VK_EXT_memory_priority

VK_EXT_memory_priority is a device extension that allows to pass additional "priority" value to Vulkan memory allocations that the implementation may use prefer certain buffers and images that are critical for performance to stay in device-local memory in cases when the memory is over-subscribed, while some others may be moved to the system memory.

VMA offers convenient usage of this extension. If you enable it, you can pass "priority" parameter when creating allocations or custom pools and the library automatically passes the value to Vulkan using this extension.

If you want to use this extension in connection with VMA, follow these steps:

Initialization

1) Call vkEnumerateDeviceExtensionProperties for the physical device. Check if the extension is supported - if returned array of VkExtensionProperties contains "VK_EXT_memory_priority".

2) Call vkGetPhysicalDeviceFeatures2 for the physical device instead of old vkGetPhysicalDeviceFeatures. Attach additional structure VkPhysicalDeviceMemoryPriorityFeaturesEXT to VkPhysicalDeviceFeatures2::pNext to be returned. Check if the device feature is really supported - check if VkPhysicalDeviceMemoryPriorityFeaturesEXT::memoryPriority is true.

3) While creating device with vkCreateDevice, enable this extension - add "VK_EXT_memory_priority" to the list passed as VkDeviceCreateInfo::ppEnabledExtensionNames.

4) While creating the device, also don't set VkDeviceCreateInfo::pEnabledFeatures. Fill in VkPhysicalDeviceFeatures2 structure instead and pass it as VkDeviceCreateInfo::pNext. Enable this device feature - attach additional structure VkPhysicalDeviceMemoryPriorityFeaturesEXT to VkPhysicalDeviceFeatures2::pNext chain and set its member memoryPriority to VK_TRUE.

5) While creating VmaAllocator with vmaCreateAllocator() inform VMA that you have enabled this extension and feature - add VMA_ALLOCATOR_CREATE_EXT_MEMORY_PRIORITY_BIT to VmaAllocatorCreateInfo::flags.

Usage

When using this extension, you should initialize following member:

• VmaAllocationCreateInfo::priority when creating a dedicated allocation with VMA_ALLOCATION_CREATE_DEDICATED_MEMORY_BIT.
• VmaPoolCreateInfo::priority when creating a custom pool.

It should be a floating-point value between 0.0f and 1.0f, where recommended default is 0.5f. Memory allocated with higher value can be treated by the Vulkan implementation as higher priority and so it can have lower chances of being pushed out to system memory, experiencing degraded performance.

It might be a good idea to create performance-critical resources like color-attachment or depth-stencil images as dedicated and set high priority to them. For example:

VkImageCreateInfo imgCreateInfo = { VK_STRUCTURE_TYPE_IMAGE_CREATE_INFO };
imgCreateInfo.imageType = VK_IMAGE_TYPE_2D;
imgCreateInfo.extent.width = 3840;
imgCreateInfo.extent.height = 2160;
imgCreateInfo.extent.depth = 1;
imgCreateInfo.mipLevels = 1;
imgCreateInfo.arrayLayers = 1;
imgCreateInfo.format = VK_FORMAT_R8G8B8A8_UNORM;
imgCreateInfo.tiling = VK_IMAGE_TILING_OPTIMAL;
imgCreateInfo.initialLayout = VK_IMAGE_LAYOUT_UNDEFINED;
imgCreateInfo.usage = VK_IMAGE_USAGE_SAMPLED_BIT | VK_IMAGE_USAGE_COLOR_ATTACHMENT_BIT;
imgCreateInfo.samples = VK_SAMPLE_COUNT_1_BIT;

VmaAllocationCreateInfo allocCreateInfo = {};
allocCreateInfo.usage = VMA_MEMORY_USAGE_AUTO;
allocCreateInfo.flags = VMA_ALLOCATION_CREATE_DEDICATED_MEMORY_BIT;
allocCreateInfo.priority = 1.0f;

VkImage img;
VmaAllocation alloc;
vmaCreateImage(allocator, &imgCreateInfo, &allocCreateInfo, &img, &alloc, nullptr);

priority member is ignored in the following situations:

• Allocations created in custom pools: They inherit the priority, along with all other allocation parameters from the parametrs passed in VmaPoolCreateInfo when the pool was created.
• Allocations created in default pools: They inherit the priority from the parameters VMA used when creating default pools, which means priority == 0.5f.