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DX12 short cmdlist vs. long cmdlist

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Hey Guys,

 

For recommendations of building commandlist in dx12, I remembered one which said that a good strategy is to have 12~20 draw calls per commandlist. Without wondering the reason behind it, I blindly target to fill around 12 draw/dispatch calls before submit my cmdlist to GPU in my 'single-thread' renderer.... Today after I fixed a GPU time stamp bug, I realized that my GPU always idled half of frame time.... GPU is waiting CPU 'accumulating enough draw calls' to hand the cmdlist to GPU...

 

Now I totally changed my strategy: as long as I know some draw/dispatch will take reasonable time for GPU, I submit it immediately to GPU even there may only be 1 draw/dispatch in that cmdlist, so while GPU is working on the job, CPU is building new jobs for GPU... (please let me know if that strategy is also not recommended...)

 

But why it is recommend to have 12~20 draw/dispatch per commandlist?   what's the difference between short and long cmdlist in terms of CPU/GPU overhead?

 

Thanks in advance 

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If you've noticed a difference in GPU idling between those two cases, I would guess that it's because you're syncing every frame instead of the recommended practice of having 1 frame of latency between CPU and GPU. This is mosly down to your presentation code.

You should have quite a few draws per submission because there's a large CPU cost involved in submission. One of D3D12's advantages is that draws are cheap, but doing one submit per draw-call will negate this benefit.

In my tests I found that ~500 draws per command list performed best in my game.

Thanks Hodgman,  could you elaborate on how to do the 1 frame of latency? or point me some resource about that? I have 5 frame buffers but I guess I get confused and lost somewhere. My engine is based on Microsoft's MiniEngine link though I modified lots of things, but the main framework is almost the same: So basically I record commandlist and submit it before present within the same frame.... 

My current project is more like academic research project so don't have ton of stuffs to draw, so typically I only have around 50 draw/dispatch calls per frame, I mean currently I can have CPU wait for GPU but definitely not the other way around. So what's your suggestion? 

 

also since my project have the following logic per frame, I feel it's very tricky to adapt '1frame of latency' strategy though.

do{
    m = CPU_ICPSolver( result ); // Nothing to do with GPU inside

    GPU_PrepareWorkingBuffer(
        depth_and_normalmap1, // input as SRV
        depth_and_normalmap2, // input as SRV
        matrix,               // input as CBV
        workingBuf);          // output as UAV (all 7 buffer)
    
    for (int i = 0; i < 7; ++i) {
        GPU_Reduction::Process(workingBuf[i]); // reduction to 1 float4 value inside GPU, but not copied to ReadBack buffer
    }
    GPU_Reduction::Readback( result ); // Read the reduction result, copy from default heap to readback heap, need to wait GPU inside

    reprojection_error = GetReprojectionError( result );
}while(iterations < 20 && reprojection_error > threshold) 

so my project has a long GPU, CPU work dependency chain here, any suggestions? thanks 

Edited by Mr_Fox

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I remembered one which said that a good strategy is to have 12~20 draw calls per commandlist.

Actually IIRC the suggestion is 12-20 draw calls per command list minimum.  The document Practical_DX12_Programming_Model_and_Hardware_Capabilities.pdf (on page 7) actually suggests to aim for  15 to 30 command lists per frame split across 5 to 10 execute command lists invocations.  I think you can do more command lists then the recommendation but I think I remember reading or someone saying don't exceed 10 execute command lists calls per frame.

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I assume you have compute tasks with GPU<->CPU dependencies, but maybe while waiting you can do some draw calls based on the compute results from the previous frame.

This way you might be able to fill the bubbles and utilize Hodgmans suggestion, but probably at the cost of double buffering some memory.

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The problem you have is that readback - you are going to stall the GPU while you wait for the tasks pre-readback to complete, then do the readback, and then loop again.
Every time you wait on the result you will cause the CPU and GPU to sync - this is bad voodoo.

If you are only doing that loop in your application, well, you'll have to suck it up.

Games, however, will typically run something like that over a few frames or find some other method of keeping it all on the GPU for the whole loop in order to remove that readback stall, or at least minimise the impact; say do all the work, issue a readback via the copy queue and at the last moment stall for the result if it isn't ready while doing as much work as possible on the CPU/GPU to cover the copy time and avoid stalling. As you've a CPU-GPU data dependency this could be tricky without reworking the algorithm somewhat.

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I remembered one which said that a good strategy is to have 12~20 draw calls per commandlist.

Actually IIRC the suggestion is 12-20 draw calls per command list minimum.  The document Practical_DX12_Programming_Model_and_Hardware_Capabilities.pdf (on page 7) actually suggests to aim for  15 to 30 command lists per frame split across 5 to 10 execute command lists invocations.  I think you can do more command lists then the recommendation but I think I remember reading or someone saying don't exceed 10 execute command lists calls per frame.

 

Thanks infinisearch, what I curious is how this overhead affect execution time line:  Is this overhead totally on CPU side and doesn't stall GPU? so GPU can still work on previous tasks while CPU is finishing up sending cmdlist? I think if that is the case, I do have tons of spare CPU cycles to saturate GPU by paying such overhead. But if there are any GPU-CPU sync points in this overhead, I guess I probably have to rethink my algorithm.... :(

Thanks

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I assume you have compute tasks with GPU<->CPU dependencies, but maybe while waiting you can do some draw calls based on the compute results from the previous frame. This way you might be able to fill the bubbles and utilize Hodgmans suggestion, but probably at the cost of double buffering some memory.

If you are only doing that loop in your application, well, you'll have to suck it up. Games, however, will typically run something like that over a few frames or find some other method of keeping it all on the GPU for the whole loop in order to remove that readback stall, or at least minimise the impact; say do all the work, issue a readback via the copy queue and at the last moment stall for the result if it isn't ready while doing as much work as possible on the CPU/GPU to cover the copy time and avoid stalling. As you've a CPU-GPU data dependency this could be tricky without reworking the algorithm somewhat.
 

 

Thanks JoeJ and phantom. Sadly that GPU<->CPU dependency chain is very sensitive to latency, so can't spread it into multiple frame to hide the GPU stall. The best bit is replacing CPU_ICPSolver with GPU_ICPSolver....  the main task of that function is solving a 6x6 linear system, so if you guys know any GPU solver exist already, that will be my silver bullet! And given the matrix size is fixed 6x6 I think it should totally be doable.... but please do let me know if you think a GPU linear solver is super slow, unstable and not worth it, thanks

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Thanks infinisearch, what I curious is how this overhead affect execution time line:  Is this overhead totally on CPU side and doesn't stall GPU? so GPU can still work on previous tasks while CPU is finishing up sending cmdlist? I think if that is the case, I do have tons of spare CPU cycles to saturate GPU by paying such overhead. But if there are any GPU-CPU sync points in this overhead, I guess I probably have to rethink my algorithm.... Thanks

I think the number of command lists is a CPU optimization and the number of execute command lists is a GPU optimization.  For the latter read towards the end of this thread: https://www.gamedev.net/topic/677701-d3d12-resource-barriers-in-multiple-command-lists/

 

However I doubt this is causing you significant performance problems.

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However I doubt this is causing you significant performance problems.

Thanks Infinisearch, could you give me some suggestions where else should I look at?  I found the GPU idle time by placing timestamps before and after dispatch/draw and related PSO setting, descriptor moving and transition command with in each commandlist. But since I only have one thread generate commandlist, I also use cross commandlist timestamp pairs (one at the end of previous cmdlist, and one at the begin of current cmdlist) thus this cross cmdlist timestamps pair could effectively tell me the GPU idle time between this two cmdlist (one thing I think worth noting is that present call may between such timestamp pair, so I have no idea how that affect the timing.....)  Also I understand it is possible that other GPU task (from other application) may get inserted between my two consecutive cmdlist, so GPU may not be idle during such 'idle time' (please correct me if such thing is trickier than I thought). But what I noticed is around 5ms 'GPU idle' time with only Kinect service (which I believe use GPU to perform some work, but definitly not 5ms) runing in background, so I guess at least there must be something wrong with the way I generate cmdlist....

 

Please let me know is it safe to use cross cmdlist timestamp to measure GPU idle time (especially no present call get inbetween), and it will be great if you could list some other thing which may possibly cause such GPU idle time.

 

Thanks

Edited by Mr_Fox

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Cross command list time stamps are fine under SetStablePowerState or the gpu clock can vary between the command list ( in practice, as you start pushing load on the gpu or forcing clock in a control panel, should not be a problem ), and if stable power is on, the gpu may run at a slower speed than optimal. It is worth to mention that command list order is not guarantee in a single Execute call, the driver can reorder if he think it is better for him.

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It is worth to mention that command list order is not guarantee in a single Execute call, the driver can reorder if he think it is better for him.

 

What makes you think this? I don't believe that to be true. If they executed out of order then any resource transitions within the 2 (or more) command lists could be executed in any order, potentially conflicting with the current state of the resource.

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However I doubt this is causing you significant performance problems.

Thanks Infinisearch, could you give me some suggestions where else should I look at?  I found the GPU idle time by placing timestamps before and after dispatch/draw and related PSO setting, descriptor moving and transition command with in each commandlist. But since I only have one thread generate commandlist, I also use cross commandlist timestamp pairs (one at the end of previous cmdlist, and one at the begin of current cmdlist) thus this cross cmdlist timestamps pair could effectively tell me the GPU idle time between this two cmdlist (one thing I think worth noting is that present call may between such timestamp pair, so I have no idea how that affect the timing.....)  Also I understand it is possible that other GPU task (from other application) may get inserted between my two consecutive cmdlist, so GPU may not be idle during such 'idle time' (please correct me if such thing is trickier than I thought). But what I noticed is around 5ms 'GPU idle' time with only Kinect service (which I believe use GPU to perform some work, but definitly not 5ms) runing in background, so I guess at least there must be something wrong with the way I generate cmdlist....

 

Please let me know is it safe to use cross cmdlist timestamp to measure GPU idle time (especially no present call get inbetween), and it will be great if you could list some other thing which may possibly cause such GPU idle time.

 

Thanks

 

First off let me clarify why I said I doubt its causing you significant performance problems.  Didn't you say in one of your threads that you have like 50 draw/dispatch calls total?  Also in this thread you said you submit 10-12 calls per executecommandlists call.  This means you'd have 5 calls to executecommandlists which is well within the guidelines.  But the real question is what does your profiler tell you?  I am not sure if your timestamp method of measurement is valid or not so I can't really help you there.  I myself am only beginning to learn to use a GPU profiler.  Can you try this on a machine with a working GPU profiler to confirm your results?  As far as suggestion as to what is wrong I'd suggest making sure your performance results are correct first.

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      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 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 turanszkij
      I am doing a DX12 graphics wrapper, and I would like to update constant buffers. I found the ID3D12GraphicsCommandList2::WriteBufferImmediate method, which is apparently available from a Windows 10 Creators update only. I couldn't really find any info about this (and couldn't try it yet), am I correct to assume this would be useful for writing to constant buffers without much need to do synchronization? It seems to me like this method copies data to the command list itself and then that data will be copied into the DEFAULT resource address which I provided? The only synchronization needed here would be transition barriers to COPY_DEST before WriteBufferImmediate() and back to GENERIC_READ afterwards? I could be totally off though, I'm still wrapping my head around a lot of things.
      What other use cases would this method allow for?
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