VkDescriptorPoolCreateInfo

Additional information about the pool is passed in a VkDescriptorPoolCreateInfo structure:

typedef struct VkDescriptorPoolCreateInfo {
    VkStructureType sType;
    const void* pNext;
    VkDescriptorPoolCreateFlags flags;
    uint32_t maxSets;
    uint32_t poolSizeCount;
    const VkDescriptorPoolSize* pPoolSizes;
} VkDescriptorPoolCreateInfo;

• sType is a VkStructureType value identifying this structure.
• pNext is NULL or a pointer to a structure extending this structure.
• flags is a bitmask of VkDescriptorPoolCreateFlagBits specifying certain supported operations on the pool.
• maxSets is the maximum number of descriptor sets that can be allocated from the pool.
• poolSizeCount is the number of elements in pPoolSizes.
• pPoolSizes is a pointer to an array of VkDescriptorPoolSize structures, each containing a descriptor type and number of descriptors of that type to be allocated in the pool.

If multiple VkDescriptorPoolSize structures containing the same descriptor type appear in the pPoolSizes array then the pool will be created with enough storage for the total number of descriptors of each type.

Fragmentation of a descriptor pool is possible and may lead to descriptor set allocation failures. A failure due to fragmentation is defined as failing a descriptor set allocation despite the sum of all outstanding descriptor set allocations from the pool plus the requested allocation requiring no more than the total number of descriptors requested at pool creation. Implementations provide certain guarantees of when fragmentation must not cause allocation failure, as described below.

If a descriptor pool has not had any descriptor sets freed since it was created or most recently reset then fragmentation must not cause an allocation failure (note that this is always the case for a pool created without the VK_DESCRIPTOR_POOL_CREATE_FREE_DESCRIPTOR_SET_BIT bit set). Additionally, if all sets allocated from the pool since it was created or most recently reset use the same number of descriptors (of each type) and the requested allocation also uses that same number of descriptors (of each type), then fragmentation must not cause an allocation failure.

If an allocation failure occurs due to fragmentation, an application can create an additional descriptor pool to perform further descriptor set allocations.

If flags has the VK_DESCRIPTOR_POOL_CREATE_UPDATE_AFTER_BIND_BIT bit set, descriptor pool creation may fail with the error VK_ERROR_FRAGMENTATION if the total number of descriptors across all pools (including this one) created with this bit set exceeds maxUpdateAfterBindDescriptorsInAllPools, or if fragmentation of the underlying hardware resources occurs.

If a pPoolSizes[i]::type is VK_DESCRIPTOR_TYPE_MUTABLE_EXT, a VkMutableDescriptorTypeCreateInfoEXT structure in the pNext chain can be used to specify which mutable descriptor types can be allocated from the pool. If included in the pNext chain, VkMutableDescriptorTypeCreateInfoEXT::pMutableDescriptorTypeLists[i] specifies which kind of VK_DESCRIPTOR_TYPE_MUTABLE_EXT descriptors can be allocated from this pool entry. If VkMutableDescriptorTypeCreateInfoEXT does not exist in the pNext chain, or VkMutableDescriptorTypeCreateInfoEXT::pMutableDescriptorTypeLists[i] is out of range, the descriptor pool allocates enough memory to be able to allocate a VK_DESCRIPTOR_TYPE_MUTABLE_EXT descriptor with any supported VkDescriptorType as a mutable descriptor. A mutable descriptor can be allocated from a pool entry if the type list in VkDescriptorSetLayoutCreateInfo is a subset of the type list declared in the descriptor pool, or if the pool entry is created without a descriptor type list. Multiple pPoolSizes entries with VK_DESCRIPTOR_TYPE_MUTABLE_EXTcan be declared. When multiple such pool entries are present in pPoolSizes, they specify sets of supported descriptor types which either fully overlap, partially overlap, or are disjoint. Two sets fully overlap if the sets of supported descriptor types are equal. If the sets are not disjoint they partially overlap. A pool entry without a VkMutableDescriptorTypeListEXT assigned to it is considered to partially overlap any other pool entry which has a VkMutableDescriptorTypeListEXT assigned to it. The application must ensure that partial overlap does not exist in pPoolSizes.