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.

For each pair of elements X and Y returned in memoryTypes, X must be placed at a lower index position than Y if:

  • either the set of bit flags returned in the propertyFlags member of X is a strict subset of the set of bit flags returned in the propertyFlags member of Y.

  • or the propertyFlags members of X and Y are equal, and X belongs to a memory heap with greater performance (as determined in an implementation-specific manner).

Note

There is no ordering requirement between X and Y elements for the case their propertyFlags members are not in a subset relation. That potentially allows more than one possible way to order the same set of memory types. Notice that the list of all allowed memory property flag combinations is written in the required order. But if instead VK_MEMORY_PROPERTY_DEVICE_LOCAL_BIT was before VK_MEMORY_PROPERTY_HOST_VISIBLE_BIT | VK_MEMORY_PROPERTY_HOST_COHERENT_BIT, the list would still be in the required order.

This ordering requirement enables applications to use a simple search loop to select the desired memory type along the lines of:

// Find a memory in `memoryTypeBitsRequirement` that includes all of `requiredProperties`
int32_t findProperties(const VkPhysicalDeviceMemoryProperties* pMemoryProperties,
                       uint32_t memoryTypeBitsRequirement,
                       VkMemoryPropertyFlags requiredProperties) {
    const uint32_t memoryCount = pMemoryProperties->memoryTypeCount;
    for (uint32_t memoryIndex = 0; memoryIndex < memoryCount; ++memoryIndex) {
        const uint32_t memoryTypeBits = (1 << memoryIndex);
        const bool isRequiredMemoryType = memoryTypeBitsRequirement & memoryTypeBits;

        const VkMemoryPropertyFlags properties =
            pMemoryProperties->memoryTypes[memoryIndex].propertyFlags;
        const bool hasRequiredProperties =
            (properties & requiredProperties) == requiredProperties;

        if (isRequiredMemoryType && hasRequiredProperties)
            return static_cast<int32_t>(memoryIndex);
    }

    // failed to find memory type
    return -1;
}

// Try to find an optimal memory type, or if it does not exist try fallback memory type
// `device` is the VkDevice
// `image` is the VkImage that requires memory to be bound
// `memoryProperties` properties as returned by vkGetPhysicalDeviceMemoryProperties
// `requiredProperties` are the property flags that must be present
// `optimalProperties` are the property flags that are preferred by the application
VkMemoryRequirements memoryRequirements;
vkGetImageMemoryRequirements(device, image, &memoryRequirements);
int32_t memoryType =
    findProperties(&memoryProperties, memoryRequirements.memoryTypeBits, optimalProperties);
if (memoryType == -1) // not found; try fallback properties
    memoryType =
        findProperties(&memoryProperties, memoryRequirements.memoryTypeBits, requiredProperties);

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.