NV_context_priority_realtime

EGL_NV_context_priority_realtime

Sandeep Shinde
Kirill Artamonov
Sami Kiminki
Donghan Ryu
Daniel Koch
Mark Kilgard

Sandeep Shinde, NVIDIA (sashinde 'at' nvidia 'dot' com)

Complete

Version 4 - November 21, 2017

EGL Extension #124

Requires EGL 1.0.

Requires EGL_IMG_context_priority

This extension is written against the wording of the EGL 1.5
Specification - August 27, 2014 (but may be implemented against earlier
versions).

This extension allows an EGLContext to be created with one extra
priority level in addition to three priority levels that are part of
EGL_IMG_context_priority extension.

This new level has extra privileges that are not available to other three
levels. Some of the privileges may include:
- Allow realtime priority to only few contexts
- Allow realtime priority only to trusted applications
- Make sure realtime priority contexts are executed immediately
- Preempt any current context running on GPU on submission of
  commands for realtime context

The mechanism for determining which EGL context is allowed to use this
priority level is platform dependent.

None

None

New attribute value accepted for the EGL_CONTEXT_PRIORITY_LEVEL_IMG
attribute in the <attrib_list> argument of eglCreateContext:

    EGL_CONTEXT_PRIORITY_REALTIME_NV         0x3357

Add a NEW section "3.7.1.7 Context Priority" to specify the context
priority attribute for EGL_IMG_context_priority and values:

"3.7.1.7 Context Priority

The attribute EGL_CONTEXT_PRIORITY_LEVEL_IMG specifies a context
priority hint for a context supporting context priority.  This
attribute's value may be one of EGL_CONTEXT_PRIORITY_HIGH_IMG,
EGL_CONTEXT_PRIORITY_MEDIUM_IMG, EGL_CONTEXT_PRIORITY_LOW_IMG,
or EGL_CONTEXT_PRIORITY_REALTIME_NV.  The default value for
EGL_CONTEXT_PRIORITY_LEVEL_IMG is EGL_CONTEXT_PRIORITY_MEDIUM_IMG.

This attribute is a hint, as an implementation may not support
multiple contexts at some priority levels and system policy may limit
access to high priority contexts to appropriate system privilege
level.

The value EGL_CONTEXT_PRIORITY_REALTIME_NV requests the created
context run at the highest possible priority and be capable of
preempting the current executing context when commands are flushed
by such a realtime context.

This attribute is supported only for OpenGL and OpenGL ES contexts."

Within section 3.7.4 "Context Queries" amend the eglQueryContext
discussion as follows:

Change the sentence describing the attribute parameter to include
EGL_CONTEXT_PRIORITY_LEVEL_IMG so it reads:

"attribute must be set to EGL_CONFIG_ID, EGL_CONTEXT_CLIENT_TYPE,
EGL_CONTEXT_CLIENT_VERSION, EGL_RENDER_BUFFER, or
 EGL_CONTEXT_PRIORITY_LEVEL_IMG."

After the discussion of "Querying EGL_RENDER_BUFFER", add:

"Querying EGL_CONTEXT_PRIORITY_LEVEL_IMG returns the priority this
context was actually created with. Note: this may not be the same
as specified at context creation time, due to implementation limits
on the number of contexts that can be created at a specific priority
level in the system."

1)  The EGL_IMG_context_priority extension is written to amend the EGL
    1.4 specification.  Should this extension amend EGL 1.5 or 1.4?

    RESOLVED:  EGL 1.5 because it is newer and better organized to
    extend context attributes.

    EGL 1.5 rewrites 3.7.1 "Creating Rendering Contexts" to have subsections
    for different context attributes.  This extension adds a new such section
    that includes the EGL_IMG_context_priority attribute and values too.

2)  Is context priority hint supported for both OpenGL and OpenGL ES contexts?

    RESOLVED:  Yes.

3)  What is the intended application of the realtime priority level?

    RESOLVED:  One anticipated application is the system compositor
    for a Head Mounted Display (HMD) requires realtime recomposition
    for time-warping.

4)  What action causes a context with realtime priority to preempt
    other contexts?

    RESOLVED:  Preemption by a context with realtime priority should
    occur when there are pending rendering commands and an implicit or
    explicit flush (i.e. glFlush or glFinish) occurs.

5)  What does "trusted" or "appropriate system privilege level"
    mean in practice for a Linux-based operating system such as Android?

    RESOLVED: Trusted means an application that has higher privileges
    than other apps such as having CAP_SYS_NICE capability. On Android
    such applications have to be registered in advance with the OS;
    unpriviledged third party app cannot acquire this capability.

    This restriction exists so arbitrary applications do not starve or
    otherwise compromise the interactivity of the system overall.

6)  In practice how many realtime priority contexts can exist in a system to
    get best performance?

    RESOLVED: Only one realtime priority context should be active at a given
    moment to get best performance.

7)  Can a context created with a realtime priority hint that is
    in fact given a realtime priority, subsequently find that realtime
    priority revoked and, if revoked, can it be restored?

    RESOLVED: No, once a context is created with specific priority level, the
    priority will not change for lifetime of the context. This means there will
    not be revoking or restoring of realtime priority to already created context.

8)  The attrib_list for eglCreateContext could list the attribute
    EGL_CONTEXT_PRIORITY_LEVEL_IMG multiple times with different valid values.
    What happens in this case?

    RESOLVED: Behavior is undefined in this case.

    NVIDIA's EGL implementation handles such case by using the last (valid) attribute
    value listed in the attrib_list array as the effective attribute value for
    creating the context.

    The EGL specification is unfortunately silent on this issue.