << stat.second << '\n';         }     } private:     std::unordered_map<std::string, size_t> words_; };
34. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 34/110 string frequency example class histogram { public:     void add(const std::string& word) {         ++words_[word];     }     void print_top(size_t n, std::ostream& os) const {         using count = std::pair<std::string, size_t>;         std::vector<count> popular(words_.begin(), words_.end());         std::ranges::partial_sort(popular,                                                   popular.begin() + n, std::greater{}, &count::second);         for (const auto& stat : popular | std::ranges::views::take(n)) {             os << stat.first << '\t' << stat.second << '\n';         }     } private:     std::unordered_map<std::string, size_t> words_; };
35. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 35/110 string frequency example class histogram { public:     void add(const std::string& word) {         ++words_[word];     }     void print_top(size_t n, std::ostream& os) const {         using count = std::pair<std::string, size_t>;         std::vector<count> popular(words_.begin(), words_.end());         std::ranges::partial_sort(popular,                                                   popular.begin() + n, std::greater{}, &count::second);         for (const auto& stat : popular | std::ranges::views::take(n)) {             os << stat.first << '\t' << stat.second << '\n';         }     } private:     std::unordered_map<std::string, size_t> words_; };
36. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 36/110 string frequency example class histogram { public:     void add(const std::string& word) {         ++words_[word];     }     void print_top(size_t n, std::ostream& os) const {         using count = std::pair<std::string, size_t>;         std::vector<count> popular(words_.begin(), words_.end());         std::ranges::partial_sort(popular,                                                   popular.begin() + n, std::greater{}, &count::second);         for (const auto& stat : popular | std::ranges::views::take(n)) {             os << stat.first << '\t' << stat.second << '\n';         }     } private:     std::unordered_map<std::string, size_t> words_; };
37. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 37/110 string frequency example class histogram { public:     void add(const std::string& word) {         ++words_[word];     }     void print_top(size_t n, std::ostream& os) const {         using count = std::pair<std::string, size_t>;         std::vector<count> popular(words_.begin(), words_.end());         std::ranges::partial_sort(popular,                                                   popular.begin() + n, std::greater{}, &count::second);         for (const auto& stat : popular | std::ranges::views::take(n)) {             os << stat.first << '\t' << stat.second << '\n';         }     } private:     std::unordered_map<std::string, size_t> words_; };
38. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 38/110 string frequency example class histogram { public:     void add(const std::string& word) {         ++words_[word];     }     void print_top(size_t n, std::ostream& os) const {         using count = std::pair<std::string, size_t>;         std::vector<count> popular(words_.begin(), words_.end());         std::ranges::partial_sort(popular,                                                   popular.begin() + n, std::greater{}, &count::second);         for (const auto& stat : popular | std::ranges::views::take(n)) {             os << stat.first << '\t' << stat.second << '\n';         }     } private:     std::unordered_map<std::string, size_t> words_; }; int main() { std::string word; histogram hist; while (std::cin >> word) { hist.add(word); } hist.print_top(5, std::cout); }
39. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 39/110 Live Demo
40. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 40/110 Most Malleable Memory Management Method Nothing surprising here, but good to have a baseline and a tool Let’s PMR it!
41. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 41/110 PMR histogram class histogram { public: histogram(const std::pmr::polymorphic_allocator<>& alloc = {}): words_(alloc){} void add(const std::pmr::string& word) { ++words_[word]; } void print_top(size_t n, std::ostream& os) const { using count = std::pair<std::pmr::string, size_t>; std::vector<count> popular(words_.begin(), words_.end()); std::ranges::partial_sort(popular, popular.begin() + n, std::greater{}, &count::second); for (const auto& stat : popular | std::ranges::views::take(n)) { os << stat.first << '\t' << stat.second << '\n'; } } private: std::pmr::unordered_map<std::pmr::string, size_t> words_; };
42. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 42/110 PMR histogram class histogram { public: histogram(const std::pmr::polymorphic_allocator<>& alloc = {}): words_(alloc){} void add(const std::pmr::string& word) { ++words_[word]; } void print_top(size_t n, std::ostream& os) const { using count = std::pair<std::pmr::string, size_t>; std::vector<count> popular(words_.begin(), words_.end()); std::ranges::partial_sort(popular, popular.begin() + n, std::greater{}, &count::second); for (const auto& stat : popular | std::ranges::views::take(n)) { os << stat.first << '\t' << stat.second << '\n'; } } private: std::pmr::unordered_map<std::pmr::string, size_t> words_; }; PMR-ify the container and strings
43. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 43/110 PMR histogram class histogram { public: histogram(const std::pmr::polymorphic_allocator<>& alloc = {}): words_(alloc){} void add(const std::pmr::string& word) { ++words_[word]; } void print_top(size_t n, std::ostream& os) const { using count = std::pair<std::pmr::string, size_t>; std::vector<count> popular(words_.begin(), words_.end()); std::ranges::partial_sort(popular, popular.begin() + n, std::greater{}, &count::second); for (const auto& stat : popular | std::ranges::views::take(n)) { os << stat.first << '\t' << stat.second << '\n'; } } private: std::pmr::unordered_map<std::pmr::string, size_t> words_; }; Get an allocator to use
44. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 44/110 PMR histogram class histogram { public: histogram(const std::pmr::polymorphic_allocator<>& alloc = {}): words_(alloc){} void add(const std::pmr::string& word) { ++words_[word]; } void print_top(size_t n, std::ostream& os) const { using count = std::pair<std::pmr::string, size_t>; std::vector<count> popular(words_.begin(), words_.end()); std::ranges::partial_sort(popular, popular.begin() + n, std::greater{}, &count::second); for (const auto& stat : popular | std::ranges::views::take(n)) { os << stat.first << '\t' << stat.second << '\n'; } } private: std::pmr::unordered_map<std::pmr::string, size_t> words_; }; In C++17 it was necessary to express a type here, but in C++20 onwards, just use the diamond
45. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 45/110 PMR histogram class histogram { public: histogram(const std::pmr::polymorphic_allocator<>& alloc = {}): words_(alloc){} void add(const std::pmr::string& word) { ++words_[word]; } void print_top(size_t n, std::ostream& os) const { using count = std::pair<std::pmr::string, size_t>; std::vector<count> popular(words_.begin(), words_.end()); std::ranges::partial_sort(popular, popular.begin() + n, std::greater{}, &count::second); for (const auto& stat : popular | std::ranges::views::take(n)) { os << stat.first << '\t' << stat.second << '\n'; } } private: std::pmr::unordered_map<std::pmr::string, size_t> words_; }; int main() { std::pmr::string word; histogram hist; while (std::cin >> word) { hist.add(word); } hist.print_top(5, std::cout); }
46. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 46/110 Live Demo
47. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 47/110 PMR histogram class histogram { public: histogram(const std::pmr::polymorphic_allocator<>& alloc = {}): words_(alloc){} void add(const std::pmr::string& word) { ++words_[word]; } void print_top(size_t n, std::ostream& os) const { using count = std::pair<std::pmr::string, size_t>; std::vector<count> popular(words_.begin(), words_.end()); std::ranges::partial_sort(popular, popular.begin() + n, std::greater{}, &count::second); for (const auto& stat : popular | std::ranges::views::take(n)) { os << stat.first << '\t' << stat.second << '\n'; } } private: std::pmr::unordered_map<std::pmr::string, size_t> words_; }; int main() { std::pmr::string word; histogram hist; while (std::cin >> word) { hist.add(word); } hist.print_top(5, std::cout); }
48. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 48/110 PMR histogram class histogram { public: histogram(const std::pmr::polymorphic_allocator<>& alloc = {}): words_(alloc){} void add(const std::pmr::string& word) { ++words_[word]; } void print_top(size_t n, std::ostream& os) const { using count = std::pair<std::pmr::string, size_t>; std::vector<count> popular(words_.begin(), words_.end()); std::ranges::partial_sort(popular, popular.begin() + n, std::greater{}, &count::second); for (const auto& stat : popular | std::ranges::views::take(n)) { os << stat.first << '\t' << stat.second << '\n'; } } private: std::pmr::unordered_map<std::pmr::string, size_t> words_; }; int main() { std::pmr::string word; histogram hist; while (std::cin >> word) { hist.add(word); } hist.print_top(5, std::cout); } Not much new, because a default constructed polymorphic_allocator uses the default memory_resource, which is new_delete_resource.
49. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 49/110 PMR histogram class histogram { public: histogram(const std::pmr::polymorphic_allocator<>& alloc = {}): words_(alloc){} void add(const std::pmr::string& word) { ++words_[word]; } void print_top(size_t n, std::ostream& os) const { using count = std::pair<std::pmr::string, size_t>; std::vector<count> popular(words_.begin(), words_.end()); std::ranges::partial_sort(popular, popular.begin() + n, std::greater{}, &count::second); for (const auto& stat : popular | std::ranges::views::take(n)) { os << stat.first << '\t' << stat.second << '\n'; } } private: std::pmr::unordered_map<std::pmr::string, size_t> words_; }; int main() { std::pmr::string word; histogram hist; while (std::cin >> word) { hist.add(word); } hist.print_top(5, std::cout); } But we did see it being used!
50. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 50/110 Choosing a memory resource The header <memory_resource> offers: std::pmr::new_delete_resource std::pmr::null_resource std::pmr::synchronized_pool_resource std::pmr::unsynchronized_pool_resource std::pmr::monotonic_buffer_resource
51. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 51/110 Choosing a memory resource The header <memory_resource> offers: std::pmr::new_delete_resource std::pmr::null_resource std::pmr::synchronized_pool_resource std::pmr::unsynchronized_pool_resource std::pmr::monotonic_buffer_resource You’ve seen this one. Plain operator new and operator delete.
52. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 52/110 Choosing a memory resource The header <memory_resource> offers: std::pmr::new_delete_resource std::pmr::null_resource std::pmr::synchronized_pool_resource std::pmr::unsynchronized_pool_resource std::pmr::monotonic_buffer_resource A resource that always fails to allocate. Can be useful when testing.
53. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 53/110 Choosing a memory resource The header <memory_resource> offers: std::pmr::new_delete_resource std::pmr::null_resource std::pmr::synchronized_pool_resource std::pmr::unsynchronized_pool_resource std::pmr::monotonic_buffer_resource A thread safe pool of memory blocks, in which allocations are simple indexing operations into an available slot in a block.
54. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 54/110 Choosing a memory resource The header <memory_resource> offers: std::pmr::new_delete_resource std::pmr::null_resource std::pmr::synchronized_pool_resource std::pmr::unsynchronized_pool_resource std::pmr::monotonic_buffer_resource Same as above, but without the overhead of thread synchronization. Very fast when you know allocation and deallocation will happen in the same thread.
55. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 55/110 Choosing a memory resource The header <memory_resource> offers: std::pmr::new_delete_resource std::pmr::null_resource std::pmr::synchronized_pool_resource std::pmr::unsynchronized_pool_resource std::pmr::monotonic_buffer_resource Very fast resource, where each allocation is at a monotonically increasing space in the available buffer. Deallocation only happens when the resource is destroyed. Useful in very local scopes.
56. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 56/110 Choosing a memory resource The header <memory_resource> offers: std::pmr::new_delete_resource std::pmr::null_resource std::pmr::synchronized_pool_resource std::pmr::unsynchronized_pool_resource std::pmr::monotonic_buffer_resource You can cascade memory resources. Construct with a pointer to an upstream resource
57. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 57/110 Choosing a memory resource The header <memory_resource> offers: std::pmr::new_delete_resource std::pmr::null_resource std::pmr::synchronized_pool_resource std::pmr::unsynchronized_pool_resource std::pmr::monotonic_buffer_resource void do_work(std::pmr::polymorphic_allocator<> alloc); auto heap = std::pmr::new_delete_resource(); std::pmr::monotonic_buffer_resource monotonic_buffers(heap); std::pmr::unsynchronized_pool_resource pool(&monotonic_buffers); do_work(&pool);
58. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 58/110 Choosing a memory resource The header <memory_resource> offers: std::pmr::new_delete_resource std::pmr::null_resource std::pmr::synchronized_pool_resource std::pmr::unsynchronized_pool_resource std::pmr::monotonic_buffer_resource void do_work(std::pmr::polymorphic_allocator<> alloc); auto heap = std::pmr::new_delete_resource(); std::pmr::monotonic_buffer_resource monotonic_buffers(heap); std::pmr::unsynchronized_pool_resource pool(&monotonic_buffers); do_work(&pool); Construct a polymorphic allocator using pool.
59. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 59/110 Choosing a memory resource The header <memory_resource> offers: std::pmr::new_delete_resource std::pmr::null_resource std::pmr::synchronized_pool_resource std::pmr::unsynchronized_pool_resource std::pmr::monotonic_buffer_resource void do_work(std::pmr::polymorphic_allocator<> alloc); auto heap = std::pmr::new_delete_resource(); std::pmr::monotonic_buffer_resource monotonic_buffers(heap); std::pmr::unsynchronized_pool_resource pool(&monotonic_buffers); do_work(&pool); Constrct a pool resource using monotonic_buffers.
60. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 60/110 Choosing a memory resource The header <memory_resource> offers: std::pmr::new_delete_resource std::pmr::null_resource std::pmr::synchronized_pool_resource std::pmr::unsynchronized_pool_resource std::pmr::monotonic_buffer_resource void do_work(std::pmr::polymorphic_allocator<> alloc); auto heap = std::pmr::new_delete_resource(); std::pmr::monotonic_buffer_resource monotonic_buffers(heap); std::pmr::unsynchronized_pool_resource pool(&monotonic_buffers); do_work(&pool); Construct a monotonic buffer resource using the heap.
61. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 61/110 Choosing a memory resource The header <memory_resource> offers: std::pmr::new_delete_resource std::pmr::null_resource std::pmr::synchronized_pool_resource std::pmr::unsynchronized_pool_resource std::pmr::monotonic_buffer_resource void do_work(std::pmr::polymorphic_allocator<> alloc); auto heap = std::pmr::new_delete_resource(); std::pmr::monotonic_buffer_resource monotonic_buffers(heap); std::pmr::unsynchronized_pool_resource pool(&monotonic_buffers); do_work(&pool); Get the new_delete_allocator.
62. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 62/110 Choosing a memory resource The header <memory_resource> offers: std::pmr::new_delete_resource std::pmr::null_resource std::pmr::synchronized_pool_resource std::pmr::unsynchronized_pool_resource std::pmr::monotonic_buffer_resource void do_work(std::pmr::polymorphic_allocator<> alloc); auto heap = std::pmr::new_delete_resource(); std::pmr::monotonic_buffer_resource monotonic_buffers(heap); std::pmr::unsynchronized_pool_resource pool(&monotonic_buffers); do_work(&pool);
63. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 63/110 Choosing a memory resource The header <memory_resource> offers: std::pmr::new_delete_resource std::pmr::null_resource std::pmr::synchronized_pool_resource std::pmr::unsynchronized_pool_resource std::pmr::monotonic_buffer_resource void do_work(std::pmr::polymorphic_allocator<> alloc); auto heap = std::pmr::new_delete_resource(); std::pmr::monotonic_buffer_resource monotonic_buffers(heap); std::pmr::unsynchronized_pool_resource pool(&monotonic_buffers); do_work(&pool); The allocator will use the pool to allocate memory.
64. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 64/110 Choosing a memory resource The header <memory_resource> offers: std::pmr::new_delete_resource std::pmr::null_resource std::pmr::synchronized_pool_resource std::pmr::unsynchronized_pool_resource std::pmr::monotonic_buffer_resource void do_work(std::pmr::polymorphic_allocator<> alloc); auto heap = std::pmr::new_delete_resource(); std::pmr::monotonic_buffer_resource monotonic_buffers(heap); std::pmr::unsynchronized_pool_resource pool(&monotonic_buffers); do_work(&pool); If the pool does not have any free memory, it allocates more via the monotonic buffer.
65. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 65/110 Choosing a memory resource The header <memory_resource> offers: std::pmr::new_delete_resource std::pmr::null_resource std::pmr::synchronized_pool_resource std::pmr::unsynchronized_pool_resource std::pmr::monotonic_buffer_resource void do_work(std::pmr::polymorphic_allocator<> alloc); auto heap = std::pmr::new_delete_resource(); std::pmr::monotonic_buffer_resource monotonic_buffers(heap); std::pmr::unsynchronized_pool_resource pool(&monotonic_buffers); do_work(&pool); If the monotonic buffer doesn’t have any memory, it gets more from the heap
66. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 66/110 A slight detour class tracing_resource final : public std::pmr::memory_resource { public: tracing_resource(std::ostream& os) : os_(os) {} private: void* do_allocate(size_t bytes, size_t align) override    { auto addr = operator new (bytes, std::align_val_t(align)); os_ << "allocate(" << bytes << ", " << align << ") -> " << addr << '\n'; return addr; } void do_deallocate(void* addr, size_t bytes, size_t align) override { os_ << "deallocate(" << addr << ", " << bytes << ", " << align << ")\n"; operator delete(addr, bytes, std::align_val_t(align)); } bool do_is_equal(const std::pmr::memory_resource& other) const noexcept override { return &other == this; } std::ostream& os_; };
67. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 67/110 A slight detour class tracing_resource final : public std::pmr::memory_resource { public: tracing_resource(std::ostream& os) : os_(os) {} private: void* do_allocate(size_t bytes, size_t align) override    { auto addr = operator new (bytes, std::align_val_t(align)); os_ << "allocate(" << bytes << ", " << align << ") -> " << addr << '\n'; return addr; } void do_deallocate(void* addr, size_t bytes, size_t align) override { os_ << "deallocate(" << addr << ", " << bytes << ", " << align << ")\n"; operator delete(addr, bytes, std::align_val_t(align)); } bool do_is_equal(const std::pmr::memory_resource& other) const noexcept override { return &other == this; } std::ostream& os_; }; Maybe going directly to the heap isn’t such a great idea?
68. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 68/110 Improving the tracing resource class tracing_resource final : public std::pmr::memory_resource { public: tracing_resource(std::ostream& os, std::pmr::memory_resource* next = std::pmr::get_default_resource()) : os_(os), next_(next) {} private: void* do_allocate(size_t bytes, size_t align) override    {   auto* addr = next_->allocate(bytes, align); os_ << "allocate(" << bytes << ", " << align << ") -> " << addr << '\n'; return addr; } void do_deallocate(void* addr, size_t bytes, size_t align) override { os_ << "deallocate(" << addr << ", " << bytes << ", " << align << ")\n"; next_->deallocate(addr, bytes, align); } bool do_is_equal(const std::pmr::memory_resource& other) const noexcept override { auto p = dynamic_cast<const tracing_resource*>(&other); return p && next_ == p->next_; } std::ostream& os_; std::pmr::memory_resource* next_; };
69. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 69/110 Improving the tracing resource class tracing_resource final : public std::pmr::memory_resource { public: tracing_resource(std::ostream& os, std::pmr::memory_resource* next = std::pmr::get_default_resource()) : os_(os), next_(next) {} private: void* do_allocate(size_t bytes, size_t align) override    {   auto* addr = next_->allocate(bytes, align); os_ << "allocate(" << bytes << ", " << align << ") -> " << addr << '\n'; return addr; } void do_deallocate(void* addr, size_t bytes, size_t align) override { os_ << "deallocate(" << addr << ", " << bytes << ", " << align << ")\n"; next_->deallocate(addr, bytes, align); } bool do_is_equal(const std::pmr::memory_resource& other) const noexcept override { auto p = dynamic_cast<const tracing_resource*>(&other); return p && next_ == p->next_; } std::ostream& os_; std::pmr::memory_resource* next_; };
70. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 70/110 Improving the tracing resource class tracing_resource final : public std::pmr::memory_resource { public: tracing_resource(std::ostream& os, std::pmr::memory_resource* next = std::pmr::get_default_resource()) : os_(os), next_(next) {} private: void* do_allocate(size_t bytes, size_t align) override    {   auto* addr = next_->allocate(bytes, align); os_ << "allocate(" << bytes << ", " << align << ") -> " << addr << '\n'; return addr; } void do_deallocate(void* addr, size_t bytes, size_t align) override { os_ << "deallocate(" << addr << ", " << bytes << ", " << align << ")\n"; next_->deallocate(addr, bytes, align); } bool do_is_equal(const std::pmr::memory_resource& other) const noexcept override { auto p = dynamic_cast<const tracing_resource*>(&other); return p && next_ == p->next_; } std::ostream& os_; std::pmr::memory_resource* next_; }; Get an underlying memory resource
71. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 71/110 Improving the tracing resource class tracing_resource final : public std::pmr::memory_resource { public: tracing_resource(std::ostream& os, std::pmr::memory_resource* next = std::pmr::get_default_resource()) : os_(os), next_(next) {} private: void* do_allocate(size_t bytes, size_t align) override    {   auto* addr = next_->allocate(bytes, align); os_ << "allocate(" << bytes << ", " << align << ") -> " << addr << '\n'; return addr; } void do_deallocate(void* addr, size_t bytes, size_t align) override { os_ << "deallocate(" << addr << ", " << bytes << ", " << align << ")\n"; next_->deallocate(addr, bytes, align); } bool do_is_equal(const std::pmr::memory_resource& other) const noexcept override { auto p = dynamic_cast<const tracing_resource*>(&other); return p && next_ == p->next_; } std::ostream& os_; std::pmr::memory_resource* next_; }; Use the default memory resource if none is given
72. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 72/110 Improving the tracing resource class tracing_resource final : public std::pmr::memory_resource { public: tracing_resource(std::ostream& os, std::pmr::memory_resource* next = std::pmr::get_default_resource()) : os_(os), next_(next) {} private: void* do_allocate(size_t bytes, size_t align) override    {   auto* addr = next_->allocate(bytes, align); os_ << "allocate(" << bytes << ", " << align << ") -> " << addr << '\n'; return addr; } void do_deallocate(void* addr, size_t bytes, size_t align) override { os_ << "deallocate(" << addr << ", " << bytes << ", " << align << ")\n"; next_->deallocate(addr, bytes, align); } bool do_is_equal(const std::pmr::memory_resource& other) const noexcept override { auto p = dynamic_cast<const tracing_resource*>(&other); return p && next_ == p->next_; } std::ostream& os_; std::pmr::memory_resource* next_; }; Use it to allocate and deallocate memory
73. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 73/110 PMR histogram class histogram { public: histogram(const std::pmr::polymorphic_allocator<>& alloc = {}): words_(alloc){} void add(const std::pmr::string& word) { ++words_[word]; } void print_top(size_t n, std::ostream& os) const { using count = std::pair<std::pmr::string, size_t>; std::vector<count> popular(words_.begin(), words_.end()); std::ranges::partial_sort(popular, popular.begin() + n, std::greater{}, &count::second); for (const auto& stat : popular | std::ranges::views::take(n)) { os << stat.first << '\t' << stat.second << '\n'; } } private: std::pmr::unordered_map<std::pmr::string, size_t> words_; };
74. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 74/110 PMR histogram class histogram { public: histogram(const std::pmr::polymorphic_allocator<>& alloc = {}): words_(alloc){} void add(const std::pmr::string& word) { ++words_[word]; } void print_top(size_t n, std::ostream& os) const { using count = std::pair<std::pmr::string, size_t>; std::vector<count> popular(words_.begin(), words_.end()); std::ranges::partial_sort(popular, popular.begin() + n, std::greater{}, &count::second); for (const auto& stat : popular | std::ranges::views::take(n)) { os << stat.first << '\t' << stat.second << '\n'; } } private: std::pmr::unordered_map<std::pmr::string, size_t> words_; }; Back to chosing an allocator for the word frequency histogram
75. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 75/110 PMR histogram class histogram { public: histogram(const std::pmr::polymorphic_allocator<>& alloc = {}): words_(alloc){} void add(const std::pmr::string& word) { ++words_[word]; } void print_top(size_t n, std::ostream& os) const { using count = std::pair<std::pmr::string, size_t>; std::vector<count> popular(words_.begin(), words_.end()); std::ranges::partial_sort(popular, popular.begin() + n, std::greater{}, &count::second); for (const auto& stat : popular | std::ranges::views::take(n)) { os << stat.first << '\t' << stat.second << '\n'; } } private: std::pmr::unordered_map<std::pmr::string, size_t> words_; }; int main() { std::pmr::string word; std::pmr::unsynchronized_pool_resource r; histogram hist(&r); while (std::cin >> word) { hist.add(word); } hist.print_top(5, std::cout); }
76. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 76/110 PMR histogram class histogram { public: histogram(const std::pmr::polymorphic_allocator<>& alloc = {}): words_(alloc){} void add(const std::pmr::string& word) { ++words_[word]; } void print_top(size_t n, std::ostream& os) const { using count = std::pair<std::pmr::string, size_t>; std::vector<count> popular(words_.begin(), words_.end()); std::ranges::partial_sort(popular, popular.begin() + n, std::greater{}, &count::second); for (const auto& stat : popular | std::ranges::views::take(n)) { os << stat.first << '\t' << stat.second << '\n'; } } private: std::pmr::unordered_map<std::pmr::string, size_t> words_; }; int main() { std::pmr::string word; std::pmr::unsynchronized_pool_resource r; histogram hist(&r); while (std::cin >> word) { hist.add(word); } hist.print_top(5, std::cout); } I choose an unsynchronized pool resource.
77. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 77/110 Live Demo
78. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 78/110 PMR histogram class histogram { public: histogram(const std::pmr::polymorphic_allocator<>& alloc = {}): words_(alloc){} void add(const std::pmr::string& word) { ++words_[word]; } void print_top(size_t n, std::ostream& os) const { using count = std::pair<std::pmr::string, size_t>; std::pmr::vector<count> popular(words_.begin(), words_.end(), words_.get_allocator()); std::ranges::partial_sort(popular, popular.begin() + n, std::greater{}, &count::second); for (const auto& stat : popular | std::ranges::views::take(n)) { os << stat.first << '\t' << stat.second << '\n'; } } private: std::pmr::unordered_map<std::pmr::string, size_t> words_; };
79. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 79/110 PMR histogram class histogram { public: histogram(const std::pmr::polymorphic_allocator<>& alloc = {}): words_(alloc){} void add(const std::pmr::string& word) { ++words_[word]; } void print_top(size_t n, std::ostream& os) const { using count = std::pair<std::pmr::string, size_t>; std::pmr::vector<count> popular(words_.begin(), words_.end(), words_.get_allocator()); std::ranges::partial_sort(popular, popular.begin() + n, std::greater{}, &count::second); for (const auto& stat : popular | std::ranges::views::take(n)) { os << stat.first << '\t' << stat.second << '\n'; } } private: std::pmr::unordered_map<std::pmr::string, size_t> words_; }; 84 allocations down from 133000
80. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 80/110 PMR histogram class histogram { public: histogram(const std::pmr::polymorphic_allocator<>& alloc = {}): words_(alloc){} void add(const std::pmr::string& word) { ++words_[word]; } void print_top(size_t n, std::ostream& os) const { using count = std::pair<std::pmr::string, size_t>; std::pmr::vector<count> popular(words_.begin(), words_.end(), words_.get_allocator()); std::ranges::partial_sort(popular, popular.begin() + n, std::greater{}, &count::second); for (const auto& stat : popular | std::ranges::views::take(n)) { os << stat.first << '\t' << stat.second << '\n'; } } private: std::pmr::unordered_map<std::pmr::string, size_t> words_; }; But you really need tools like heaptrack to find the places you’ve missed.
81. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 81/110 PMR for your types class counted_string { public: counted_string(size_t count, std::pmr::string str) : str_(std::move(str)) , count_(count)   {} std::string_view str() const { return str_; } size_t count() const { return count_; } friend bool operator<(const counted_string& lh, const counted_string& rh) { return lh.count_ < rh.count_; } protected: std::pmr::string str_; size_t count_; };
82. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 82/110 https://godbolt.org/z/8c4eKcxhh
83. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 83/110 PMR for your types A PMR enabled type needs:
84. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 84/110 PMR for your types A PMR enabled type needs: – A public allocator_type type
85. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 85/110 PMR for your types A PMR enabled type needs: – A public allocator_type type – Constructors with the allocator type as the last argument
86. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 86/110 PMR for your types A PMR enabled type needs: – A public allocator_type type – Constructors with the allocator type as the last argument – Variadic constructors with std::allocator_arg_t and allocator_type as the first parameters
87. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 87/110 PMR for your types A PMR enabled type needs: – A public allocator_type type – Constructors with the allocator type as the last argument – Variadic constructors with std::allocator_arg_t and allocator_type as the first parameters – And a copy-like constructor with allocator
88. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 88/110 PMR for your types A PMR enabled type needs: – A public allocator_type type – Constructors with the allocator type as the last argument – Variadic constructors with std::allocator_arg_t and allocator_type as the first parameters – And a copy-like constructor with allocator – And move-like constructor with allocator
89. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 89/110 PMR for your types class counted_string { public: using allocator_type = std::pmr::polymorphic_allocator<>; counted_string(size_t count,std::pmr::string str, const allocator_type& alloc = {}); template <typename ... Ts> counted_string(std::allocator_arg_t,  const allocator_type& alloc, size_t count, Ts&& ... ts); counted_string(const counted_string& orig); counted_string(const counted_string& orig, const allocator_type& alloc); counted_string(counted_string&& orig) noexcept; counted_string(counted_string&& orig, const allocator_type& alloc) noexcept; std::string_view str() const { return str_; } size_t count() const { return count_; } friend bool operator<(const counted_string& lh, const counted_string& rh); protected: std::pmr::string str_; size_t count_; };
90. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 90/110 PMR for your types class counted_string { public: using allocator_type = std::pmr::polymorphic_allocator<>; counted_string(size_t count,std::pmr::string str, const allocator_type& alloc = {}); template <typename ... Ts> counted_string(std::allocator_arg_t,  const allocator_type& alloc, size_t count, Ts&& ... ts); counted_string(const counted_string& orig); counted_string(const counted_string& orig, const allocator_type& alloc); counted_string(counted_string&& orig) noexcept; counted_string(counted_string&& orig, const allocator_type& alloc) noexcept; std::string_view str() const { return str_; } size_t count() const { return count_; } friend bool operator<(const counted_string& lh, const counted_string& rh); protected: std::pmr::string str_; size_t count_; }; Signal that this type uses a PMR allocator.
91. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 91/110 PMR for your types class counted_string { public: using allocator_type = std::pmr::polymorphic_allocator<>; counted_string(size_t count,std::pmr::string str, const allocator_type& alloc = {}); template <typename ... Ts> counted_string(std::allocator_arg_t,  const allocator_type& alloc, size_t count, Ts&& ... ts); counted_string(const counted_string& orig); counted_string(const counted_string& orig, const allocator_type& alloc); counted_string(counted_string&& orig) noexcept; counted_string(counted_string&& orig, const allocator_type& alloc) noexcept; std::string_view str() const { return str_; } size_t count() const { return count_; } friend bool operator<(const counted_string& lh, const counted_string& rh); protected: std::pmr::string str_; size_t count_; }; Constructor with allocator last
92. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 92/110 PMR for your types class counted_string { public: using allocator_type = std::pmr::polymorphic_allocator<>; counted_string(size_t count,std::pmr::string str, const allocator_type& alloc = {}); template <typename ... Ts> counted_string(std::allocator_arg_t,  const allocator_type& alloc, size_t count, Ts&& ... ts); counted_string(const counted_string& orig); counted_string(const counted_string& orig, const allocator_type& alloc); counted_string(counted_string&& orig) noexcept; counted_string(counted_string&& orig, const allocator_type& alloc) noexcept; std::string_view str() const { return str_; } size_t count() const { return count_; } friend bool operator<(const counted_string& lh, const counted_string& rh); protected: std::pmr::string str_; size_t count_; }; Variadic constructor with non-allocator args at end
93. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 93/110 PMR for your types class counted_string { public: using allocator_type = std::pmr::polymorphic_allocator<>; counted_string(size_t count,std::pmr::string str, const allocator_type& alloc = {}); template <typename ... Ts> counted_string(std::allocator_arg_t,  const allocator_type& alloc, size_t count, Ts&& ... ts); counted_string(const counted_string& orig); counted_string(const counted_string& orig, const allocator_type& alloc); counted_string(counted_string&& orig) noexcept; counted_string(counted_string&& orig, const allocator_type& alloc) noexcept; std::string_view str() const { return str_; } size_t count() const { return count_; } friend bool operator<(const counted_string& lh, const counted_string& rh); protected: std::pmr::string str_; size_t count_; }; Copy-like constructor with allocator, used by containers
94. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 94/110 PMR for your types class counted_string { public: using allocator_type = std::pmr::polymorphic_allocator<>; counted_string(size_t count,std::pmr::string str, const allocator_type& alloc = {}); template <typename ... Ts> counted_string(std::allocator_arg_t,  const allocator_type& alloc, size_t count, Ts&& ... ts); counted_string(const counted_string& orig); counted_string(const counted_string& orig, const allocator_type& alloc); counted_string(counted_string&& orig) noexcept; counted_string(counted_string&& orig, const allocator_type& alloc) noexcept; std::string_view str() const { return str_; } size_t count() const { return count_; } friend bool operator<(const counted_string& lh, const counted_string& rh); protected: std::pmr::string str_; size_t count_; }; Move-like constructor with allocator, used by containers
95. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 95/110 https://godbolt.org/z/aoYPWGxP8
96. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 96/110 In conclusion
97. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 97/110 In conclusion • Making your own types use PMR allocators is not hard, but it’s easy to make mistakes
98. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 98/110 In conclusion • Making your own types use PMR allocators is not hard, but it’s easy to make mistakes • Using PMR types with standard containers is easy
99. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 99/110 In conclusion • Making your own types use PMR allocators is not hard, but it’s easy to make mistakes • Using PMR types with standard containers is easy • PMR is very flexible because the memory resources and the types that uses them are decoupled
100. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 100/110 In conclusion • Making your own types use PMR allocators is not hard, but it’s easy to make mistakes • Using PMR types with standard containers is easy • PMR is very flexible because the memory resources and the types that uses them are decoupled • Memory usage is quite opaque, so it’s not easy to see where you’ve made mistakes
101. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 101/110 In conclusion • Making your own types use PMR allocators is not hard, but it’s easy to make mistakes • Using PMR types with standard containers is easy • PMR is very flexible because the memory resources and the types that uses them are decoupled • Memory usage is quite opaque, so it’s not easy to see where you’ve made mistakes • PMR is still not a panacea for your memory management needs, but it’s often sufficient
102. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 102/110 Just one more thing... http://mojtv.hr/images/fb2de057-6db9-45f8-ba8a-6891149a4a87.jpg
103. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 103/110 Just one more thing... http://mojtv.hr/images/fb2de057-6db9-45f8-ba8a-6891149a4a87.jpg A pmr container never changes its memory resource.
104. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 104/110 Just one more thing... http://mojtv.hr/images/fb2de057-6db9-45f8-ba8a-6891149a4a87.jpg A pmr container never changes its memory resource. Not even when move-assigning.
105. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 105/110 Just one more thing... http://mojtv.hr/images/fb2de057-6db9-45f8-ba8a-6891149a4a87.jpg A pmr container never changes its memory resource. Not even when move-assigning. • If resource operator== says the memory resources are the same, then the internal pointers are taken over, otherwise it’s element-wise move assignment to newly allocated memory.
106. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 106/110 Just one more thing... http://mojtv.hr/images/fb2de057-6db9-45f8-ba8a-6891149a4a87.jpg A pmr container never changes its memory resource. Not even when move-assigning. • If resource operator== says the memory resources are the same, then the internal pointers are taken over, otherwise it’s element-wise move assignment to newly allocated memory.
107. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 107/110 Just one more thing... http://mojtv.hr/images/fb2de057-6db9-45f8-ba8a-6891149a4a87.jpg A pmr container never changes its memory resource. Not even when move-assigning. • If resource operator== says the memory resources are the same, then the internal pointers are taken over, otherwise it’s element-wise move assignment to newly allocated memory.
108. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 108/110 Just one more thing... http://mojtv.hr/images/fb2de057-6db9-45f8-ba8a-6891149a4a87.jpg A pmr container never changes its memory resource. Not even when move-assigning. • If resource operator== says the memory resources are the same, then the internal pointers are taken over, otherwise it’s element-wise move assignment to newly allocated memory.
109. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 109/110 Learn more https://tinyurl.com/cppweekly-on-pmr YouTube playlist with 6 episodes of Jason Turner’s C++Weekly covering PMR https://youtu.be/RLezJuqNcEQ CppConn 2019 – Pablo Halpern and Alisdair Meredith, “Congratulations! You survived C++11 allocators!”
110. Mmmmm – NDC{TechTown} 2023 © Björn Fahller @bjorn_fahller@mastodon.social 110/110 bjorn@fahller.se @bjorn_fahller@mastodon.social @rollbear Björn Fahller Most Malleable Memory Management Method