DISCLAIMER: This code is a playground, it was not thorougly tested and most probably contains some nasty errors. DO NOT USE IT IN PRODUCTION!

Table of Contents

• MemPool
  • Details
    • Interface
  • How to build and run tests
    • Disclaimer
    • Toolset
    • Build steps

MemPool

This header-only C++17 library provides an implementation of a simple concurent lock-free memory pool of elements of a single type. The element type is passed as a first template parameter.

Details

Interface

template 
    < typename ElemT
    , unsigned short ChunkSize = 65534
    , typename Allocator = std::allocator<ElemT>
    >
class MemoryPool final
{
public:
    // construction
    MemoryPool();
    MemoryPool(Allocator& alloc);
    explicit MemoryPool(size_t initialCapacity, Allocator& alloc = Allocator());
    
    ~MemoryPool();

    // the pool is not copyable and not movable
    MemoryPool(const MemoryPool&) = delete;
    MemoryPool& operator=(const MemoryPool&) = delete;
    MemoryPool(MemoryPool&& other) = delete;
    MemoryPool& operator=(MemoryPool&&) = delete;

    // allocation (concurrent, lock-free[*])
    template<typename... Args> [[nodiscard]] 
    ElemT* alloc(Args&& ...args);

    // deallocation (concurrent, lock-free)
    void free(ElemT* elem) noexcept;

    // diagnostics (non-concurrent)
    static size_t chunk_size() noexcept;
    size_t chunk_count() const noexcept;
    size_t capacity() const noexcept;
    size_t allocated() const noexcept;
};

The memory pool is implemented as a single-linked list of fixed size chunks, where each of them contains a preallocated arena of elements. The number of elements in each chunk is passed as a second template parameter. The maximum (and default) chunk size if 65534 elements. The minimum chunk size is 2 elements, but you should avoid too small chunks as they have relatively large overhead.

When the pool's current capacity is exceeded, it automatically grows adding a new chunk. The pool never shrinks until being destroyed.

Arena chunks are allocated using a standard allocator. A custom allocator can be passed as a third template parameter.

Concurrency is supported on a chunk level by a lock-free free list (a ring buffer of indices).

[*] NOTE: allocation of a new chunk for pool growth is not lock-free, though it should happen not very often, and only once per a new chunk allocation.

The pool does not require the element type to be default constructible. The pool's alloc(Args&&...) method uses a variadic template to pass constructor parameters and returns a fully constructed element (or throws). The pool's free() method calls an element't destructor (the destructor must be noexcept).

Example:

#include "mempool.h"

class Widget { 
public:
    explicit Widget(std::string name);
    ...
};


...

// create a pool with a chunk size 1024, and a capacity reserved for 4096 elements (i.e. 4 chunks)
auto pool = MemoryPool<Shape, 1024> { 4096 };

Widget* widget = pool.alloc("Hello World!"s); // will call Widged(std::string)
...
pool.free(widget); // will call Widget::~Widget();

---

How to build and run tests

The library is header-only and does not require a build.

Unit tests (Google Test) can be built using CMake. It seems Google Test is not well integrated with MSBuild yet, hence no traditional .vcxproj/.sln are provided.

Disclaimer

This is my first experince using CMake and Google Test/Benchmark for Visual Studio, and Visual Studio's support of them is still flaky. Within a limited time frame for the project, I was not able to make Google Benchmark to automatically download with dependencies and build in VS, so the project uses Google Test only.*

Toolset

The project was created with the Microsoft Visual Studio 2017 v15.9.3.

Following Visual Studio 20017 components must be installed:

• Visual C++ tools for CMake and Linux
• Visual C++ for Linux Development
• Windows SDK 10 for Desktop [x86 and x64]

Build steps

To open the project in Visual Studio, run:

> cd MemPool
> devenv .

or start Visual Studio and use the FILE -> Open -> Folder main menu command.

When project is opened, select the x64-Release build configuration.

To configure the project and make InteliSense working, regenerate CMake cache when Visual Studio asks it or with the CMAKE -> Cache -> Generate [x64-Release] -> MemPool main menu command. Note that configuraton will download the Google Test library from their Gir repository.

To build the project use the CMAKE -> [Re]Build All main menu command, or right-click on a CMakeLists.txt and select Build menu command from a popup menu.

To run tests, right-click on tests/unit/UnitTest.cpp and select Set as startup item. Then run the UnitTest with the DEBUG -> Start [Without Debugging] main menu command (F5 or Ctrl-F5).