Name EXT_disjoint_timer_query Name Strings GL_EXT_disjoint_timer_query Contact Maurice Ribble, Qualcomm (mribble 'at' qualcomm.com) Contributors Matt Trusten Maurice Ribble Daniel Koch Jan-Harald Fredriksen Contributors to ANGLE_timer_query Contributors to ARB_timer_query Contributors to EXT_timer_query Contributors to EXT_occlusion_query_boolean This extension is based on the ARB_timer_query and ANGLE_timer_query IP Status No known IP claims. Status Complete Version Version 9, November 20, 2020 Number OpenGL ES Extension #150 Dependencies OpenGL ES 2.0 or greater required. The extension is written against the OpenGL ES 2.0 specification. The GetInteger64vEXT command is required only if OpenGL ES 3.0 or later is not supported (see the Interactions section for details). 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. New Procedures and Functions void GenQueriesEXT(sizei n, uint *ids); void DeleteQueriesEXT(sizei n, const uint *ids); boolean IsQueryEXT(uint id); void BeginQueryEXT(enum target, uint id); void EndQueryEXT(enum target); void QueryCounterEXT(uint id, enum target); void GetQueryivEXT(enum target, enum pname, int *params); void GetQueryObjectivEXT(uint id, enum pname, int *params); void GetQueryObjectuivEXT(uint id, enum pname, uint *params); void GetQueryObjecti64vEXT(uint id, enum pname, int64 *params); void GetQueryObjectui64vEXT(uint id, enum pname, uint64 *params); void GetInteger64vEXT(enum pname, int64 *data) New Tokens Accepted by the parameter of GetQueryivEXT: QUERY_COUNTER_BITS_EXT 0x8864 CURRENT_QUERY_EXT 0x8865 Accepted by the parameter of GetQueryObjectivEXT, GetQueryObjectuivEXT, GetQueryObjecti64vEXT, and GetQueryObjectui64vEXT: QUERY_RESULT_EXT 0x8866 QUERY_RESULT_AVAILABLE_EXT 0x8867 Accepted by the parameter of BeginQueryEXT, EndQueryEXT, and GetQueryivEXT: TIME_ELAPSED_EXT 0x88BF Accepted by the parameter of GetQueryiv and QueryCounter. Accepted by the parameter of GetBooleanv, GetIntegerv, GetInteger64v, and GetFloatv: TIMESTAMP_EXT 0x8E28 Accepted by the parameter of GetBooleanv, GetIntegerv, GetInteger64v, and GetFloatv: GPU_DISJOINT_EXT 0x8FBB Additions to Chapter 2 of the OpenGL ES 2.0 Specification (OpenGL ES Operation) (Modify table 2.1, Correspondence of command suffix letters to GL argument) Add two new types: Letter Corresponding GL Type ------ --------------------- i64 int64 ui64 uint64 (Modify table 2.2, GL data types) Add two new types: GL Type Minimum Bit Width Description ------- ----------------- ----------------------------- int64 64 Signed 2's complement integer uint64 64 Unsigned binary integer Additions to Chapter 5 of the OpenGL ES 2.0 Specification (Special Functions) Add a new section 5.3 "Timer Queries": "5.3 Timer Queries 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. Timer queries are associated with query objects. The command void GenQueriesEXT(sizei n, uint *ids); returns previously unused query object names in . These names are marked as used, but no object is associated with them until the first time they are used by BeginQueryEXT or QueryCounterEXT. Query objects contain one piece of state, an integer result value. This result value is initialized to zero when the object is created. Any positive integer except for zero (which is reserved for the GL) is a valid query object name. Query objects are deleted by calling void DeleteQueriesEXT(sizei n, const uint *ids); contains names of query objects to be deleted. After a query object is deleted, its name is again unused. Unused names in are silently ignored. If an active query object is deleted its name immediately becomes unused, but the underlying object is not deleted until it is no longer active. A timer query can be started and finished by calling void BeginQueryEXT(enum target, uint id); void EndQueryEXT(enum target); where is TIME_ELAPSED_EXT. If BeginQueryEXT is called with an unused , that name is marked as used and associated with a new query object. If BeginQueryEXT is called with an of zero, if the active query object name for is non-zero, if is the name of an existing query object whose type does not match , or if is the active query object name for any query type, the error INVALID_OPERATION is generated. If EndQueryEXT is called while no query with the same target is in progress, an INVALID_OPERATION error is generated. When BeginQueryEXT and EndQueryEXT are called with a of TIME_ELAPSED_EXT, 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. On some architectures framebuffer can have different meanings (specifically on some tiling GPUs fully realized might refer to the framebuffer being in tile memory before it gets copied out to system memory). The BeginQueryEXT and EndQueryEXT 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, , 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. The necessary state is a single bit indicating whether a timer query is active, the identifier of the currently active timer query, and a counter keeping track of the time that has passed. When the command void QueryCounterEXT(uint id, enum target); is called with TIMESTAMP_EXT, 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. On some architectures framebuffer can have different meanings (specifically on some tiling GPUs fully realized might refer to the framebuffer being in tile memory before it gets copied out to system memory). When the time is recorded, the query result for that object is marked available. QueryCounterEXT timer queries can be used within a BeginQueryEXT / EndQueryEXT block where the is TIME_ELAPSED_EXT and it does not affect the result of that query object. The error INVALID_OPERATION is generated if the is already in use within a BeginQueryEXT/EndQueryEXT block. The current time of the GL may be queried by calling GetIntegerv or GetInteger64v with the symbolic constant TIMESTAMP_EXT. 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. In order to know if the value returned from GetIntegerv or GetQuery is valid GPU_DISJOINT_EXT needs to be used to make sure the GPU did not perform any disjoint operation. This can be done through GetIntegerv by using GPU_- DISJOINT_EXT for . will be filled with a non-zero value if a disjoint operation occurred since the last time GetIntegerv was used with GPU_DISJOINT_EXT. A zero value will be returned if no disjoint operation occurred, indicating the values returned by this extension that are found in-between subsequent GetIntegerv calls will be valid for performance metrics. Disjoint operations occur whenever a change in the GPU occurs that will make the values returned by this extension unusable for performance metrics. An example can be seen with how mobile GPUs need to proactively try to conserve power, which might cause the GPU to go to sleep at the lower levers. This means disjoint states will occur at different times on different platforms and are implementation dependent. When the returned value is non-zero, all time values that were filled since the previous disjoint check should be considered undefined." Additions to Chapter 6 of the OpenGL ES 2.0 Specification (State and State Requests) Add GetInteger64vEXT to section 6.1.1 "Simple Queries", following the prototype for GetIntegerv: " void GetInteger64vEXT(enum pname, int64 *data); void GetFloatv(enum value, float *data); The commands obtain boolean, integer, 64-bit integer, or floating-point variables..." Add a new section 6.1.9 "Timer Queries": "The command boolean IsQueryEXT(uint id); returns TRUE if is the name of a query object. If is zero, or if is a non-zero value that is not the name of a query object, IsQueryEXT returns FALSE. Information about a query target can be queried with the command void GetQueryivEXT(enum target, enum pname, int *params); identifies the query target and can be TIME_ELAPSED_EXT or TIMESTAMP_EXT for timer queries. If is CURRENT_QUERY_EXT, the name of the currently active query for , or zero if no query is active, will be placed in . If is QUERY_COUNTER_BITS_EXT, the implementation-dependent number of bits used to hold the query result for will be placed in . The number of query counter bits may be zero, in which case the counter contains no useful information. For timer queries (TIME_ELAPSED_EXT and TIMESTAMP_EXT), 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 GetQueryObjectivEXT(uint id, enum pname, int *params); void GetQueryObjectuivEXT(uint id, enum pname, uint *params); void GetQueryObjecti64vEXT(uint id, enum pname, int64 *params); void GetQueryObjectui64vEXT(uint id, enum pname, uint64 *params); If is not the name of a query object, or if the query object named by is currently active, then an INVALID_OPERATION error is generated. If is QUERY_RESULT_EXT, then the query object's result value is returned as a single integer in . 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 is QUERY_RESULT_AVAILABLE_EXT, 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 a given timer query forces that timer query to complete within a finite amount of time. If multiple queries are issued on the same target and id prior to calling GetQueryObject[u]i[64]vEXT, the result returned will always be from the last query issued. The results from any queries before the last one will be lost if the results are not retrieved before starting a new query on the same and . Errors The error INVALID_VALUE is generated if GenQueriesEXT is called where is negative. The error INVALID_VALUE is generated if DeleteQueriesEXT is called where is negative. The error INVALID_OPERATION is generated if BeginQueryEXT is called when a query of the given is already active. The error INVALID_OPERATION is generated if EndQueryEXT is called when a query of the given is not active. The error INVALID_OPERATION is generated if BeginQueryEXT is called where is zero. The error INVALID_OPERATION is generated if BeginQueryEXT is called where is the name of a query currently in progress. The error INVALID_OPERATION is generated if BeginQueryEXT is called where is the name of an existing query object whose type does not match . The error INVALID_ENUM is generated if BeginQueryEXT or EndQueryEXT is called where is not TIME_ELAPSED_EXT. The error INVALID_ENUM is generated if GetQueryivEXT is called where is not TIME_ELAPSED_EXT or TIMESTAMP_EXT. The error INVALID_ENUM is generated if GetQueryivEXT is called where is not QUERY_COUNTER_BITS_EXT or CURRENT_QUERY_EXT. The error INVALID_ENUM is generated if QueryCounterEXT is called where is not TIMESTAMP_EXT. The error INVALID_OPERATION is generated if QueryCounterEXT is called on a query object that is already in use inside a BeginQueryEXT/EndQueryEXT. The error INVALID_OPERATION is generated if GetQueryObjectivEXT, GetQueryObjectuivEXT, GetQueryObjecti64vEXT, or GetQueryObjectui64vEXT is called where is not the name of a query object. The error INVALID_OPERATION is generated if GetQueryObjectivEXT, GetQueryObjectuivEXT, GetQueryObjecti64vEXT, or GetQueryObjectui64vEXT is called where is the name of a currently active query object. The error INVALID_ENUM is generated if GetQueryObjectivEXT, GetQueryObjectuivEXT, GetQueryObjecti64vEXT, or GetQueryObjectui64vEXT is called where is not QUERY_RESULT_EXT or QUERY_RESULT_AVAILABLE_EXT. New State (Add a new table 6.xx, "Query Operations") Get Value Type Get Command Initial Value Description Sec --------- ---- ----------- ------------- ----------- ------ - B - FALSE query active 5.3 CURRENT_QUERY_EXT Z+ GetQueryivEXT 0 active query ID 5.3 QUERY_RESULT_EXT Z+ GetQueryObjectuivEXT, 0 samples-passed count 5.3 GetQueryObjectui64vEXT QUERY_RESULT_AVAILABLE_EXT B GetQueryObjectivEXT FALSE query result available 5.3 New Implementation Dependent State (Add the following entry to table 6.18): Get Value Type Get Command Minimum Value Description Sec -------------------------- ---- ----------- ------------- ---------------- ------ QUERY_COUNTER_BITS_EXT Z+ GetQueryivEXT see 6.1.9 Number of bits in 6.1.9 query counter Interactions with OpenGL ES 2.0 and OpenGL ES 3.x If only OpenGL ES 2.0 is supported, then GetInteger64vEXT is defined, and is used instead of the GetInteger64v command defined by OpenGL ES 3.x. If OpenGL ES 3.0 or later is supported, GetInteger64vEXT is not required by an implementation of this extension, and the changes to section 6.1.1 are ignored. Examples (1) Here is some rough sample code that demonstrates the intended usage of this extension. GLint queries[N]; GLint available = 0; GLint disjointOccurred = 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); /* Clear disjoint error */ glGetIntegerv(GL_GPU_DISJOINT_EXT, &disjointOccurred); /* Start query 1 */ glBeginQuery(GL_TIME_ELAPSED_EXT, queries[0]); /* Draw object 1 */ .... /* End query 1 */ glEndQuery(GL_TIME_ELAPSED_EXT); ... /* Start query N */ glBeginQuery(GL_TIME_ELAPSED_EXT, queries[N-1]); /* Draw object N */ .... /* End query N */ glEndQuery(GL_TIME_ELAPSED_EXT); /* Wait for all results to become available */ while (!available) { glGetQueryObjectiv(queries[N-1], GL_QUERY_RESULT_AVAILABLE, &available); } /* Check for disjoint operation for all queries within the last disjoint check. This way we can only check disjoint once for all queries between, and once the last is filled we know all previous will have been filled as well */ glGetIntegerv(GL_GPU_DISJOINT_EXT, &disjointOccurred); /* If a disjoint operation occurred, all timer queries in between the last two disjoint checks that were filled are invalid, continue without reading the the values */ if (!disjointOccurred) { for (i = 0; i < N; i++) { /* See how much time the rendering of object i took in nanoseconds. */ glGetQueryObjectui64vEXT(queries[i], GL_QUERY_RESULT, &timeElapsed); /* Do something useful with the time if a disjoint operation did not occur. 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 uses QueryCounter. GLint queries[2]; GLint available = 0; GLint disjointOccurred = 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(2, queries); /* Clear disjoint error */ glGetIntegerv(GL_GPU_DISJOINT_EXT, &disjointOccurred); /* Query current timestamp before drawing */ glQueryCounterEXT(queries[0], GL_TIMESTAMP_EXT); /* Draw full rendertarget of objects */ /* Query current timestamp after drawing */ glQueryCounterEXT(queries[1], GL_TIMESTAMP_EXT); /* Do some other work so you don't stall waiting for available */ /* Wait for the query result to become available */ while (!available) { glGetQueryObjectiv(queries[1], GL_QUERY_RESULT_AVAILABLE, &available); } /* Check for disjoint operation. */ glGetIntegerv(GL_GPU_DISJOINT_EXT, &disjointOccurred); /* If a disjoint operation occurred, continue without reading the the values */ if (!disjointOccurred) { /* Get timestamp for when rendertarget started. */ glGetQueryObjectui64vEXT(queries[0], GL_QUERY_RESULT, &timeStart); /* Get timestamp for when rendertarget finished. */ glGetQueryObjectui64vEXT(queries[1], GL_QUERY_RESULT, &timeEnd); /* See how much time the rendering took in nanoseconds. */ timeElapsed = timeEnd - timeStart; /* Do something useful with the time if a disjoint operation did not occur. Note that care should be taken to use all significant bits of the result, not just the least significant 32 bits. */ AdjustObjectLODBasedOnDrawTime(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. */ /* Clear disjoint error */ glGetIntegerv(GL_GPU_DISJOINT_EXT, &disjointOccurred); /* Queue a query to find out when the frame finishes on the GL */ glQueryCounterEXT(endFrameQuery, GL_TIMESTAMP_EXT); /* Get the current GL time without stalling the GL */ glGetIntegerv(GL_TIMESTAMP_EXT, &flushTime); /* Finish the frame, submitting outstanding commands to the GL */ SwapBuffers(); /* Render another frame */ /* Later, compare the query result of and to measure the latency of the frame. A disjoint error still needs to be checked for in order to verify these values are valid. */ Issues from EXT_timer_query (1) What time interval is being measured? RESOLVED: The timer starts when all commands prior to BeginQueryEXT() 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 EndQueryEXT() 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, int64 and uint64, and their corresponding GetQueryObject entry points. These types hold integer values and have a minimum bit width of 64. (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) If multiple query types are supported, can multiple query types be active simultaneously? RESOLVED: Yes; an application may perform a timer query and another type of query simultaneously. An application can not perform multiple timer queries or multiple queries of other types simultaneously. An application also can not use the same query object for another 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 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 GLint64/GLuint64. 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. Issues from ARB_timer_query (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. Issues (12) What should we call this extension? RESOLVED: EXT_disjoint_timer_query (13) Why is this done as a separate extension instead of just supporting ARB_timer_query? ARB_timer_query is written against OpenGL 3.2, which includes a lot of the required support for dealing with query objects. None of these functions or tokens exist in OpenGL ES, and as such have to be added in this specification. (14) How does this extension differ from ARB_timer_query? This extension contains most ARB_timer_query behavior unchanged as well as adds the ability to detect GPU issues using GPU_DISJOINT_EXT. (15) Are query objects shareable between multiple contexts? RESOLVED: No. Query objects are lightweight and we normally share large data across contexts. Also, being able to share query objects across contexts is not particularly useful. In order to do the async query across contexts, a query on one context would have to be finished before the other context could query it. (16) How does this extension interact with EXT_occlusion_query_boolean? This extension redefines the Query Api originally defined in the EXT- _occlusion_query_boolean. If both EXT_disjoint_timer_query and EXT- _occlusion_query_boolean are supported, the rules and specification regarding any overlap will be governed by the EXT_occlusion_query_boolean extension. This extension should redefine the functionality in the same way, but if some discrepancy is found and both are supported EXT_disjoint_timer_query will yield to the rules and specifications governing the overlap in the order above. (17) How does this extension interact with the OpenGL ES 3.0 specification? Some of the functionality and requirements described here overlap with the OpenGL ES 3.0 specification. Any overlap for the functions or tokens in this extension were meant to complement each other, but the OpenGL ES 3.0 spec takes precedence. If the implementation supports OpenGL ES 3.0 then it should support both the core non-decorated functions and the EXT decorated functions. (18) How do times from BeginQueryEXT/EndQueryEXT with a of TIME_ELAPSED_EXT and QueryCounterEXT with a of TIMESTAMP_EXT compare on some Qualcomm and ARM tiling GPUs? This does not describe all tiling GPUs, but it is how some tiling GPUs from ARM, Qualcomm, and possibly other vendors work. This is just an implementation note and there is no guarantee all ARM and Qualcomm implementations will work this way. TIME_ELAPSED_EXT will be a summation of all the time spent on the workload between begin and end. Tiling architectures might split this work up over a binning pass and rendering many different tiles. It is up to the hardware and/or driver to add up all the time spent on the work between begin and end and report a single number making the implementation transparent to developers using this feature. If the binning pass happens in parallel to rendering pass this time would not be counted twice. On some implementations this does not include the time to copy tile memory to or from the frame buffer in system memory, and on other implementations this time is included. TIMESTAMP_EXT is the time when all the commands are complete and copied out of tile memory to the framebuffer in system memory. This can result in a courser grain timestamp than developers familiar with immediate GPUs expect. For example all the draws to an FBO can often all get the same timestamp, or even all the draw calls to multiple FBOs can end up with the same timestamp. If some operation causes a midframe store/load (such as flush or readPixels) then that would create another point for timestamps, but is a lot of extra work for the GPU so it should be avoided. If a preemption event happens before the TIMESTAMP_EXT is reported then that time will include the time for preemption. With TIME_ELAPSED_EXT it is undefined if the preemption time is counted or not. Some hardware will count the preemption time (even though it is from a different context). For this behavior GPU_DISJOINT_EXT will be set so you know there was an event from a different context affecting results. Other hardware will not count the time spent in the preempting context and for this cases GPU_DISJOINT_EXT will not be set. Revision History Revision 9, 2020/11/20 (xndcn) - Minor fix of code sample Revision 8, 2019/12/11 (Jon Leech) - Add actual spec language defining GetInteger64vEXT (github OpenGL-Registry issue 326) Revision 7, 2016/9/2 (Maurice Ribble) - Clarify language dealing with GetInteger64v Revision 6, 2016/7/15 (Maurice Ribble) - Clarified some outstanding questions about tiling GPUs - Added issue 18 Revision 5, 2013/6/5 - Minor cleanup to match new gl2ext.h Revision 4, 2013/4/25 (Jon Leech) - Cleanup for publication - Fix value assigned to GPU_DISJOINT_EXT Revision 3, 2013/4/8 - Minor cleanup of code sample and re-wording Revision 2, 2013/4/2 - Minor cleanup Revision 1, 2013/1/2 - Copied from revision 1 of ANGLE_timer_query - Added TIMESTAMP_EXT and GPU_DISJOINT_EXT