• Advertisement
Sign in to follow this  

OpenGL Overhead with gl* Calls

This topic is 1252 days old which is more than the 365 day threshold we allow for new replies. Please post a new topic.

If you intended to correct an error in the post then please contact us.

Recommended Posts

This question regards OpenGL in general, but I think it's more specific to the mobile side of things were efficiency is especially important. Over the years, I've developed my OpenGL ES games with the mindset that making any sort of gl* call will almost-always query the GPU as mobile implementations of OpenGL are completely hardware-accelerated, like modern implementations of OpenGL. That being said, any thread making a gl* call will be halted while the CPU queries the GPU, waiting for a response. Is this correct so far?

 

I understand that cutting down on glEnable/glDisable calls by calling by sorting renderable elements with similar states, and also wrapping those two in my own state manager is important. I also do the same when binding buffers and textures. You could call SetTexture(GL_TEXTURE_2D, 0, &tex->name) to bind to the first texture unit in 2D. If that particular texture name, for that target at that texture unit has already been bound, then it won't do it again. This would come in handy when rendering multiple instances of the same model multiple times because it'd call glBindTexture(), glBindBuffer(), etc once for the first model, but all subsequent calls wouldn't because they're all using the same texture/buffer parameters that's common to the loaded model they share. Same for checking shaders. It's pretty common that multiple models might use the same shader in a scene. Image rendering dozens of individual models to the screen, but only having to call glUseProgram() twice each frame instead of once per instance rendered. I mean, since I'm still using OpenGL 2.1 (OpenGL ES 2.0 for mobile), glDrawElements() is called once per mesh per instance of the model drawn. For example, drawing 12 instances of a model with 5 meshes would be 60 draw calls. This could be heavy on mobile until I learn about instancing in higher versions of OpenGL and support OpenGL ES 3.0 on mobile.

 

My question is: is my managing OpenGL contexts internally in my engine worthwhile? Is it a huge performance hit to call glBindTexture() constantly (especially on mobile), or do OpenGL implementations usually check this already. Should I just focus on keeping draw calls down, or is the way I'm managing my states pretty important too?

 

From what I've read about OpenGL 4.5, it's going through a significant rewrite to be closer to vender-specific implementations such as AMD's Mantle, NVIDIA's CUDA and even iOS's soon-to-be Metal API (ok, so that one's OS-specific working on providing efficient OpenGL drivers under-the-hood) so we could set a texture at a specified target at a specific active texture unit in one function call instead of 2.

Share this post


Link to post
Share on other sites
Advertisement


That being said, any thread making a gl* call will be halted while the CPU queries the GPU, waiting for a response. Is this correct so far?
No. Most gl calls will just do CPU work in a driver and not communicate with the GPU at all.

The GPU usually lags behind the CPU by about a whole frame (or more), so GPU->CPU data readback is terrible for performance (can instantly halve your framerate). glGet* functions are the scary ones that can cause this kind of thing.

 

Most gl functions are just setting a small amount of data inside the driver, doing some error checking on the arguments, and setting a dirty flag.

The glDraw* functions then check all of the dirty flags, and generate any required actual native GPU commands (bind this texture, bind this shader, draw these triangles...), telling the GPU how to draw things. This is why draw-calls are expensive on the CPU-side; the driver has to do a lot of work inside the glDraw* functions to figure out what commands need to be written into the command buffer.

These commands aren't sent to the GPU synchronously -- instead they're written into a "command buffer". The GPU asynchronously reads commands from this buffer and executes them, but like I said above, the GPU will usually have about a whole frame's worth of commands buffered up at once, so there's a big delay between the CPU writing commands and the GPU executing them.

Share this post


Link to post
Share on other sites

Ok, so that being said, is it ok to make common, repetitive calls to glEnable(), glDisable(), glUseProgram(), glBindTexture(), etc with the same parameter values, or should I continue to to provide extra logic to reduce the amount of gl* calls being made. I never use glGet* calls unless it's glGetUniformLocation(), and that's just once when my shader is successfully compiled, and loaded.

 

Apple's docs have stated that it's important to provide our own state machines for GL states in the past, but now I'm starting to think it's meant only to be an alternative to constantly querying the GPU for what states are enabled. It's been years since I've read that, anyway... I learned earlier this year that GL_TEXTURE_2D not longer needs to be called in OpenGL ES 2.0, which I always assumed was necessary as I came from using OpenGL ES 1.1.

Share this post


Link to post
Share on other sites

I think it's considered good practice to remove unnecessary gl calls by shadowing the state on the application side. Certainly Apple's tools (OpenGLES analyser for instance) explicitly warn you about each and every redundant state change you make, so while we can't know what exactly their driver is doing, it'd be reasonable to assume that each redundant state change you make is causing the driver to actually add extra stuff into the command buffer.

Share this post


Link to post
Share on other sites


Apple's docs have stated that it's important to provide our own state machines for GL states

I'm no expert when it comes to OpenGL, but I think they recommend this mostly because of context resets (from https://www.khronos.org/registry/gles/extensions/EXT/EXT_robustness.txt):

 

    If the reset notification behavior is NO_RESET_NOTIFICATION_EXT,
    then the implementation will never deliver notification of reset
    events, and GetGraphicsResetStatusEXT will always return
    NO_ERROR[fn1].
       [fn1: In this case it is recommended that implementations should
        not allow loss of context state no matter what events occur.
        However, this is only a recommendation, and cannot be relied
        upon by applications.]

Share this post


Link to post
Share on other sites

I have a had issues in the past where enable and disable calls had a significant effect on performance.

 

You have to remember when working in the mobile world that not all devices are created equal.

 

Even devices with the same exact chipset will probably have a different software stack, and hence different performance.

 

A classic case is the nightmare of compiling shaders on mobile devices. I have had a case with two devices with the same GPU (Mali) and very similar hardware, one compiled the shader into 317 instructions. The other failed to compile the shader at all as the instruction limit went over 512.

 

Doing things in the best possible way from day one can really help you down the line. Honestly, it may be boring and a pain in posterior but it is worth all the effort when the game "just runs" on every device you test it on.

 

There is nothing worse than sitting there trying to figure out why the game crashes on a device that you don't own smile.png

Share this post


Link to post
Share on other sites
Sign in to follow this  

  • Advertisement
  • Advertisement
  • Popular Now

  • Advertisement
  • Similar Content

    • By reenigne
      For those that don't know me. I am the individual who's two videos are listed here under setup for https://wiki.libsdl.org/Tutorials
      I also run grhmedia.com where I host the projects and code for the tutorials I have online.
      Recently, I received a notice from youtube they will be implementing their new policy in protecting video content as of which I won't be monetized till I meat there required number of viewers and views each month.

      Frankly, I'm pretty sick of youtube. I put up a video and someone else learns from it and puts up another video and because of the way youtube does their placement they end up with more views.
      Even guys that clearly post false information such as one individual who said GLEW 2.0 was broken because he didn't know how to compile it. He in short didn't know how to modify the script he used because he didn't understand make files and how the requirements of the compiler and library changes needed some different flags.

      At the end of the month when they implement this I will take down the content and host on my own server purely and it will be a paid system and or patreon. 

      I get my videos may be a bit dry, I generally figure people are there to learn how to do something and I rather not waste their time. 
      I used to also help people for free even those coming from the other videos. That won't be the case any more. I used to just take anyone emails and work with them my email is posted on the site.

      I don't expect to get the required number of subscribers in that time or increased views. Even if I did well it wouldn't take care of each reoccurring month.
      I figure this is simpler and I don't plan on putting some sort of exorbitant fee for a monthly subscription or the like.
      I was thinking on the lines of a few dollars 1,2, and 3 and the larger subscription gets you assistance with the content in the tutorials if needed that month.
      Maybe another fee if it is related but not directly in the content. 
      The fees would serve to cut down on the number of people who ask for help and maybe encourage some of the people to actually pay attention to what is said rather than do their own thing. That actually turns out to be 90% of the issues. I spent 6 hours helping one individual last week I must have asked him 20 times did you do exactly like I said in the video even pointed directly to the section. When he finally sent me a copy of the what he entered I knew then and there he had not. I circled it and I pointed out that wasn't what I said to do in the video. I didn't tell him what was wrong and how I knew that way he would go back and actually follow what it said to do. He then reported it worked. Yea, no kidding following directions works. But hey isn't alone and well its part of the learning process.

      So the point of this isn't to be a gripe session. I'm just looking for a bit of feed back. Do you think the fees are unreasonable?
      Should I keep the youtube channel and do just the fees with patreon or do you think locking the content to my site and require a subscription is an idea.

      I'm just looking at the fact it is unrealistic to think youtube/google will actually get stuff right or that youtube viewers will actually bother to start looking for more accurate videos. 
    • By Balma Alparisi
      i got error 1282 in my code.
      sf::ContextSettings settings; settings.majorVersion = 4; settings.minorVersion = 5; settings.attributeFlags = settings.Core; sf::Window window; window.create(sf::VideoMode(1600, 900), "Texture Unit Rectangle", sf::Style::Close, settings); window.setActive(true); window.setVerticalSyncEnabled(true); glewInit(); GLuint shaderProgram = createShaderProgram("FX/Rectangle.vss", "FX/Rectangle.fss"); float vertex[] = { -0.5f,0.5f,0.0f, 0.0f,0.0f, -0.5f,-0.5f,0.0f, 0.0f,1.0f, 0.5f,0.5f,0.0f, 1.0f,0.0f, 0.5,-0.5f,0.0f, 1.0f,1.0f, }; GLuint indices[] = { 0,1,2, 1,2,3, }; GLuint vao; glGenVertexArrays(1, &vao); glBindVertexArray(vao); GLuint vbo; glGenBuffers(1, &vbo); glBindBuffer(GL_ARRAY_BUFFER, vbo); glBufferData(GL_ARRAY_BUFFER, sizeof(vertex), vertex, GL_STATIC_DRAW); GLuint ebo; glGenBuffers(1, &ebo); glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, ebo); glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices), indices,GL_STATIC_DRAW); glVertexAttribPointer(0, 3, GL_FLOAT, false, sizeof(float) * 5, (void*)0); glEnableVertexAttribArray(0); glVertexAttribPointer(1, 2, GL_FLOAT, false, sizeof(float) * 5, (void*)(sizeof(float) * 3)); glEnableVertexAttribArray(1); GLuint texture[2]; glGenTextures(2, texture); glActiveTexture(GL_TEXTURE0); glBindTexture(GL_TEXTURE_2D, texture[0]); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR); sf::Image* imageOne = new sf::Image; bool isImageOneLoaded = imageOne->loadFromFile("Texture/container.jpg"); if (isImageOneLoaded) { glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, imageOne->getSize().x, imageOne->getSize().y, 0, GL_RGBA, GL_UNSIGNED_BYTE, imageOne->getPixelsPtr()); glGenerateMipmap(GL_TEXTURE_2D); } delete imageOne; glActiveTexture(GL_TEXTURE1); glBindTexture(GL_TEXTURE_2D, texture[1]); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR); sf::Image* imageTwo = new sf::Image; bool isImageTwoLoaded = imageTwo->loadFromFile("Texture/awesomeface.png"); if (isImageTwoLoaded) { glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, imageTwo->getSize().x, imageTwo->getSize().y, 0, GL_RGBA, GL_UNSIGNED_BYTE, imageTwo->getPixelsPtr()); glGenerateMipmap(GL_TEXTURE_2D); } delete imageTwo; glUniform1i(glGetUniformLocation(shaderProgram, "inTextureOne"), 0); glUniform1i(glGetUniformLocation(shaderProgram, "inTextureTwo"), 1); GLenum error = glGetError(); std::cout << error << std::endl; sf::Event event; bool isRunning = true; while (isRunning) { while (window.pollEvent(event)) { if (event.type == event.Closed) { isRunning = false; } } glClear(GL_COLOR_BUFFER_BIT); if (isImageOneLoaded && isImageTwoLoaded) { glActiveTexture(GL_TEXTURE0); glBindTexture(GL_TEXTURE_2D, texture[0]); glActiveTexture(GL_TEXTURE1); glBindTexture(GL_TEXTURE_2D, texture[1]); glUseProgram(shaderProgram); } glBindVertexArray(vao); glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT, nullptr); glBindVertexArray(0); window.display(); } glDeleteVertexArrays(1, &vao); glDeleteBuffers(1, &vbo); glDeleteBuffers(1, &ebo); glDeleteProgram(shaderProgram); glDeleteTextures(2,texture); return 0; } and this is the vertex shader
      #version 450 core layout(location=0) in vec3 inPos; layout(location=1) in vec2 inTexCoord; out vec2 TexCoord; void main() { gl_Position=vec4(inPos,1.0); TexCoord=inTexCoord; } and the fragment shader
      #version 450 core in vec2 TexCoord; uniform sampler2D inTextureOne; uniform sampler2D inTextureTwo; out vec4 FragmentColor; void main() { FragmentColor=mix(texture(inTextureOne,TexCoord),texture(inTextureTwo,TexCoord),0.2); } I was expecting awesomeface.png on top of container.jpg

    • By khawk
      We've just released all of the source code for the NeHe OpenGL lessons on our Github page at https://github.com/gamedev-net/nehe-opengl. code - 43 total platforms, configurations, and languages are included.
      Now operated by GameDev.net, NeHe is located at http://nehe.gamedev.net where it has been a valuable resource for developers wanting to learn OpenGL and graphics programming.

      View full story
    • By TheChubu
      The Khronos™ Group, an open consortium of leading hardware and software companies, announces from the SIGGRAPH 2017 Conference the immediate public availability of the OpenGL® 4.6 specification. OpenGL 4.6 integrates the functionality of numerous ARB and EXT extensions created by Khronos members AMD, Intel, and NVIDIA into core, including the capability to ingest SPIR-V™ shaders.
      SPIR-V is a Khronos-defined standard intermediate language for parallel compute and graphics, which enables content creators to simplify their shader authoring and management pipelines while providing significant source shading language flexibility. OpenGL 4.6 adds support for ingesting SPIR-V shaders to the core specification, guaranteeing that SPIR-V shaders will be widely supported by OpenGL implementations.
      OpenGL 4.6 adds the functionality of these ARB extensions to OpenGL’s core specification:
      GL_ARB_gl_spirv and GL_ARB_spirv_extensions to standardize SPIR-V support for OpenGL GL_ARB_indirect_parameters and GL_ARB_shader_draw_parameters for reducing the CPU overhead associated with rendering batches of geometry GL_ARB_pipeline_statistics_query and GL_ARB_transform_feedback_overflow_querystandardize OpenGL support for features available in Direct3D GL_ARB_texture_filter_anisotropic (based on GL_EXT_texture_filter_anisotropic) brings previously IP encumbered functionality into OpenGL to improve the visual quality of textured scenes GL_ARB_polygon_offset_clamp (based on GL_EXT_polygon_offset_clamp) suppresses a common visual artifact known as a “light leak” associated with rendering shadows GL_ARB_shader_atomic_counter_ops and GL_ARB_shader_group_vote add shader intrinsics supported by all desktop vendors to improve functionality and performance GL_KHR_no_error reduces driver overhead by allowing the application to indicate that it expects error-free operation so errors need not be generated In addition to the above features being added to OpenGL 4.6, the following are being released as extensions:
      GL_KHR_parallel_shader_compile allows applications to launch multiple shader compile threads to improve shader compile throughput WGL_ARB_create_context_no_error and GXL_ARB_create_context_no_error allow no error contexts to be created with WGL or GLX that support the GL_KHR_no_error extension “I’m proud to announce OpenGL 4.6 as the most feature-rich version of OpenGL yet. We've brought together the most popular, widely-supported extensions into a new core specification to give OpenGL developers and end users an improved baseline feature set. This includes resolving previous intellectual property roadblocks to bringing anisotropic texture filtering and polygon offset clamping into the core specification to enable widespread implementation and usage,” said Piers Daniell, chair of the OpenGL Working Group at Khronos. “The OpenGL working group will continue to respond to market needs and work with GPU vendors to ensure OpenGL remains a viable and evolving graphics API for all its customers and users across many vital industries.“
      The OpenGL 4.6 specification can be found at https://khronos.org/registry/OpenGL/index_gl.php. The GLSL to SPIR-V compiler glslang has been updated with GLSL 4.60 support, and can be found at https://github.com/KhronosGroup/glslang.
      Sophisticated graphics applications will also benefit from a set of newly released extensions for both OpenGL and OpenGL ES to enable interoperability with Vulkan and Direct3D. These extensions are named:
      GL_EXT_memory_object GL_EXT_memory_object_fd GL_EXT_memory_object_win32 GL_EXT_semaphore GL_EXT_semaphore_fd GL_EXT_semaphore_win32 GL_EXT_win32_keyed_mutex They can be found at: https://khronos.org/registry/OpenGL/index_gl.php
      Industry Support for OpenGL 4.6
      “With OpenGL 4.6 our customers have an improved set of core features available on our full range of OpenGL 4.x capable GPUs. These features provide improved rendering quality, performance and functionality. As the graphics industry’s most popular API, we fully support OpenGL and will continue to work closely with the Khronos Group on the development of new OpenGL specifications and extensions for our customers. NVIDIA has released beta OpenGL 4.6 drivers today at https://developer.nvidia.com/opengl-driver so developers can use these new features right away,” said Bob Pette, vice president, Professional Graphics at NVIDIA.
      "OpenGL 4.6 will be the first OpenGL release where conformant open source implementations based on the Mesa project will be deliverable in a reasonable timeframe after release. The open sourcing of the OpenGL conformance test suite and ongoing work between Khronos and X.org will also allow for non-vendor led open source implementations to achieve conformance in the near future," said David Airlie, senior principal engineer at Red Hat, and developer on Mesa/X.org projects.

      View full story
    • By _OskaR
      Hi,
      I have an OpenGL application but without possibility to wite own shaders.
      I need to perform small VS modification - is possible to do it in an alternative way? Do we have apps or driver modifictions which will catch the shader sent to GPU and override it?
  • Advertisement