November 2018

# "Co działa szybko, a co wolno w grafice 3D?" - talk at Collegium da Vinci

Tue
20
Nov 2018

(EN) This post will be in Polish because it's an invitation for my talk, which will happen 7th December 2018 in Collegium da Vinci in Poznań, and it will be in Polish.

(PL) Wszystkich zainteresowanych tworzeniem gier (także w Unity czy Unreal Engine, niekoniecznie zaawansowanym programowaniem grafiki w C++) mam przyjemność zaprosić na mój wykład pt. „Co działa szybko, a co wolno w grafice 3D?”, który odbędzie się 7 grudnia 2018 na uczelni Collegium Da Vinci w Poznaniu, w ramach cyklu spotkań „INTERAKCJE”.

Opis: Grafika 3D jest istotną częścią współczesnych gier video, a jej wydajne renderowanie jest niezbędne do płynnego działania gier w czasie rzeczywistym. Znajomość podstaw tej dziedziny jest przydatna niezależnie od wybranej technologii (np. Unity, Unreal Engine czy własny silnik pisany w C++). Wykład stanowi przegląd technik stosowanych w grafice renderowanej z użyciem współczesnych GPU z podkreśleniem, które z nich mogą stanowić problem wydajnościowy oraz jakimi sposobami można uzyskać lepszą wydajność.

Comments | #teaching #graphics #events Share

# Vulkan with DXGI - experiment results

Mon
19
Nov 2018

In my previous post, I’ve described a way to get GPU memory usage in Windows Vulkan app by using DXGI. This API, designed for Direct3D, seems to work with Vulkan as well. In this post I would like to share detailed results of my experiment on two different platforms with two graphics cards from different vendors. But before that, a disclaimer:

Read full entry > | Comments | #windows #graphics #directx #vulkan Share

# There is a way to query GPU memory usage in Vulkan - use DXGI

Thu
15
Nov 2018

In my GDC 2018 talk “Memory management in Vulkan and DX12” (slides freely available, video behind GDC Vault paywall) I said that in Direct3D 12 you can query for the exact amount of GPU memory used and available, while in Vulkan there is no way to do that, so I recommend to just query for memory capacity (VkMemoryHeap::​size) and limit your usage to around 80% of it. It turns out that I wasn’t quite right. If you code for Windows, there is a way to do this. I assumed that the mentioned function IDXGIAdapter3::​QueryVideoMemoryInfo is part of Direct3D 12 interface, while it is actually part of DirectX Graphics Infrastructure (DXGI). This is a more generic, higher level Windows API that allows you to enumerate adapters (graphics cards) installed in the system, query for their parameters and outputs (monitors) connected to them. Direct3D refers to this API, but it’s not the same.

Key question is: Can you use DXGI to query for GPU memory usage while doing graphics using Vulkan, not D3D11 or D3D12? Would it return some reasonable data and not all zeros? Short answer is: YES! I’ve made an experiment - I wrote a simple app that creates various Vulkan objects and queries DXGI for memory usage. Results look very promising. But before I move on to the details, here is a short primer of how to use this DXGI interface, for all non-DirectX developers:

1. Use C++ in Visual Studio. You may also use some other compiler for Windows or other programming language, but it will be probably harder to setup.

2. Install relatively new Windows SDK.

3. #include <dxgi1_4.h> and <atlbase.h>

4. Link with “dxgi.lib”.

5. Create Factory object:

IDXGIFactory4* dxgiFactory = nullptr;
CreateDXGIFactory1(IID_PPV_ARGS(&dxgiFactory));

Don’t forget to release it at the end:

dxgiFactory->Release();

6. Write a loop to enumerate available adapters. Choose and remember suitable one.

IDXGIAdapter3* dxgiAdapter = nullptr;
IDXGIAdapter1* tmpDxgiAdapter = nullptr;
UINT adapterIndex = 0;
while(m_DxgiFactory->EnumAdapters1(adapterIndex, &tmpDxgiAdapter) != DXGI_ERROR_NOT_FOUND)
{
    DXGI_ADAPTER_DESC1 desc;
    tmpDxgiAdapter>GetDesc1(&desc);
    if(!dxgiAdapter && desc.Flags == 0)
    {
        tmpDxgiAdapter->QueryInterface(IID_PPV_ARGS(&dxgiAdapter));
    }
    tmpDxgiAdapter->Release();
    ++adapterIndex;
}

At the end, don’t forget to release it:

dxgiAdapter->Release();

Please note that using new version of DXGI interfaces like DXGIFactory4 and DXGIAdapter3 requires some relatively new version (I’m not sure which one) of both Windows SDK on developer’s side (otherwise it won’t compile) and updated Windows system on user’s side (otherwise function calls with fail with appropriate returned HRESULT).

7. To query for GPU memory usage at the moment, use this code:

DXGI_QUERY_VIDEO_MEMORY_INFO info = {};
dxgiAdapter->QueryVideoMemoryInfo(0, DXGI_MEMORY_SEGMENT_GROUP_LOCAL, &info);

There are two possible options:

Among members of the returned structure, the most interesting is CurrentUsage. It seems to precisely reflect the use of GPU memory - it increases when I allocate a new VkDeviceMemory object, as well as when I use some implicit memory by creating other Vulkan resources, like a swap chain, descriptor pools and descriptor sets, command pools and command buffers, query pools etc.

Other DXGI features for video memory - callback for budget change notification (IDXGIAdapter3::​RegisterVideoMemoryBudgetChangeNotificationEvent) and reservation (IDXGIAdapter3::​SetVideoMemoryReservation) may also work with Vulkan, but I didn’t check them.

As an example, on my system with GPU = AMD Radeon RX 580 8 GB and 16 GB of system RAM, on program startup and before any Vulkan or D3D initialization, DXGI reports following data:

Local:
 Budget=7252479180 CurrentUsage=0
 AvailableForReservation=3839547801 CurrentReservation=0
Nonlocal:
 Budget=7699177267 CurrentUsage=0
 AvailableForReservation=4063454668 CurrentReservation=0

8. You may want to choose correct DXGI adapter to match the physical device used in Vulkan. Even on the system with just one discrete GPU there are two adapters reported, one of them being software renderer. I exclude it by comparing desc.Flags == 0, which means this is a real, hardware-accelerated GPU, not DXGI_ADAPTER_FLAG_REMOTE or DXGI_ADAPTER_FLAG_SOFTWARE.

Good news is that even when there are more such adapters in the system, there is a way to match them between DXGI and Vulkan. Both APIs return something called Locally Unique Identifier (LUID). In DXGI it’s in DXGI_ADAPTER_DESC1::​AdapterLuid. In Vulkan it’s in VkPhysicalDeviceIDProperties::​deviceLUID. They are of different types - two 32-bit numbers versus array of bytes, but it seems that simple, raw memory compare works correctly. So the way to find DXGI adapter matching Vulkan physical device is:

// After obtaining VkPhysicalDevice of your choice:
VkPhysicalDeviceIDProperties physDeviceIDProps = {
    VK_STRUCTURE_TYPE_PHYSICAL_DEVICE_ID_PROPERTIES };
VkPhysicalDeviceProperties2 physDeviceProps = {
    VK_STRUCTURE_TYPE_PHYSICAL_DEVICE_PROPERTIES_2 };
physDeviceProps.pNext = &physDeviceIDProps;
vkGetPhysicalDeviceProperties2(physicalDevice, &physDeviceProps);

// While enumerating DXGI adapters, replace condition:
// if(!dxgiAdapter && desc.Flags == 0)
// With this:
if(memcmp(&desc.AdapterLuid, physDeviceIDProps.deviceLUID, VK_LUID_SIZE) == 0)

Please note that function vkGetPhysicalDeviceProperties2 requires Vulkan 1.1, so set VkApplicationInfo::​apiVersion = VK_API_VERSION_1_1. Otherwise the call results in “Access Violation” error.

In my next blog post, I present detailed results of my experiment with DXGI used with Vulkan application, tested on 2 different GPUs.

Comments | #windows #graphics #vulkan #directx Share

# Two most obscure bugs in my life

Wed
14
Nov 2018

Fixing bugs is a significant part of software development, and a very emotional one - from frustration when you have no idea what is happening to euphoria when you find the cause and you know how to fix it. There are many different kinds of bugs and reasons why they are introduced to the code. (Conducting a deep investigation of how each bug happened might be an interesting idea - I will write a separate blog post about it someday...) But the most frustrating ones are probably these that occur only in some specific conditions, like only on one of many supported platforms or only in "Release" and not in "Debug" configuration. Below you will find description of the two most obscure bugs I've found and fixed in my life, for your education and amusement :) Both were in large, multiplatform, C++ codebases.

1. There was this bug reported causing incorrect program behavior, occurring on only one of many platforms where the program was built and used. It was probably Mac, but I can't remember exactly. It was classified as a regression - it used to work before and stopped working, caught by automated tests, after some specific code change. The strange thing was that the culprit submit to the code repository introduced only new comments and nothing else. How can change in a code comment introduce a new bug?!

It turned out that the author of this change wanted to draw a nice "ASCII art" in his comment, so he's put something like this:

// HERE STARTS AN IMPORTANT SECTION
//==================================================================\\
Function1();
Function2();
(...)

Have you noticed anything suspicious?...

A backslash '\' at the end of line in C/C++ means that the logical line is continued to next line. This feature is used mostly when writing complex preprocessor macros. But a code starting from two slashes '//' begins a comment that spans until the end of line. Now the question is: Does the single-line comment span to next line as well? In other words: Is the first function call commented out, or not?

I don't know and I don't really care that much about what would "language lawyers" read from the C++ specification and define as a proper behavior. The real situation was that compilers used on some platforms considered the single-line comment to span to the next line, thus commenting out call to Function1(), while others didn't. That caused the bug to occur on some platforms only.

The solution was obviously to change the comment not to contain backslash at the end of line, even at the expense of aesthetics and symmetry of this little piece of art :)

2. I was assigned a "stack overflow" bug occurring only in "Debug" and not in "Release" configuration of a Visual Studio project. At first, I was pretty sure it would be easy to find. After all, "stack overflow" usually means infinite recursion, right? The stack is the piece of memory where local function variables are allocated, along with return addresses from nested function calls. They tend not to be too big, while the stack is 1 MB by default, so there must be unreasonable call depth involved to hit that error.

It turned out not to be true. After few debugging sessions and reading the code involved, I understood that there was no infinite recursion there. It was a traversal of a tree structure, but the depth of its hierarchy was not large enough to be of a concern. It took me a while to realize that the stack was bloated to the extent that exceeded its capacity by one function. It was a very long function - you know, this kind of function that you can see in corporate environment which defies any good practices, but no one feels responsible to refactor. It must have grown over time with more and more code added gradually until it reached many hundreds of lines. It was just one big switch with lots of code in each case.

void Function(Option op)
{
  switch(op)
  {
  case OPTION_FIRST:
    // Lots of code, local variables, and stuff...
    break;
  case OPTION_SECOND:
    // Lots of code, local variables, and stuff...
    break;
  case OPTION_THIRD:
    // Lots of code, local variables, and stuff...
    break;
  ...
  }
}

What really caused the bug was the number and size of local variables used in this function. Each of the cases involved many variables, some big like fixed-size arrays or objects of some classes, defined by value on the stack. It was enough to call this function recursively just few times to exhaust the stack capacity.

Why "stack overflow" occurred in "Debug" configuration only and not in "Release"? Apparently Visual Studio compiler can lazily allocate or alias local variables used in different cases of a switch instruction, while "Debug" configuration has all the optimizations disabled and so it allocates all these variables with every function call.

The solution was just to refactor this long function with a switch - to place the code from each case in a separate, new function.

What are the most obscure bugs you've met in your coding practice? Let me know in the comments below.

Comments | #debugger #visual studio #c++ Share

# Technical debt is a good metaphor

Mon
12
Nov 2018

There is this term "technical debt" used in the world of software development as a metaphor to describe poor quality of source code (or the lack of documentation, tests etc.), whether it's a bad overall architecture, quick and dirty hacks in some places, or any kinds of software anti-patterns. It makes the code harder, slower, and less pleasant to maintain, fix, and expand. It's called "debt" because, just as the financial debt, it causes some "interest" that needs to be paid, in form of increased maintenance cost and more bugs to fix, until it's repaid (refactoring or rewrite of bad code, completing work that was missing and left "TODO"). It is most frequently caused by lack of time due to deadlines imposed by business people - lack of time to fully understand the requirements and design good solution, to read and understand the existing codebase, to refactor or rewrite if the code is already bad.

Some people say that "technical debt" is a bad metaphor. There was this one article I can't find now which argued that bad code is unlike real-world debt because we don't need to pay "interest" until we actually need to go back to and modify the specific piece of code where we made a dirty hack. I don't agree with it. Sure things in software development are harder to measure and estimate quantitatively than in the business world, but I think that "technical debt" is a good metaphor. Just like in real life, there are different kinds of debt. It depends on who do you borrow money from:

- Leaving bad code in a place where you and everyone else in the team may never need to look again is like borrowing money from you family or best friend. You know the debt is there, but you may postpone paying it off indefinitely and you will still be fine.

- Leaving bad code in a place where you periodically need to make fixes and other changes is like borrowing money from a bank. You have to periodically pay interest until you repay it all.

- Finally, making ugly hack in a critical piece of code that you touch constantly is like being in so desperate need for money to borrow from mafia. You better repay it quickly or you will get in trouble.

Comments | #philosophy #software engineering Share

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