Name

    ARB_timer_query

Name Strings

    GL_ARB_timer_query

Contact

    Piers Daniell, NVIDIA Corporation (pdaniell 'at' nvidia.com)

Contributors

    Axel Mamode, Sony
    Brian Paul, Tungsten Graphics
    Bruce Merry, ARM
    James Jones, NVIDIA Corporation
    Pat Brown, NVIDIA
    Remi Arnaud, Sony

Notice

    Copyright (c) 2010-2013 The Khronos Group Inc. Copyright terms at
        http://www.khronos.org/registry/speccopyright.html

Status

    Complete. Approved by the ARB at the 2010/01/22 F2F meeting.
    Approved by the Khronos Board of Promoters on March 10, 2010.

Version

    Last Modified Date: August 9, 2014
    Revision: 13

Number

    ARB Extension #85

Dependencies

    This extension is written against the OpenGL 3.2 specification.

Overview

    Applications can benefit from accurate timing information in a number of
    different ways.  During application development, timing information can
    help identify application or driver bottlenecks.  At run time,
    applications can use timing information to dynamically adjust the amount
    of detail in a scene to achieve constant frame rates.  OpenGL
    implementations have historically provided little to no useful timing
    information.  Applications can get some idea of timing by reading timers
    on the CPU, but these timers are not synchronized with the graphics
    rendering pipeline.  Reading a CPU timer does not guarantee the completion
    of a potentially large amount of graphics work accumulated before the
    timer is read, and will thus produce wildly inaccurate results.
    glFinish() can be used to determine when previous rendering commands have
    been completed, but will idle the graphics pipeline and adversely affect
    application performance.

    This extension provides a query mechanism that can be used to determine
    the amount of time it takes to fully complete a set of GL commands, and
    without stalling the rendering pipeline.  It uses the query object
    mechanisms first introduced in the occlusion query extension, which allow
    time intervals to be polled asynchronously by the application.

IP Status

    No known IP claims.

New Procedures and Functions

     void QueryCounter(uint id, enum target);

     void GetQueryObjecti64v(uint id, enum pname, int64 *params);
     void GetQueryObjectui64v(uint id, enum pname, uint64 *params);

New Tokens

    Accepted by the <target> parameter of BeginQuery, EndQuery, and
    GetQueryiv:

        TIME_ELAPSED                                   0x88BF

    Accepted by the <target> parameter of GetQueryiv and QueryCounter.
    Accepted by the <value> parameter of GetBooleanv, GetIntegerv,
    GetInteger64v, GetFloatv, and GetDoublev:

        TIMESTAMP                                      0x8E28

Additions to Chapter 2 of the OpenGL 3.2 (Core Profile) Specification
(OpenGL Operation)

    (Modify table 2.1, Correspondence of command suffix letters to GL argument
     types, p. 14) Add one new type and suffix:

    Letter Corresponding GL Type
    ------ ---------------------
    ui64   uint64

    (Modify Section 2.14, Asynchronous Queries, p. 89)

    Asynchronous queries provide a mechanism to return information about the
    processing of a sequence of GL commands. There are three query types
    supported by the GL. Transform feedback queries (see section 2.16) return
    information on the number of vertices and primitives processed by the GL
    and written to one or more buffer objects. Occlusion queries (see section
    4.1.6) count the number of fragments or samples that pass the depth test.
    Timer queries (section 5.4) record the amount of time needed to fully
    process these commands or the current time of the GL.

Additions to Chapter 3 of the OpenGL 3.2 Specification (Rasterization)

    None.

Additions to Chapter 4 of the OpenGL 3.2 Specification (Per-Fragment
Operations and the Framebuffer)

    None.

Additions to Chapter 5 of the OpenGL 3.2 Specification (Special Functions)

    (Add new Section 5.4, Timer Queries, p. 246)

    Timer queries use query objects to track the amount of time needed to
    fully complete a set of GL commands, or to determine the current time
    of the GL.

    When BeginQuery and EndQuery are called with a <target> of
    TIME_ELAPSED, the GL prepares to start and stop the timer used for
    timer queries.  The timer is started or stopped when the effects from all
    previous commands on the GL client and server state and the framebuffer
    have been fully realized.  The BeginQuery and EndQuery commands may return
    before the timer is actually started or stopped.  When the timer query
    timer is finally stopped, the elapsed time (in nanoseconds) is written to
    the corresponding query object as the query result value, and the query
    result for that object is marked as available.

    If the elapsed time overflows the number of bits, <n>, available to hold
    elapsed time, its value becomes undefined.  It is recommended, but not
    required, that implementations handle this overflow case by saturating at
    2^n - 1.

    A timer query object is created with the command

         void QueryCounter(uint id, enum target);

    <target> must be TIMESTAMP. If <id> is an unused query object name, the
    name is marked as used and associated with a new query object of type
    TIMESTAMP. Otherwise <id> must be the name of an existing query object
    of that type.

    When QueryCounter is called, the GL records the current time into
    the corresponding query object. The time is recorded after all previous
    commands on the GL client and server state and the framebuffer have been
    fully realized. When the time is recorded, the query result for that
    object is marked available. QueryCounter timer queries can be used
    within a BeginQuery / EndQuery block where the <target> is TIME_ELAPSED
    and it does not affect the result of that query object.

** core profile only
    QueryCounter fails and an INVALID\_OPERATION error is generated if <id>
    is not a name returned from a previous call to GenQueries, or if such a
    name has since been deleted with DeleteQueries.
** end core profile only

    If <id> is already in use within a BeginQuery / EndQuery block, or if
    <id> is the name of an existing query object whose type does not match
    <target>, an INVALID_OPERATION error is generated.

    The current time of the GL may be queried by calling GetIntegerv or
    GetInteger64v with the symbolic constant TIMESTAMP. This will return the
    GL time after all previous commands have reached the GL server but have
    not yet necessarily executed. By using a combination of this synchronous
    get command and the asynchronous timestamp query object target,
    applications can measure the latency between when commands reach the GL
    server and when they are realized in the framebuffer.

Additions to Chapter 6 of the OpenGL 2.0 Specification (State and State
Requests)

    (Modify Section 6.1.6, Asynchronous Queries, p. 255)

    Section 6.1.6, Asynchronous Queries

    The command

      boolean IsQuery(uint id);

    returns TRUE if <id> is the name of a query object. If <id> is zero, or if
    <id> is a non-zero value that is not the name of a query object, IsQuery
    returns FALSE.

    Information about a query target can be queried with the command

      void GetQueryiv(enum target, enum pname, int *params);

    <target> identifies the query target and can be SAMPLES_PASSED for
    occlusion queries, PRIMITIVES_GENERATED and
    TRANSFORM_FEEDBACK_PRIMITIVES_WRITTEN for primitive queries, or
    TIME_ELAPSED or TIMESTAMP for timer queries.

    If <pname> is CURRENT_QUERY, the name of the currently active query for
    <target>, or zero if no query is active, will be placed in <params>.

    If <pname> is QUERY_COUNTER_BITS, the implementation-dependent number of
    bits used to hold the query result for <target> will be placed in
    <params>.  The number of query counter bits may be zero, in which case
    the counter contains no useful information.

    For primitive queries (PRIMITIVES_GENERATED and
    TRANSFORM_FEEDBACK_PRIMITIVES_WRITTEN) if the number of bits is non-zero,
    the minimum number of bits allowed is 32.

    For occlusion queries (SAMPLES_PASSED), if the number of bits is
    non-zero, the minimum number of bits allowed is a function of the
    implementation's maximum viewport dimensions (MAX_VIEWPORT_DIMS). The
    counter must be able to represent at least two overdraws for every pixel
    in the viewport. The formula to compute the allowable minimum value
    (where <n> is the minimum number of bits) is:

        n = min(32, ceil(log_2(maxViewportWidth * maxViewportHeight * 2))).

    For timer queries (TIME_ELAPSED and TIMESTAMP), if the number
    of bits is non-zero, the minimum number of bits allowed is 30 which
    will allow at least 1 second of timing.

    The state of a query object can be queried with the commands

        void GetQueryObjectiv(uint id, enum pname, int *params);
        void GetQueryObjectuiv(uint id, enum pname, uint *params);
        void GetQueryObjecti64v(uint id, enum pname, int64 *params);
        void GetQueryObjectui64v(uint id, enum pname, uint64 *params);

    If <id> is not the name of a query object, or if the query object named
    by <id> is currently active, then an INVALID_OPERATION error is
    generated.

    If <pname> is QUERY_RESULT, then the query object's result
    value is returned as a single integer in <params>. If the value is so
    large in magnitude that it cannot be represented with the requested type,
    then the nearest value representable using the requested type is
    returned. If the number of query counter bits for target is zero, then
    the result is returned as a single integer with the value zero.

    There may be an indeterminate delay before the above query returns. If
    <pname> is QUERY_RESULT_AVAILABLE, FALSE is returned if such a delay
    would be required; otherwise TRUE is returned. It must always be true
    that if any query object returns a result available of TRUE, all queries
    of the same type issued prior to that query must also return TRUE.

    Querying the state for any given query object forces that occlusion
    query to complete within a finite amount of time.

    If multiple queries are issued using the same object name prior to
    calling GetQueryObject[u]i[64]v, the result and availability information
    returned will always be from the last query issued. The results from any
    queries before the last one will be lost if they are not retrieved before
    starting a new query on the same <target> and <id>.

Interactions with NV_present_video and NV_video_capture

    The GL timer recorded by this extension is the same timer as that used
    by the NV_present_video and NV_video_capture extensions. This allows
    the timer to be used with any of these extensions interchangeably.

Interactions with the Compatibility Profile

    In the compatibility profile, query objects support application-provided
    names, and the language requiring an error is <id> is not a name
    returned from GenQueries is removed. This is noted in the body text
    above.

Errors

    The error INVALID_ENUM is generated if BeginQuery or EndQuery is called
    where <target> is not SAMPLES_PASSED,
    TRANSFORM_FEEDBACK_PRIMITIVES_WRITTEN or TIME_ELAPSED.

    The error INVALID_ENUM is generated if GetQueryiv is called where
    <target> is not SAMPLES_PASSED, TRANSFORM_FEEDBACK_PRIMITIVES_WRITTEN,
    TIME_ELAPSED or TIMESTAMP.

    The error INVALID_ENUM is generated if QueryCounter is called where
    <target> is not TIMESTAMP.

    The error INVALID_OPERATION is generated if QueryCounter is called
    on a query object that is already in use inside a BeginQuery/EndQuery.

    The error INVALID_OPERATION is generated if QueryCounter is called on
    a query object whose type is not TIMESTAMP.

    (in the core profile only)
    The error INVALID_OPERATION is generated if QueryCounter is called
    where <id> is not a name returned from a previous call to GenQueries,
    or if such a name has since been deleted with DeleteQueries.

    The error INVALID_OPERATION is generated if GetQueryObjecti64v or
    GetQueryObjectui64v is called where <id> is not the name of a query
    object.

    The error INVALID_OPERATION is generated if GetQueryObjecti64v or
    GetQueryObjectui64v is called where <id> is the name of a currently
    active query object.

    The error INVALID_ENUM is generated if GetQueryObjecti64v or
    GetQueryObjectui64v is called where <pname> is not QUERY_RESULT or
    QUERY_RESULT_AVAILABLE.

New State

    None.

Examples

    (1) Here is some rough sample code that demonstrates the intended usage
        of this extension.

        GLuint queries[N];
        GLint available = 0;
        // timer queries can contain more than 32 bits of data, so always
        // query them using the 64 bit types to avoid overflow
        GLuint64 timeElapsed = 0;

        // Create a query object.
        glGenQueries(N, queries);

        // Start query 1
        glBeginQuery(GL_TIME_ELAPSED, queries[0]);

        // Draw object 1
        ....

        // End query 1
        glEndQuery(GL_TIME_ELAPSED);

        ...

        // Start query N
        glBeginQuery(GL_TIME_ELAPSED, queries[N-1]);

        // Draw object N
        ....

        // End query N
        glEndQuery(GL_TIME_ELAPSED);

        // Wait for all results to become available
        while (!available) {
            glGetQueryObjectiv(queries[N-1], GL_QUERY_RESULT_AVAILABLE, &available);
        }

        for (i = 0; i < N; i++) {
            // See how much time the rendering of object i took in nanoseconds.
            glGetQueryObjectui64v(queries[i], GL_QUERY_RESULT, &timeElapsed);

            // Do something useful with the time.  Note that care should be
            // taken to use all significant bits of the result, not just the
            // least significant 32 bits.
            AdjustObjectLODBasedOnDrawTime(i, timeElapsed);
        }

        This example is sub-optimal in that it stalls at the end of every
        frame to wait for query results.  Ideally, the collection of results
        would be delayed one frame to minimize the amount of time spent
        waiting for the GPU to finish rendering.

    (2) This example is basically the same as the example above but uses
        QueryCounter instead.

        GLuint queries[N+1];
        GLint available = 0;
        // timer queries can contain more than 32 bits of data, so always
        // query them using the 64 bit types to avoid overflow
        GLuint64 timeStart, timeEnd, timeElapsed = 0;

        // Create a query object.
        glGenQueries(N+1, queries);

        // Query current timestamp 1
        glQueryCounter(queries[0], GL_TIMESTAMP);

        // Draw object 1
        ....

        // Query current timestamp N
        glQueryCounter(queries[N-1], GL_TIMESTAMP);

        // Draw object N
        ....

        // Query current timestamp N+1
        glQueryCounter(queries[N], GL_TIMESTAMP);

        // Wait for all results to become available
        while (!available) {
            glGetQueryObjectiv(queries[N], GL_QUERY_RESULT_AVAILABLE, &available);
        }

        for (i = 0; i < N; i++) {
            // See how much time the rendering of object i took in nanoseconds.
            glGetQueryObjectui64v(queries[i], GL_QUERY_RESULT, &timeStart);
            glGetQueryObjectui64v(queries[i+1], GL_QUERY_RESULT, &timeEnd);
            timeElapsed = timeEnd - timeStart;

            // Do something useful with the time.  Note that care should be
            // taken to use all significant bits of the result, not just the
            // least significant 32 bits.
            AdjustObjectLODBasedOnDrawTime(i, timeElapsed);
        }

    (3) This example demonstrates how to measure the latency between GL
        commands reaching the server and being realized in the framebuffer.

        /* Submit a frame of rendering commands */
        while (!doneRendering) {
          ...
          glDrawElements(...);
        }

        /*
         * Measure rendering latency:
         *
         * Some commands may have already been submitted to hardware,
         * and some of those may have already completed.  The goal is
         * to measure the time it takes for the remaining commands to
         * complete, thereby measuring how far behind the app the GPU
         * is lagging, but without synchronizing the GPU with the CPU.
         */

        /* Queue a query to find out when the frame finishes on the GL */
        glQueryCounter(endFrameQuery, GL_TIMESTAMP);

        /* Get the current GL time without stalling the GL */
        glGet(GL_TIMESTAMP, &flushTime);

        /* Finish the frame, submitting outstanding commands to the GL */
        SwapBuffers();

        /* Render another frame */

        /*
         * Later, compare the query result of <endFrameQuery>
         * and <flushTime> to measure the latency of the frame
         */


Issues from EXT_timer_query

    (1) What time interval is being measured?

    RESOLVED:  The timer starts when all commands prior to BeginQuery() have
    been fully executed.  At that point, everything that should be drawn by
    those commands has been written to the framebuffer.  The timer stops
    when all commands prior to EndQuery() have been fully executed.

    (2) What unit of time will time intervals be returned in?

    RESOLVED:  Nanoseconds (10^-9 seconds).  This unit of measurement allows
    for reasonably accurate timing of even small blocks of rendering
    commands.  The granularity of the timer is implementation-dependent.  A
    32-bit query counter can express intervals of up to approximately 4
    seconds.

    (3) What should be the minimum number of counter bits for timer queries?

    RESOLVED:  30 bits, which will allow timing sections that take up to 1
    second to render.

    (4) How are counter results of more than 32 bits returned?

    RESOLVED:  Via two new datatypes, int64EXT and uint64EXT, and their
    corresponding GetQueryObject entry points.  These types hold integer
    values and have a minimum bit width of 64.

    UPDATE: This resolution was relevant for EXT_timer_query and OpenGL 2.0.
    OpenGL 3.2 now has int64 and uint64 datatypes as part of the core spec.

    (5) Should the extension measure total time elapsed between the full
        completion of the BeginQuery and EndQuery commands, or just time
        spent in the graphics library?

    RESOLVED:  This extension will measure the total time elapsed between
    the full completion of these commands.  Future extensions may implement
    a query to determine time elapsed at different stages of the graphics
    pipeline.

    (6) This extension introduces a second query type supported by
        BeginQuery/EndQuery.  Can multiple query types be active
        simultaneously?

    RESOLVED:  Yes; an application may perform an occlusion query and a
    timer query simultaneously.  An application can not perform multiple
    occlusion queries or multiple timer queries simultaneously.  An
    application also can not use the same query object for an occlusion
    query and a timer query simultaneously.

    (7) Do query objects have a query type permanently associated with them?

    RESOLVED:  No.  A single query object can be used to perform different
    types of queries, but not at the same time.

    Having a fixed type for each query object simplifies some aspects of the
    implementation -- not having to deal with queries with different result
    sizes, for example.  It would also mean that BeginQuery() with a query
    object of the "wrong" type would result in an INVALID_OPERATION error.

    UPDATE: This resolution was relevant for EXT_timer_query and OpenGL 2.0.
    Since EXT_transform_feedback has since been incorporated into the core,
    the resolution is that BeginQuery will generate error INVALID_OPERATION
    if <id> represents a query object of a different type.

    (8) How predictable/repeatable are the results returned by the timer
        query?

    RESOLVED:  In general, the amount of time needed to render the same
    primitives should be fairly constant.  But there may be many other
    system issues (e.g., context switching on the CPU and GPU, virtual
    memory page faults, memory cache behavior on the CPU and GPU) that can
    cause times to vary wildly.

    Note that modern GPUs are generally highly pipelined, and may be
    processing different primitives in different pipeline stages
    simultaneously.  In this extension, the timers start and stop when the
    BeginQuery/EndQuery commands reach the bottom of the rendering pipeline.
    What that means is that by the time the timer starts, the GL driver on
    the CPU may have started work on GL commands issued after BeginQuery,
    and the higher pipeline stages (e.g., vertex transformation) may have
    started as well.

   (9) What should the new 64 bit integer type be called?

    RESOLVED: The new types will be called GLint64EXT/GLuint64EXT  The new
    command suffixes will be i64 and ui64.  These names clearly convey the
    minimum size of the types.  These types are similar to the C99 standard
    type int_least64_t, but we use names similar to the C99 optional type
    int64_t for simplicity.

    UPDATE: This resolution was relevant for EXT_timer_query and OpenGL 2.0.
    OpenGL 3.2 now has int64 and uint64 datatypes as part of the core spec.
    The i64 suffix already exists in OpenGL 3.2 and the ui64 suffix has been
    added as part of this extension.

Issues

   (10) What about tile-based implementations? The effects of a command are
        not complete until the frame is completely rendered. Timing recorded
        before the frame is complete may not be what developers expect. Also
        the amount of time needed to render the same primitives is not
        consistent, which conflicts with issue (8) above. The time depends on
        how early or late in the scene it is placed.

    RESOLVED: The current language supports tile-based rendering okay as it
    is written. Developers are warned that using timers on tile-based
    implementation may not produce results they expect since rendering is not
    done in a linear order. Timing results are calculated when the frame is
    completed and may depend on how early or late in the scene it is placed.

   (11) Can the GL implementation use different clocks to implement the
        TIME_ELAPSED and TIMESTAMP queries?

    RESOLVED: Yes, the implementation can use different internal clocks to
    implement TIME_ELAPSED and TIMESTAMP. If different clocks are
    used it is possible there is a slight discrepancy when comparing queries
    made from TIME_ELAPSED and TIMESTAMP; they may have slight
    differences when both are used to measure the same sequence. However, this
    is unlikely to affect real applications since comparing the two queries is
    not expected to be useful.

   (12) Why do BeginQuery and QueryCounter have the same arguments in the
        opposite order?

    RESOLVED: Due to an unfortunate oversight, which cannot be fixed at
    this point.


Revision History

    Rev.  Date          Author    Changes
    ----  ------------  --------  -------------------------------------------
    13    Aug 9, 2014   Jon Leech Fix typo in example 3 (bug 12552).

    12    Jul 11, 2013  Jon Leech Change type of queries[] in sample code to
                                  GLuint (public bug 432).

    11    Apr 13, 2012  Jon Leech Clean up error language, add error for
                                  query objects which are not of type
                                  TIMESTAMP, and add issue 12 (Khronos
                                  internal bug 7662)

    10    June 3, 2011  dkoch     Add INVALID_OPERATION error when calling
                                  QueryCounter with a non-generated <id> in
                                  the core profile (Khronos internal bug 7662).

     9    Dec 18, 2009  pdaniell  Remove ambiguous language about "interuptions
                                  to the GL". Rename CURRENT_TIME to TIMESTAMP.

     8    Dec 10, 2009  Jon Leech Improve description of QueryCounter command.

     7    Dec 10, 2009  Jon Leech Replace non-ASCII punctuation.

     6    Dec 07, 2009  pdaniell  Remove ARB suffix from new tokens for core.

     5    Oct 29, 2009  pdaniell  TIMESTAMP_ARB renamed to CURRENT_TIME_ARB.
                                  Issue (11) raised about using different
                                  clocks to implement CURRENT_TIME and
                                  TIME_ELAPSED queries. Add example (3) for
                                  calculating the GL latency.

     4    Oct 23, 2009  pdaniell  Add support for TIMESTAMP_ARB as a <value>
                                  to Get* to allow synchronous time query.

     3    Oct 15, 2009  pdaniell  Resolved Issue (10). Added Interactions
                                  with NV_present_video and NV_video_capture
                                  section.

     2    Oct 15, 2009  pdaniell  Clarified some of the old EXT_timer_query
                                  Issues wrt OpenGL 3.2. Added specification
                                  for the TIMESTAMP_ARB time. Added new Issue
                                  for tile-based implementations. Issue 3
                                  resolution added to the spec.

     1    Oct 13, 2009  pdaniell  Initial revision based on EXT_timer_query