Vulkan-Docs/doc/specs/vulkan/chapters/queries.txt

// Copyright (c) 2015-2016 The Khronos Group Inc.
// Copyright notice at https://www.khronos.org/registry/speccopyright.html

[[queries]]
= Queries

_Queries_ provide a mechanism to return information about the processing of
a sequence of {apiname} commands. Query operations are asynchronous, and as
such, their results are not returned immediately. Instead, their results,
and their availability status, are stored in a <<queries-pools, Query
Pool>>. The state of these queries can: be read back on the host, or copied
to a buffer object on the device.

The supported query types are <<queries-occlusion,Occlusion Queries>>,
<<queries-pipestats,Pipeline Statistics Queries>>, and
<<queries-timestamps, Timestamp Queries>>.


[[queries-pools]]
== Query Pools

Queries are managed using _query pool_ objects. Each query pool is a
collection of a specific number of queries of a particular type.

To create a query pool, call:

include::../protos/vkCreateQueryPool.txt[]

  * pname:device is the logical device that creates the query pool.
  * pname:pCreateInfo is a pointer to an instance of the
    sname:VkQueryPoolCreateInfo structure containing the number and type of
    queries to be managed by the pool.
  * pname:pAllocator controls host memory allocation as described in the
    <<memory-allocation, Memory Allocation>> chapter.
  * pname:pQueryPool is a pointer to a sname:VkQueryPool handle in which the
    resulting query pool object is returned.

include::../validity/protos/vkCreateQueryPool.txt[]

The definition of sname:VkQueryPoolCreateInfo is:

include::../structs/VkQueryPoolCreateInfo.txt[]

The members of sname:VkQueryPoolCreateInfo have the following meanings:

  * 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:queryType is the type of queries managed by the pool,
    and must: be one of the values
+
include::../enums/VkQueryType.txt[]
  * pname:queryCount is the number of queries managed by the pool.
  * pname:pipelineStatistics is a bitmask indicating which counters will
    be returned in queries on the new pool, as described below in
    <<queries-pipestats>>. pname:pipelineStatistics is ignored if
    pname:queryType is not ename:VK_QUERY_TYPE_PIPELINE_STATISTICS.

include::../validity/structs/VkQueryPoolCreateInfo.txt[]

To destroy a query pool, call:

include::../protos/vkDestroyQueryPool.txt[]

  * pname:device is the logical device that destroys the query pool.
  * pname:queryPool is the query pool to destroy.
  * pname:pAllocator controls host memory allocation as described in the
    <<memory-allocation, Memory Allocation>> chapter.

include::../validity/protos/vkDestroyQueryPool.txt[]


[[queries-operation]]
== Query Operation

The operation of queries is controlled by the commands
flink:vkCmdBeginQuery, flink:vkCmdEndQuery, flink:vkCmdResetQueryPool,
flink:vkCmdCopyQueryPoolResults, and flink:vkCmdWriteTimestamp.

In order for a sname:VkCommandBuffer to record query management commands,
the queue family for which its sname:VkCommandPool was created must:
support the appropriate type of operations (graphics, compute) suitable
for the query type of a given query pool.

Each query in a query pool has a status that is either _unavailable_ or
_available_, and also has state to store the numerical results of a query
operation of the type requested when the query pool was created. Resetting a
query via flink:vkCmdResetQueryPool sets the status to unavailable and
makes the numerical results undefined. Performing a query operation with
flink:vkCmdBeginQuery and flink:vkCmdEndQuery changes the status to
available when the query <<queries-operation-finished,finishes>>,
and updates the numerical results.
Both the availability status and numerical results are retrieved by calling
either flink:vkGetQueryPoolResults or flink:vkCmdCopyQueryPoolResults.

All query commands execute in order and are guaranteed to see the effects of
each other's memory accesses, with one significant exception:
fname:vkCmdCopyQueryPoolResults may: execute before the results of
fname:vkCmdEndQuery are available. However, if
ename:VK_QUERY_RESULT_WAIT_BIT is used, then fname:vkCmdCopyQueryPoolResults
must: reflect the result of any previously executed queries. Other sequences
of commands, such as fname:vkCmdResetQueryPool followed by
fname:vkCmdBeginQuery, must: make the effects of the first command visible
to the second command.

After query pool creation, each query is in an undefined state and must: be
reset prior to use. Queries must: also be reset between uses. Using a
query that has not been reset will result in undefined behavior.

To reset a range of queries in a query pool, call:

include::../protos/vkCmdResetQueryPool.txt[]

  * pname:commandBuffer is the command buffer into which this command will
    be recorded.
  * pname:queryPool is the handle of the query pool managing the queries
    being reset.
  * pname:firstQuery is the initial query index to reset.
  * pname:queryCount is the number of queries to reset.

When executed on a queue, this command sets the status of query indices
latexmath:[$firstQuery,firstQuery+queryCount-1$] to unavailable.

include::../validity/protos/vkCmdResetQueryPool.txt[]

Once queries are reset and ready for use, query commands can: be
issued to a command buffer. Occlusion queries and pipeline statistics
queries count events - drawn samples and pipeline stage invocations,
respectively - resulting from commands that are recorded between a
flink:vkCmdBeginQuery command and a flink:vkCmdEndQuery command within
a specified command buffer, effectively scoping a set of drawing and/or
compute commands. Timestamp queries write timestamps to a query pool.

A query must: begin and end in the same command buffer, although if it is a
primary command buffer, and the
<<features-features-inherited-queries,inherited queries>> feature is
enabled, it can: execute secondary command buffers during the query
operation. For a secondary command buffer to be executed while a query is
active, it must: set the pname:occlusionQueryEnable, pname:queryFlags,
and/or pname:pipelineStatistics members of slink:VkCommandBufferBeginInfo to
conservative values, as described in the <<commandbuffers-recording, Command
Buffer Recording>> section. A query must: either begin and end inside the
same subpass of a render pass instance, or must: both begin and end outside
of a render pass instance (i.e. contain entire render pass instances).

Begin a query by calling:

include::../protos/vkCmdBeginQuery.txt[]

  * pname:commandBuffer is the command buffer into which this command will
    be recorded.
  * pname:queryPool is the query pool that will manage the results of the
    query.
  * pname:query is the query index within the query pool that will contain
    the results.
  * pname:flags is a bitmask indicating constraints on the types of queries
    that can: be performed. Valid bits in pname:flags include:
+
include::../enums/VkQueryControlFlagBits.txt[]

If the pname:queryType of the pool is ename:VK_QUERY_TYPE_OCCLUSION and
pname:flags contains ename:VK_QUERY_CONTROL_PRECISE_BIT, an implementation
must: return a result that matches the actual number of samples passed. This
is described in more detail in <<queries-occlusion,Occlusion Queries>>.

include::../validity/protos/vkCmdBeginQuery.txt[]

[[queries-operation-active]]
After beginning a query, that query is considered active within the command
buffer it was called in until that same query is ended. Queries active in a
primary command buffer when secondary command buffers are executed are
considered active for those secondary command buffers.

After the set of desired draw or dispatch commands, end a query by calling:

include::../protos/vkCmdEndQuery.txt[]

  * pname:commandBuffer is the command buffer into which this command will
    be recorded.
  * pname:queryPool is the query pool that is managing the results of the
    query.
  * pname:query is the query index within the query pool where the result is
    stored.

include::../validity/protos/vkCmdEndQuery.txt[]

[[queries-operation-finished]]
As queries operate asynchronously, ending a query does not immediately set
the query's status to available. A query is considered _finished_
when the final results of the query are ready to be retrieved by
flink:vkGetQueryPoolResults and flink:vkCmdCopyQueryPoolResults, and this
is when the query's status is set to available.

Once a query is ended the query must: finish in finite time, unless the
state of the query is changed using other commands, e.g. by issuing a reset
of the query.

[[queries-operation-memorylayout]]
An application can: retrieve results either by requesting they be written
into application-provided memory, or by requesting they be copied into a
sname:VkBuffer. In either case, the layout in memory is defined as follows:

  * The first query's result is written starting at the first byte requested
    by the command, and each subsequent query's result begins pname:stride
    bytes later.
  * Each query's result is a tightly packed array of unsigned integers,
    either 32- or 64-bits as requested by the command, storing the numerical
    results and, if requested, the availability status.
  * If ename:VK_QUERY_RESULT_WITH_AVAILABILITY_BIT is used, the final
    element of each query's result is an integer indicating whether the
    query's result is available, with any non-zero value indicating that
    it is available.
  * Occlusion queries write one integer value - the number of samples
    passed. Pipeline statistics queries write one integer value for each bit
    that is enabled in the pname:pipelineStatistics when the pool is
    created, and the statistics values are written in bit order starting
    from the least significant bit. Timestamps write one integer value.
  * If more than one query is retrieved and pname:stride is not at least as
    large as the size of the array of integers corresponding to a single
    query, the values written to memory are undefined.

To retrieve status and results for a set of queries, call:

include::../protos/vkGetQueryPoolResults.txt[]

  * pname:device is the logical device that owns the query pool.
  * pname:queryPool is the query pool managing the queries containing the
    desired results.
  * pname:firstQuery is the initial query index.
  * pname:queryCount is the number of queries. pname:firstQuery and
    pname:queryCount together define a range of queries.
  * pname:dataSize is the size in bytes of the buffer pointed to by
    pname:pData.
  * pname:pData is a pointer to a user-allocated buffer
    where the results will be written
  * pname:stride is the stride in bytes between results for individual
    queries within pname:pData.
  * pname:flags is a bitmask of elink:VkQueryResultFlagBits specifying how
    and when results are returned.

Valid bits in pname:flags include:

include::../enums/VkQueryResultFlagBits.txt[]

These bits have the following meanings:

  * ename:VK_QUERY_RESULT_64_BIT indicates the results will be written as an
    array of 64-bit unsigned integer values. If this bit is not set, the
    results will be written as an array of 32-bit unsigned integer values.
  * ename:VK_QUERY_RESULT_WAIT_BIT indicates that {apiname} will wait for
    each query's status to become available before retrieving its results.
  * ename:VK_QUERY_RESULT_WITH_AVAILABILITY_BIT indicates that the
    availability status accompanies the results.
  * ename:VK_QUERY_RESULT_PARTIAL_BIT indicates that returning partial
    results is acceptable.

If no bits are set in pname:flags, and all requested queries are
in the available state, results are written as an array of
32-bit unsigned integer values. The behavior when not all queries
are available, is described <<queries-wait-bit-not-set, below>>.

If ename:VK_QUERY_RESULT_64_BIT is not set and the result overflows a
32-bit value, the value may: either wrap or saturate. Similarly, if
ename:VK_QUERY_RESULT_64_BIT is set and the result overflows a 64-bit
value, the value may: either wrap or saturate.

If ename:VK_QUERY_RESULT_WAIT_BIT is set, {apiname} will wait for each
query to be in the available state before retrieving the numerical
results for that query. In this case, fname:vkGetQueryPoolResults is
guaranteed to succeed and return ename:VK_SUCCESS if the queries
become available in a finite time (i.e. if they have been issued and not
reset). If queries will never finish (e.g. due to being reset but not
issued), then fname:vkGetQueryPoolResults maynot: return in finite time.

[[queries-wait-bit-not-set]]
If ename:VK_QUERY_RESULT_WAIT_BIT and ename:VK_QUERY_RESULT_PARTIAL_BIT
are both not set then no result values are written to pname:pData for
queries that are in the unavailable state at the time of the call,
and fname:vkGetQueryPoolResults returns ename:VK_NOT_READY.
However, availability state is still written to pname:pData for those
queries if ename:VK_QUERY_RESULT_WITH_AVAILABILITY_BIT is set.

[NOTE]
.Note
====
Applications must: take care to ensure that use of the
ename:VK_QUERY_RESULT_WAIT_BIT bit has the desired effect.

For example, if a query has been used previously and a command buffer
records the commands fname:vkCmdResetQueryPool, fname:vkCmdBeginQuery, and
fname:vkCmdEndQuery for that query, then the query will remain
in the available state until the fname:vkCmdResetQueryPool command
executes on a queue. Applications can: use fences or events to ensure that
an query has already been reset before checking for its results or
availability status. Otherwise, a stale value could be returned from a
previous use of the query.

The above also applies when ename:VK_QUERY_RESULT_WAIT_BIT is used in
combination with ename:VK_QUERY_RESULT_WITH_AVAILABILITY_BIT. In this
case, the returned availability status may: reflect the result of a
previous use of the query unless the fname:vkCmdResetQueryPool command
has been executed since the last use of the query.
====

[NOTE]
.Note
====
Applications can: double-buffer query pool usage, with a pool per
frame, and reset queries at the end of the frame in which they
are read.
====

If ename:VK_QUERY_RESULT_PARTIAL_BIT is set, ename:VK_QUERY_RESULT_WAIT_BIT
is not set, and the query's status is unavailable, an intermediate
result value between zero and the final result value is written to
pname:pData for that query.

ename:VK_QUERY_RESULT_PARTIAL_BIT mustnot: be used if the pool's
pname:queryType is ename:VK_QUERY_TYPE_TIMESTAMP.

If ename:VK_QUERY_RESULT_WITH_AVAILABILITY_BIT is set, the final integer
value written for each query is non-zero if the query's status was
available or zero if the status was unavailable. When
ename:VK_QUERY_RESULT_WITH_AVAILABILITY_BIT is used, implementations must:
guarantee that if they return a non-zero availability value then the
numerical results must: be valid, assuming the results are not reset by a
subsequent command.

[NOTE]
.Note
====
Satisfying this guarantee may: require careful ordering by the application,
e.g. to read the availability status before reading the results.
====

include::../validity/protos/vkGetQueryPoolResults.txt[]

To copy query statuses and numerical results directly to buffer memory,
call:

include::../protos/vkCmdCopyQueryPoolResults.txt[]

  * pname:commandBuffer is the command buffer into which this command will
    be recorded.
  * pname:queryPool is the query pool managing the queries containing the
    desired results.
  * pname:firstQuery is the initial query index.
  * pname:queryCount is the number of queries. pname:firstQuery and
    pname:queryCount together define a range of queries.
  * pname:dstBuffer is a sname:VkBuffer object that will receive the results
    of the copy command.
  * pname:dstOffset is an offset into pname:dstBuffer.
  * pname:stride is the stride in bytes between results for individual
    queries within pname:dstBuffer. The required size of the backing memory
    for pname:dstBuffer is determined as described above for
    flink:vkGetQueryPoolResults.
  * pname:flags is a bitmask of elink:VkQueryResultFlagBits specifying how
    and when results are returned.

fname:vkCmdCopyQueryPoolResults is guaranteed to see the effect of previous
uses of fname:vkCmdResetQueryPool in the same queue, without any additional
synchronization. Thus, the results will always reflect the most
recent use of the query.

pname:flags has the same possible values described above for the param:flags
parameter of flink:vkGetQueryPoolResults, but the different style of
execution causes some subtle behavioral differences. Because
fname:vkCmdCopyQueryPoolResults executes in order with respect to other
query commands, there is less ambiguity about which use of a query is being
requested.

If no bits are set in pname:flags, results for all requested queries in the
available state are written as 32-bit unsigned integer values, and nothing
is written for queries in the unavailable state.

If ename:VK_QUERY_RESULT_64_BIT is set, the results are written as an array
of 64-bit unsigned integer values as described for
flink:vkGetQueryPoolResults.

If ename:VK_QUERY_RESULT_WAIT_BIT is set, the implementation will wait for
each query's status to be in the available state before retrieving the
numerical results for that query. This is guaranteed to reflect the most
recent use of the query on the same queue, assuming that the query is
not being simultaneously used by other queues. If the query does not become
available in a finite amount of time (e.g. due to not issuing a query
since the last reset), a ename:VK_ERROR_DEVICE_LOST error may: occur.

Similarly, if ename:VK_QUERY_RESULT_WITH_AVAILABILITY_BIT is set and
ename:VK_QUERY_RESULT_WAIT_BIT is not set, the availability is guaranteed to
reflect the most recent use of the query on the same queue, assuming
that the query is not being simultaneously used by other queues. As with
fname:vkGetQueryPoolResults, implementations must: guarantee that if they
return a non-zero availability value, then the numerical results are valid.

If ename:VK_QUERY_RESULT_PARTIAL_BIT is set, ename:VK_QUERY_RESULT_WAIT_BIT
is not set, and the query's status is unavailable, an intermediate
result value between zero and the final result value is written for that
query.

ename:VK_QUERY_RESULT_PARTIAL_BIT mustnot: be used if the pool's
pname:queryType is ename:VK_QUERY_TYPE_TIMESTAMP.

fname:vkCmdCopyQueryPoolResults is considered to be a transfer operation,
and its writes to buffer memory must: be synchronized using
ename:VK_PIPELINE_STAGE_TRANSFER_BIT and
ename:VK_ACCESS_TRANSFER_WRITE_BIT before using the results.

include::../validity/protos/vkCmdCopyQueryPoolResults.txt[]


[[queries-operation-undefined]]
Rendering operations such as clears, MSAA resolves, attachment load/store
operations, and blits may: or maynot: count towards the results of queries.
This behavior is implementation-dependent and may: vary depending on the
path used within an implementation. For example, some implementations have
several types of clears, some of which may: include vertices and some not.


[[queries-occlusion]]
== Occlusion Queries

Occlusion queries track the number of samples that pass the per-fragment
tests for a set of drawing commands. As such, occlusion queries are only
available on queue families supporting graphics operations. The application
can: then use these results to inform future rendering decisions. An
occlusion query is begun and ended by calling fname:vkCmdBeginQuery and
fname:vkCmdEndQuery, respectively. When an occlusion query begins, the count
of passing samples always starts at zero. For each drawing command, the
count is incremented as described in <<fragops-samplecount,Sample
Counting>>. If pname:flags does not contain
ename:VK_QUERY_CONTROL_PRECISE_BIT an implementation may: generate any
non-zero result value for the query if the count of passing samples is
non-zero.

[NOTE]
.Note
====
Not setting ename:VK_QUERY_CONTROL_PRECISE_BIT mode may: be more efficient
on some implementations, and should: be used where it is sufficient to know
a boolean result on whether any samples passed the per-fragment tests. In
this case, some implementations may: only return zero or one, indifferent to
the actual number of samples passing the per-fragment tests.
====

When an occlusion query finishes, the result for that query is marked
as available. The application can: then either copy the result to a buffer
(via fname:vkCmdCopyQueryPoolResults) or request it be put into host memory
(via fname:vkGetQueryPoolResults).

[NOTE]
.Note
====
If occluding geometry is not drawn first, samples can: pass the depth test,
but still not be visible in a final image.
====


[[queries-pipestats]]
== Pipeline Statistics Queries

Pipeline statistics queries allow the application to sample a specified set
of sname:VkPipeline counters. These counters are accumulated by {apiname}
for a set of either draw or dispatch commands while a pipeline statistics
query is active. As such, pipeline statistics queries are available on
queue families supporting either graphics or compute operations. Further,
the availability of pipeline statistics queries is indicated by the
pname:pipelineStatisticsQuery member of the sname:VkPhysicalDeviceFeatures
object (see fname:vkGetPhysicalDeviceFeatures and fname:vkCreateDevice for
detecting and requesting this query type on a sname:VkDevice).

A pipeline statistics query is begun and ended by calling
fname:vkCmdBeginQuery and fname:vkCmdEndQuery, respectively. When a pipeline
statistics query begins, all statistics counters are set to zero. While the
query is active, the pipeline type determines which set of statistics are
available, but these must: be configured on the query pool when it is
created. If a statistic counter is issued on a command buffer that does
not support the corresponding operation, the value of that counter is
undefined after the query has finished. At least one statistic counter
relevant to the operations supported on the recording command buffer
must: be enabled.

pname:pipelineStatisticsQuery is a bitmask indicating different
possible pipeline statistics.

Valid bits in pname:flags include:

include::../enums/VkQueryPipelineStatisticFlagBits.txt[]

These bits have the following meanings:

  * If ename:VK_QUERY_PIPELINE_STATISTIC_INPUT_ASSEMBLY_VERTICES_BIT is set,
    queries managed by the pool will count the number of vertices processed
    by the <<drawing,input assembly>> stage. Vertices
    corresponding to incomplete primitives may: or maynot: contribute to the
    count.
  * If ename:VK_QUERY_PIPELINE_STATISTIC_INPUT_ASSEMBLY_PRIMITIVES_BIT is
    set, queries managed by the pool will count the number of primitives
    processed by the <<drawing,input assembly>> stage. If
    primitive restart is enabled, restarting the primitive topology has no
    effect on the count. Incomplete primitives may: or maynot: be counted.
  * If ename:VK_QUERY_PIPELINE_STATISTIC_VERTEX_SHADER_INVOCATIONS_BIT is
    set, queries managed by the pool will count the number of vertex shader
    invocations. This counter's value is incremented each time a vertex
    shader is <<shaders-vertex-execution,invoked>>.
  * If ename:VK_QUERY_PIPELINE_STATISTIC_GEOMETRY_SHADER_INVOCATIONS_BIT is
    set, queries managed by the pool will count the number of geometry
    shader invocations. This counter's value is incremented each time a
    geometry shader is <<shaders-geometry-execution,invoked>>. In the case
    of <<geometry-invocations,instanced geometry shaders>>, the geometry
    shader invocations count is incremented for each separate instanced
    invocation.
  * If ename:VK_QUERY_PIPELINE_STATISTIC_GEOMETRY_SHADER_PRIMITIVES_BIT is
    set, queries managed by the pool will count the number of primitives
    generated by geometry shader invocations. The counter's value is
    incremented each time the geometry shader emits a primitive. Restarting
    primitive topology using the SPIR-V instructions code:OpEndPrimitive or
    code:OpEndStreamPrimitive has no effect on the geometry shader output
    primitives count.
  * If ename:VK_QUERY_PIPELINE_STATISTIC_CLIPPING_INVOCATIONS_BIT is set,
    queries managed by the pool will count the number of primitives
    processed by the <<vertexpostproc-clipping,Primitive Clipping>> stage of
    the pipeline. The counter's value is incremented each time a primitive
    reaches the primitive clipping stage.
  * If ename:VK_QUERY_PIPELINE_STATISTIC_CLIPPING_PRIMITIVES_BIT is set,
    queries managed by the pool will count the number of primitives output
    by the <<vertexpostproc-clipping,Primitive Clipping>> stage of the
    pipeline. The counter's value is incremented each time a primitive
    passes the primitive clipping stage. The actual number of primitives
    output by the primitive clipping stage for a particular input primitive
    is implementation-dependent but must: satisfy the following conditions:
  ** If at least one vertex of the input primitive lies inside the clipping
     volume, the counter is incremented by one or more.
  ** Otherwise, the counter is incremented by zero or more.
  * If ename:VK_QUERY_PIPELINE_STATISTIC_FRAGMENT_SHADER_INVOCATIONS_BIT is
    set, queries managed by the pool will count the number of fragment
    shader invocations. The counter's value is incremented each time the
    fragment shader is <<shaders-fragment-execution,invoked>>.
  * If
    ename:VK_QUERY_PIPELINE_STATISTIC_TESSELLATION_CONTROL_SHADER_PATCHES_BIT
    is set, queries managed by the pool will count the number of patches
    processed by the tessellation control shader. The counter's value is
    incremented once for each patch for which a tessellation control shader
    is <<shaders-tessellation-control-execution,invoked>>.
  * If
    ename:VK_QUERY_PIPELINE_STATISTIC_TESSELLATION_EVALUATION_SHADER_INVOCATIONS_BIT
    is set, queries managed by the pool will count the number of invocations
    of the tessellation evaluation shader. The counter's value is
    incremented each time the tessellation evaluation shader is
    <<shaders-tessellation-evaluation-execution,invoked>>.
  * If ename:VK_QUERY_PIPELINE_STATISTIC_COMPUTE_SHADER_INVOCATIONS_BIT is
    set, queries managed by the pool will count the number of compute shader
    invocations. The counter's value is incremented every time the compute
    shader is invoked. Implementations may: skip the execution of certain
    compute shader invocations or execute additional compute shader
    invocations for implementation-dependent reasons as long as the results
    of rendering otherwise remain unchanged.


These values are intended to measure relative statistics on one
implementation. Various device architectures will count these values
differently. Any or all counters may: be affected by the issues described in
<<queries-operation-undefined,Query Operation>>.

[NOTE]
.Note
====
For example, tile-based rendering devices may: need to replay the scene
multiple times, affecting some of the counts.
====

If a pipeline has pname:rasterizerDiscardEnable enabled, implementations
may: discard primitives after the final vertex processing stage. As a
result, if pname:rasterizerDiscardEnable is enabled, the clipping input and
output primitives counters maynot: be incremented.

When a pipeline statistics query finishes, the result for that query is
marked as available. The application can: copy the result to a
buffer (via fname:vkCmdCopyQueryPoolResults), or request it be put into host
memory (via fname:vkGetQueryPoolResults).


[[queries-timestamps]]
== Timestamp Queries

_Timestamps_ provide applications with a mechanism for timing the execution
of commands. A timestamp is an integer value generated by the
sname:VkPhysicalDevice. Unlike other queries, timestamps do not operate over
a range, and so do not use flink:vkCmdBeginQuery or flink:vkCmdEndQuery. The
mechanism is built around a set of commands that allow the application to
tell the sname:VkPhysicalDevice to write timestamp values to a
<<queries-pools,query pool>> and then either read timestamp values on the
host (using flink:vkGetQueryPoolResults) or copy timestamp values to a
sname:VkBuffer (using flink:vkCmdCopyQueryPoolResults). The application can:
then compute differences between timestamps to determine execution time.

The number of valid bits in a timestamp value is determined by the
sname:VkQueueFamilyProperties::pname:timestampValidBits property of the
queue on which the timestamp is written. Timestamps are supported on any
queue which reports a non-zero value for pname:timestampValidBits via
flink:vkGetPhysicalDeviceQueueFamilyProperties.
If the <<features-limits-timestampComputeAndGraphics,
pname:timestampComputeAndGraphics>> limit is ename:VK_TRUE, timestamps
are supported by every queue family that supports either
graphics or compute operations (see slink:VkQueueFamilyProperties).

The number of nanoseconds it takes for a timestamp value to be
incremented by 1 can: be obtained from
sname:VkPhysicalDeviceLimits::pname:timestampPeriod after a call to
fname:vkGetPhysicalDeviceProperties.

A timestamp is requested by calling:

include::../protos/vkCmdWriteTimestamp.txt[]

  * pname:commandBuffer is the command buffer into which the command will be
    recorded.
  * pname:pipelineStage is one of the elink:VkPipelineStageFlagBits,
    specifying a stage of the pipeline.
  * pname:queryPool is the query pool that will manage the timestamp.
  * pname:query is the query within the query pool that will contain the
    timestamp.

fname:vkCmdWriteTimestamp latches the value of the timer when all previous
commands have completed executing as far as the specified pipeline stage,
and writes the timestamp value to memory. When the timestamp value is
written, the availability status of the query is set to available.

[NOTE]
.Note
====
If an implementation is unable to detect completion and latch the timer at
any specific stage of the pipeline, it may: instead do so at any logically
later stage.
====

flink:vkCmdCopyQueryPoolResults can: then be called to copy the timestamp
value from the query pool into buffer memory, with ordering and
synchronization behavior equivalent to how other queries operate. Timestamp
values can: also be retrieved from the query pool using
flink:vkGetQueryPoolResults. As with other queries, the query must: be reset
using flink:vkCmdResetQueryPool before requesting the timestamp value be
written to it.

While fname:vkCmdWriteTimestamp can: be called inside or outside of a render
pass instance, flink:vkCmdCopyQueryPoolResults must: only be called outside
of a render pass instance.

include::../validity/protos/vkCmdWriteTimestamp.txt[]