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// Copyright (c) 2015-2016 The Khronos Group Inc.
// Copyright notice at https://www.khronos.org/registry/speccopyright.html
[[devsandqueues]]
= Devices and Queues
Once Vulkan is initialized, devices and queues are the primary objects used
to interact with a Vulkan implementation.
// refBegin VkPhysicalDevice Opaque handle to a physical device object
Vulkan separates the concept of _physical_ and _logical_ devices.
A physical device usually represents a single device in a system (perhaps
made up of several individual hardware devices working together), of which
there are a finite number.
A logical device represents an application's view of the device.
Physical devices are represented by sname:VkPhysicalDevice handles:
include::../api/handles/VkPhysicalDevice.txt[]
// refEnd VkPhysicalDevice
[[devsandqueues-physical-device-enumeration]]
== Physical Devices
// refBegin vkEnumeratePhysicalDevices Enumerates the physical devices accessible to a Vulkan instance
To retrieve a list of physical device objects representing the physical
devices installed in the system, call:
include::../api/protos/vkEnumeratePhysicalDevices.txt[]
* pname:instance is a handle to a Vulkan instance previously created with
fname:vkCreateInstance.
* pname:pPhysicalDeviceCount is a pointer to an integer related to the
number of physical devices available or queried, as described below.
* pname:pPhysicalDevices is either `NULL` or a pointer to an array of
sname:VkPhysicalDevice handles.
If pname:pPhysicalDevices is `NULL`, then the number of physical devices
available is returned in pname:pPhysicalDeviceCount.
Otherwise, pname:pPhysicalDeviceCount must: point to a variable set by the
user to the number of elements in the pname:pPhysicalDevices array, and on
return the variable is overwritten with the number of structures actually
written to pname:pPhysicalDevices.
If pname:pPhysicalDeviceCount is less than the number of physical devices
available, at most pname:pPhysicalDeviceCount structures will be written.
If pname:pPhysicalDeviceCount is smaller than the number of physical devices
available, ename:VK_INCOMPLETE will be returned instead of ename:VK_SUCCESS,
to indicate that not all the available physical devices were returned.
include::../validity/protos/vkEnumeratePhysicalDevices.txt[]
// refBegin vkGetPhysicalDeviceProperties Returns properties of a physical device
To query general properties of physical devices once enumerated, call:
include::../api/protos/vkGetPhysicalDeviceProperties.txt[]
* pname:physicalDevice is the handle to the physical device whose
properties will be queried.
* pname:pProperties points to an instance of the
slink:VkPhysicalDeviceProperties structure, that will be filled with
returned information.
include::../validity/protos/vkGetPhysicalDeviceProperties.txt[]
// refBegin VkPhysicalDeviceProperties Structure specifying physical device properties
The sname:VkPhysicalDeviceProperties structure is defined as:
include::../api/structs/VkPhysicalDeviceProperties.txt[]
* pname:apiVersion is the version of Vulkan supported by the device,
encoded as described in the <<fundamentals-versionnum,API Version
Numbers and Semantics>> section.
* pname:driverVersion is the vendor-specified version of the driver.
* pname:vendorID is a unique identifier for the _vendor_ (see below) of
the physical device.
* pname:deviceID is a unique identifier for the physical device among
devices available from the vendor.
* pname:deviceType is a elink:VkPhysicalDeviceType specifying the type of
device.
* pname:deviceName is a null-terminated UTF-8 string containing the name
of the device.
* pname:pipelineCacheUUID is an array of size ename:VK_UUID_SIZE,
containing 8-bit values that represent a universally unique identifier
for the device.
* pname:limits is the slink:VkPhysicalDeviceLimits structure which
specifies device-specific limits of the physical device.
See <<features-limits,Limits>> for details.
* pname:sparseProperties is the slink:VkPhysicalDeviceSparseProperties
structure which specifies various sparse related properties of the
physical device.
See <<sparsememory-physicalprops,Sparse Properties>> for details.
The pname:vendorID and pname:deviceID fields are provided to allow
applications to adapt to device characteristics that are not adequately
exposed by other Vulkan queries.
These may: include performance profiles, hardware errata, or other
characteristics.
In PCI-based implementations, the low sixteen bits of pname:vendorID and
pname:deviceID must: contain (respectively) the PCI vendor and device IDs
associated with the hardware device, and the remaining bits must: be set to
zero.
In non-PCI implementations, the choice of what values to return may: be
dictated by operating system or platform policies.
It is otherwise at the discretion of the implementer, subject to the
following constraints and guidelines:
* For purposes of physical device identification, the _vendor_ of a
physical device is the entity responsible for the most salient
characteristics of the hardware represented by the physical device
handle.
In the case of a discrete GPU, this should: be the GPU chipset vendor.
In the case of a GPU or other accelerator integrated into a
system-on-chip (SoC), this should: be the supplier of the silicon IP
used to create the GPU or other accelerator.
* If the vendor of the physical device has a valid PCI vendor ID issued by
https://pcisig.com/[PCI-SIG], that ID should: be used to construct
pname:vendorID as described above for PCI-based implementations.
Implementations that do not return a PCI vendor ID in pname:vendorID
must: return a valid Khronos vendor ID, obtained as described in the
<<vulkan-styleguide,Vulkan Documentation and Extensions>> document in
the section ``Registering a Vendor ID with Khronos''.
Khronos vendor IDs are allocated starting at 0x10000, to distinguish
them from the PCI vendor ID namespace.
* The vendor of the physical device is responsible for selecting
pname:deviceID.
The value selected should: uniquely identify both the device version and
any major configuration options (for example, core count in the case of
multicore devices).
The same device ID should: be used for all physical implementations of
that device version and configuration.
For example, all uses of a specific silicon IP GPU version and
configuration should: use the same device ID, even if those uses occur
in different SoCs.
include::../validity/structs/VkPhysicalDeviceProperties.txt[]
// refBegin VkPhysicalDeviceType Supported physical device types
The physical devices types are:
include::../api/enums/VkPhysicalDeviceType.txt[]
* ename:VK_PHYSICAL_DEVICE_TYPE_OTHER The device does not match any other
available types.
* ename:VK_PHYSICAL_DEVICE_TYPE_INTEGRATED_GPU The device is typically one
embedded in or tightly coupled with the host.
* ename:VK_PHYSICAL_DEVICE_TYPE_DISCRETE_GPU The device is typically a
separate processor connected to the host via an interlink.
* ename:VK_PHYSICAL_DEVICE_TYPE_VIRTUAL_GPU The device is typically a
virtual node in a virtualization environment.
* ename:VK_PHYSICAL_DEVICE_TYPE_CPU The device is typically running on the
same processors as the host.
The physical device type is advertised for informational purposes only, and
does not directly affect the operation of the system.
However, the device type may: correlate with other advertised properties or
capabilities of the system, such as how many memory heaps there are.
// refEnd VkPhysicalDeviceType
// refBegin vkGetPhysicalDeviceQueueFamilyProperties Reports properties of the queues of the specified physical device
To query properties of queues available on a physical device, call:
include::../api/protos/vkGetPhysicalDeviceQueueFamilyProperties.txt[]
* pname:physicalDevice is the handle to the physical device whose
properties will be queried.
* pname:pQueueFamilyPropertyCount is a pointer to an integer related to
the number of queue families available or queried, as described below.
* pname:pQueueFamilyProperties is either `NULL` or a pointer to an array
of slink:VkQueueFamilyProperties structures.
If pname:pQueueFamilyProperties is `NULL`, then the number of queue families
available is returned in pname:pQueueFamilyPropertyCount.
Otherwise, pname:pQueueFamilyPropertyCount must: point to a variable set by
the user to the number of elements in the pname:pQueueFamilyProperties
array, and on return the variable is overwritten with the number of
structures actually written to pname:pQueueFamilyProperties.
If pname:pQueueFamilyPropertyCount is less than the number of queue families
available, at most pname:pQueueFamilyPropertyCount structures will be
written.
include::../validity/protos/vkGetPhysicalDeviceQueueFamilyProperties.txt[]
// refBegin VkQueueFamilyProperties Structure providing information about a queue family
The sname:VkQueueFamilyProperties structure is defined as:
include::../api/structs/VkQueueFamilyProperties.txt[]
* pname:queueFlags contains flags indicating the capabilities of the
queues in this queue family.
* pname:queueCount is the unsigned integer count of queues in this queue
family.
* pname:timestampValidBits is the unsigned integer count of meaningful
bits in the timestamps written via fname:vkCmdWriteTimestamp.
The valid range for the count is 36..64 bits, or a value of 0,
indicating no support for timestamps.
Bits outside the valid range are guaranteed to be zeros.
* pname:minImageTransferGranularity is the minimum granularity supported
for image transfer operations on the queues in this queue family.
The bits specified in pname:queueFlags are:
// refBegin VkQueueFlagBits Bitmask specifying capabilities of queues in a queue family
include::../api/enums/VkQueueFlagBits.txt[]
* if ename:VK_QUEUE_GRAPHICS_BIT is set, then the queues in this queue
family support graphics operations.
* if ename:VK_QUEUE_COMPUTE_BIT is set, then the queues in this queue
family support compute operations.
* if ename:VK_QUEUE_TRANSFER_BIT is set, then the queues in this queue
family support transfer operations.
* if ename:VK_QUEUE_SPARSE_BINDING_BIT is set, then the queues in this
queue family support sparse memory management operations (see
<<sparsememory,Sparse Resources>>).
If any of the sparse resource features are enabled, then at least one
queue family must: support this bit.
If an implementation exposes any queue family that supports graphics
operations, at least one queue family of at least one physical device
exposed by the implementation must: support both graphics and compute
operations.
[NOTE]
.Note
====
All commands that are allowed on a queue that supports transfer operations
are also allowed on a queue that supports either graphics or compute
operations thus if the capabilities of a queue family include
ename:VK_QUEUE_GRAPHICS_BIT or ename:VK_QUEUE_COMPUTE_BIT then reporting the
ename:VK_QUEUE_TRANSFER_BIT capability separately for that queue family is
optional:.
====
For further details see <<devsandqueues-queues,Queues>>.
The value returned in pname:minImageTransferGranularity has a unit of
compressed texel blocks for images having a block-compressed format, and a
unit of texels otherwise.
Possible values of pname:minImageTransferGranularity are:
* [eq]#(0,0,0)# which indicates that only whole mip levels must: be
transferred using the image transfer operations on the corresponding
queues.
In this case, the following restrictions apply to all offset and extent
parameters of image transfer operations:
** The pname:x, pname:y, and pname:z members of a slink:VkOffset3D
parameter must: always be zero.
** The pname:width, pname:height, and pname:depth members of a
slink:VkExtent3D parameter must: always match the width, height, and
depth of the image subresource corresponding to the parameter,
respectively.
* [eq]#(A~x~, A~y~, A~z~)# where [eq]#A~x~#, [eq]#A~y~#, and [eq]#A~z~#
are all integer powers of two.
In this case the following restrictions apply to all image transfer
operations:
** pname:x, pname:y, and pname:z of a slink:VkOffset3D parameter must: be
integer multiples of [eq]#A~x~#, [eq]#A~y~#, and [eq]#A~z~#,
respectively.
** pname:width of a slink:VkExtent3D parameter must: be an integer
multiple of [eq]#A~x~#, or else [eq]#pname:x + pname:width# must: equal
the width of the image subresource corresponding to the parameter.
** pname:height of a slink:VkExtent3D parameter must: be an integer
multiple of [eq]#A~y~#, or else [eq]#pname:y + pname:height# must:
equal the height of the image subresource corresponding to the
parameter.
** pname:depth of a slink:VkExtent3D parameter must: be an integer
multiple of [eq]#A~z~#, or else [eq]#pname:z + pname:depth# must: equal
the depth of the image subresource corresponding to the parameter.
** If the format of the image corresponding to the parameters is one of
the block-compressed formats then for the purposes of the above
calculations the granularity must: be scaled up by the compressed texel
block dimensions.
Queues supporting graphics and/or compute operations must: report
[eq]#(1,1,1)# in pname:minImageTransferGranularity, meaning that there are
no additional restrictions on the granularity of image transfer operations
for these queues.
Other queues supporting image transfer operations are only required: to
support whole mip level transfers, thus pname:minImageTransferGranularity
for queues belonging to such queue families may: be [eq]#(0,0,0)#.
include::../validity/structs/VkQueueFamilyProperties.txt[]
The <<memory-device,Device Memory>> section describes memory properties
queried from the physical device.
For physical device feature queries see the <<features, Features>> chapter.
[[devsandqueues-devices]]
== Devices
Device objects represent logical connections to physical devices.
Each device exposes a number of _queue families_ each having one or more
_queues_.
All queues in a queue family support the same operations.
As described in <<devsandqueues-physical-device-enumeration,Physical
Devices>>, a Vulkan application will first query for all physical devices in
a system.
Each physical device can: then be queried for its capabilities, including
its queue and queue family properties.
Once an acceptable physical device is identified, an application will create
a corresponding logical device.
An application must: create a separate logical device for each physical
device it will use.
The created logical device is then the primary interface to the physical
device.
How to enumerate the physical devices in a system and query those physical
devices for their queue family properties is described in the
<<devsandqueues-physical-device-enumeration, Physical Device Enumeration>>
section above.
[[devsandqueues-device-creation]]
=== Device Creation
// refBegin VkDevice Opaque handle to a device object
Logical devices are represented by sname:VkDevice handles:
include::../api/handles/VkDevice.txt[]
// refEnd VkDevice
// refBegin vkCreateDevice Create a new device instance
A logical device is created as a _connection_ to a physical device.
To create a logical device, call:
include::../api/protos/vkCreateDevice.txt[]
* pname:physicalDevice must: be one of the device handles returned from a
call to fname:vkEnumeratePhysicalDevices (see
<<devsandqueues-physical-device-enumeration, Physical Device
Enumeration>>).
* pname:pCreateInfo is a pointer to a slink:VkDeviceCreateInfo structure
containing information about how to create the device.
* pname:pAllocator controls host memory allocation as described in the
<<memory-allocation, Memory Allocation>> chapter.
* pname:pDevice points to a handle in which the created sname:VkDevice is
returned.
Multiple logical devices can: be created from the same physical device.
Logical device creation may: fail due to lack of device-specific resources
(in addition to the other errors).
If that occurs, fname:vkCreateDevice will return
ename:VK_ERROR_TOO_MANY_OBJECTS.
include::../validity/protos/vkCreateDevice.txt[]
// refBegin VkDeviceCreateInfo Structure specifying parameters of a newly created device
The sname:VkDeviceCreateInfo structure is defined as:
include::../api/structs/VkDeviceCreateInfo.txt[]
* pname:sType is the type of this structure.
* pname:pNext is `NULL` or a pointer to an extension-specific structure.
* pname:flags is reserved for future use.
* pname:queueCreateInfoCount is the unsigned integer size of the
pname:pQueueCreateInfos array.
Refer to the <<devsandqueues-queue-creation,Queue Creation>> section
below for further details.
* pname:pQueueCreateInfos is a pointer to an array of
slink:VkDeviceQueueCreateInfo structures describing the queues that are
requested to be created along with the logical device.
Refer to the <<devsandqueues-queue-creation,Queue Creation>> section
below for further details.
* pname:enabledLayerCount is deprecated and ignored.
* pname:ppEnabledLayerNames is deprecated and ignored.
See <<extended-functionality-device-layer-deprecation,Device Layer
Deprecation>>.
* pname:enabledExtensionCount is the number of device extensions to
enable.
* pname:ppEnabledExtensionNames is a pointer to an array of
pname:enabledExtensionCount null-terminated UTF-8 strings containing the
names of extensions to enable for the created device.
See the <<extended-functionality-extensions,Extensions>> section for
further details.
* pname:pEnabledFeatures is `NULL` or a pointer to a
slink:VkPhysicalDeviceFeatures structure that contains boolean
indicators of all the features to be enabled.
Refer to the <<features-features,Features>> section for further details.
.Valid Usage
****
* The pname:queueFamilyIndex member of any given element of
pname:pQueueCreateInfos must: be unique within pname:pQueueCreateInfos
****
include::../validity/structs/VkDeviceCreateInfo.txt[]
[[devsandqueues-use]]
=== Device Use
The following is a high-level list of sname:VkDevice uses along with
references on where to find more information:
* Creation of queues.
See the <<devsandqueues-queues,Queues>> section below for further
details.
* Creation and tracking of various synchronization constructs.
See <<synchronization,Synchronization and Cache Control>> for further
details.
* Allocating, freeing, and managing memory.
See <<memory,Memory Allocation>> and <<resources,Resource Creation>> for
further details.
* Creation and destruction of command buffers and command buffer pools.
See <<commandbuffers,Command Buffers>> for further details.
* Creation, destruction, and management of graphics state.
See <<pipelines,Pipelines>> and <<descriptorsets,Resource Descriptors>>,
among others, for further details.
[[devsandqueues-lost-device]]
=== Lost Device
A logical device may: become _lost_ because of hardware errors, execution
timeouts, power management events and/or platform-specific events.
This may: cause pending and future command execution to fail and cause
hardware resources to be corrupted.
When this happens, certain commands will return ename:VK_ERROR_DEVICE_LOST
(see <<fundamentals-errorcodes,Error Codes>> for a list of such commands).
After any such event, the logical device is considered _lost_.
It is not possible to reset the logical device to a non-lost state, however
the lost state is specific to a logical device (sname:VkDevice), and the
corresponding physical device (sname:VkPhysicalDevice) may: be otherwise
unaffected.
In some cases, the physical device may: also be lost, and attempting to
create a new logical device will fail, returning ename:VK_ERROR_DEVICE_LOST.
This is usually indicative of a problem with the underlying hardware, or its
connection to the host.
If the physical device has not been lost, and a new logical device is
successfully created from that physical device, it must: be in the non-lost
state.
[NOTE]
.Note
====
Whilst logical device loss may: be recoverable, in the case of physical
device loss, it is unlikely that an application will be able to recover
unless additional, unaffected physical devices exist on the system.
The error is largely informational and intended only to inform the user that
their hardware has probably developed a fault or become physically
disconnected, and should: be investigated further.
In many cases, physical device loss may: cause other more serious issues
such as the operating system crashing; in which case it may: not be reported
via the Vulkan API.
====
[NOTE]
.Note
====
Undefined behavior caused by an application error may: cause a device to
become lost.
However, such undefined behavior may: also cause unrecoverable damage to the
process, and it is then not guaranteed that the API objects, including the
sname:VkPhysicalDevice or the sname:VkInstance are still valid or that the
error is recoverable.
====
When a device is lost, its child objects are not implicitly destroyed and
their handles are still valid.
Those objects must: still be destroyed before their parents or the device
can: be destroyed (see the <<fundamentals-objectmodel-lifetime,Object
Lifetime>> section).
The host address space corresponding to device memory mapped using
flink:vkMapMemory is still valid, and host memory accesses to these mapped
regions are still valid, but the contents are undefined.
It is still legal to call any API command on the device and child objects.
Once a device is lost, command execution may: fail, and commands that return
a elink:VkResult may: return ename:VK_ERROR_DEVICE_LOST.
Commands that do not allow run-time errors must: still operate correctly for
valid usage and, if applicable, return valid data.
Commands that wait indefinitely for device execution (namely
flink:vkDeviceWaitIdle, flink:vkQueueWaitIdle, flink:vkWaitForFences
ifdef::VK_KHR_swapchain[]
or flink:vkAcquireNextImageKHR
endif::VK_KHR_swapchain[]
with a maximum pname:timeout, and flink:vkGetQueryPoolResults with the
ename:VK_QUERY_RESULT_WAIT_BIT bit set in pname:flags) must: return in
finite time even in the case of a lost device, and return either
ename:VK_SUCCESS or ename:VK_ERROR_DEVICE_LOST.
For any command that may: return ename:VK_ERROR_DEVICE_LOST, for the purpose
of determining whether a command buffer is pending execution, or whether
resources are considered in-use by the device, a return value of
ename:VK_ERROR_DEVICE_LOST is equivalent to ename:VK_SUCCESS.
ifdef::editing-notes[]
[NOTE]
.editing-note
====
TODO (piman) - I do not think we are very clear about what ``in-use by the
device'' means.
====
endif::editing-notes[]
[[devsandqueues-destruction]]
=== Device Destruction
// refBegin vkDestroyDevice Destroy a logical device
To destroy a device, call:
include::../api/protos/vkDestroyDevice.txt[]
* pname:device is the logical device to destroy.
* pname:pAllocator controls host memory allocation as described in the
<<memory-allocation, Memory Allocation>> chapter.
To ensure that no work is active on the device, flink:vkDeviceWaitIdle can:
be used to gate the destruction of the device.
Prior to destroying a device, an application is responsible for
destroying/freeing any Vulkan objects that were created using that device as
the first parameter of the corresponding ftext:vkCreate* or
ftext:vkAllocate* command.
[NOTE]
.Note
====
The lifetime of each of these objects is bound by the lifetime of the
sname:VkDevice object.
Therefore, to avoid resource leaks, it is critical that an application
explicitly free all of these resources prior to calling
fname:vkDestroyDevice.
====
.Valid Usage
****
* All child objects created on pname:device must: have been destroyed
prior to destroying pname:device
* If sname:VkAllocationCallbacks were provided when pname:device was
created, a compatible set of callbacks must: be provided here
* If no sname:VkAllocationCallbacks were provided when pname:device was
created, pname:pAllocator must: be `NULL`
****
include::../validity/protos/vkDestroyDevice.txt[]
[[devsandqueues-queues]]
== Queues
[[devsandqueues-queueprops]]
=== Queue Family Properties
As discussed in the <<devsandqueues-physical-device-enumeration,Physical
Device Enumeration>> section above, the
flink:vkGetPhysicalDeviceQueueFamilyProperties command is used to retrieve
details about the queue families and queues supported by a device.
Each index in the pname:pQueueFamilyProperties array returned by
flink:vkGetPhysicalDeviceQueueFamilyProperties describes a unique queue
family on that physical device.
These indices are used when creating queues, and they correspond directly
with the pname:queueFamilyIndex that is passed to the flink:vkCreateDevice
command via the slink:VkDeviceQueueCreateInfo structure as described in the
<<devsandqueues-queue-creation,Queue Creation>> section below.
Grouping of queue families within a physical device is
implementation-dependent.
[NOTE]
.Note
====
The general expectation is that a physical device groups all queues of
matching capabilities into a single family.
However, this is a recommendation to implementations and it is possible that
a physical device may: return two separate queue families with the same
capabilities.
====
Once an application has identified a physical device with the queue(s) that
it desires to use, it will create those queues in conjunction with a logical
device.
This is described in the following section.
[[devsandqueues-queue-creation]]
=== Queue Creation
// refBegin VkQueue Opaque handle to a queue object
Creating a logical device also creates the queues associated with that
device.
The queues to create are described by a set of slink:VkDeviceQueueCreateInfo
structures that are passed to flink:vkCreateDevice in
pname:pQueueCreateInfos.
Queues are represented by sname:VkQueue handles:
include::../api/handles/VkQueue.txt[]
// refEnd VkQueue
// refBegin VkDeviceQueueCreateInfo Structure specifying parameters of a newly created device queue
The sname:VkDeviceQueueCreateInfo structure is defined as:
include::../api/structs/VkDeviceQueueCreateInfo.txt[]
* pname:sType is the type of this structure.
* pname:pNext is `NULL` or a pointer to an extension-specific structure.
* pname:flags is reserved for future use.
* pname:queueFamilyIndex is an unsigned integer indicating the index of
the queue family to create on this device.
This index corresponds to the index of an element of the
pname:pQueueFamilyProperties array that was returned by
fname:vkGetPhysicalDeviceQueueFamilyProperties.
* pname:queueCount is an unsigned integer specifying the number of queues
to create in the queue family indicated by pname:queueFamilyIndex.
* pname:pQueuePriorities is an array of pname:queueCount normalized
floating point values, specifying priorities of work that will be
submitted to each created queue.
See <<devsandqueues-priority,Queue Priority>> for more information.
.Valid Usage
****
* pname:queueFamilyIndex must: be less than
pname:pQueueFamilyPropertyCount returned by
fname:vkGetPhysicalDeviceQueueFamilyProperties
* pname:queueCount must: be less than or equal to the pname:queueCount
member of the sname:VkQueueFamilyProperties structure, as returned by
fname:vkGetPhysicalDeviceQueueFamilyProperties in the
pname:pQueueFamilyProperties[pname:queueFamilyIndex]
* Each element of pname:pQueuePriorities must: be between `0.0` and `1.0`
inclusive
****
include::../validity/structs/VkDeviceQueueCreateInfo.txt[]
// refBegin vkGetDeviceQueue Get a queue handle from a device
To retrieve a handle to a VkQueue object, call:
include::../api/protos/vkGetDeviceQueue.txt[]
* pname:device is the logical device that owns the queue.
* pname:queueFamilyIndex is the index of the queue family to which the
queue belongs.
* pname:queueIndex is the index within this queue family of the queue to
retrieve.
* pname:pQueue is a pointer to a sname:VkQueue object that will be filled
with the handle for the requested queue.
.Valid Usage
****
* pname:queueFamilyIndex must: be one of the queue family indices
specified when pname:device was created, via the
sname:VkDeviceQueueCreateInfo structure
* pname:queueIndex must: be less than the number of queues created for the
specified queue family index when pname:device was created, via the
pname:queueCount member of the sname:VkDeviceQueueCreateInfo structure
****
include::../validity/protos/vkGetDeviceQueue.txt[]
[[devsandqueues-index]]
=== Queue Family Index
The queue family index is used in multiple places in Vulkan in order to tie
operations to a specific family of queues.
When retrieving a handle to the queue via fname:vkGetDeviceQueue, the queue
family index is used to select which queue family to retrieve the
sname:VkQueue handle from as described in the previous section.
When creating a sname:VkCommandPool object (see
<<commandbuffers-pools,Command Pools>>), a queue family index is specified
in the slink:VkCommandPoolCreateInfo structure.
Command buffers from this pool can: only be submitted on queues
corresponding to this queue family.
When creating sname:VkImage (see <<resources-images,Images>>) and
sname:VkBuffer (see <<resources-buffers,Buffers>>) resources, a set of queue
families is included in the slink:VkImageCreateInfo and
slink:VkBufferCreateInfo structures to specify the queue families that can:
access the resource.
When inserting a slink:VkBufferMemoryBarrier or slink:VkImageMemoryBarrier
(see <<synchronization-events>>) a source and destination queue family index
is specified to allow the ownership of a buffer or image to be transferred
from one queue family to another.
See the <<resources-sharing,Resource Sharing>> section for details.
[[devsandqueues-priority]]
=== Queue Priority
Each queue is assigned a priority, as set in the
slink:VkDeviceQueueCreateInfo structures when creating the device.
The priority of each queue is a normalized floating point value between 0.0
and 1.0, which is then translated to a discrete priority level by the
implementation.
Higher values indicate a higher priority, with 0.0 being the lowest priority
and 1.0 being the highest.
Within the same device, queues with higher priority may: be allotted more
processing time than queues with lower priority.
The implementation makes no guarantees with regards to ordering or
scheduling among queues with the same priority, other than the constraints
defined by explicit scheduling primitives.
The implementation make no guarantees with regards to queues across
different devices.
An implementation may: allow a higher-priority queue to starve a
lower-priority queue on the same sname:VkDevice until the higher-priority
queue has no further commands to execute.
The relationship of queue priorities must: not cause queues on one VkDevice
to starve queues on another sname:VkDevice.
No specific guarantees are made about higher priority queues receiving more
processing time or better quality of service than lower priority queues.
[[devsandqueues-submission]]
=== Queue Submission
Work is submitted to a queue via _queue submission_ commands such as
flink:vkQueueSubmit.
Queue submission commands define a set of _queue operations_ to be executed
by the underlying physical device, including synchronization with semaphores
and fences.
Submission commands take as parameters a target queue, zero or more
_batches_ of work, and an optional fence to signal upon completion.
Each batch consists of three distinct parts:
. Zero or more semaphores to wait on before execution of the rest of the
batch.
** If present, these describe a <<synchronization-semaphores-waiting,
semaphore wait operation>>.
. Zero or more work items to execute.
** If present, these describe a _queue operation_ matching the work
described.
. Zero or more semaphores to signal upon completion of the work items.
** If present, these describe a <<synchronization-semaphores-signaling,
semaphore signal operation>>.
If a fence is present in a queue submission, it describes a
<<synchronization-fences-signaling, fence signal operation>>.
All work described by a queue submission command must: be submitted to the
queue before the command returns.
[[devsandqueues-sparsebinding]]
==== Sparse Memory Binding
In Vulkan it is possible to sparsely bind memory to buffers and images as
described in the <<sparsememory,Sparse Resource>> chapter.
Sparse memory binding is a queue operation.
A queue whose flags include the ename:VK_QUEUE_SPARSE_BINDING_BIT must: be
able to support the mapping of a virtual address to a physical address on
the device.
This causes an update to the page table mappings on the device.
This update must: be synchronized on a queue to avoid corrupting page table
mappings during execution of graphics commands.
By binding the sparse memory resources on queues, all commands that are
dependent on the updated bindings are synchronized to only execute after the
binding is updated.
See the <<synchronization,Synchronization and Cache Control>> chapter for
how this synchronization is accomplished.
[[devsandqueues-queuedestruction]]
=== Queue Destruction
Queues are created along with a logical device during fname:vkCreateDevice.
All queues associated with a logical device are destroyed when
fname:vkDestroyDevice is called on that device.
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