GL_SGIX_async

XXX - Not complete yet!!!

Name

    SGIX_async

Name Strings

    GL_SGIX_async

Version

    $Date: 1998/06/04 20:57:36 $ $Revision: 1.8 $

Number

    132

Dependencies

    None.

Overview

    This extension provides a framework for asynchronous OpenGL
    commands.  It also provides commands allowing a program to wait
    for the completion of asynchronous commands.
    
    Asynchronous commands have two properties:
    
    1) Asynchronous commands are non-blocking.  For example, an
    asynchronous ReadPixels command returns control to the program
    immediately rather than blocking until the command completes.
    This property allows the program to issue other OpenGL commands in
    parallel with the execution of commands that normally block.
    
    2) Asynchronous commands may complete out-of-order with respect to
    other OpenGL commands.  For example, an asynchronous TexImage
    command may complete after subsequent OpenGL commands issued by
    the program rather than maintaining the normal serial order of the
    OpenGL command stream.  This property allows the graphics
    accelerator to execute asynchronous commands in parallel with the
    normal command stream, for instance using a secondary path to
    transfer data from or to the host, without doing any dependency
    checking.
    
    Programs that issue asynchronous commands must also be able to
    determine when the commands have completed.  The completion status
    may be needed so that results can be retrieved (e.g. the image
    data from a ReadPixels command) or so that dependent commands can
    be issued (e.g. drawing commands that use texture data downloaded
    by an earlier asynchronous command).  This extension provides
    fine-grain control over asynchronous commands by introducing a
    mechanism for determining the status of individual commands.

    Each invocation of an asynchronous command is associated with an
    integer called a "marker."  A program specifies a marker before it
    issues an asynchronous command.  The program may later issue a
    command to query if any asynchronous commands have completed.  The
    query commands return a marker to identify the command that
    completed.  This extension provides both blocking and non-blocking
    query commands.

    This extension does not define any asynchronous commands.
    See SGIX_async_pixel for the asynchronous pixel commands.

Issues

    * Should all asynchronous operations share the same marker name space?

      Ans: Yes.  The argument for separate name spaces is that it
      makes modularizing an application easier.  The proposal was to
      use a separate name space for each category of commands,
      e.g. readpixels, teximage, drawpixels, and histogram. That works
      if each name space is used by only one part of the application
      (e.g. one part of the application issues asynchronous readpixels
      and some other part issues asynchronous teximage), but it
      doesn't work if there are multiple uses of one name space
      (e.g. two libraries that both use asynchronous readpixels).  It
      doesn't seem worth complicating the API to provide such a
      limited benefit.  The better solution is for a higher-level API
      to provide a mechanism for registering an interest in particular
      markers, similar to the X Windows mechanism for distributing
      window events.

    * Should the order of asynchronous operations be preserved, even
      if they are executed out-of-order with respect to normal commands?
    
      Ans: No, let the extensions that define the async operations choose
      whether or not to preserve order.

    * Should the extension also define a WaitAsyncSGIX command that
      guarantees completion of a specified asynchronous command before
      subsequent commands are executed?  This command wouldn't require
      the marker to make a round trip back to the host like
      FinishAsyncSGIX does, so it could be implemented more efficiently.

      Ans: No for now (no compelling application).

    * When are errors associated with an asynchronous command
      reported?

      Ans: A call to GetError after an asynchronous command always
      reports any errors associated with the command.  The behavior
      is as if errors are detected at the time a command is issued rather
      than when it is executed.  In this respect asynchronous commands
      are no different from synchronous commands.
      
    * Should a call to MakeCurrent implicitly force all pending
      asynchronous commands to complete?

      Ans: This is an implementation issue.  It does not affect the
      semantics of this extension.

    * Should there be an implementation-dependent limit on the
      number of outstanding asynchronous commands before the
      application calls FinishAsync?
      
      Ans: This extension does not impose such a limit, but the
      extensions that introduce asynchronous commands must address the
      issue.  An asynchronous command that uses an application-allocated
      buffer for return results (e.g. instruments) lets the application
      determine the limit by sizing the buffer appropriately.

    * Should this extension modify the instruments extension to use
      the new PollAsyncSGIX command instead of PollInstrumentsSGIX?

      Ans: No for now.

    * Should there be a generic command to set the current marker
      instead of a different mechanism for each asynchronous command?

      Ans: Yes, this has been added (AsyncMarkerSGIX).

    * Should there be a command to reserve a set of markers, like
      GenLists?

      Ans: Yes, this has been added (GenAsyncMarkers and
      DeleteAsyncMarkers).

    * Should name spaces be specified using enums instead of bitmasks?

      Ans: Yes, but name spaces have now been removed (see above).

    * Would it be better to define an extension that provides non-blocking
      commands rather than asynchronous commands?

      Ans: No.  Non-blocking commands would require the GL to preserve
      the order of commands, but that limits the potential for
      optimization.  The GL would have to check for dependencies
      between a non-blocking command and all subsequent commands
      (which is hard to do efficiently), or stall all subsequent
      commands until the non-blocking command completes, or maintain
      multiple sets of state.  By defining an asynchronous style of
      execution we place the burden of detecting dependencies on the
      application.  There is a large class of applications which can
      easily guarantee that there are no dependencies.

New Procedures and Functions

    void AsyncMarkerSGIX(uint marker)

    int FinishAsyncSGIX(uint *markerp)

    int PollAsyncSGIX(uint *markerp)

    uint GenAsyncMarkersSGIX(sizei range)

    void DeleteAsyncMarkersSGIX(uint marker, sizei range)

    boolean IsAsyncMarkerSGIX(uint marker)

New Tokens

    Accepted by the <pname> parameters of GetBooleanv, GetIntegerv,
    GetFloatv, and GetDoublev:

	ASYNC_MARKER_SGIX		0x8329

Additions to Chapter 2 of the 1.1 Specification (OpenGL Operation)

    None

Additions to Chapter 3 of the 1.1 Specification (Rasterization)

    None

Additions to Chapter 4 of the 1.1 Specification (Fragments and the Frame Buffer)

    None

Additions to Chapter 5 of the 1.1 Specification (Special Functions)

    Add to section 5.4 (Display Lists):
    
    The following commands are not included in display lists:

	FinishAsyncSGIX
	PollAsyncSGIX
        GenAsyncMarkersSGIX
        DeleteAsyncMarkersSGIX
	IsAsyncMarkerSGIX

    Add a section 5.X prior to the Instruments section entitled
    "Asynchronous Commands":

    Asynchronous commands are commands that may complete out-of-order
    with respect to other OpenGL commands.  An asynchronous command
    samples the OpenGL state vector when it is issued.  The sampled
    state includes the results from the complete execution of all
    synchronous (normal) commands issued prior to the asynchronous
    command.  However, the results of an asynchronous command (state
    changes or framebuffer updates) are not necessarily committed in
    order.  An asynchronous command may commit its results any time
    after the completion of all synchronous commands issued prior to
    the asynchronous command.  An implementation may choose to execute
    asynchronous commands in parallel with subsequent commands or at
    some convenient time in the future.

    Implementations of asynchronous commands may also be non-blocking.
    For example, a non-blocking query command returns control to the
    program immediately rather than stalling the program until the
    results of the query are available.

    When an asynchronous command is issued the current value of
    ASYNC_MARKER_SGIX is associated with the particular invocation of
    the command.  This value is an integer called a "marker" which can
    be used by the application to identify a particular command when
    it completes as described later in this section.  The value of
    ASYNC_MARKER_SGIX is specified by calling AsyncMarkerSGIX with the
    <marker> parameter set to the marker value.  It is not an error to
    invoke multiple asynchronous commands without changing
    ASYNC_MARKER_SGIX, although such a situation may make it
    impossible for the application to distinguish the completion
    status of the commands.

    OpenGL provides two commands for determining that
    previously-issued asynchronous commands have completed:
    FinishAsyncSGIX, which blocks until an asynchronous command
    completes, and PollAsyncSGIX, which is non-blocking.  Both
    commands write the marker associated with the completed
    asynchronous command into the integer referred to by the <markerp>
    parameter.

    FinishAsyncSGIX returns 1 when an asynchronous command has
    completed.  If there are pending asynchronous commands but none
    have completed then FinishAsyncSGIX blocks until at least one
    command completes.  If there are no pending asynchronous commands
    then FinishAsyncSGIX immediately returns 0 and does not modify the
    integer referred to by <markerp>.

    PollAsyncSGIX is identical to FinishAsyncSGIX except that if there
    are no asynchronous commands that have completed then
    PollAsyncSGIX immediately returns 0 rather than blocking.

    Calls to FinishAsyncSGIX and PollAsyncSGIX may be freely
    intermixed.  Once OpenGL has reported the completion of an
    asynchronous command via either FinishAsyncSGIX or PollAsyncSGIX
    the completion status of the command will not be reported again.
    If several commands have the same marker associated with them, the
    completion status of each command will be reported separately,
    although these reports will be indistinguishable.  The order in
    which asynchronous commands are reported is undefined by this
    extension, although the extensions that define asynchronous
    commands may guarantee a particular order.
    
    A program may guarantee that all outstanding asynchronous commands
    have completed by calling Finish.  A call to Finish does not cause
    the completion status of outstanding commands to be reported, so
    subsequent calls to FinishAsyncSGIX or PollAsyncSGIX will report
    the status of such commands.
    
    Three commands are provided to manage asynchronous command
    markers.  A marker is "in use" if it is the current value of
    ASYNC_MARKER_SGIX or if it is associated with any asynchronous
    command that has been issued but has not been queried using
    FinishAsyncSGIX or PollAsyncSGIX.  A marker may also be "reserved"
    by calling GenAsyncMarkersSGIX.  GenAsyncMarkersSGIX returns an
    integer n such that the markers n, n+1, n+2, ..., n+s-1 are not
    previously reserved or in use, where s is the value of the <range>
    parameter, or it returns 0 if no contiguous range of the specified
    length could be reserved.  The command also records that each of
    the markers in the returned range is reserved.
    
    DeleteAsyncMarkersSGIX records that each of the markers in the
    range n, n+1, n+2, ..., n+s-1 is no longer reserved, where n is
    the value of the <marker> parameter and s is the value of the
    <range> parameter.  It is not an error if some of the markers in
    the range are not reserved or are currently in use, but a marker
    that is in use will remain so even if DeleteAsyncMarkersSGIX is
    called.  IsAsyncMarkerSGIX returns 1 if the parameter <marker>
    identifies a marker that is currently in use or reserved, and the
    command returns 0 otherwise.

    Note that the marker name space is never shared across contexts,
    as the name spaces for texture objects and display lists may be.
    

Additions to Chapter 6 of the 1.1 Specification (State and State Requests)

    None

Additions to the GLX Specification

    Add to the end of Section 4.4 (Sequentiality):
    
    Asynchronous OpenGL commands do not obey the normal sequentiality
    guarantees for OpenGL commands.  Any command issued asynchronously
    may be inserted into the GLX protocol stream at any command
    boundary following the command that was issued before it.

GLX Protocol

    XXX Not complete.  Asynchronous commands should generate events
    when they complete.  The current GLX spec says that no new events
    are defined, so this will have to be changed.

Errors

    INVALID_OPERATION is generated if AsyncMarkerSGIX,
    FinishAsyncSGIX, PollAsyncSGIX, GenAsyncMarkersSGIX,
    DeleteAsyncMarkersSGIX or IsAsyncMarkerSGIX is called between
    execution of Begin and the corresponding execution of End.

New State

    Get Value			Get Command	Type	Value	Attrib
    ---------			-----------	----	-------	------
    ASYNC_MARKER_SGIX           GetIntegerv     Z+      0       -

New Implementation Dependent State

    None

Implementation Support

   List of OpenGL implementations supporting the GL_SGIX_async extension

Original File

   Original text file for the GL_SGIX_async extension