include/boost/capy/ex/recycling_memory_resource.hpp

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1			//
2			// Copyright (c) 2025 Vinnie Falco (vinnie.falco@gmail.com)
3			//
4			// Distributed under the Boost Software License, Version 1.0. (See accompanying
5			// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
6			//
7			// Official repository: https://github.com/cppalliance/capy
8			//
9
10			#ifndef BOOST_CAPY_RECYCLING_MEMORY_RESOURCE_HPP
11			#define BOOST_CAPY_RECYCLING_MEMORY_RESOURCE_HPP
12
13			#include <boost/capy/detail/config.hpp>
14
15			#include <bit>
16			#include <cstddef>
17			#include <memory_resource>
18			#include <mutex>
19
20			namespace boost {
21			namespace capy {
22
23			/** Recycling memory resource with size-class buckets.
24
25			This memory resource recycles memory blocks using power-of-two
26			size classes for O(1) allocation lookup. It maintains a thread-local
27			pool for fast lock-free access and a global pool for cross-thread
28			block sharing.
29
30			Size classes: 64, 128, 256, 512, 1024, 2048 bytes.
31			Allocations larger than 2048 bytes bypass the pools entirely.
32
33			This is the default allocator used by run_async when no allocator
34			is specified.
35
36			@par Thread Safety
37			Thread-safe. The thread-local pool requires no synchronization.
38			The global pool uses a mutex for cross-thread access.
39
40			@par Example
41			@code
42			auto* mr = get_recycling_memory_resource();
43			run_async(ex, mr)(my_task());
44			@endcode
45
46			@see get_recycling_memory_resource
47			@see run_async
48			*/
49			BOOST_CAPY_MSVC_WARNING_PUSH
50			BOOST_CAPY_MSVC_WARNING_DISABLE(4275) // non dll-interface base class
51			class BOOST_CAPY_DECL recycling_memory_resource : public std::pmr::memory_resource
52			{
53			static constexpr std::size_t num_classes = 6;
54			static constexpr std::size_t min_class_size = 64; // 2^6
55			static constexpr std::size_t max_class_size = 2048; // 2^11
56			static constexpr std::size_t bucket_capacity = 16;
57
58			static std::size_t
59		9930x	round_up_pow2(std::size_t n) noexcept
60			{
61	1/2 ✓ Branch 0 taken 9930 times. ✗ Branch 1 not taken.	9930x	return n <= min_class_size ? min_class_size : std::bit_ceil(n);
62			}
63
64			static std::size_t
65		9930x	get_class_index(std::size_t rounded) noexcept
66			{
67		9930x	std::size_t idx = std::countr_zero(rounded) - 6; // 64 = 2^6
68	1/2 ✓ Branch 0 taken 9930 times. ✗ Branch 1 not taken.	9930x	return idx < num_classes ? idx : num_classes;
69			}
70
71			struct bucket
72			{
73			std::size_t count = 0;
74			void* ptrs[bucket_capacity];
75
76		5072x	void* pop() noexcept
77			{
78	2/2 ✓ Branch 0 taken 107 times. ✓ Branch 1 taken 4965 times.	5072x	if(count == 0)
79		107x	return nullptr;
80		4965x	return ptrs[--count];
81			}
82
83			// Peter Dimov's idea
84		107x	void* pop(bucket& b) noexcept
85			{
86	1/2 ✓ Branch 0 taken 107 times. ✗ Branch 1 not taken.	107x	if(count == 0)
87		107x	return nullptr;
88		✗	for(std::size_t i = 0; i < count; ++i)
89		✗	b.ptrs[i] = ptrs[i];
90		✗	b.count = count - 1;
91		✗	count = 0;
92		✗	return b.ptrs[b.count];
93			}
94
95		4969x	bool push(void* p) noexcept
96			{
97	2/2 ✓ Branch 0 taken 4 times. ✓ Branch 1 taken 4965 times.	4969x	if(count >= bucket_capacity)
98		4x	return false;
99		4965x	ptrs[count++] = p;
100		4965x	return true;
101			}
102			};
103
104			struct pool
105			{
106			bucket buckets[num_classes];
107
108		67x	~pool()
109			{
110	2/2 ✓ Branch 0 taken 402 times. ✓ Branch 1 taken 67 times.	469x	for(auto& b : buckets)
111	2/2 ✓ Branch 0 taken 107 times. ✓ Branch 1 taken 402 times.	509x	while(b.count > 0)
112		107x	::operator delete(b.pop());
113		67x	}
114			};
115
116		10037x	static pool& local() noexcept
117			{
118	2/2 ✓ Branch 0 taken 34 times. ✓ Branch 1 taken 10003 times.	10037x	static thread_local pool p;
119		10037x	return p;
120			}
121
122			static pool& global() noexcept;
123			static std::mutex& global_mutex() noexcept;
124
125			void* allocate_slow(std::size_t rounded, std::size_t idx);
126			void deallocate_slow(void* p, std::size_t idx);
127
128			public:
129			~recycling_memory_resource();
130
131			/** Allocate without virtual dispatch.
132
133			Handles the fast path inline (thread-local bucket pop)
134			and falls through to the slow path for global pool or
135			heap allocation.
136			*/
137			void*
138		4965x	allocate_fast(std::size_t bytes, std::size_t)
139			{
140		4965x	std::size_t rounded = round_up_pow2(bytes);
141		4965x	std::size_t idx = get_class_index(rounded);
142	1/2 ✗ Branch 0 not taken. ✓ Branch 1 taken 4965 times.	4965x	if(idx >= num_classes)
143		✗	return ::operator new(bytes);
144		4965x	auto& lp = local();
145	2/2 ✓ Branch 1 taken 4858 times. ✓ Branch 2 taken 107 times.	4965x	if(auto* p = lp.buckets[idx].pop())
146		4858x	return p;
147		107x	return allocate_slow(rounded, idx);
148			}
149
150			/** Deallocate without virtual dispatch.
151
152			Handles the fast path inline (thread-local bucket push)
153			and falls through to the slow path for global pool or
154			heap deallocation.
155			*/
156			void
157		4965x	deallocate_fast(void* p, std::size_t bytes, std::size_t)
158			{
159		4965x	std::size_t rounded = round_up_pow2(bytes);
160		4965x	std::size_t idx = get_class_index(rounded);
161	1/2 ✗ Branch 0 not taken. ✓ Branch 1 taken 4965 times.	4965x	if(idx >= num_classes)
162			{
163		✗	::operator delete(p);
164		✗	return;
165			}
166		4965x	auto& lp = local();
167	2/2 ✓ Branch 1 taken 4961 times. ✓ Branch 2 taken 4 times.	4965x	if(lp.buckets[idx].push(p))
168		4961x	return;
169		4x	deallocate_slow(p, idx);
170			}
171
172			protected:
173			void*
174			do_allocate(std::size_t bytes, std::size_t) override;
175
176			void
177			do_deallocate(void* p, std::size_t bytes, std::size_t) override;
178
179			bool
180		✗	do_is_equal(const memory_resource& other) const noexcept override
181			{
182		✗	return this == &other;
183			}
184			};
185			BOOST_CAPY_MSVC_WARNING_POP
186
187			/** Returns pointer to the default recycling memory resource.
188
189			The returned pointer is valid for the lifetime of the program.
190			This is the default allocator used by run_async.
191
192			@return Pointer to the recycling memory resource.
193
194			@see recycling_memory_resource
195			@see run_async
196			*/
197			BOOST_CAPY_DECL
198			std::pmr::memory_resource*
199			get_recycling_memory_resource() noexcept;
200
201			} // namespace capy
202			} // namespace boost
203
204			#endif
205