General considerations

Thread safety

• The library has no global state, so separate VmaAllocator objects can be used independently. There should be no need to create multiple such objects though - one per VkDevice is enough.
• By default, all calls to functions that take VmaAllocator as first parameter are safe to call from multiple threads simultaneously because they are synchronized internally when needed.
• When the allocator is created with VMA_ALLOCATOR_CREATE_EXTERNALLY_SYNCHRONIZED_BIT flag, calls to functions that take such VmaAllocator object must be synchronized externally.
• Access to a VmaAllocation object must be externally synchronized. For example, you must not call vmaGetAllocationInfo() and vmaMapMemory() from different threads at the same time if you pass the same VmaAllocation object to these functions.

Validation layer warnings

When using this library, you can meet following types of warnings issued by Vulkan validation layer. They don't necessarily indicate a bug, so you may need to just ignore them.

• vkBindBufferMemory(): Binding memory to buffer 0xeb8e4 but vkGetBufferMemoryRequirements() has not been called on that buffer.
  • It happens when VK_KHR_dedicated_allocation extension is enabled. vkGetBufferMemoryRequirements2KHR function is used instead, while validation layer seems to be unaware of it.
• Mapping an image with layout VK_IMAGE_LAYOUT_DEPTH_STENCIL_ATTACHMENT_OPTIMAL can result in undefined behavior if this memory is used by the device. Only GENERAL or PREINITIALIZED should be used.
  • It happens when you map a buffer or image, because the library maps entire VkDeviceMemory block, where different types of images and buffers may end up together, especially on GPUs with unified memory like Intel.
• Non-linear image 0xebc91 is aliased with linear buffer 0xeb8e4 which may indicate a bug.
  • It happens when you use lost allocations, and a new image or buffer is created in place of an existing object that bacame lost.
  • It may happen also when you use defragmentation.

Allocation algorithm

The library uses following algorithm for allocation, in order:

1. Try to find free range of memory in existing blocks.
2. If failed, try to create a new block of VkDeviceMemory, with preferred block size.
3. If failed, try to create such block with size/2, size/4, size/8.
4. If failed and VMA_ALLOCATION_CREATE_CAN_MAKE_OTHER_LOST_BIT flag was specified, try to find space in existing blocks, possilby making some other allocations lost.
5. If failed, try to allocate separate VkDeviceMemory for this allocation, just like when you use VMA_ALLOCATION_CREATE_DEDICATED_MEMORY_BIT.
6. If failed, choose other memory type that meets the requirements specified in VmaAllocationCreateInfo and go to point 1.
7. If failed, return VK_ERROR_OUT_OF_DEVICE_MEMORY.

Features not supported

Features deliberately excluded from the scope of this library:

• Data transfer. Uploading (straming) and downloading data of buffers and images between CPU and GPU memory and related synchronization is responsibility of the user. Defining some "texture" object that would automatically stream its data from a staging copy in CPU memory to GPU memory would rather be a feature of another, higher-level library implemented on top of VMA.
• Allocations for imported/exported external memory. They tend to require explicit memory type index and dedicated allocation anyway, so they don't interact with main features of this library. Such special purpose allocations should be made manually, using vkCreateBuffer() and vkAllocateMemory().
• Sub-allocation of parts of one large buffer. Although recommended as a good practice, it is the user's responsibility to implement such logic on top of VMA.
• Recreation of buffers and images. Although the library has functions for buffer and image creation (vmaCreateBuffer(), vmaCreateImage()), you need to recreate these objects yourself after defragmentation. That's because the big structures VkBufferCreateInfo, VkImageCreateInfo are not stored in VmaAllocation object.
• Handling CPU memory allocation failures. When dynamically creating small C++ objects in CPU memory (not Vulkan memory), allocation failures are not checked and handled gracefully, because that would complicate code significantly and is usually not needed in desktop PC applications anyway. Success of an allocation is just checked with an assert.
• Code free of any compiler warnings. Maintaining the library to compile and work correctly on so many different platforms is hard enough. Being free of any warnings, on any version of any compiler, is simply not feasible.
• This is a C++ library with C interface. Bindings or ports to any other programming languages are welcomed as external projects and are not going to be included into this repository.