Finding the correct alignment for host visible memory

Question

ppData points to a pointer in which is returned a host-accessible pointer to the beginning of the mapped range. This pointer minus offset must be aligned to at least VkPhysicalDeviceLimits::minMemoryMapAlignment.

I want to allocate a Vec3 float in a uniform buffer. A Vec3 float is 12bytes big.

VkMemoryRequirements { size: 16, alignment: 16, memory_type_bits: 15 }

Vulkan reports that it has to be aligned to 16 bytes, which means that the size of the allocation is now 16 instead of 12. So Vulkan already handled this for me.

minMemoryMapAlignment on my GPU is 64 bytes. What exactly does this mean for my allocation? Does this mean that I can not use the size from a VkMemoryRequirements for my allocation? And instead of allocating 16bytes here, I would have to allocate 64bytes?

Update:

For a 12 byte allocation with a 16 byte alignment and 64 bytes minMemoryMapAlignment. I would still allocate only 16 bytes and then call:

vkMapMemory(device, memory, 0, 16, 0, &mapped);

But the ptr returned from vkMapMemory is actually not 16 bytes but 64 bytes wide? And all the relevant data is in the first 12 bytes and the rest is just "padded" memory? So in practice this basically means that I don't need to use minMemoryMapAlignment at all?

minMemoryMapAlignment is a promise the driver makes to you, not a requirement you have to satisfy. It is a guarantee that when mapped, allocations will be at least aligned to this value. That way, you can safely use things like SSE vector load/store instructions that require 16-byte alignment. So you don't have to use this value, and when you do, you only need to check that it's at least as large as whatever alignment your code needs. — Jesse Hall

ratchet freak ratchet freak · Accepted Answer · 2017-06-03T10:38:18

There is nothing in the spec that restricts the size of the allocation like that. The paragraph you quoted means that the mapping will be aligned to minMemoryMapAlignment and you can then tell the compiler to use aligned memory accesses when accessing it. What will happen is that when the memory is mapped the later 48 bytes are wasted space in the host's memory space. That is unlikely to matter though.

This is why people keep saying to allocate larger blocks and subdivide them as needed. That way you can put 4 of those vkBuffers into a single 64 byte allocation (which you will need if you want to pipeline the rendering).

It's highly unlikely that that single vec3 is the only thing you need memory for, so take a look at your other allocations and see which ones you can combine.

Finding the correct alignment for host visible memory

1 Answers