Interview: Advanced Memory Management and Optimization
===

https://chatgpt.com/share/c4dfd43c-15f6-4201-a318-7dde06c8e5e0
## Background

In this interview, we aim to assess the candidate's proficiency in advanced memory management and optimization techniques in C++. The focus will be on their ability to implement and use custom memory allocators in the STL, manage memory pools, understand and exploit memory models and alignment for performance, and apply advanced garbage collection techniques.
## Interview Questions

### Task 1: Custom Allocators in STL

1. **Question**: Implement a custom allocator for an STL container, such as `std::vector`, that allocates memory from a pre-allocated memory pool.

We define the arena's shared interface, which is following the cppreference: https://en.cppreference.com/w/cpp/memory/allocator

namespace scc
{
    namespace detail
    {
        void *allocate(std::size_t n) noexcept;
        void deallocate(void *p, std::size_t n) noexcept;
    }

    template <typename T>
    struct allocator
    {
        using value_type = T;
        using size_type = std::size_t;
        using difference_type = std::ptrdiff_t;
        using propagate_on_container_move_assignment = std::true_type;
        allocator() noexcept {}

        template <typename U>
        allocator(const allocator<U> &) noexcept {}

        ~allocator() = default;

        T *allocate(std::size_t n) noexcept { return static_cast<T *>(detail::allocate(n * sizeof(T))); }
        void deallocate(T *p, std::size_t n) noexcept { detail::deallocate(p, n * sizeof(T)); }

        template <typename U>
        struct rebind
        {
            using other = allocator<U>;
        };
    };
}

Please notice that we discard the deprecated interfaces. We are not expecting the user or the standard library use those deprecated interfaces.
And, the easiest custom allocator is just a trivial wrapper function, which helps us invoke the standard new/free operator indirectly.

Note that we invoke the `detail` functions to encapsulate the implementation details of real functions. This approach allows us to present a clean interface for interaction, treating the internal components as part of the library.

So we skip that trivial wrapper functions for this task.

### Task 2: Pool Allocation Techniques

2. **Question**: Design and implement a memory pool allocator. The allocator should be able to allocate and deallocate memory efficiently for objects of a fixed size.

We define a five-level arena, which works as a memory pool allocator and fulfills the requirements of efficiently allocating and deallocating memory for objects of a fixed size.

#### Overview of the 5-Level Arena Implementation
The implementation uses a hierarchical structure of arenas, each responsible for allocating fixed-size blocks of memory. The top-level arena manages multiple lower-level arenas, each with a decreasing order of block sizes. This structure allows efficient allocation and deallocation for varying sizes of objects within a certain range.

1. **Top-Level Arena**:
- Defined as `Arena<TopArenaOrder>` with `TopArenaOrder` set to 5.
- Manages multiple sub-arenas of order 4, which in turn manage sub-arenas of order 3, and so on, down to order 0.

2. **Order-Based Arena Structure**:
- Each `Arena<Order>` can allocate and deallocate memory blocks of size `1 << (Order * 4)` bytes.
- For example, `Arena<5>` manages blocks of size `1MB`, `Arena<4>` manages blocks of size `64KB`, and so on.

3. **Allocation Mechanism**:
- When memory is requested via `allocate()`, the arena checks if there are free blocks in the current arena.
- If no free blocks are available, a new sub-arena is created and added to the free list.
- The `acquire(size)` function is used to obtain a block of the requested size, traversing the sub-arenas if necessary.

4. **Deallocation Mechanism**:
- The `deallocate(p)` function returns the block to the free list of the corresponding arena.
- The `release(p)` function helps manage the memory by releasing the specified block back to the free list.

5. **Leaf Arena (Order 0)**:
- Special case where the arena directly manages individual blocks.
- Uses a `std::bitset` to track free and used blocks within the arena.

Here is the implementation:

// About 32 x 1MB, 32 x 64KB, 32 x 4KB, 32 x 256B, 32 x 16B, 32 x 8B
constexpr std::size_t TopArenaOrder = 5; 

template <size_t Order>
struct Arena
{
public:
    using ContainedType = Arena<Order - 1>;
    constexpr static std::size_t ContainedCount = 32;
    constexpr static std::size_t SlotSize = 1 << (Order * 4);
    constexpr static bool is_leaf = false;

    struct SubArena
    {
        Arena<Order - 1> *head = nullptr;
        Arena<Order - 1> *tail = nullptr;
        std::size_t size = 0;
    };

private:
    std::mutex mutex;
    SubArena used;
    SubArena free;

public:
    Arena() noexcept = default;
    ~Arena() { /*...*/ }
    ContainedType *allocate() noexcept
    { /*...*/ }
    void deallocate(ContainedType *p) noexcept
    { /*...*/ }
    bool contains(void *p) const noexcept
    { /*...*/ }
    void *acquire(size_t size) noexcept
    { /*...*/ }
    bool release(void *p) noexcept
    { /*...*/ }

    Arena<Order> *prev = nullptr;
    Arena<Order> *next = nullptr;
};

The reader may notice that our implementation extensively uses the `noexcept` specifier. This is because we aim to ensure that users do not encounter any unexpected exceptions. This is particularly critical in scenarios where failures are not acceptable, such as in powerhouse infrastructure.

#### Implementation Details

- **allocate**

ContainedType *allocate() noexcept
{
    std::lock_guard<std::mutex> lock(mutex);

    if (free.size == 0)
    {
        auto newArena = new (std::nothrow) ContainedType();
        if (!newArena)
            return nullptr;

        free.head = free.tail = newArena;
        free.size++;
    }

    auto allocated = free.head;
    free.head = free.head->next;
    if (free.head)
    {
        free.head->prev = nullptr;
    }
    free.size--;

    if (used.size == 0)
    {
        used.head = allocated;
        used.tail = allocated;
    }
    else
    {
        used.tail->next = allocated;
        allocated->prev = used.tail;
        used.tail = allocated;
    }
    used.size++;

    return allocated;
}

- The key points constrained the allocate member functionm:
- Locks the arena to ensure thread-safety.
- Checks for available free blocks or creates new sub-arenas if necessary.
- Moves allocated blocks to the used list and returns a pointer to the block.

- **deallocate**

void deallocate(ContainedType *p) noexcept
{
    if (!p)
        return;

    std::lock_guard<std::mutex> lock(mutex);
    if (p->prev)
        p->prev->next = p->next;
    if (p->next)
        p->next->prev = p->prev;

    if (used.head == p)
        used.head = p->next;
    if (used.tail == p)
        used.tail = p->prev;

    if (free.size == 0)
    {
        free.head = p;
        free.tail = p;
    }
    else
    {
        free.tail->next = p;
        p->prev = free.tail;
        free.tail = p;
    }
    free.size++;
}

- **contains**

bool contains(void *p) const noexcept
{
    std::lock_guard<std::mutex> lock(mutex);
    auto current = used.head;
    while (current)
    {
        if (current->contains(p))
            return true;
        current = current->next;
    }
    return false;
}

The function, `contains`, is a helper function to make the following `release` early return.

- **acquire**

void *acquire(size_t size) noexcept
{
    // it is locked by the allocate() and deallocate()
    if (size > SlotSize)
        return nullptr;

    {
        std::lock_guard<std::mutex> lock(mutex);
        auto current = used.head;
        while (current)
        {
            auto result = current->acquire(size);
            if (result)
                return result;
            current = current->next;
        }
    }

    if (auto allocated = allocate())
        return allocated->acquire(size);

    return nullptr;
}

- The acquire function's key points are:
- Checks if the requested size fits within the block size of the arena.
- Traverses sub-arenas to find a suitable block.
- Allocates a new block if necessary and returns a pointer to it.
- We should use this function inside the

### Task 3: Memory Models and Alignment

3. **Question**: Write a structure that is optimized for memory alignment. Explain the choices made to ensure alignment and how it impacts performance.

### Task 4: Advanced Garbage Collection Techniques

4. **Question**: Implement a custom smart pointer with a custom reference-counting scheme. The smart pointer should manage the lifetime of an object using advanced garbage collection techniques.