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Name Strings
Sandeep Shinde
Kirill Artamonov
Sami Kiminki
Donghan Ryu
Daniel Koch
Mark Kilgard
Sandeep Shinde, NVIDIA (sashinde 'at' nvidia 'dot' com)
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
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.
New Types
New Procedures and Functions
New Tokens
New attribute value accepted for the EGL_CONTEXT_PRIORITY_LEVEL_IMG
attribute in the <attrib_list> argument of eglCreateContext:
Additions to Chapter 3 of the EGL 1.5 Specification (EGL Functions and Errors)
Add a NEW section " Context Priority" to specify the context
priority attribute for EGL_IMG_context_priority and values:
" 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,
or EGL_CONTEXT_PRIORITY_REALTIME_NV. The default value for
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
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
"attribute must be set to EGL_CONFIG_ID, EGL_CONTEXT_CLIENT_TYPE,
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?
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.
Revision History
Version 1, 2016/11/23 (Sandeep Shinde)
- Initial version
Version 2, 2017/10/13 (Mark Kilgard)
- Complete and convert to NV extension
Version 3, 2017/10/31 (Sandeep Shinde)
- Few minor corrections. Issue 6 resolved.
Version 4, 2017/11/21 (Sandeep Shinde)
- Update enum value and add extension number