Physical devices and queue families

Selecting a physical device

After initializing the Vulkan library through a VkInstance we need to look for and select a graphics card in the system that supports the features we need. In fact, we can select any number of graphics cards and use them simultaneously, but in this tutorial we’ll stick to the first graphics card that suits our needs.

We’ll add a function pickPhysicalDevice and add a call to it in the initVulkan function.

void initVulkan() {
    createInstance();
    setupDebugMessenger();
    pickPhysicalDevice();
}

void pickPhysicalDevice() {

}

The graphics card that we’ll end up selecting will be stored in a VkPhysicalDevice handle added as a new class member.

vk::raii::PhysicalDevice physicalDevice;

Listing the graphics cards is very similar to listing extensions and starts with querying just the number.

auto devices = instance.enumeratePhysicalDevices()

If there are no devices with Vulkan support, then there is no point going further.

if (devices.empty()) {
    throw std::runtime_error("failed to find GPUs with Vulkan support!");
}

Now we need to evaluate each of them and check if they are suitable for the operations we want to perform, because not all graphics cards are created equal. We’ll check if any of the physical devices meet the requirements that we’ll add to that function.

for (const auto& device : devices) {
    physicalDevice = std::make_unique<vk::raii::PhysicalDevice>(device);
    break;
}

Base device suitability checks

To evaluate the suitability of a device, we can start by querying for some details. Basic device properties like the name, type and supported Vulkan version can be queried using vkGetPhysicalDeviceProperties.

auto deviceProperties = device.getProperties();

The support for optional features like texture compression, 64-bit floats and multi viewport rendering (useful for VR) can be queried using vkGetPhysicalDeviceFeatures:

auto deviceFeatures = device.getFeatures();

There are more details that can be queried from devices that we’ll discuss later concerning device memory and queue families (see the next section).

As an example, let’s say we consider our application only usable for dedicated graphics cards that support geometry shaders. Then the isDeviceSuitable function would look like this:

bool isDeviceSuitable(VkPhysicalDevice device) {
    auto deviceProperties = device.getProperties();
    auto deviceFeatures = device.getFeatures();
    if (deviceProperties.deviceType == vk::PhysicalDeviceType::eDiscreteGpu &&
           deviceFeatures.geometryShader) {
        physicalDevice = std::make_unique<vk::raii::PhysicalDevice>(device);
        break;
   }
}

Instead of just checking if a device is suitable or not and going with the first one, you could also give each device a score and pick the highest one. That way you could favor a dedicated graphics card by giving it a higher score, but fall back to an integrated GPU if that’s the only available one. You could implement something like that as follows:

#include <map>

...

void pickPhysicalDevice() {
    auto devices = vk::raii::PhysicalDevices( *instance );
    if (devices.empty()) {
        throw std::runtime_error( "failed to find GPUs with Vulkan support!" );
    }
    // Use an ordered map to automatically sort candidates by increasing score
    std::multimap<int, vk::raii::PhysicalDevice> candidates;

    for (const auto& device : devices) {
        auto deviceProperties = device.getProperties();
        auto deviceFeatures = device.getFeatures();
        uint32_t score = 0;

        // Discrete GPUs have a significant performance advantage
        if (deviceProperties.deviceType == vk::PhysicalDeviceType::eDiscreteGpu) {
            score += 1000;
        }

        // Maximum possible size of textures affects graphics quality
        score += deviceProperties.limits.maxImageDimension2D;

        // Application can't function without geometry shaders
        if (!deviceFeatures.geometryShader) {
           continue;
        }
        candidates.insert(std::make_pair(score, device));
    }

    // Check if the best candidate is suitable at all
    if (candidates.rbegin()->first > 0) {
        physicalDevice = std::make_unique<vk::raii::PhysicalDevice>(candidates.rbegin()->second);
    } else {
        throw std::runtime_error("failed to find a suitable GPU!");
    }
}

You don’t need to implement all that for this tutorial, but it’s to give you an idea of how you could design your device selection process. Of course, you can also display the names of the choices and allow the user to select.

Because we’re just starting out, Vulkan 1.3 support is the only thing we need, and therefore we’ll search for that and the extensions that we actually are going to be demonstrating:

std::vector<const char*> deviceExtensions = {
    vk::KHRSwapchainExtensionName,
    vk::KHRSpirv14ExtensionName,
    vk::KHRSynchronization2ExtensionName,
    vk::KHRCreateRenderpass2ExtensionName
};

void pickPhysicalDevice() {
    std::vector<vk::raii::PhysicalDevice> devices = instance.enumeratePhysicalDevices();
    const auto devIter = std::ranges::find_if(devices,
    [&](auto const & device) {
            auto queueFamilies = device.getQueueFamilyProperties();
            bool isSuitable = device.getProperties().apiVersion >= VK_API_VERSION_1_3;
            const auto qfpIter = std::ranges::find_if(queueFamilies,
            []( vk::QueueFamilyProperties const & qfp )
                    {
                        return (qfp.queueFlags & vk::QueueFlagBits::eGraphics) != static_cast<vk::QueueFlags>(0);
                    } );
            isSuitable = isSuitable && ( qfpIter != queueFamilies.end() );
            auto extensions = device.enumerateDeviceExtensionProperties( );
            bool found = true;
            for (auto const & extension : deviceExtensions) {
                auto extensionIter = std::ranges::find_if(extensions, [extension](auto const & ext) {return strcmp(ext.extensionName, extension) == 0;});
                found = found &&  extensionIter != extensions.end();
            }
            isSuitable = isSuitable && found;
            printf("\n");
            if (isSuitable) {
                physicalDevice = device;
            }
            return isSuitable;
    });
    if (devIter == devices.end()) {
        throw std::runtime_error("failed to find a suitable GPU!");
    }
}

In the next section, we’ll discuss the first real required feature to check for.

Queue families

It has been briefly touched upon before that almost every operation in Vulkan, anything from drawing to uploading textures, requires commands to be submitted to a queue. There are different types of queues that originate from different queue families, and each family of queues allows only a subset of commands. For example, there could be a queue family that only allows processing of compute commands or one that only allows memory transfer related commands.

We need to check which queue families are supported by the device and which one of these supports the commands that we want to use. Right now we are only going to look for a queue that supports graphics commands, so the code could look like this:

uint32_t findQueueFamilies(VkPhysicalDevice device) {
    // find the index of the first queue family that supports graphics
    std::vector<vk::QueueFamilyProperties> queueFamilyProperties = physicalDevice->getQueueFamilyProperties();

    // get the first index into queueFamilyProperties which supports graphics
    auto graphicsQueueFamilyProperty =
      std::find_if( queueFamilyProperties.begin(),
                    queueFamilyProperties.end(),
                    []( vk::QueueFamilyProperties const & qfp ) { return qfp.queueFlags & vk::QueueFlagBits::eGraphics; } );

    return static_cast<uint32_t>( std::distance( queueFamilyProperties.begin(), graphicsQueueFamilyProperty ) );
}

Great, that’s all we need for now to find the right physical device! The next step is to create a logical device to interface with it.

C++ code