Sign in to follow this  

OpenGL Overhead with gl* Calls

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

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


    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
       [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

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

Sign in to follow this  

  • Announcements

  • Forum Statistics

    • Total Topics
    • Total Posts
  • Similar Content

    • By test opty
      Hi all,
      I'm starting OpenGL using a tut on the Web. But at this point I would like to know the primitives needed for creating a window using OpenGL. So on Windows and using MS VS 2017, what is the simplest code required to render a window with the title of "First Rectangle", please?
    • By DejayHextrix
      Hi, New here. 
      I need some help. My fiance and I like to play this mobile game online that goes by real time. Her and I are always working but when we have free time we like to play this game. We don't always got time throughout the day to Queue Buildings, troops, Upgrades....etc.... 
      I was told to look into DLL Injection and OpenGL/DirectX Hooking. Is this true? Is this what I need to learn? 
      How do I read the Android files, or modify the files, or get the in-game tags/variables for the game I want? 
      Any assistance on this would be most appreciated. I been everywhere and seems no one knows or is to lazy to help me out. It would be nice to have assistance for once. I don't know what I need to learn. 
      So links of topics I need to learn within the comment section would be SOOOOO.....Helpful. Anything to just get me started. 
      Dejay Hextrix 
    • By mellinoe
      Hi all,
      First time poster here, although I've been reading posts here for quite a while. This place has been invaluable for learning graphics programming -- thanks for a great resource!
      Right now, I'm working on a graphics abstraction layer for .NET which supports D3D11, Vulkan, and OpenGL at the moment. I have implemented most of my planned features already, and things are working well. Some remaining features that I am planning are Compute Shaders, and some flavor of read-write shader resources. At the moment, my shaders can just get simple read-only access to a uniform (or constant) buffer, a texture, or a sampler. Unfortunately, I'm having a tough time grasping the distinctions between all of the different kinds of read-write resources that are available. In D3D alone, there seem to be 5 or 6 different kinds of resources with similar but different characteristics. On top of that, I get the impression that some of them are more or less "obsoleted" by the newer kinds, and don't have much of a place in modern code. There seem to be a few pivots:
      The data source/destination (buffer or texture) Read-write or read-only Structured or unstructured (?) Ordered vs unordered (?) These are just my observations based on a lot of MSDN and OpenGL doc reading. For my library, I'm not interested in exposing every possibility to the user -- just trying to find a good "middle-ground" that can be represented cleanly across API's which is good enough for common scenarios.
      Can anyone give a sort of "overview" of the different options, and perhaps compare/contrast the concepts between Direct3D, OpenGL, and Vulkan? I'd also be very interested in hearing how other folks have abstracted these concepts in their libraries.
    • By aejt
      I recently started getting into graphics programming (2nd try, first try was many years ago) and I'm working on a 3d rendering engine which I hope to be able to make a 3D game with sooner or later. I have plenty of C++ experience, but not a lot when it comes to graphics, and while it's definitely going much better this time, I'm having trouble figuring out how assets are usually handled by engines.
      I'm not having trouble with handling the GPU resources, but more so with how the resources should be defined and used in the system (materials, models, etc).
      This is my plan now, I've implemented most of it except for the XML parts and factories and those are the ones I'm not sure of at all:
      I have these classes:
      For GPU resources:
      Geometry: holds and manages everything needed to render a geometry: VAO, VBO, EBO. Texture: holds and manages a texture which is loaded into the GPU. Shader: holds and manages a shader which is loaded into the GPU. For assets relying on GPU resources:
      Material: holds a shader resource, multiple texture resources, as well as uniform settings. Mesh: holds a geometry and a material. Model: holds multiple meshes, possibly in a tree structure to more easily support skinning later on? For handling GPU resources:
      ResourceCache<T>: T can be any resource loaded into the GPU. It owns these resources and only hands out handles to them on request (currently string identifiers are used when requesting handles, but all resources are stored in a vector and each handle only contains resource's index in that vector) Resource<T>: The handles given out from ResourceCache. The handles are reference counted and to get the underlying resource you simply deference like with pointers (*handle).  
      And my plan is to define everything into these XML documents to abstract away files:
      Resources.xml for ref-counted GPU resources (geometry, shaders, textures) Resources are assigned names/ids and resource files, and possibly some attributes (what vertex attributes does this geometry have? what vertex attributes does this shader expect? what uniforms does this shader use? and so on) Are reference counted using ResourceCache<T> Assets.xml for assets using the GPU resources (materials, meshes, models) Assets are not reference counted, but they hold handles to ref-counted resources. References the resources defined in Resources.xml by names/ids. The XMLs are loaded into some structure in memory which is then used for loading the resources/assets using factory classes:
      Factory classes for resources:
      For example, a texture factory could contain the texture definitions from the XML containing data about textures in the game, as well as a cache containing all loaded textures. This means it has mappings from each name/id to a file and when asked to load a texture with a name/id, it can look up its path and use a "BinaryLoader" to either load the file and create the resource directly, or asynchronously load the file's data into a queue which then can be read from later to create the resources synchronously in the GL context. These factories only return handles.
      Factory classes for assets:
      Much like for resources, these classes contain the definitions for the assets they can load. For example, with the definition the MaterialFactory will know which shader, textures and possibly uniform a certain material has, and with the help of TextureFactory and ShaderFactory, it can retrieve handles to the resources it needs (Shader + Textures), setup itself from XML data (uniform values), and return a created instance of requested material. These factories return actual instances, not handles (but the instances contain handles).
      Is this a good or commonly used approach? Is this going to bite me in the ass later on? Are there other more preferable approaches? Is this outside of the scope of a 3d renderer and should be on the engine side? I'd love to receive and kind of advice or suggestions!
    • By nedondev
      I 'm learning how to create game by using opengl with c/c++ coding, so here is my fist game. In video description also have game contain in Dropbox. May be I will make it better in future.
  • Popular Now