# VkPhysicalDeviceMemoryProperties(3) Manual Page

## Name

VkPhysicalDeviceMemoryProperties - Structure specifying physical device memory properties

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## C Specification

The `VkPhysicalDeviceMemoryProperties` structure is defined as:

    // Provided by VK_VERSION_1_0
    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 `VK_MAX_MEMORY_TYPES` 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 `VK_MAX_MEMORY_HEAPS` 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`

- `VK_MEMORY_PROPERTY_PROTECTED_BIT`

- `VK_MEMORY_PROPERTY_PROTECTED_BIT` | `VK_MEMORY_PROPERTY_DEVICE_LOCAL_BIT`

- `VK_MEMORY_PROPERTY_HOST_VISIBLE_BIT` |
  `VK_MEMORY_PROPERTY_HOST_COHERENT_BIT` |
  `VK_MEMORY_PROPERTY_DEVICE_COHERENT_BIT_AMD`

- `VK_MEMORY_PROPERTY_HOST_VISIBLE_BIT` |
  `VK_MEMORY_PROPERTY_HOST_CACHED_BIT` |
  `VK_MEMORY_PROPERTY_HOST_COHERENT_BIT` |
  `VK_MEMORY_PROPERTY_DEVICE_COHERENT_BIT_AMD`

- `VK_MEMORY_PROPERTY_DEVICE_LOCAL_BIT` |
  `VK_MEMORY_PROPERTY_DEVICE_COHERENT_BIT_AMD`

- `VK_MEMORY_PROPERTY_DEVICE_LOCAL_BIT` |
  `VK_MEMORY_PROPERTY_HOST_VISIBLE_BIT` |
  `VK_MEMORY_PROPERTY_HOST_COHERENT_BIT` |
  `VK_MEMORY_PROPERTY_DEVICE_COHERENT_BIT_AMD`

- `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_COHERENT_BIT_AMD`

- `VK_MEMORY_PROPERTY_HOST_VISIBLE_BIT` |
  `VK_MEMORY_PROPERTY_HOST_COHERENT_BIT` |
  `VK_MEMORY_PROPERTY_DEVICE_COHERENT_BIT_AMD` |
  `VK_MEMORY_PROPERTY_DEVICE_UNCACHED_BIT_AMD`

- `VK_MEMORY_PROPERTY_HOST_VISIBLE_BIT` |
  `VK_MEMORY_PROPERTY_HOST_CACHED_BIT` |
  `VK_MEMORY_PROPERTY_HOST_COHERENT_BIT` |
  `VK_MEMORY_PROPERTY_DEVICE_COHERENT_BIT_AMD` |
  `VK_MEMORY_PROPERTY_DEVICE_UNCACHED_BIT_AMD`

- `VK_MEMORY_PROPERTY_DEVICE_LOCAL_BIT` |
  `VK_MEMORY_PROPERTY_DEVICE_COHERENT_BIT_AMD` |
  `VK_MEMORY_PROPERTY_DEVICE_UNCACHED_BIT_AMD`

- `VK_MEMORY_PROPERTY_DEVICE_LOCAL_BIT` |
  `VK_MEMORY_PROPERTY_HOST_VISIBLE_BIT` |
  `VK_MEMORY_PROPERTY_HOST_COHERENT_BIT` |
  `VK_MEMORY_PROPERTY_DEVICE_COHERENT_BIT_AMD` |
  `VK_MEMORY_PROPERTY_DEVICE_UNCACHED_BIT_AMD`

- `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_COHERENT_BIT_AMD` |
  `VK_MEMORY_PROPERTY_DEVICE_UNCACHED_BIT_AMD`

- `VK_MEMORY_PROPERTY_DEVICE_LOCAL_BIT` |
  `VK_MEMORY_PROPERTY_RDMA_CAPABLE_BIT_NV`

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`. If the `deviceCoherentMemory` feature is enabled, there must be at least one memory type with the `VK_MEMORY_PROPERTY_DEVICE_COHERENT_BIT_AMD` 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:

- 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) ; or

- the `propertyFlags` members of Y includes `VK_MEMORY_PROPERTY_DEVICE_COHERENT_BIT_AMD` or `VK_MEMORY_PROPERTY_DEVICE_UNCACHED_BIT_AMD` and X does not

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 a valid 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 a valid order. There may be a performance penalty for using device coherent or uncached device memory types, and using these accidentally is undesirable. In order to avoid this, memory types with these properties always appear at the end of the list; but are subject to the same rules otherwise.

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

VK_VERSION_1_0, VkMemoryHeap, VkMemoryType, VkPhysicalDeviceMemoryProperties2, vkGetPhysicalDeviceMemoryProperties

## Document Notes

For more information, see the Vulkan Specification

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

## Copyright

Copyright 2014-2024 The Khronos Group Inc.

SPDX-License-Identifier: CC-BY-4.0