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DX11 DirectX 11 Buffer Headache

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The topic says it all. In DirectX 9 you can create a Vertex/Index buffer and then when you map it you give it an hint to tell it how you are going to access the buffer, easy to use. In DirectX 11 on the other hand we can no longer specify the map type unless CPUAccessResource is set to either Read,Write or Read/Write which is ok. But the problem comes from whenever you have a READ/WRITE resource the resource cannot be bind to anything. DX11 Provide different Access flags, but there is no flags that I can set where I can access the resource as a Read/Write on the CPU and at the same time have it bind to an vertex or index buffer. So how in DirectX11 are you supposed to create a buffer that you want to READ/Write from on the CPU side and be able to use that same buffer to render onto the screen. The buffer system in DirectX11 is definitely a step backward. If anyone has any idea, please let me know. Because right now the only way I can see to do that is to create an Staging resource and a Dynamic resource. Then copy staging resource data to the dynamic resource using an map. Doing that would force me to duplicate buffers, so I know there has to be a better way to do this.

Edited by BornToCode

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The topic says it all. In DirectX 9 you can create a Vertex/Index buffer and then when you map it you give it an hint to tell it how you are going to access the buffer, easy to use. In DirectX 11 on the other hand we can no longer specify the map type unless CPUAccessResource is set to either Read,Write or Read/Write which is ok. But the problem comes from whenever you have a READ/WRITE resource the resource cannot be bind to anything. DX11 Provide different Access flags, but there is no flags that I can set where I can access the resource as a Read/Write on the CPU and at the same time have it bind to an vertex or index buffer. So how in DirectX11 are you supposed to create a buffer that you want to READ/Write from on the CPU side and be able to use that same buffer to render onto the screen. The buffer system in DirectX11 is definitely a step backward. If anyone has any idea, please let me know. Because right now the only way I can see to do that is to create an Staging resource and a Dynamic resource. Then copy staging resource data to the dynamic resource using an map. Doing that would force me to duplicate buffers, so I know there has to be a better way to do this.

It'd be helpful if we knew why you're reading back data from the buffer on the CPU?  That sounds a little counterproductive to me.

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Are you saying that the issue is that you can't map the buffer while it is bound to the pipeline?  If so, why don't you just un-bind it by setting a null to that buffer slot?

 

Vertex and index buffers most certainly can be mapped and directly used, so I'm not really sure what the issue is that you are facing...

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The old D9D9 way of dealing with resources was completely broken in terms of how GPU's actually work. When you put a resource in GPU memory it's no longer accessible to the CPU, since it's a completely different memory pool that's not accessible to userspace code. In order to support the old (broken) D3D9 semantics drivers had to do crazy things behind the scenes, and the D3D runtime usually had to keep a separate copy of the resource contents in CPU memory. Starting with D3D10 they cleaned all of this up in order to better reflect the way CPU's work, and to also force programs to use the "fast path" by default by not giving them traps to fall into that would cause performance degradation or excessive memory allocation by the runtime or driver. Part of this is that you can no longer just grab GPU resources on the CPU, and you have explicitly specify up-front what behavior you want from a resource.

That said, why would you ever need to read back vertex buffer data? If you've provided the data, then you surely already have access to that data and you can keep it around for later. You wouldn't be wasting any memory compared to the old D3D9 behavior or using a staging buffer, and it would be more efficient to boot.

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Are you saying that the issue is that you can't map the buffer while it is bound to the pipeline?  If so, why don't you just un-bind it by setting a null to that buffer slot?

 

Vertex and index buffers most certainly can be mapped and directly used, so I'm not really sure what the issue is that you are facing...

I am not talking about when I am binding it through the binding stage. I mean at creation time when setting the Bind Flags stages the buffer will mapped to. The Bind

Flag stage can only be set if the buffer will not be read from the CPU which means staging. Therefore preventing me from creating any type of resource view using the buffer.

 

The old D9D9 way of dealing with resources was completely broken in terms of how GPU's actually work. When you put a resource in GPU memory it's no longer accessible to the CPU, since it's a completely different memory pool that's not accessible to userspace code. In order to support the old (broken) D3D9 semantics drivers had to do crazy things behind the scenes, and the D3D runtime usually had to keep a separate copy of the resource contents in CPU memory. Starting with D3D10 they cleaned all of this up in order to better reflect the way CPU's work, and to also force programs to use the "fast path" by default by not giving them traps to fall into that would cause performance degradation or excessive memory allocation by the runtime or driver. Part of this is that you can no longer just grab GPU resources on the CPU, and you have explicitly specify up-front what behavior you want from a resource.

That said, why would you ever need to read back vertex buffer data? If you've provided the data, then you surely already have access to that data and you can keep it around for later. You wouldn't be wasting any memory compared to the old D3D9 behavior or using a staging buffer, and it would be more efficient to boot.

I know that CPUVM cannot see GPUVM, but that does not matter in that case. Currently the drivers just copies the GPU memory over to the CPU memory whenever you do an map, then copy it back to GPUVM whenever you do Unmap. So those flags are just bogus, all they do is give the driver some hint of which memory pool to put it in.. Even with the implementation of this new system. The driver side has not change on how the buffers are handle today, because CPU still cannot see GPUVM. The only reason they are those restriction is because Microsoft introduced them in the spec. The reason I know about the how the driver works is because last year I work on them. So the two copy in memory in D3D9 like you said is a false statement.

 

Now to answer your other question, the reason why I want to access it. Let's say I have a buffer X that has been updated and outputted in an OuputStream. How do I read that output stream back on the CPU if I want to that since the CPU cannot read the resource.

Edited by BornToCode

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Now to answer your other question, the reason why I want to access it. Let's say I have a buffer X that has been updated and outputted in an OuputStream. How do I read that output stream back on the CPU if I want to that since the CPU cannot read the resource.

You use a staging buffer - that is its whole purpose for existing, just to give  you CPU read access to the GPU based resource objects.  You can write to buffers directly with the appropriate flags, but reading requires a separate buffer.  If you don't like hassling with the copy, then just write a small wrapper for your buffers that handles the copying and mirroring of the buffer data for you.

 

However, I would invest in a technique that could just directly use the stream output buffers without CPU intervention.  That will make the whole thing much faster, reducing bandwidth requirements, and eliminating the need for a staging buffer altogether.  This of course assumes that you don't need to store the data for whatever reason, but if you are only going to consume the contents then it would be far better just to use it directly.

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Now to answer your other question, the reason why I want to access it. Let's say I have a buffer X that has been updated and outputted in an OuputStream. How do I read that output stream back on the CPU if I want to that since the CPU cannot read the resource.

You use a staging buffer - that is its whole purpose for existing, just to give  you CPU read access to the GPU based resource objects.  You can write to buffers directly with the appropriate flags, but reading requires a separate buffer.  If you don't like hassling with the copy, then just write a small wrapper for your buffers that handles the copying and mirroring of the buffer data for you.

 

However, I would invest in a technique that could just directly use the stream output buffers without CPU intervention.  That will make the whole thing much faster, reducing bandwidth requirements, and eliminating the need for a staging buffer altogether.  This of course assumes that you don't need to store the data for whatever reason, but if you are only going to consume the contents then it would be far better just to use it directly.

That is what i meant in my first post. I would need to have a staging buffer and and dynamic buffer. Why do i need t o create two buffers. If i did not care about reading the ouput stream, then what you mention would have work and i would not have posted this in the first place. But i want to be able to access the buffer on the CPU for Read Access with a Bind Flag, without having to duplicate anything similar to how DirectX9 works. But it seems like there is no way to do what i want to do in DX11 without having the darn staging buffer. I need to read that buffer on the CPU because after the ouputStream is filled up i am doing something with it on the CPU side. For now i will just do the two buffer route just to get this working. Thanks everyone for the help.

Edited by BornToCode

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I'm not sure if you saw it yet or not, but in the new features for Direct3D 11.2 there is a new feature which allows mapping of default usage buffers.  This fits what you are talking about, and bypasses the staging buffer require in between.

 

I don't know if Windows 8.1 is a target system for you, but it seems to be in the cards to have this ability.  They mentioned in the BUILD presentation that it only works for buffers right now (due to considerations in using the compute shader) but one could foresee this coming to all resources eventually (that's my guess anyways).

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      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 two samples, asteroids performance benchmark and example Unity project that uses Diligent Engine in native plugin.
      AntTweakBar sample is Diligent Engine’s “Hello World” example.

       
      Atmospheric scattering sample is a more advanced example. It 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 and Android platforms. Direct3D11, Direct3D12, OpenGL/GLES backends are now feature complete. Vulkan backend is coming next, and support for more platforms is planned.
    • By kan123
      Hello,
      DX9Ex. I have the problem with driver stability in time of serial renderings, which i try to use for image processing in memory with fragment shaders. For big bitmaps the video driver sometimes becomes unstable ("Display driver stopped responding and has recovered") and, for instance, if the media player runs video in background, it sometimes freezes and distorts. I tried to use next methods of IDirect3DDevice9Ex:
      SetGPUThreadPriority(-7);
      WaitForVBlank(0);
      EvictManagedResources();
      with purpose to give some time for GPU between scenes, but it seems to be has not notable effect in this case. I don't want to reinitilialize subsystem for every step to avoid performance loss.
      So, my question is next: does some common practice exists to avoid overloading of GPU by running tasks? Many thanks in advance.
       
    • By AxeGuywithanAxe
      I wanted to see how others are currently handling descriptor heap updates and management.
      I've read a few articles and there tends to be three major strategies :
      1 ) You split up descriptor heaps per shader stage ( i.e one for vertex shader , pixel , hull, etc)
      2) You have one descriptor heap for an entire pipeline
      3) You split up descriptor heaps for update each update frequency (i.e EResourceSet_PerInstance , EResourceSet_PerPass , EResourceSet_PerMaterial, etc)
      The benefits of the first two approaches is that it makes it easier to port current code, and descriptor / resource descriptor management and updating tends to be easier to manage, but it seems to be not as efficient.
      The benefits of the third approach seems to be that it's the most efficient because you only manage and update objects when they change.
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