Memory mapping

To "map memory" in Vulkan means to obtain a CPU pointer to VkDeviceMemory, to be able to read from it or write to it in CPU code. Mapping is possible only of memory allocated from a memory type that has VK_MEMORY_PROPERTY_HOST_VISIBLE_BIT flag. Functions vkMapMemory(), vkUnmapMemory() are designed for this purpose. You can use them directly with memory allocated by this library, but it is not recommended because of following issue: Mapping the same VkDeviceMemory block multiple times is illegal - only one mapping at a time is allowed. This includes mapping disjoint regions. Mapping is not reference-counted internally by Vulkan. Because of this, Vulkan Memory Allocator provides following facilities:

Mapping functions

The library provides following functions for mapping of a specific VmaAllocation: vmaMapMemory(), vmaUnmapMemory(). They are safer and more convenient to use than standard Vulkan functions. You can map an allocation multiple times simultaneously - mapping is reference-counted internally. You can also map different allocations simultaneously regardless of whether they use the same VkDeviceMemory block. They way it's implemented is that the library always maps entire memory block, not just region of the allocation. For further details, see description of vmaMapMemory() function. Example:

// Having these objects initialized:

struct ConstantBuffer
{
    ...
};
ConstantBuffer constantBufferData;

VmaAllocator allocator;
VmaBuffer constantBuffer;
VmaAllocation constantBufferAllocation;

// You can map and fill your buffer using following code:

void* mappedData;
vmaMapMemory(allocator, constantBufferAllocation, &mappedData);
memcpy(mappedData, &constantBufferData, sizeof(constantBufferData));
vmaUnmapMemory(allocator, constantBufferAllocation);

Persistently mapped memory

Kepping your memory persistently mapped is generally OK in Vulkan. You don't need to unmap it before using its data on the GPU. The library provides a special feature designed for that: Allocations made with VMA_ALLOCATION_CREATE_MAPPED_BIT flag set in VmaAllocationCreateInfo::flags stay mapped all the time, so you can just access CPU pointer to it any time without a need to call any "map" or "unmap" function. Example:

VkBufferCreateInfo bufCreateInfo = { VK_STRUCTURE_TYPE_BUFFER_CREATE_INFO };
bufCreateInfo.size = sizeof(ConstantBuffer);
bufCreateInfo.usage = VK_BUFFER_USAGE_TRANSFER_SRC_BIT;

VmaAllocationCreateInfo allocCreateInfo = {};
allocCreateInfo.usage = VMA_MEMORY_USAGE_CPU_ONLY;
allocCreateInfo.flags = VMA_ALLOCATION_CREATE_MAPPED_BIT;

VkBuffer buf;
VmaAllocation alloc;
VmaAllocationInfo allocInfo;
vmaCreateBuffer(allocator, &bufCreateInfo, &allocCreateInfo, &buf, &alloc, &allocInfo);

// Buffer is already mapped. You can access its memory.
memcpy(allocInfo.pMappedData, &constantBufferData, sizeof(constantBufferData));

There are some exceptions though, when you should consider mapping memory only for a short period of time:

• When operating system is Windows 7 or 8.x (Windows 10 is not affected because it uses WDDM2), device is discrete AMD GPU, and memory type is the special 256 MiB pool of DEVICE_LOCAL + HOST_VISIBLE memory (selected when you use VMA_MEMORY_USAGE_CPU_TO_GPU), then whenever a memory block allocated from this memory type stays mapped for the time of any call to vkQueueSubmit() or vkQueuePresentKHR(), this block is migrated by WDDM to system RAM, which degrades performance. It doesn't matter if that particular memory block is actually used by the command buffer being submitted.
• Keeping many large memory blocks mapped may impact performance or stability of some debugging tools.

Cache control

Memory in Vulkan doesn't need to be unmapped before using it on GPU, but unless a memory types has VK_MEMORY_PROPERTY_HOST_COHERENT_BIT flag set, you need to manually invalidate cache before reading of mapped pointer using function vkvkInvalidateMappedMemoryRanges() and flush cache after writing to mapped pointer using function vkFlushMappedMemoryRanges(). Example:

memcpy(allocInfo.pMappedData, &constantBufferData, sizeof(constantBufferData));

VkMemoryPropertyFlags memFlags;
vmaGetMemoryTypeProperties(allocator, allocInfo.memoryType, &memFlags);
if((memFlags & VK_MEMORY_PROPERTY_HOST_COHERENT_BIT) == 0)
{
    VkMappedMemoryRange memRange = { VK_STRUCTURE_TYPE_MAPPED_MEMORY_RANGE };
    memRange.memory = allocInfo.deviceMemory;
    memRange.offset = allocInfo.offset;
    memRange.size   = allocInfo.size;
    vkFlushMappedMemoryRanges(device, 1, &memRange);
}

Please note that memory allocated with VMA_MEMORY_USAGE_CPU_ONLY is guaranteed to be host coherent.

Also, Windows drivers from all 3 PC GPU vendors (AMD, Intel, NVIDIA) currently provide VK_MEMORY_PROPERTY_HOST_COHERENT_BIT flag on all memory types that are VK_MEMORY_PROPERTY_HOST_VISIBLE_BIT, so on this platform you may not need to bother.