Entries for tag "gpu", ordered from most recent. Entry count: 15.
# Differences in memory management between Direct3D 12 and Vulkan
Since July 2017 I develop Vulkan Memory Allocator (VMA) – a C++ library that helps with memory management in games and other applications using Vulkan. But because I deal with both Vulkan and DirectX 12 in my everyday work, I think it’s a good idea to compare them.
This is an article about a very specific topic. It may be useful to you if you are a programmer working with both graphics APIs – Direct3D 12 and Vulkan. These two APIs offer a similar set of features and performance. Both are the new generation, explicit, low-level interfaces to the modern graphics hardware (GPUs), so we could compare them back-to-back to show similarities and differences, e.g. in naming things. For example,
ID3D12CommandQueue::ExecuteCommandLists function has Vulkan equivalent in form of
vkQueueSubmit function. However, this article focuses on just one aspect – memory management, which means the rules and limitation of GPU memory allocation and the creation of resources – images (textures, render targets, depth-stencil surfaces etc.) and buffers (vertex buffers, index buffers, constant/uniform buffers etc.) Chapters below describe pretty much all the aspects of memory management that differ between the two APIs.
# Programming FreeSync 2 support in Direct3D
AMD just showed Oasis demo, presenting usage of its FreeSync 2 HDR technology. If you wonder how could you implement same features in your Windows DirectX program or game (it doesn’t matter if you use D3D11 or D3D12), here is an article for you.
But first, a disclaimer: Although I already put it on my “About” page, I’d like to stress that this is my personal blog, so all opinions presented here are my own and do not reflect that of my employer.
Radeon FreeSync (its new, official web page is here: Radeon™ FreeSync™ Technology | FreeSync™ 2 HDR Games) is an AMD technology that covers two different things, which may cause some confusion. First is variable refresh rate, second is HDR. Both of them need to be supported by a monitor. The database of FreeSync compatible monitors and their parameters is: Freesync Monitors.
# Programming HDR monitor support in Direct3D
I got an HDR supporting monitor (LG 32GK850F), so I started learning how I can use its capabilities programatically. I still have much to learn, as there is a lot of theory to be ingested about color spaces etc., but in this blog post I’d like to go straight to the point: How to enable HDR in your C++ DirectX program? To test this, I used 3 graphics chips from 3 different PC GPU vendors. Below you can see results of my experiments.
# How to design API of a library for Vulkan?
In my previous blog post yesterday, I shared my thoughts on graphics APIs and libraries. Another problem that brought me to these thoughts is a question: How do you design an API for a library that implements a single algorithm, pass, or graphics effect, using Vulkan or DX12? It may seem trivial at first, like a task that just needs to be designed and implemented, but if you think about it more, it turns out to be a difficult issue. They are few software libraries like this in existence. I don’t mean here a complex library/framework/engine that “horizontally” wraps the entire graphics API and takes it to a higher level, like V-EZ, Nvidia Falcor, or Google Filament. I mean just a small, “vertical”, plug-in library doing one thing, e.g. implementing ambient occlusion effect, efficient texture mipmap down-sampling, rendering UI, or simulating particle physics on the GPU. Such library needs to interact efficiently with the rest of the user’s code to be part of a large program or game. Vulkan Memory Allocator is also not a good example of this, because it only manages memory, implements no render passes, involves no shaders, and it interacts with a command buffer only in its part related to memory defragmentation.
I met this problem at my work. Later I also discussed it in details with my colleague. There are multiple questions to consider:
VK_IMAGE_USAGE_flags. If the library should allocate them internally, how should it do it? By just grabbing new pieces of
VkDeviceMemoryblocks? What if the user prefers all GPU memory to be allocated using a complex allocator of his choice, like Vulkan Memory Allocator?
VkDescriptorPool, but what if the user prefers all the descriptors to be allocated from his own descriptor pool?
vkGetDeviceProcAddr, possibly with help of something like volk? The library should be able to use those.
VkAllocationCallbacks) to all Vulkan functions? The library should be able to do it.
This is a problem similar to what we have with any C++ libraries. There is no consensus about the implementation of various basic facilities, like strings, containers, asserts, mutexes etc., so every major framework or game engine implements its own. Even something so simple as min/max function is defined is multiple places. It is defined once in
<algorithm> header, but some developers don’t use STL.
<Windows.h> provides its own, but these are defined as macros, so they break any other, unless you
#define NOMINMAX before the include… A typical C++ nightmare. Smaller libraries are better just configurable or define their own everything, like the Vulkan Memory Allocator having its own assert, vector (can be switched to standard STL one), and 3 versions of read-write mutex.
All these issues make it easier for developers to just write a paper, describe their algorithm, possibly share a piece of code, pseudo-code or a shader, rather than provide ready to use library. This is a very bad situation. I hope that over time patterns emerge of how the API of a library implementing a single pass or effect using Vulkan/DX12 should look like. Recently my colleague shared an idea with me that if there was some higher-level API that would implement all these interactions between various parts (like resource allocation, image barriers) and we all commonly agreed on using it, then authoring libraries and stitching them together on top of it would be way easier. That’s another argument for the need of such new, higher-level graphics API.
# Thoughts on graphics APIs and libraries
Warning: This is a long rant. I’d like to share my personal thoughts and opinions on graphics APIs like Vulkan, Direct3D 12.
Some time ago I came up with a diagram showing how the graphics software technologies evolved over last decades – see my blog post “Lower-Level Graphics API - What Does It Mean?”. The new graphics APIs (Direct3D 12, Vulkan, Metal) are not only a clean start, so they abandon all the legacy garbage going back to ‘90s (like
glVertex), but they also take graphics programming to a new level. It is a lower level – they are more explicit, closer to the hardware, and better match how modern GPUs work. At least that’s the idea. It means simpler, more efficient, and less error-prone drivers. But they don’t make the game or engine programming simpler. Quite the opposite – more responsibilities are now moved to engine developers (e.g. memory management/allocation). Overall, it is commonly considered a good thing though, because the engine has higher-level knowledge of its use cases (e.g. which textures are critically important and which can be unloaded when GPU memory is full), so it can get better performance by doing it properly. All this is hidden in the engines anyway, so developers making their games don’t notice the difference.
Those of you, who – just like me – deal with those low-level graphics APIs in their everyday work, may wonder if these APIs provide the right level of abstraction. I know it will sound controversial, but sometimes I get a feeling they are at the exactly worst possible level – so low they are difficult to learn and use properly, while so high they still hide some implementation details important for getting a good performance. Let’s take image/texture barriers as an example. They were non-existent in previous APIs. Now we have to do them, which is a major pain point when porting old code to a new API. Do too few of them and you get graphical corruptions on some GPUs and not on the others. Do too many and your performance can be worse than it has been on DX11 or OGL. At the same time, they are an abstract concept that still hides multiple things happening under the hood. You can never be sure which barrier will flush some caches, stall the whole graphics pipeline, or convert your texture between internal compression formats on a specific GPU, unless you use some specialized, vendor-specific profiling tool, like Radeon GPU Profiler (RGP).
It’s the same with memory. In DX11 you could just specify intended resource usage (
D3D11_USAGE_DYNAMIC) and the driver chose preferred place for it. In Vulkan you have to query for memory heaps available on the current GPU and explicitly choose the one you decide best for your resource, based on low-level flags like
VK_MEMORY_PROPERTY_HOST_VISIBLE_BIT etc. AMD exposes 4 memory types and 3 memory heaps. Nvidia has 11 types and 2 heaps. Intel integrated graphics exposes just 1 heap and 2 types, showing the memory is really unified, while AMD APU, also integrated, has same memory model as the discrete card. If you try to match these to what you know about physically existing video RAM and system RAM, it doesn’t make any sense. You could just pick the first
DEVICE_LOCAL memory for the fastest GPU access, but even then, you cannot be sure your resource will stay in video RAM. It may be silently migrated to system RAM without your knowledge and consent (e.g. if you go out of memory), which will degrade performance. What is more, there is no way to query for the amount of free GPU memory in Vulkan, unless you do hacks like using DXGI.
Hardware queues are no better. Vulkan claims to give explicit access to the pieces of GPU hardware, so you need to query for queues that are available. For example, Intel exposes only a single graphics queue. AMD lets you create up to 3 additional compute-only queues and 2 transfer queues. Nvidia has 8 compute queues and 1 transfer queue. Do they all really map to silicon that can work in parallel? I doubt it. So how many of them to use to get the best performance? There is no way to tell by just using Vulkan API. AMD promotes doing compute work in parallel with 3D rendering while Nvidia diplomatically advises to be “conscious” with it.
It's the same with presentation modes. You have to enumerate
VkPresentModeKHR-s available on the machine and choose the right one, along with number of images in the swapchain. These don't map intuitively to a typical user-facing setting of V-sync = on/off, as they are intended to be low level. Still you have no control and no way to check whether the driver does "blit" or "flip".
One could say the new APIs don’t deliver to their promise of being low level, explicit, and having predictable performance. It is impossible to deliver, unless the API is specific to one GPU, like there is on consoles. A common API over different GPUs is always high level, things happen under the hood, and there are still fast and slow paths. Isn’t all this complexity just for nothing? It may be true that comparing to previous generation APIs, drivers for the new ones need not launch additional threads in the background or perform shader compilation on first draw call, which greatly reduces chances of major hitching. (We will see how long this state will persist as the APIs and drivers evolve.) * Still there is no way to predict or ensure minimum FPS/maximum frame time. We are talking about systems where multiple processes compete for resources. On modern PCs there is even no way to know how many cycles will a single instruction take! Cache memory, branch prediction, out-of-order execution – all of these mechanisms are there in the CPU to speed up average cases, but there can always be cases when it works slowly (e.g. cache miss). It’s the same with graphics. I think we should abandon the false hope of predictable performance as a thing of the past, just like rendering graphics pixel-perfect. We can optimize for the average, but we cannot ensure the minimum. After all, games are “soft real-time systems”.
Based on that, I am thinking if there is a room for a new graphics API or top of DX12 or Vulkan. I don’t mean whole game engine with physical simulation, handling sound, input controllers and all, like Unity or UE4. I mean an API just like DX11 or OGL, on a similar or higher abstraction level (if higher level, maybe the concept of persistent “frame graph” with explicit pass and resource dependencies is the way to go?). I also don’t think it’s enough to just reimplement any of those old APIs. The new one should take advantage of features of the explicit APIs (like parallel command buffer recording), while hiding the difficult parts (e.g. queues, memory types, descriptors, barriers), so it’s easier to use and harder to misuse. (An existing library similar to this concept is V-EZ from AMD.) I think it may still have good performance. The key thing needed for creation of such library is abandoning the assumption that developer must define everything up-front, with nothing allocated, created, or transferred on first use.
See also next post: "How to design API of a library for Vulkan?"
Update 2019-02-12: I want to thank all of you for the amazing feedback I received after publishing this post, especially on Twitter. Many projects have been mentioned that try to provide an API better than Vulkan or DX12 - e.g. Apple Metal, WebGPU, The Forge by Confetti.
* Update 2019-04-16: Microsoft just announced they are adding background shader optimizations to D3D12, so driver can recompile and optimize shaders in the background on its own threads. Congratulations! We are back at D3D11 :P
# Vulkan API - my talk at Warsaw University of Technology
On Wednesday 16 April, around 8 PM, at Warsaw University of Technology, during weekly meeting of KNTG Polygon, I will give a talk about "Vulkan API" (in Polish). Come if you want to hear about new generation of graphics APIs, see how Vulkan API looks like, what tools are there to support it, what are advantages and disadvantages of using such API and finally decide whethere learning Vulkan is a good idea for you.
Event on Facebook: https://www.facebook.com/events/185314825611839/
# Switchable graphics versus D3D11 adapters
When you have a laptop with so called "switchable graphics" (like I do in my Lenovo IdeaPad G50-80), you effectively have two GPUs. In my case, these are: integrated Intel i7-5500U and AMD Radeon R5 M330. While programming in DirectX 11, you can enumerate these two adapters and choose any of them while creating a
ID3D11Device object. For quite some time I was wondering how various settings of this "switchable graphics" affect my app? Today I finally figured it out. Long story short: They just change order of these adapters as visible to my program, so that the appropriate one is visible as adapter 0. Here is the full story:
It looks like the base setting is the one that can be found in Windows Settings > Power options > edit your power plan > Switchable Dynamic Graphics. (Not to confuse with "AMD Graphics Power Settings"!) When you set it to "Optimize power savings" or "Optimize performance", application sees Intel GPU as first adapter:
When you choose "Maximize performance", application sees AMD GPU as first adapter:
I also found that Radeon Settings (the app that comes with AMD graphics driver) overrides this system setting. If you go to System > Switchable Graphics and make configuration for your specific executable, then again: choosing "Power Saving" makes your app see Intel GPU as first adapter, while choosing "High Performance" makes AMD graphics first.
It's as simple as that. Basically if you always use the first adapter you find, then you follow recommended settings of the system. You are still free to use the other adapter while creating your D3D11 device. I checked that - it works and it really uses that one.
It's especially important if you meet a strange bug where your app hangs on one of these GPUs.
Update 2018-05-02: Microsoft plans to add an API for enumerating adapters based on a given GPU preference (minimum power or high performance). See IDXGIFactory6::EnumAdapterByGpuPreference.
Update 2018-08-23: See also related article: Selecting the Best Graphics Device to Run a 3D Intensive Application - GPUOpen.
# When integrated graphics works better
In RPG games the more powerful your character is, the more tough and scary are the monsters you have to fight. I sometimes get a feeling that the same applies to real life - bugs you meet when you are a programmer. I recently blogged about the issue when QueryPerformanceCounter call takes long time. I've just met another weird problem. Here is my story:
I have Lenovo IdeaPad G50-80 (80E502ENPB) laptop. It has switchable graphics: integrated Intel i7-5500U and dedicated AMD Radeon R5 M330. Of course I used to choose AMD dedicated graphics, because it's more powerful. My application is a music visualization program. It renders graphics using Direct3D 11. It uses one
ID3D11Device object and one thread for rendering, but two windows displayed on two outputs: output 1 (laptop screen) contains window with GUI and preview, while output 2 (projector connected via VGA or HDMI) shows main view using borderless, topmost window covering whole screen (but not real fullscreen as in
IDXGISwapChain::SetFullscreenState). I tend to enable V-sync on output 1 (
IDXGISwapChain::Present SyncInterval = 1) and disable it on output 0 (
SyncInterval = 0). My rendering algorithm looks like this:
Loop over frames: Render scene to MainRenderTarget Render MainRenderTarget to OutputBackBuffer, covering whole screen Render MainRenderTarget to PreviewBackBuffer, on a quad Render ImGui to PreviewBackBuffer OutputSwapChain->Present() PreviewSwapChain->Present()
So far I had just one problem with it: my framerate decreased over time. It used to drop very quickly after launching the app from 60 to 30 FPS and stabilize there, but after few hours it was steadily decreasing to 20 FPS or even less. I couldn't identify the reason for it in my code, like a memory leak. It seemed to be related to rendering. I could somehow live with this issue - low framerate was not that noticable.
Suddenly this Thursday, when I wanted to test new version of the program, I realized it hangs after around a minute from launching. It was a strange situation in which the app seemed to be running normally, but it was just not rendering any new frames. I could see it still works by inspecting CPU usage and thread list with Process Hacker. I could minimize its windows or cover them by other windows and they preserved their content after restoring. I even captured trace in GPUView, only to notice that the app is filling DirectX command queue and AMD GPU is working. Still, nothing was rendered.
That was a frightening situation for me, because I need to have it working for this weekend. After I checked that restarting app or the whole system doesn't help, I tried to identify the cause and fix it in various ways:
1. I thought that maybe there is just some bug in the new version of my program, so I launched the previous version - one that successfully worked before, reaching more than 10 hours of uptime. Unfortunately, the problem still occured.
2. I thought that maybe it's a bug in the new AMD graphics driver, so I downloaded and installed previous version, performing "Clean install". It didn't help either.
3. In desperation, I formatted whole hard drive and reinstalled operating system. I planned to to it anyway, because it was a 3-year-old system, upgraded from Windows 8 and I had some other problems with it (that I don't describe here because they were unrelated to graphics). I installed the latest, clean Windows 10 with latest updates and all the drivers. Even that didn't solve my problem. The program still hung soon after every launch.
I finally came up with an idea to switch my app to using Intel integrated graphics. It can be done in Radeon Settings > "Switchable Graphics" tab. In a popup menu for a specific executable, "High Performance" means choosing dedicated AMD GPU and "Power Saving" means choosing integrated Intel GPU. See article Configuring Laptop Switchable Graphics... for details.
It solved my problem! The program not only doesn't hang any longer, but it also maintains stable 60 FPS now (at least it did during my 2h test). Framerate drops only when there is a scene that blends many layers together on a FullHD output - apparently this GPU cannot keep up with drawing so many pixels per second. Anyway, this is the situation where using integrated Intel graphics turns out work better than a faster, dedicated GPU.
I still don't know what is the cause of this strange bug. Is it something in the way my app uses D3D11? Or is it a bug in graphics driver (one of the two I need to have installed)? I'd like to investigate it further when I find some time. For now, I tend to believe that:
- The only thing that might have changed recently and break my app was some Windows updated pushed by Microsoft.
- The two issues: the one that I had before with framerate decreasing over time and the new one with total image freeze are related. They may have something to do with switchable graphics - having two different GPUs in the system, both enabled at the same time. I suspect that maybe when I want to use Radeon, the outputs (or one of them) are connected to Intel anyway, so the image needs to be copied and synchronized with Intel driver.
Update 2018-02-21: Later after I published this post, I tried few other things to fix the problem. For example, I updated AMD graphics driver to latest version 18.2.2. It didn't help. Suddently, the problem disappeared as mysteriously as it appeared. It happened during a single system launch, without a restart. My application was hunging, and later it started working properly. The only thing that I can remember doing in between was downloading and launching UIforETW - a GUI tool for capturing Event Tracing for Windows (ETW) traces, like the ones for GPUView. I know that it automatically installs GPUView and other necessary tools on first launch, so that may have changed something in my system. Either way, now my program works on AMD graphics without a hang, reaching few hours of uptime and maintaining 60 FPS, which only sometimes drops to 30 FPS, but it also go back up.