C Specification

The VkPhysicalDeviceMemoryProperties structure is defined as:

typedef struct VkPhysicalDeviceMemoryProperties {
    uint32_t        memoryTypeCount;
    VkMemoryType    memoryTypes[VK_MAX_MEMORY_TYPES];
    uint32_t        memoryHeapCount;
    VkMemoryHeap    memoryHeaps[VK_MAX_MEMORY_HEAPS];
} VkPhysicalDeviceMemoryProperties;

Members

  • memoryTypeCount is the number of valid elements in the memoryTypes array.

  • memoryTypes is an array of VkMemoryType structures describing the memory types that can be used to access memory allocated from the heaps specified by memoryHeaps.

  • memoryHeapCount is the number of valid elements in the memoryHeaps array.

  • memoryHeaps is an array of VkMemoryHeap structures describing the memory heaps from which memory can be allocated.

Description

The VkPhysicalDeviceMemoryProperties structure describes a number of memory heaps as well as a number of memory types that can be used to access memory allocated in those heaps. Each heap describes a memory resource of a particular size, and each memory type describes a set of memory properties (e.g. host cached vs uncached) that can be used with a given memory heap. Allocations using a particular memory type will consume resources from the heap indicated by that memory type’s heap index. More than one memory type may share each heap, and the heaps and memory types provide a mechanism to advertise an accurate size of the physical memory resources while allowing the memory to be used with a variety of different properties.

The number of memory heaps is given by memoryHeapCount and is less than or equal to VK_MAX_MEMORY_HEAPS. Each heap is described by an element of the memoryHeaps array, as a VkMemoryHeap structure. The number of memory types available across all memory heaps is given by memoryTypeCount and is less than or equal to VK_MAX_MEMORY_TYPES. Each memory type is described by an element of the memoryTypes array, as a VkMemoryType structure.

At least one heap must include VK_MEMORY_HEAP_DEVICE_LOCAL_BIT in VkMemoryHeap::flags. If there are multiple heaps that all have similar performance characteristics, they may all include VK_MEMORY_HEAP_DEVICE_LOCAL_BIT. In a unified memory architecture (UMA) system, there is often only a single memory heap which is considered to be equally “local” to the host and to the device, and such an implementation must advertise the heap as device-local.

Each memory type returned by vkGetPhysicalDeviceMemoryProperties must have its propertyFlags set to one of the following values:

  • 0

  • VK_MEMORY_PROPERTY_HOST_VISIBLE_BIT | VK_MEMORY_PROPERTY_HOST_COHERENT_BIT

  • VK_MEMORY_PROPERTY_HOST_VISIBLE_BIT | VK_MEMORY_PROPERTY_HOST_CACHED_BIT

  • VK_MEMORY_PROPERTY_HOST_VISIBLE_BIT | VK_MEMORY_PROPERTY_HOST_CACHED_BIT | VK_MEMORY_PROPERTY_HOST_COHERENT_BIT

  • VK_MEMORY_PROPERTY_DEVICE_LOCAL_BIT

  • VK_MEMORY_PROPERTY_DEVICE_LOCAL_BIT | VK_MEMORY_PROPERTY_HOST_VISIBLE_BIT | VK_MEMORY_PROPERTY_HOST_COHERENT_BIT

  • VK_MEMORY_PROPERTY_DEVICE_LOCAL_BIT | VK_MEMORY_PROPERTY_HOST_VISIBLE_BIT | VK_MEMORY_PROPERTY_HOST_CACHED_BIT

  • VK_MEMORY_PROPERTY_DEVICE_LOCAL_BIT | VK_MEMORY_PROPERTY_HOST_VISIBLE_BIT | VK_MEMORY_PROPERTY_HOST_CACHED_BIT | VK_MEMORY_PROPERTY_HOST_COHERENT_BIT

  • VK_MEMORY_PROPERTY_DEVICE_LOCAL_BIT | VK_MEMORY_PROPERTY_LAZILY_ALLOCATED_BIT

There must be at least one memory type with both the VK_MEMORY_PROPERTY_HOST_VISIBLE_BIT and VK_MEMORY_PROPERTY_HOST_COHERENT_BIT bits set in its propertyFlags. There must be at least one memory type with the VK_MEMORY_PROPERTY_DEVICE_LOCAL_BIT bit set in its propertyFlags.

The memory types are sorted according to a preorder which serves to aid in easily selecting an appropriate memory type. Given two memory types X and Y, the preorder defines X ≤ Y if:

  • the memory property bits set for X are a strict subset of the memory property bits set for Y. Or,

  • the memory property bits set for X are the same as the memory property bits set for Y, and X uses a memory heap with greater or equal performance (as determined in an implementation-specific manner).

Memory types are ordered in the list such that X is assigned a lesser memoryTypeIndex than Y if (X ≤ Y) ∧ ¬ (Y ≤ X) according to the preorder. Note that the list of all allowed memory property flag combinations above satisfies this preorder, but other orders would as well. The goal of this ordering is to enable applications to use a simple search loop in selecting the proper memory type, along the lines of:

// Find a memory type in "memoryTypeBits" that includes all of "properties"
int32_t FindProperties(uint32_t memoryTypeBits, VkMemoryPropertyFlags properties)
{
    for (int32_t i = 0; i < memoryTypeCount; ++i)
    {
        if ((memoryTypeBits & (1 << i)) &&
            ((memoryTypes[i].propertyFlags & properties) == properties))
            return i;
    }
    return -1;
}

// Try to find an optimal memory type, or if it does not exist
// find any compatible memory type
VkMemoryRequirements memoryRequirements;
vkGetImageMemoryRequirements(device, image, &memoryRequirements);
int32_t memoryType = FindProperties(memoryRequirements.memoryTypeBits, optimalProperties);
if (memoryType == -1)
    memoryType = FindProperties(memoryRequirements.memoryTypeBits, requiredProperties);

The loop will find the first supported memory type that has all bits requested in properties set. If there is no exact match, it will find a closest match (i.e. a memory type with the fewest additional bits set), which has some additional bits set but which are not detrimental to the behaviors requested by properties. The application can first search for the optimal properties, e.g. a memory type that is device-local or supports coherent cached accesses, as appropriate for the intended usage, and if such a memory type is not present can fallback to searching for a less optimal but guaranteed set of properties such as "0" or "host-visible and coherent".

See Also

Document Notes

For more information, see the Vulkan Specification at URL

This page is extracted from the Vulkan Specification. Fixes and changes should be made to the Specification,not directly.

Copyright (c) 2014-2017 Khronos Group. This work is licensed under a Creative Commons Attribution 4.0 International License.