Memory Management for C++

Both modern glibc and gperftools malloc implementations are quite fast and may show even better results for some use cases. However, FutoIn API provides futoin::IMemPool interface to support implementation-defined way to optimize object allocation and/or control memory usage.

IMemPool::Allocator<T> is compatible with standard C++ library allocator interface. It can be used in any container and standalone. Unless explicitly provided, a global thread-local pointer to IMemPool instance is used. It is set to standard heap implementation by default.

General idea is that IAsyncTool instance can set own custom memory pool for its event loop thread. Such memory pool may disable any synchronization overhead (~20-30% boost in some tests) and/or use dedicated pools per object size to minimize fragmentation of heap and improve performance.

It’s possible to create static object of IMemPool::Allocator<T>::EnsureOptimized type in any part of the program. This will trigger special logic to try to use special memory pools optimize for sizeof(T), if supported by implementation.

The minor drawback is that each container instance grows for sizeof(void*) as allocator instance holds pointer to associated memory pool instance. That’s not critical in most cases.

Usage

#include <futoin/imempool.hpp>

class MyMemPool : public IMemPool {};

// Define static object or call anywhere in program.
// All Allocators created after this initialization will try to
// request optimized memory pool.
IMemPool::Allocator<int>::EnsureOptimized ensure_int_allocation_is_optimized;

void usage_example() {
    MyMemPool my_mem_pool;
    futoin::GlobalMemPool::set_thread_default(my_mem_pool);

    using Alloc = futoin::IMemPool::Allocator<int>;
    
    // Use thread default 
    std::vector<int, Alloc> vec;
    
    // Other mem pool
    MyMemPool other_mem_pool;
    std::vector<int, Alloc> vec{Alloc(other_mem_pool)};
    
    // No RAII helper as it should not be used by business logic
    futoin::GlobalMemPool::reset_thread_default();
}

API

namespace futoin {
    struct IMemPool {
        
        //...

        virtual void* allocate(
                std::size_t object_size, std::size_t count) noexcept = 0;

        virtual void deallocate(
                void* ptr,
                std::size_t object_size,
                std::size_t count) noexcept = 0;

        // act as shrink_to_fit()
        virtual void release_memory() noexcept = 0;

        // Allocator uses it on construct
        virtual IMemPool& mem_pool(
                std::size_t /*object_size*/ = 1,
                bool /*optimize*/ = false) noexcept
        {
            return *this;
        }
        
        template<typename T>
        struct Allocator;
        
        //...
    };

    // Interface to manipulate threadlocal pointer.
    struct GlobalMemPool {
        inline static IMemPool& get_default() noexcept;
        inline static IMemPool& get_common() noexcept;
        inline static void set_thread_default(IMemPool& mem_pool) noexcept;
        inline static void reset_thread_default() noexcept;
    };
}