CODEWITHSUNDEEP
vulkan
Maik Klein
Maik Klein

Reputation: 16148

Finding the correct alignment for host visible memory

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?

Upvotes: 4

Views: 954

Answers (1)

ratchet freak
ratchet freak

Reputation: 48196

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.

Upvotes: 6

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