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OpenGL VRAM with VBO's and VAR's (or D3D equivalent)

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I'm reading up on exactly how the system and video card interacts in terms of memory. I'm trying to figure how to decide when to upload data to VRAM, how to do it, etc. I have a pretty good grasp on vertex buffer objects. I've never really worried about memory, but I want to start building a game that might need to manage it a little. Tell if this is a bad idea. With VBO's, you can figure out if you've ran out of memory by detecting the OUT_OF_MEMORY error from the BufferDataARB call. However, it seems risky to wait until you totally filled up the memory. Anyway, I guess you could detect that when you run out, delete the least recently used buffer, and try to upload the data again. But that seems inefficient and risky. You could get a kind of thrashing if you have a big buffer that is constantly deleted and reuploaded, which would make many other things reupload as well. That kind of takes that point out of the vertex buffer object, where you can upload data once and not worry about it again. What's a simple uploading/caching scheme? Do you usually use vertex arrays in combination with vertex buffer objects? What exactly happens in VRAM with vertex arrays? You upload the data once, and then is it cached at all? If so, how do you detect it, or does the system do that for you so that when you call a {Vertex}Pointer() function it finds the cached data? edit: I just realized that I'm talking about OpenGL structures. I hope this is still applicable in this forum. I guess I just wanted a general answer as to how you decide when/how to upload data to VRAM (all of it once at the beginning, if it all fits? how do you know if it fits?) [Edited by - okonomiyaki on July 7, 2004 11:47:59 AM]

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Hi...

Quote:

With VBO's, you can figure out if you've ran out of memory by detecting the OUT_OF_MEMORY error from the BufferDataARB call.


I'm not really sure that this is true. If BufferDataARB can't allocate the requested buffer on Video Memory, it will try AGP Memory and if it fails again it will try to allocate it on system memory. Don't take this too seriously though, because there is sometime since i read the specs. But i think this is the behavior for VBO at least.
With VAR it can be different because of the frequency and priority flags.

Any feedback on this?

Quote:

However, it seems risky to wait until you totally filled up the memory. Anyway, I guess you could detect that when you run out, delete the least recently used buffer, and try to upload the data again.


What happens if the 90% of the memory is allocated by textures, display lists, and other things that go to the Video/AGP memory? When you are at this situation, how can you be sure that deallocating the LRU vertex buffer will solve the problem?

What i thought for preventing situation like these, is precalculate the maximum texture/vbo/index etc. memory a certain level will use, and try to minimize the requirements "a priori" (e.g. compress textures to free some memory, use 16-bit indices where applicable, etc.). Or, if we are talking for really big levels (from the memory point of view), try to calculate the mean memory usage for some positions in it, and try to minimize this. This stands for situations where you stream data (almost) continuesly from the HD. Something like FarCry's terrain renderer (it keeps unloading and loading terrain chunks all the time, when you move fast of course (see editor)). BUT, what happens if you can't minimize memory requirements? Then you need somekind of memory manager for that. But, don't wait running out of memory to start unloading data. Because in OGL, i don't there is anyway for requesting availiable mem of any type (Vid/AGP).

Quote:

What's a simple uploading/caching scheme? Do you usually use vertex arrays in combination with vertex buffer objects? What exactly happens in VRAM with vertex arrays? You upload the data once, and then is it cached at all? If so, how do you detect it, or does the system do that for you so that when you call a {Vertex}Pointer() function it finds the cached data?


What do you mean by caching? If your data were uploaded to the Video RAM, then (i want to believe that) they will stay there as long I don't remove them. If you mean Post TnL caching it has nothing to do with where the data is stored. It works for system memory VAs or VBOs or VAR, as long as you are using indexed arrays.

About the uploading/caching scheme you said, sorry but i can't comment. I haven't experimented with anything like this before, so...

Hope i didn't confused you, or said anything that isn't true. If something isn't true, someone please correct me.

HellRaiZer

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Original post by HellRaiZer
Hi...

Quote:

With VBO's, you can figure out if you've ran out of memory by detecting the OUT_OF_MEMORY error from the BufferDataARB call.


I'm not really sure that this is true. If BufferDataARB can't allocate the requested buffer on Video Memory, it will try AGP Memory and if it fails again it will try to allocate it on system memory. Don't take this too seriously though, because there is sometime since i read the specs. But i think this is the behavior for VBO at least.
With VAR it can be different because of the frequency and priority flags.

Any feedback on this?


"OUT_OF_MEMORY may be generated if the data store of a buffer object
cannot be allocated because the <size> argument of BufferDataARB is
too large."

Maybe I misinterpreted that. Now that I think about it, you're probably right. The system would probably fall back on some kind of paging scheme. Oh well, I never liked my idea in the first place anyway.

Quote:
Quote:

However, it seems risky to wait until you totally filled up the memory. Anyway, I guess you could detect that when you run out, delete the least recently used buffer, and try to upload the data again.


What happens if the 90% of the memory is allocated by textures, display lists, and other things that go to the Video/AGP memory? When you are at this situation, how can you be sure that deallocating the LRU vertex buffer will solve the problem?

What i thought for preventing situation like these, is precalculate the maximum texture/vbo/index etc. memory a certain level will use, and try to minimize the requirements "a priori" (e.g. compress textures to free some memory, use 16-bit indices where applicable, etc.). Or, if we are talking for really big levels (from the memory point of view), try to calculate the mean memory usage for some positions in it, and try to minimize this. This stands for situations where you stream data (almost) continuesly from the HD. Something like FarCry's terrain renderer (it keeps unloading and loading terrain chunks all the time, when you move fast of course (see editor)). BUT, what happens if you can't minimize memory requirements? Then you need somekind of memory manager for that. But, don't wait running out of memory to start unloading data. Because in OGL, i don't there is anyway for requesting availiable mem of any type (Vid/AGP).


Well, I meant that in a general sense. Free a texture, free a buffer, whatever is needed.
The more I think about it though, the more I'm inclined to forget about vertex arrays altogether and use VBO's for everything. OpenGL and drivers should do memory management for me with many small buffers. I shouldn't have to really worry about it.
Though, of course it's good to minimize memory requirements. But I should be able to upload everything to VRAM and let the drivers worry about the rest, right?

Quote:

What do you mean by caching?


I meant in the sense where you upload data one frame, and then come around the next frame, and the data is still cached in memory (VRAM). Yes, you are right that it should stay there. But somewhere later on you might delete the buffer for various reasons. That's kind of what I was asking, if a typical scheme consisted of managing buffers or anything uploaded to VRAM.


I was just searching around and I found some stuff in the Materials/Shaders thread. In the described effect system, they always check to see if the data is already "cached" (uploaded to the video card). If it's not, they reupload it. That's my question, why would it ever end up not being cached?

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The whole caching subject is down to the fact that you dont want to make lots of little VBOs or even one big VBO, because the driver cant manage them properly (ATI and NV both agree on this point).
Instead you can create a bunch of VBOs which are filled as need and the size taken in each one and what is allocated to it is tracked. Then, when you come to draw your model you see if its already inserted into a VBO, if it is then render away, if it isnt then upload the data to the VBO and then render.

For the most part, yes the driver will handle VBO memory management using VRAM, AGP and even System as needed, however you can help it by not allocating one huge buffer or loads and loads of little ones.

As to how big is too big and how small is too small, i asked that question in the OGL forum a while back and got some answers, although i dont recall atm what they were [smile]

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Original post by _the_phantom_
For the most part, yes the driver will handle VBO memory management using VRAM, AGP and even System as needed, however you can help it by not allocating one huge buffer or loads and loads of little ones.


though that part is still more than confusing to me. as a simple question:

if i can easily allocate the required memory as chunks of system memory, shouldnt the driver be able to allocate the SAME memory as vbo? instead it just seems to fail when its out of video memory, forcing me to fall back to vertex arrays for the remaining few chunks (which will kill performance, because for a weird reason my indices are in video memory and _usually_ squeeze out a few percent more speed). i begin to wonder if there is an upper limit to how much memory can be managed by the driver for vbo (as a sum, not a big single chunk).

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For the most part, yes the driver will handle VBO memory management using VRAM, AGP and even System as needed, however you can help it by not allocating one huge buffer or loads and loads of little ones.


ok, if this is true, I can understand it. It's better to handle yourself. But I assume it can kind of be a loose system, where it's a simple design that isn't bulletproof, because the drivers should be able to do some management.

Quote:

if i can easily allocate the required memory as chunks of system memory, shouldnt the driver be able to allocate the SAME memory as vbo? instead it just seems to fail when its out of video memory, forcing me to fall back to vertex arrays for the remaining few chunks (which will kill performance, because for a weird reason my indices are in video memory and _usually_ squeeze out a few percent more speed). i begin to wonder if there is an upper limit to how much memory can be managed by the driver for vbo (as a sum, not a big single chunk).


That's exactly what I'm trying to figure out. The specs don't really go into it. So you've encountered the OUT_OF_MEMORY error. I suppose there could be an upper limit to the amount of memory, though I don't see how this could be hardcoded with the variable amount of memory on different systems. Maybe it doesn't try to allocate any memory other than VRAM.

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In theory, yeah the driver should fall back from vram to agp and then system, however I guess that the implimentations still arent perfect.

The drivers do perform some management, but its hard for it to manage large cunks of ram allocated effectively and for small chunks the overhead of buffer switching and management out weights any gains you might have gotten anyway.

I'll have to do some experiments as to how much VBO space i can reserve etc at some point, if its silly for my setup then I guess its time to poke some driver dev people [smile]

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Original post by _the_phantom_
I'll have to do some experiments as to how much VBO space i can reserve etc at some point, if its silly for my setup then I guess its time to poke some driver dev people [smile]


on my radeon about 17mb in a single chunk worked, 70mb didnt. if i allocate a bunch of chunks i can get 12 of these 17mb chunks (the remaining 4 that i would need fail). that would be about 200mb, but seeing all the textures, frame/depth/whatever buffers thats obviously where i run out of video memory. unfortunately it refuses to use anything else, even if i try a usage like stream_read.

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I guess I'll just reserve a certain amount of memory for vertices and all their attributes, and when rendering, use a kind of sliding slot system. I don't think it should be that hard to implement, and I guess it gives me more control on exactly what's going on, and lets me track what's being uploaded and when, etc.
Should be fun. Question is how much to reserve :D

edit: also, I'm think I just reserve that as one big buffer and then map into it? Seems wrong for some reason. I'll try it out.

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Original post by _the_phantom_
yeah, I'd have thought that would fall back into AGP/system ram at that point as well... hmmm intresting...


especially since having to fall back to vertex arrays for the remaining data creates an ugly mess and the constant need to check if vbo needs to be disabled (not to mention that vbo index buffers dont play nice with va vertex data). i wouldnt mind if vbo could completely replace va and become a single solution, but with all the little issues they seem to be more complicated than one would have expected.

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      PipelineStateDesc PSODesc; RasterizerStateDesc &RasterizerDesc = PSODesc.GraphicsPipeline.RasterizerDesc; RasterizerDesc.FillMode = FILL_MODE_SOLID; RasterizerDesc.CullMode = CULL_MODE_NONE; RasterizerDesc.FrontCounterClockwise = True; RasterizerDesc.ScissorEnable = True; RasterizerDesc.AntialiasedLineEnable = False; Depth-stencil and blend states are defined in a similar fashion.
      Another important thing that pipeline state object encompasses is the input layout description that defines how inputs to the vertex shader, which is the very first shader stage, should be read from the memory. Input layout may define several vertex streams that contain values of different formats and sizes:
      // Define input layout InputLayoutDesc &Layout = PSODesc.GraphicsPipeline.InputLayout; LayoutElement TextLayoutElems[] = {     LayoutElement( 0, 0, 3, VT_FLOAT32, False ),     LayoutElement( 1, 0, 4, VT_UINT8, True ),     LayoutElement( 2, 0, 2, VT_FLOAT32, False ), }; Layout.LayoutElements = TextLayoutElems; Layout.NumElements = _countof( TextLayoutElems ); Finally, pipeline state defines primitive topology type. When all required members are initialized, a pipeline state object can be created by IRenderDevice::CreatePipelineState() method:
      // Define shader and primitive topology PSODesc.GraphicsPipeline.PrimitiveTopologyType = PRIMITIVE_TOPOLOGY_TYPE_TRIANGLE; PSODesc.GraphicsPipeline.pVS = pVertexShader; PSODesc.GraphicsPipeline.pPS = pPixelShader; PSODesc.Name = "My pipeline state"; m_pDev->CreatePipelineState(PSODesc, &m_pPSO); When PSO object is bound to the pipeline, the engine invokes all API-specific commands to set all states specified by the object. In case of Direct3D12 this maps directly to setting the D3D12 PSO object. In case of Direct3D11, this involves setting individual state objects (such as rasterizer and blend states), shaders, input layout etc. In case of OpenGL, this requires a number of fine-grain state tweaking calls. Diligent Engine keeps track of currently bound states and only calls functions to update these states that have actually changed.
      Binding Shader Resources
      Direct3D11 and OpenGL utilize fine-grain resource binding models, where an application binds individual buffers and textures to certain shader or program resource binding slots. Direct3D12 uses a very different approach, where resource descriptors are grouped into tables, and an application can bind all resources in the table at once by setting the table in the command list. Resource binding model in Diligent Engine is designed to leverage this new method. It introduces a new object called shader resource binding that encapsulates all resource bindings required for all shaders in a certain pipeline state. It also introduces the classification of shader variables based on the frequency of expected change that helps the engine group them into tables under the hood:
      Static variables (SHADER_VARIABLE_TYPE_STATIC) are variables that are expected to be set only once. They may not be changed once a resource is bound to the variable. Such variables are intended to hold global constants such as camera attributes or global light attributes constant buffers. Mutable variables (SHADER_VARIABLE_TYPE_MUTABLE) define resources that are expected to change on a per-material frequency. Examples may include diffuse textures, normal maps etc. Dynamic variables (SHADER_VARIABLE_TYPE_DYNAMIC) are expected to change frequently and randomly. Shader variable type must be specified during shader creation by populating an array of ShaderVariableDesc structures and initializing ShaderCreationAttribs::Desc::VariableDesc and ShaderCreationAttribs::Desc::NumVariables members (see example of shader creation above).
      Static variables cannot be changed once a resource is bound to the variable. They are bound directly to the shader object. For instance, a shadow map texture is not expected to change after it is created, so it can be bound directly to the shader:
      PixelShader->GetShaderVariable( "g_tex2DShadowMap" )->Set( pShadowMapSRV ); Mutable and dynamic variables are bound via a new Shader Resource Binding object (SRB) that is created by the pipeline state (IPipelineState::CreateShaderResourceBinding()):
      m_pPSO->CreateShaderResourceBinding(&m_pSRB); Note that an SRB is only compatible with the pipeline state it was created from. SRB object inherits all static bindings from shaders in the pipeline, but is not allowed to change them.
      Mutable resources can only be set once for every instance of a shader resource binding. Such resources are intended to define specific material properties. For instance, a diffuse texture for a specific material is not expected to change once the material is defined and can be set right after the SRB object has been created:
      m_pSRB->GetVariable(SHADER_TYPE_PIXEL, "tex2DDiffuse")->Set(pDiffuseTexSRV); In some cases it is necessary to bind a new resource to a variable every time a draw command is invoked. Such variables should be labeled as dynamic, which will allow setting them multiple times through the same SRB object:
      m_pSRB->GetVariable(SHADER_TYPE_VERTEX, "cbRandomAttribs")->Set(pRandomAttrsCB); Under the hood, the engine pre-allocates descriptor tables for static and mutable resources when an SRB objcet is created. Space for dynamic resources is dynamically allocated at run time. Static and mutable resources are thus more efficient and should be used whenever possible.
      As you can see, Diligent Engine does not expose low-level details of how resources are bound to shader variables. One reason for this is that these details are very different for various APIs. The other reason is that using low-level binding methods is extremely error-prone: it is very easy to forget to bind some resource, or bind incorrect resource such as bind a buffer to the variable that is in fact a texture, especially during shader development when everything changes fast. Diligent Engine instead relies on shader reflection system to automatically query the list of all shader variables. Grouping variables based on three types mentioned above allows the engine to create optimized layout and take heavy lifting of matching resources to API-specific resource location, register or descriptor in the table.
      This post gives more details about the resource binding model in Diligent Engine.
      Setting the Pipeline State and Committing Shader Resources
      Before any draw or compute command can be invoked, the pipeline state needs to be bound to the context:
      m_pContext->SetPipelineState(m_pPSO); Under the hood, the engine sets the internal PSO object in the command list or calls all the required native API functions to properly configure all pipeline stages.
      The next step is to bind all required shader resources to the GPU pipeline, which is accomplished by IDeviceContext::CommitShaderResources() method:
      m_pContext->CommitShaderResources(m_pSRB, COMMIT_SHADER_RESOURCES_FLAG_TRANSITION_RESOURCES); The method takes a pointer to the shader resource binding object and makes all resources the object holds available for the shaders. In the case of D3D12, this only requires setting appropriate descriptor tables in the command list. For older APIs, this typically requires setting all resources individually.
      Next-generation APIs require the application to track the state of every resource and explicitly inform the system about all state transitions. For instance, if a texture was used as render target before, while the next draw command is going to use it as shader resource, a transition barrier needs to be executed. Diligent Engine does the heavy lifting of state tracking.  When CommitShaderResources() method is called with COMMIT_SHADER_RESOURCES_FLAG_TRANSITION_RESOURCES flag, the engine commits and transitions resources to correct states at the same time. Note that transitioning resources does introduce some overhead. The engine tracks state of every resource and it will not issue the barrier if the state is already correct. But checking resource state is an overhead that can sometimes be avoided. The engine provides IDeviceContext::TransitionShaderResources() method that only transitions resources:
      m_pContext->TransitionShaderResources(m_pPSO, m_pSRB); In some scenarios it is more efficient to transition resources once and then only commit them.
      Invoking Draw Command
      The final step is to set states that are not part of the PSO, such as render targets, vertex and index buffers. Diligent Engine uses Direct3D11-syle API that is translated to other native API calls under the hood:
      ITextureView *pRTVs[] = {m_pRTV}; m_pContext->SetRenderTargets(_countof( pRTVs ), pRTVs, m_pDSV); // Clear render target and depth buffer const float zero[4] = {0, 0, 0, 0}; m_pContext->ClearRenderTarget(nullptr, zero); m_pContext->ClearDepthStencil(nullptr, CLEAR_DEPTH_FLAG, 1.f); // Set vertex and index buffers IBuffer *buffer[] = {m_pVertexBuffer}; Uint32 offsets[] = {0}; Uint32 strides[] = {sizeof(MyVertex)}; m_pContext->SetVertexBuffers(0, 1, buffer, strides, offsets, SET_VERTEX_BUFFERS_FLAG_RESET); m_pContext->SetIndexBuffer(m_pIndexBuffer, 0); Different native APIs use various set of function to execute draw commands depending on command details (if the command is indexed, instanced or both, what offsets in the source buffers are used etc.). For instance, there are 5 draw commands in Direct3D11 and more than 9 commands in OpenGL with something like glDrawElementsInstancedBaseVertexBaseInstance not uncommon. Diligent Engine hides all details with single IDeviceContext::Draw() method that takes takes DrawAttribs structure as an argument. The structure members define all attributes required to perform the command (primitive topology, number of vertices or indices, if draw call is indexed or not, if draw call is instanced or not, if draw call is indirect or not, etc.). For example:
      DrawAttribs attrs; attrs.IsIndexed = true; attrs.IndexType = VT_UINT16; attrs.NumIndices = 36; attrs.Topology = PRIMITIVE_TOPOLOGY_TRIANGLE_LIST; pContext->Draw(attrs); For compute commands, there is IDeviceContext::DispatchCompute() method that takes DispatchComputeAttribs structure that defines compute grid dimension.
      Source Code
      Full engine source code is available on GitHub and is free to use. The repository contains tutorials, sample applications, asteroids performance benchmark and an example Unity project that uses Diligent Engine in native plugin.
      Atmospheric scattering sample demonstrates how Diligent Engine can be used to implement various rendering tasks: loading textures from files, using complex shaders, rendering to multiple render targets, using compute shaders and unordered access views, etc.

      Asteroids performance benchmark is based on this demo developed by Intel. It renders 50,000 unique textured asteroids and allows comparing performance of Direct3D11 and Direct3D12 implementations. Every asteroid is a combination of one of 1000 unique meshes and one of 10 unique textures.

      Finally, there is an example project that shows how Diligent Engine can be integrated with Unity.

      Future Work
      The engine is under active development. It currently supports Windows desktop, Universal Windows, Linux, Android, MacOS, and iOS platforms. Direct3D11, Direct3D12, OpenGL/GLES backends are now feature complete. Vulkan backend is coming next, and Metal backend is in the plan.
    • By LifeArtist
      Good Evening,
      I want to make a 2D game which involves displaying some debug information. Especially for collision, enemy sights and so on ...
      First of I was thinking about all those shapes which I need will need for debugging purposes: circles, rectangles, lines, polygons.
      I am really stucked right now because of the fundamental question:
      Where do I store my vertices positions for each line (object)? Currently I am not using a model matrix because I am using orthographic projection and set the final position within the VBO. That means that if I add a new line I would have to expand the "points" array and re-upload (recall glBufferData) it every time. The other method would be to use a model matrix and a fixed vbo for a line but it would be also messy to exactly create a line from (0,0) to (100,20) calculating the rotation and scale to make it fit.
      If I proceed with option 1 "updating the array each frame" I was thinking of having 4 draw calls every frame for the lines vao, polygons vao and so on. 
      In addition to that I am planning to use some sort of ECS based architecture. So the other question would be:
      Should I treat those debug objects as entities/components?
      For me it would make sense to treat them as entities but that's creates a new issue with the previous array approach because it would have for example a transform and render component. A special render component for debug objects (no texture etc) ... For me the transform component is also just a matrix but how would I then define a line?
      Treating them as components would'nt be a good idea in my eyes because then I would always need an entity. Well entity is just an id !? So maybe its a component?
      Regards,
      LifeArtist
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