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C++: Custom memory allocation

By Tiago Costa | Published Apr 15 2013 02:07 PM in General Programming
Peer Reviewed by (GuardianX, incertia, Josh Vega)

c++ memory allocation

Fast memory allocations along with memory leak detection can have a big impact on games performance.

C++ provides two well known functions to allocate dynamic (heap) memory (malloc and new), these functions are usually very slow because they're general purpose functions and in some implementations require a context-switch from user mode into kernel mode. These functions also do not provide any kind of memory leak detection system natively.

Using custom allocators we can have well defined usage patterns and optimize the allocation process accordingly.

Full source code and performance tests in the attached file

Base Allocator


Every allocator in this articles series will be derived from the class Allocator that declares 2 virtual functions (allocate and deallocate) that must be defined by each allocator.

Allocator.h
class Allocator
{
public:
	Allocator(size_t size, void* start)
	{
		_start          = start;
		_size           = size;

		_used_memory     = 0;
		_num_allocations = 0;
	}

	virtual ~Allocator()
	{
		ASSERT(_num_allocations == 0 && _used_memory == 0);

		_start = nullptr;
		_size   = 0;
	}

	virtual void* allocate(size_t size, u8 alignment = 4) = 0;

	virtual void deallocate(void* p) = 0;

	void* getStart() const
	{
		return _start;
	}

	size_t getSize() const
	{
		return _size;
	}

	size_t getUsedMemory() const
	{
		return _used_memory;
	}

	size_t getNumAllocations() const
	{
		return _num_allocations;
	}

protected:
	void*         _start;
	size_t        _size;

	size_t        _used_memory;
	size_t        _num_allocations;
};

namespace allocator
{
	template <class T> T* allocateNew(Allocator& allocator)
	{
		return new (allocator.allocate(sizeof(T), __alignof(T))) T;
	}

	template <class T> T* allocateNew(Allocator& allocator, const T& t)
	{
		return new (allocator.allocate(sizeof(T), __alignof(T))) T(t);
	}

	template<class T> void deallocateDelete(Allocator& allocator, T& object)
	{
		object.~T();
		allocator.deallocate(&object);
	}

	template<class T> T* allocateArray(Allocator& allocator, size_t length)
	{
		ASSERT(length != 0);

		u8 headerSize = sizeof(size_t)/sizeof(T);

		if(sizeof(size_t)%sizeof(T) > 0)
			headerSize += 1;

		//Allocate extra space to store array length in the bytes before the array
		T* p = ( (T*) allocator.allocate(sizeof(T)*(length + headerSize), __alignof(T)) ) + headerSize;

		*( ((size_t*)p) - 1 ) = length;

		for(size_t i = 0; i < length; i++)
			new (&p[i]) T;

		return p;
	}

	template<class T> void deallocateArray(Allocator& allocator, T* array)
	{
		ASSERT(array != nullptr);

		size_t length = *( ((size_t*)array) - 1 );

		for(size_t i = 0; i < length; i++)
			array[i].~T();

		//Calculate how much extra memory was allocated to store the length before the array
		u8 headerSize = sizeof(size_t)/sizeof(T);

		if(sizeof(size_t)%sizeof(T) > 0)
			headerSize += 1;

		allocator.deallocate(array - headerSize);
	}
};

Memory leak detection


In the code above you can see an assert in the destructor, this is a simple and easy way to check if you forgot to deallocate any memory, that won't cause any overhead or take any extra memory.

This simple method won't tell which allocation you forgot to deallocate but it will pin point in which allocator the leak occured so you can find the leak faster (especially if you use Proxy Allocators like I suggest later in this article).

Aligned Allocations


Processors access memory in word-sized blocks, so when a processor tries to access memory in an unaligned address it might have to access more word-sized memory blocks than would be needed if the memory was aligned and perform masking/shifting to get the required data in the register.

Example:
A processor accesses memory in 4-byte words (it can only directly access the words starting at (0x00, 0x04, 0x08, 0x0C,...).

If it needs to access data (4 bytes) stored at the address 0x0B it will have to read two word-sized blocks (the address 0x08 and the address 0x0C) because the data crosses the boundary from one of the blocks to the other:

Attached Image: alignment.jpg

If the data was stored in an aligned address like 0x0C the processor could read it in a single memory access:

Attached Image: alignment2.jpg

Aligned data definition


Primitive data is said to be aligned if the memory address where it is stored is a multiple of the size of the primitive.

A data aggregate is said to be aligned if each primitive element in the aggregate is aligned.

Implementation


To n-byte align a memory address x we need to mask off the log2(n) least significant bits from x.

Simply masking off bits will return the first n-byte aligned address before x, so in order to find the first after x we just need to add alignment-1 to x and mask that address.

inline void* alignForward(void* address, u8 alignment)
{
	return (void*)( ( reinterpret_cast<uptr>(address) + static_cast<uptr>(alignment-1) ) & static_cast<uptr>(~(alignment-1)) );
}

It can be useful to calculate by how many bytes the address needs to adjusted to be aligned.

inline u8 alignForwardAdjustment(const void* address, u8 alignment)
{
    u8 adjustment =  alignment - ( reinterpret_cast<uptr>(address) & static_cast<uptr>(alignment-1) );
    
    if(adjustment == alignment)
        return 0; //already aligned
    
    return adjustment;
}

Some allocators need to store an header before each allocation so they can use the adjustment space to reduce the memory overhead caused by the headers.

inline u8 alignForwardAdjustmentWithHeader(const void* address, u8 alignment, u8 headerSize)
{
    u8 adjustment =  alignForwardAdjustment(address, alignment);
    
    u8 neededSpace = headerSize;
    
    if(adjustment < neededSpace)
    {
        neededSpace -= adjustment;
        
        //Increase adjustment to fit header
        adjustment += alignment * (neededSpace / alignment);
        
        if(neededSpace % alignment > 0)
            adjustment += alignment;
    }
    
    return adjustment;
}

Note:  The alignment must be a power of 2!


Linear Allocator


A Linear Allocator is the simplest and fastest type of allocator. Pointers to the start of the allocator, to the first free address and the total size of the allocator are maintained.

Allocations


New allocations simply move the pointer to the first free address forward.

Deallocations


Individual deallocations cannot be made in linear allocators, instead use clear() to completely clear the memory used by the allocator.

Implementation


LinearAllocator.h
#include "Allocator.h"

class LinearAllocator : public Allocator
{
public:
	LinearAllocator(size_t size, void* start);
	~LinearAllocator();

	void* allocate(size_t size, u8 alignment) override;
		
	void deallocate(void* p) override;

	void clear();

private:
	LinearAllocator(const LinearAllocator&); //Prevent copies because it might cause errors
	LinearAllocator& operator=(const LinearAllocator&);

	void* _current_pos;
};

LinearAllocator.cpp
#include "LinearAllocator.h"

LinearAllocator::LinearAllocator(size_t size, void* start) 
	: Allocator(size, start), _current_pos(start)
{
	ASSERT(size > 0);
}

LinearAllocator::~LinearAllocator()
{
	_current_pos   = nullptr;
}

void* LinearAllocator::allocate(size_t size, u8 alignment)
{
	ASSERT(size != 0);

	u8 adjustment =  pointer_math::alignForwardAdjustment(_current_pos, alignment);

	if(_used_memory + adjustment + size > _size)
		return nullptr;

	uptr aligned_address = (uptr)_current_pos + adjustment;

	_current_pos = (void*)(aligned_address + size);

	_used_memory += size + adjustment;
	_num_allocations++;

	return (void*)aligned_address;
}

void LinearAllocator::deallocate(void* p)
{
	ASSERT( false && "Use clear() instead" );
}

void LinearAllocator::clear()
{
	_num_allocations     = 0;
	_used_memory         = 0;

	_current_pos   = _start;
}

Stack Allocator


A Stack Allocator, like the name says, works like a stack. Along with the stack size, three pointers are maintained:
  • Pointer to the start of the stack.
  • Pointer to the top of the stack.
  • Pointer to the last allocation made. (This is optional in release builds)

Allocations


New allocations move the pointer up by the requested number of bytes plus the adjustment needed to align the address and store the allocation header.

The allocation header provides the following information:
  • Adjustment used in this allocation
  • Pointer to the previous allocation.

Deallocations


Note:  Memory must be deallocated in inverse order it was allocated! So if you allocate object A and then object B you must free object B memory before you can free object A memory.


To deallocate memory the allocator checks if the address to the memory that you want to deallocate corresponds to the address of the last allocation made.

If so the allocator accesses the allocation header so it also frees the memory used to align the allocation and store the allocation header, and it replaces the pointer to the last allocation made with the one in the allocation header.

Implementation


StackAllocator.h
#include "Allocator.h"

class StackAllocator : public Allocator
{
public:
	StackAllocator(size_t size, void* start);
	~StackAllocator();

	void* allocate(size_t size, u8 alignment) override;
		
	void deallocate(void* p) override;

private:
	StackAllocator(const StackAllocator&); //Prevent copies because it might cause errors
	StackAllocator& operator=(const StackAllocator&);

	struct AllocationHeader
	{
		#if _DEBUG
		void* prev_address;
		#endif
		u8 adjustment;
	};

	#if _DEBUG
	void* _prev_position;
	#endif

	void*  _current_pos;
};

StackAllocator.cpp
#include "StackAllocator.h"

StackAllocator::StackAllocator(size_t size, void* start) 
	: Allocator(size, start), _current_pos(start)
{
	ASSERT(size > 0);

	#if _DEBUG
	_prev_position    = nullptr;
	#endif
}

StackAllocator::~StackAllocator()
{
	#if _DEBUG
	_prev_position      = nullptr;
	#endif

	_current_pos   = nullptr;
}

void* StackAllocator::allocate(size_t size, u8 alignment)
{
	ASSERT(size != 0);

	u8 adjustment = pointer_math::alignForwardAdjustmentWithHeader(_current_pos, alignment, sizeof(AllocationHeader));

	if(_used_memory + adjustment + size > _size)
		return nullptr;

	void* aligned_address = pointer_math::add(_current_pos, adjustment);

	//Add Allocation Header
	AllocationHeader* header = (AllocationHeader*)(pointer_math::subtract(aligned_address, sizeof(AllocationHeader)));

	header->adjustment   = adjustment;

	#if _DEBUG
	header->prev_address = _prev_position;

	_prev_position        = aligned_address;
	#endif

	_current_pos = pointer_math::add(aligned_address, size);

	_used_memory += size + adjustment;
	_num_allocations++;

	return aligned_address;
}

void StackAllocator::deallocate(void* p)
{
	ASSERT( p == _prev_position );

	//Access the AllocationHeader in the bytes before p
	AllocationHeader* header = (AllocationHeader*)(pointer_math::subtract(p, sizeof(AllocationHeader)));

	_used_memory -= (uptr)_current_pos - (uptr)p + header->adjustment;

	_current_pos = pointer_math::subtract(p, header->adjustment);

	#if _DEBUG
	_prev_position = header->prev_address;
	#endif

	_num_allocations--;
}

Note:  Storing the last previous allocations in a list-like fashion and checking it before deallocations is not mandatory so it can be disabled in release builds. It's just helpful to prevent memory from being overwritten causing bugs.



FreeList Allocator


The FreeList allocator allows allocations of any size to be made (inside the available memory) and deallocations in any order.

A linked-list of free blocks of memory is maintained (each free block contains information about its size and a pointer to the next free block).

Allocations


The allocator tries to find a free block large enough for the allocation to fit, if it finds multiple free blocks that meet the requeriments, there's 3 simple ways to decide which free block to choose:

  • First-fit - Use the first.
  • Best-fit - Use the smallest.
  • Worst-fit - Use the largest.

Note:  The best-fit method will in most cases cause less fragmentation than the other 2 methods.



In the example implementation below I use the first-fit method.

Deallocation


The allocator keeps the free blocks orderer by the start position.

When an allocation is freed the allocator finds the right position in the free blocks list and tries to merge it with the adjacent blocks.

Implementation


FreeListAllocator.h
#include "Allocator.h"

class FreeListAllocator : public Allocator
{
public:
	FreeListAllocator(size_t size, void* start);
	~FreeListAllocator();

	void* allocate(size_t size, u8 alignment) override;
		
	void deallocate(void* p) override;

private:

	struct AllocationHeader
	{
		size_t size;
		u8     adjustment;
	};

	struct FreeBlock
	{
		size_t     size;
		FreeBlock* next;
	};

	FreeListAllocator(const FreeListAllocator&); //Prevent copies because it might cause errors
	FreeListAllocator& operator=(const FreeListAllocator&);

	FreeBlock* _free_blocks;
};

FreeListAllocator.cpp
#include "FreeListAllocator.h"

FreeListAllocator::FreeListAllocator(size_t size, void* start) 
	: Allocator(size, start), _free_blocks((FreeBlock*)start)
{
	ASSERT(size > sizeof(FreeBlock));

	_free_blocks->size  = size;
	_free_blocks->next = nullptr;
}

FreeListAllocator::~FreeListAllocator()
{
	_free_blocks        = nullptr;
}

void* FreeListAllocator::allocate(size_t size, u8 alignment)
{
	ASSERT(size != 0 && alignment != 0);

	FreeBlock* prev_free_block = nullptr;
	FreeBlock* free_block     = _free_blocks;

	while(free_block != nullptr)
	{
		//Calculate adjustment needed to keep object correctly aligned
		u8 adjustment = pointer_math::alignForwardAdjustmentWithHeader(free_block, alignment, sizeof(AllocationHeader));

		size_t total_size = size + adjustment;

		//If allocation doesn't fit in this FreeBlock, try the next
		if(free_block->size < total_size)
		{
			prev_free_block = free_block;
			free_block = free_block->next;
			continue;
		}

		static_assert(sizeof(AllocationHeader) >= sizeof(FreeBlock), "sizeof(AllocationHeader) < sizeof(FreeBlock)");

		//If allocations in the remaining memory will be impossible
		if(free_block->size - total_size <= sizeof(AllocationHeader))
		{
			//Increase allocation size instead of creating a new FreeBlock
			total_size = free_block->size;

			if(prev_free_block != nullptr)
				prev_free_block->next = free_block->next;
			else
				_free_blocks = free_block->next;
		}
		else
		{
			//Else create a new FreeBlock containing remaining memory
			FreeBlock* next_block = (FreeBlock*)( pointer_math::add(free_block, total_size) );
			next_block->size = free_block->size - total_size;
			next_block->next = free_block->next;
			
			if(prev_free_block != nullptr)
				prev_free_block->next = next_block;
			else
				_free_blocks = next_block;
		}

		uptr aligned_address = (uptr)free_block + adjustment;
	
		AllocationHeader* header = (AllocationHeader*)(aligned_address - sizeof(AllocationHeader));
		header->size             = total_size;
		header->adjustment       = adjustment;

		_used_memory += total_size;
		_num_allocations++;

		ASSERT(pointer_math::alignForwardAdjustment((void*)aligned_address, alignment) == 0);

		return (void*)aligned_address;
	}

	//ASSERT(false && "Couldn't find free block large enough!");

	return nullptr;
}

void FreeListAllocator::deallocate(void* p)
{
	ASSERT(p != nullptr);

	AllocationHeader* header = (AllocationHeader*)pointer_math::subtract(p, sizeof(AllocationHeader));

	uptr   block_start = reinterpret_cast<uptr>(p) - header->adjustment;
	size_t block_size  = header->size;
	uptr   block_end   = block_start + block_size;

	FreeBlock* prev_free_block = nullptr;
	FreeBlock* free_block = _free_blocks;

	while(free_block != nullptr)
	{
		if( (uptr) free_block >= block_end )
			break;

		prev_free_block = free_block;
		free_block = free_block->next;
	}

	if(prev_free_block == nullptr)
	{
		prev_free_block = (FreeBlock*) block_start;
		prev_free_block->size = block_size;
		prev_free_block->next = _free_blocks;

		_free_blocks = prev_free_block;
	} else if((uptr) prev_free_block + prev_free_block->size == block_start)
	{
		prev_free_block->size += block_size;
	} else
	{
		FreeBlock* temp = (FreeBlock*) block_start;
		temp->size = block_size;
		temp->next = prev_free_block->next;
		prev_free_block->next = temp;

		prev_free_block = temp;
	}

	if( free_block != nullptr && (uptr) free_block == block_end)
	{
		prev_free_block->size += free_block->size;
		prev_free_block->next = free_block->next;
	}

	_num_allocations--;
	_used_memory -= block_size;
}

Pool Allocator


This allocator only allows allocations of a fixed size and alignment to be made, this results in both fast allocations and deallocations to be made.

Like the FreeList allocator, a linked-list of free blocks is maintaied but since all blocks are the same size each free block only needs to store a pointer to the next one.

Another advantage of Pool allactors is no need to align each allocation, since all allocations have the same size/alignment only the first block has to be aligned, this results in a almost non-existant memory overhead.

Note:  
The block size of the Pool Allocator must be larger than sizeof(void*) because when blocks are free they store a pointer to the next free block in the list.



Allocations


The allocator simply returns the first free block and updates the linked list.

Deallocations


The allocator simply adds the deallocated block to the free blocks linked list.

Implementation


PoolAllocator.h
#include "Allocator.h"

class PoolAllocator : public Allocator
{
public:
	PoolAllocator(size_t objectSize, u8 objectAlignment, size_t size, void* mem);
	~PoolAllocator();

	void* allocate(size_t size, u8 alignment) override;
		
	void deallocate(void* p) override;

private:
	PoolAllocator(const PoolAllocator&); //Prevent copies because it might cause errors
	PoolAllocator& operator=(const PoolAllocator&);

	size_t     _objectSize;
	u8         _objectAlignment;

	void**     _free_list;
};

PoolAllocator.cpp
#include "PoolAllocator.h"

PoolAllocator::PoolAllocator(size_t objectSize, u8 objectAlignment, size_t size, void* mem) 
	: Allocator(size, mem), _objectSize(objectSize), _objectAlignment(objectAlignment)
{
	ASSERT(objectSize >= sizeof(void*));

	//Calculate adjustment needed to keep object correctly aligned
	u8 adjustment = pointer_math::alignForwardAdjustment(mem, objectAlignment);

	_free_list = (void**)pointer_math::add(mem, adjustment);

	size_t numObjects = (size-adjustment)/objectSize;

	void** p = _free_list;

	//Initialize free blocks list
	for(size_t i = 0; i < numObjects-1; i++)
	{
		*p = pointer_math::add(p, objectSize );
		p = (void**) *p;
	}

	*p = nullptr;
}

PoolAllocator::~PoolAllocator()
{
	_free_list = nullptr;
}

void* PoolAllocator::allocate(size_t size, u8 alignment)
{
	ASSERT(size == _objectSize && alignment == _objectAlignment);

	if(_free_list == nullptr)
		return nullptr;

	void* p = _free_list;

	_free_list = (void**)(*_free_list);

	_used_memory += size;
	_num_allocations++;

	return p;
}

void PoolAllocator::deallocate(void* p)
{
	*((void**)p) = _free_list;

	_free_list = (void**)p;

	_used_memory -= _objectSize;
	_num_allocations--;
}

Proxy Allocator


A Proxy Allocator is a special kind of allocator. It is just used to help with memory leak and subsystem memory usage tracking.

It will simply redirect all allocations/deallocations to the allocator passed as argument in the constructor while keeping track of how many allocations it made and how much memory it is "using".

Example:
Two subsystems use the same allocator A.
If you want to show in the debugging user interface how much memory each subsystem is using, you create a proxy allocator, that redirects all allocations/deallocations to A, in each subsystem and track their memory usage.

It will also help in memory leak tracking because the assert in the proxy allocator destructor of the subsystem that is leaking memory will fail.

Implementation


ProxyAllocator.h
#include "Allocator.h"

class ProxyAllocator : public Allocator
{
public:
	ProxyAllocator(Allocator& allocator);
	~ProxyAllocator();

	void* allocate(size_t size, u8 alignment) override;
		
	void deallocate(void* p) override;

private:
	ProxyAllocator(const ProxyAllocator&); //Prevent copies because it might cause errors
	ProxyAllocator& operator=(const ProxyAllocator&);

	Allocator& _allocator;
};

ProxyAllocator.cpp
#include "ProxyAllocator.h"

ProxyAllocator::ProxyAllocator(Allocator& allocator)
	: Allocator(allocator.getSize(), allocator.getStart()), _allocator(allocator)
{

}

ProxyAllocator::~ProxyAllocator()
{

}

void* ProxyAllocator::allocate(size_t size, u8 alignment)
{
	ASSERT(size != 0);
	_num_allocations++;

	size_t mem = _allocator.getUsedMemory();

	void* p = _allocator.allocate(size, alignment);

	_used_memory += _allocator.getUsedMemory() - mem;

	return p;
}

void ProxyAllocator::deallocate(void* p)
{
	_num_allocations--;

	size_t mem = _allocator.getUsedMemory();

	_allocator.deallocate(p);

	_used_memory -= mem - _allocator.getUsedMemory();
}

Allocator Managment


A large block of memory should be allocated when the program starts using malloc (and this should be the only malloc made) this large block of memory is managed by a global allocator (for example a stack allocator).

Each subsystem should then allocate the block of memory it needs to work from the global allocator, and create allocators that will manage that memory.

Example usage


  • Allocate 1GB of memory using malloc and create a FreeList allocator to manage that memory.
  • Create a Proxy allocator that redirects all allocations to the FreeList allocator.
  • Initialize the Resource Manager by passing a pointer to the Proxy allocator in the constructor.
  • Register the Proxy allocator in the memory usage tracker so it shows how much memory the Resource Manager is using.
  • Allocate 16MB of memory using the FreeList allocator and create a Linear allocator to manage that memory and register it in the memory usage tracker.
  • Use the Linear allocator to make small temporary allocations needed for game logic, etc, and clear it before the end of each frame.
  • The Resource Manager will create a Pool allocator for every ResourcePackage it loads.

Tips & Tricks


  • Depending on the type of allocator, keep the number of individual allocations to a minimum to reduce the memory wasted by allocation headers.
  • Prefer using allocateArray() to individual allocations when it makes sense. Most allocators will use extra memory in each allocation to store allocation headers and arrays will only need single header.
  • Instead of making small size allocations from allocators with large amounts of memory available, allocate a single memory block capable of holding all the small allocations and create a new allocator to manage the memory block and make the small allocations from this block.

Performance Comparison


To test the performance of each allocator compared to malloc I wrote a program that measures how long it takes to make 20000 allocations (you can download the program in the end of the article), the tests where made in release mode and the results are averages of 3 runs.

Malloc vs Linear Allocator


10k 16 bytes allocations + 1k 256 bytes allocations + 50 2Mb allocations/deallocations (allocations made using the linear allocator are deallocated in a single call to clear().
AllocatorTime (s)
Malloc0.639655
Linear0.000072

Malloc vs Stack Allocator


10k 16 bytes allocations + 1k 256 bytes allocations + 50 2Mb allocations/deallocations
AllocatorTime (s)
Malloc0.650435
Stack0.000289

Malloc vs FreeList Allocator


10k 16 bytes allocations + 1k 256 bytes allocations + 50 2Mb allocations/deallocations
AllocatorTime (s)
Malloc0.673865
FreeList0.000414

Malloc vs Pool Allocator


20k 16 bytes allocations/deallocations
AllocatorTime (s)
Malloc1.454934
Pool0.000193

Conclusion


There isn't a single best allocator - it's important to think about how the memory will be allocated/accessed/deallocated and choose the right allocator for each situation.

Full source code and performance tests in the attached file

Reference


http://bitsquid.blogspot.pt/2010/09/custom-memory-allocation-in-c.html
Game Engine Architecture, Jason Gregory 2009
http://molecularmusings.wordpress.com/2011/07/05/memory-system-part-1/


Article Update Log


04 March 2014: Rewritten FreeListAllocator, added support to x64. (Also updated code style and added a new test).
16 November 2013: Fixed two errors in FreeListAllocator.cpp lines 48 and 60
14 April 2013: Fixed an error in allocateArray
13 April 2013: FreeList and Pool allocators added
12 April 2013: Performance comparison & test project added
11 April 2013: Initial release





About the Author(s)


My name is Tiago Costa and I'm currently studying Computer Science at FCUP, Portugal and working on an indie project as a graphics/game programmer.

License


GDOL (Gamedev.net Open License)




Comments

I hate to nitpick, but the example memory addresses look like they should be hexadecimal, but aren't. 0x12 should be 0x0C, etc.

 

 

 

Otherwise, it's a decent read so far.

I hate to nitpick, but the example memory addresses look like they should be hexadecimal, but aren't. 0x12 should be 0x0C, etc.

 

 

 

Otherwise, it's a decent read so far.

 

Good catch.. 

It is forbidden to start identifier names with an underscore followed by a capital letter (e.g. _UsedMemory, _NumAllocations).

Also, leak detection (ASSERT(_NumAllocations == 0 && _UsedMemory == 0);) should be delegated to derived classes, since there is no guarantee that a derived class would update these variables.

nitpick:

the implication that "new" and "malloc" involve (always) going into the kernel isn't really correct (with most sane implementations, luckily they aren't that bad).

 

these will typically operate in-process and maintain their own free-lists, and typically grab/release "large" (say: 256kB or 1MB) chunks of memory from the OS via lower-level system calls (typically things like "mmap()" or "VirtualAlloc()").

 

system calls are not used in cases where the needed memory is already in the free-list, or is being returned to the free-list.

 

so, the only times these will usually make system calls is if more memory needs to be grabbed from the OS, or if a particularly large memory object has just been allocated or freed (such as large array or similar).

 

ADD: issue seems have been partly fixed.

The article isn't finished!

 

I still have to write about FreeList and Pool allocators, upload the code and update the performance tables (these are only placeholders), etc

 

It is forbidden to start identifier names with an underscore followed by a capital letter (e.g. _UsedMemory, _NumAllocations).

Also, leak detection (ASSERT(_NumAllocations == 0 && _UsedMemory == 0);) should be delegated to derived classes, since there is no guarantee that a derived class would update these variables.

I wasn't aware it was forbidden.

If the derived classes do not update these variables, maybe that's because they provide their own leak detection system?

 

I hate to nitpick, but the example memory addresses look like they should be hexadecimal, but aren't. 0x12 should be 0x0C, etc.

 

 

 

Otherwise, it's a decent read so far.

Good catch indeed, I can't believe I missed that...

If this is implemented via actual polymorphism as shown so far, then ~Allocator must be declared virtual. Aside from that, a lot of the 'work' done in the allocator destructors seems useless. Setting values to 0/nullptr would be at best useful as a debugging help - but the MSVC debug runtime for example would already overwrite the memory immediately afterwards anyway.

Just wanted to point out that there's two bugs in the implementation.

 

The first one is in

template<class T> T* allocateArray(uint maxNumObjects):

 

Allocating sizeof(T)*maxNumObjects is not enough for arrays of non-PODs! Most compilers (all compilers I've ever worked with, including consoles/handhelds) store the number of instances right before the start of the array. So in most cases (depending on the type T) you might need sizeof(T)*maxNumObjects + sizeof(uint32_t), but you can never be too sure about that.

 

I've written about that exact problem in more detail on my blog:

http://molecularmusings.wordpress.com/2011/07/05/memory-system-part-1/

 

If you use the implementation above, you'll overwrite memory as soon as you want to allocate an array of non-PODs. This can easily be verified using the memory window of your favourite debugger.

 

The second bug is in

template<class T> void deallocateDelete(T* pObject):

 

This one also has to do with arrays, namely that the destructors for the instances in the array aren't called. You need to call the destructors of all instances in reverse order. Again, for reasons stated above this can be tricky, because you need to know the number of instances stored in the array.

 

I know that the article isn't finished yet, just wanted to give you a heads-up :).

It is forbidden to start identifier names with an underscore followed by a capital letter (e.g. _UsedMemory, _NumAllocations).

Also, leak detection (ASSERT(_NumAllocations == 0 && _UsedMemory == 0);) should be delegated to derived classes, since there is no guarantee that a derived class would update these variables.

 

Starting functions, etc with an underscore is generally frowned upon, but it is not forbidden.  Most compilers reserve the prefix underscore.

FWIW:

in my case I am actually using spans of "cells" (currently 16-byte) and bitmaps, for small objects in my allocator (also a GC also does leak detection and sometimes helps detect overruns). free-lists are also used for small-objects (based on the objects' cell-count), before scanning for free-cells in the bitmap. currently, memory for this is allocated in chunks of 16MB on desktop.

 

this is mostly for objects < 6KiB, past 6KiB and it resorts to a more traditional allocation strategy (free lists / etc). this is partly because, at least in my case, small objects tend to dominate in terms of total memory allocations.

 

past around 6kB, and the bitmaps become expensive (and a block-carving first-fit strategy becomes preferable, up until around several MB, where raw pages become preferable).

 

IME: the 16KiB-48KiB range tends to dominate in terms of total memory usage, but drops off quickly as objects get larger.

 

granted, I guess also possible could be using larger cells (say, 512 byte), then use exact-sized free-lists up to 1MB or similar (6KiB - 1MB), before resorting to OS pages.

Also, leak detection (ASSERT(_NumAllocations == 0 && _UsedMemory == 0);) should be delegated to derived classes, since there is no guarantee that a derived class would update these variables.

 

If the derived classes do not update these variables, maybe that's because they provide their own leak detection system?

That's why the base class shouldn't assume anything about leak detection.

It is forbidden to start identifier names with an underscore followed by a capital letter (e.g. _UsedMemory, _NumAllocations).

Also, leak detection (ASSERT(_NumAllocations == 0 && _UsedMemory == 0);) should be delegated to derived classes, since there is no guarantee that a derived class would update these variables.

 

Starting functions, etc with an underscore is generally frowned upon, but it is not forbidden.  Most compilers reserve the prefix underscore.

Starting with an underscore and a lower case letter is permitted, but an underscore followed by a capital letter is reserved for the compiler implementation.

 

It is forbidden to start identifier names with an underscore followed by a capital letter (e.g. _UsedMemory, _NumAllocations).

Also, leak detection (ASSERT(_NumAllocations == 0 && _UsedMemory == 0);) should be delegated to derived classes, since there is no guarantee that a derived class would update these variables.

 

Starting functions, etc with an underscore is generally frowned upon, but it is not forbidden.  Most compilers reserve the prefix underscore.

Starting with an underscore and a lower case letter is permitted, but an underscore followed by a capital letter is reserved for the compiler implementation.

 

both "_Capital" and "__whatever" are reserved. more often though, "_Capital" is used for the language-standards to add more keywords, where it is more "__whatever" being used by the compilers (for example: "_Complex" or "_Generic" vs "__declspec" or "__attribute__").

 

 

It is forbidden to start identifier names with an underscore followed by a capital letter (e.g. _UsedMemory, _NumAllocations).

Also, leak detection (ASSERT(_NumAllocations == 0 && _UsedMemory == 0);) should be delegated to derived classes, since there is no guarantee that a derived class would update these variables.

 

Starting functions, etc with an underscore is generally frowned upon, but it is not forbidden.  Most compilers reserve the prefix underscore.

Starting with an underscore and a lower case letter is permitted, but an underscore followed by a capital letter is reserved for the compiler implementation.

 

both "_Capital" and "__whatever" are reserved. more often though, "_Capital" is used for the language-standards to add more keywords, where it is more "__whatever" being used by the compilers (for example: "_Complex" or "_Generic" vs "__declspec" or "__attribute__").

Agreed.

Your math seems to be off in allocateArray. Consider a case with e.g. sizeof(T) == 8:

headerSize will be zero, so you don't allocate extra space for storing the array length, effectively overwriting memory.

 

Why not just always allocate an extra 4 bytes in order to store the length?

You could later always make an optimization and only store the length for types T that actually need to have their destructors called (similar to how the compiler does it, POD vs. non-POD).

Your math seems to be off in allocateArray. Consider a case with e.g. sizeof(T) == 8:
headerSize will be zero, so you don't allocate extra space for storing the array length, effectively overwriting memory.

 
Yes I had a mistake in the line if(sizeof(T)%sizeof(u32) > 0) it should be if(sizeof(u32)%sizeof(T) > 0). It's now fixed. Thanks.

 

Why not just always allocate an extra 4 bytes in order to store the length?

Because if the alignment is larger than 4 bytes it would unalign the array.

Why not just always allocate an extra 4 bytes in order to store the length?

Because if the alignment is larger than 4 bytes it would unalign the array.

 

Well, you could always just make the allocate() method aware of that by passing an additional "offset" argument, so that "result ptr + offset" matches the alignment. This also has the benefit that you in general waste less space due to alignment.

Granted, it's more work for the individual allocators though.

Your math seems to be off in allocateArray. Consider a case with e.g. sizeof(T) == 8:
headerSize will be zero, so you don't allocate extra space for storing the array length, effectively overwriting memory.

 
Yes I had a mistake in the line if(sizeof(T)%sizeof(u32) > 0) it should be if(sizeof(u32)%sizeof(T) > 0). It's now fixed. Thanks.

 

You're welcome.

You should still call the destructors of the array's instances in reverse order to be correct, though. Hopefully nobody ever relies on that, but you never know :).

Hi! Thanks to the author for the article!!!
Recently, I was concerned about finding memory leaks in my code. Other programmers advised to use different  debuggers  (vld, memcheck, deleaker, insure...) Some were advised not to use dynamic memory. But the method proposed in the article is different, very simple and I hope, effective enough.
Thanks, it is very helpful to me, +1 to the author

I read an article about smart pointers. I have convinced that using of smart pointers prevents errors and memory leaks.

I read an article about smart pointers. I have convinced that using of smart pointers prevents errors and memory leaks.

Smart pointers are a useful feature, but they aren't a solution to every problem.  If every single one of your pointers is a smart pointer, you are definitly doing it wrong.  You should only use shared (i.e. reference counted) pointers when you want two different objects to both be the "owner" of a pointer (i.e. shared ownership).  This gives either object the ability to free the pointer, but only when both objects no longer reference it.  In general though, you should try to create pointers that are owned by only one object, and deleted by only one object.  This is a much better way to make sure memory is not leaked.  

 

Also, just because you use smart pointers, it doesn't mean that you will never have a memory "leak".  For instance, if you have two smart pointers, that both retain a reference to each other  unless one of them frees the reference to the other, neither pointer will ever be released.  So even with smart pointers, you still need to aware of what's going on.  

I have to agree, smart pointers are not a panacea. I have a stock of various tools that help me to deal with errors and leaks in the code
thanks!

if one of my programs does any closing of handles or freeing of memory within the program (versus at the exit point), I also have to set handle values back to INVALID_HANDLE_VALUE after closing and pointers back to NULL after freeing memory.

if one of my programs does any closing of handles or freeing of memory within the program (versus at the exit point), I also have to set handle values back to INVALID_HANDLE_VALUE after closing and pointers back to NULL after freeing memory.

In my opinion this is a good option, but not the most efficient. You can find more reliable methods higher

Hi there

 

Thanks for writing this article - not much example code out there for custom allocators.

 

Just wanted to let you know there are serious issues with the FreeListAllocator and this code should NOT be used in production as it stands. By allocating blocks till the buffer is full, then random deallocating blocks before filling the buffer again, I can make the code either crash or loop forever whilst trying to allocate/deallocate blocks.

 

This replacement main() function illustrates the issue - you'll need to remove this assert from the allocate function - ASSERT(false && "Couldn't find free block large enough!");

 

int main()
{
  char* bigbuf = (char*)malloc(1000);

  // remember our allocs
  std::list<void*> _allocs;

  FreeListAllocator allocator(1000, bigbuf);

  // lots of iterations
  for (int i = 0; i < 1000; i++)
  {
    // allocate blocks till the allocator is full
    for(;;)
    {
      size_t allocSize = 90;
      void* alloc = allocator.allocate(allocSize, 0);

      if (!alloc)
        break;

      _allocs.push_back(alloc);
    }

    // randomly mark ~50% of blocks to remove
    std::list<void*> allocsToRemove;
    for (std::list<void*>::iterator it = _allocs.begin(); it != _allocs.end(); it++)
    {
      if ((rand() % 2) == 0)
      {
        allocsToRemove.push_back(*it);
      }
    }

    // deallocate the blocks
    for (std::list<void*>::iterator it = allocsToRemove.begin(); it != allocsToRemove.end(); it++)
    {
      allocator.deallocate(*it);
      _allocs.remove(*it);
    }
  }

  // cleanup before exitting
  for (std::list<void*>::iterator it = _allocs.begin(); it != _allocs.end(); it++)
  {
    allocator.deallocate(*it);
  }

    return 0;
}

The issue above is that it can't cope with an alignment of 0. Fixing this edge condition seems to have stopped the crashies.

 

However there's another issue - it doesn't properly coalesce free blocks together. You can try this by filling the heap, randomly deleting blocks then filling it again.

 

Result - the free block is not the size of the whole buffer.

 

Edit: And there's an issue where blocks can end up pointing backwards to the front of the list...

 

Edit: And another where after aligning new allocations the previous block next pointer isn't set up correctly to the newly created aligned block...

 

Edit: And another where the free block pointers aren't correctly updated for blocks that don't result in a merge. I really don't mean to be negative but this code doesn't look like it's ever been tested with anything more rigorous than sequential allocation/deallocations :-/


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