| Name |
| |
| NV_context_priority_realtime |
| |
| Name Strings |
| |
| EGL_NV_context_priority_realtime |
| |
| Contributors |
| |
| Sandeep Shinde |
| Kirill Artamonov |
| Sami Kiminki |
| Donghan Ryu |
| Daniel Koch |
| Mark Kilgard |
| |
| Contacts |
| |
| Sandeep Shinde, NVIDIA (sashinde 'at' nvidia 'dot' com) |
| |
| Status |
| |
| Complete |
| |
| Version |
| |
| Version 4 - November 21, 2017 |
| |
| Number |
| |
| EGL Extension #124 |
| |
| Dependencies |
| |
| 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). |
| |
| Overview |
| |
| 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 |
| |
| None |
| |
| New Procedures and Functions |
| |
| None |
| |
| New Tokens |
| |
| 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 |
| |
| Additions to Chapter 3 of the EGL 1.5 Specification (EGL Functions and Errors) |
| |
| 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." |
| |
| Issues |
| |
| 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. |
| |
| |
| 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 |
