Multi-GPU Configuration

The Virtual Display Driver supports advanced multi-GPU configurations through LUID (Locally Unique Identifier) based adapter selection. This allows you to precisely control which GPU renders the virtual display.

Overview

In multi-GPU systems, the driver needs to know which GPU should handle the virtual display rendering. The driver provides multiple methods for GPU selection:

Automatic selection - Uses the GPU with the most video memory
Name-based selection - Specify GPU by friendly name
PCI bus selection - Use PCI bus number for deterministic selection
LUID-based selection - Direct LUID specification (most precise)

Basic GPU Configuration

Configure target GPU in vdd_settings.xml:

<gpu>
    <friendlyname>default</friendlyname>
</gpu>

gpu.friendlyname

string

default:"default"

Target GPU adapter specification.Valid values:

default - Automatically select GPU with most video memory
GPU name - e.g., NVIDIA GeForce RTX 4090
name,bus - GPU name with PCI bus number, e.g., NVIDIA GeForce RTX 4090,1

Automatic GPU Selection

When set to default, the driver automatically selects the best GPU:

<gpu>
    <friendlyname>default</friendlyname>
</gpu>

Selection criteria:

Enumerate all available GPUs
Sort by dedicated video memory (descending)
Select GPU with the most memory

Automatic selection works well for most systems. The driver will choose the most powerful GPU.

Name-Based GPU Selection

Specify a GPU by its exact friendly name:

<gpu>
    <friendlyname>NVIDIA GeForce RTX 4090</friendlyname>
</gpu>

Finding GPU Names

To find available GPU names, check the driver logs or use Device Manager:

Open Device Manager

Press Win + X and select “Device Manager”

Expand Display Adapters

Find the “Display adapters” section

Copy GPU name

Right-click the GPU and copy its exact name

Update configuration

Paste the name into <friendlyname> in vdd_settings.xml

Common GPU names:

NVIDIA GeForce RTX 4090
NVIDIA GeForce RTX 4080
AMD Radeon RX 7900 XTX
Intel(R) UHD Graphics 770

GPU names are case-sensitive and must match exactly, including spaces and special characters.

PCI Bus-Based Selection

For deterministic GPU selection in multi-GPU systems with identical GPUs, use PCI bus numbers:

<gpu>
    <friendlyname>NVIDIA GeForce RTX 4090,1</friendlyname>
</gpu>

Format: GPU_NAME,BUS_NUMBER

Finding PCI Bus Numbers

Use Device Manager or command-line tools:

Device Manager
Command Line
Driver Logs

Open Device Manager
Right-click GPU → Properties
Go to “Details” tab
Select “Location information”
Look for “PCI bus X”

# PowerShell command to list GPUs with bus numbers
Get-WmiObject Win32_VideoController | Select Name, PNPDeviceID

The bus number is part of the PNP Device ID.

Why Use Bus Numbers?

PCI bus numbers are essential when you have multiple identical GPUs:

<!-- System with 2x RTX 4090 -->
<gpu>
    <!-- Use GPU on PCI bus 1 -->
    <friendlyname>NVIDIA GeForce RTX 4090,1</friendlyname>
</gpu>

Without the bus number, the driver cannot distinguish between identical GPUs and may select the wrong one.

PCI bus numbers are assigned by the system and remain consistent across reboots unless hardware changes.

LUID-Based Selection

The driver uses LUIDs (Locally Unique Identifiers) internally for GPU identification. When you specify a GPU name or bus number, the driver resolves it to an LUID.

LUID Structure

An LUID is a 64-bit identifier consisting of:

LowPart - Low 32 bits (DWORD)
HighPart - High 32 bits (LONG)

LUID Resolution Process

// Simplified driver logic
if (friendlyname == "default") {
    // Load from adapter.txt or auto-select best GPU
    gpuName = selectBestGPU();
} else {
    gpuName = friendlyname;
}

// Resolve GPU name to LUID
if (contains(gpuName, ',')) {
    // Format: "name,bus"
    busNumber = extractBusNumber(gpuName);
    LUID = ResolveAdapterLuidFromPciBus(busNumber);
} else {
    // Format: "name"
    LUID = FindGPUByName(gpuName);
}

// Apply LUID to adapter
IddCxAdapterSetRenderAdapter(adapter, LUID);

Alternative: adapter.txt File

Instead of using vdd_settings.xml, you can create an adapter.txt file: Location: C:\VirtualDisplayDriver\adapter.txt Content:

NVIDIA GeForce RTX 4090

or with bus number:

NVIDIA GeForce RTX 4090,1

The adapter.txt file is only used when <friendlyname> is set to default in vdd_settings.xml.

Priority:

If friendlyname is not default: use the value from vdd_settings.xml
If friendlyname is default and adapter.txt exists: use the GPU from adapter.txt
Otherwise: auto-select best GPU

Multi-GPU Configuration Examples

Dual GPU System (Different GPUs)

<!-- System: RTX 4090 (main) + RTX 3060 (encoding) -->
<gpu>
    <!-- Use the RTX 4090 for virtual display -->
    <friendlyname>NVIDIA GeForce RTX 4090</friendlyname>
</gpu>

Dual GPU System (Identical GPUs)

<!-- System: 2x RTX 4090 in SLI/NVLink -->
<gpu>
    <!-- Use the second GPU (PCI bus 1) -->
    <friendlyname>NVIDIA GeForce RTX 4090,1</friendlyname>
</gpu>

Laptop with iGPU + dGPU

<!-- Laptop: Intel UHD 770 (iGPU) + RTX 4060 (dGPU) -->
<gpu>
    <!-- Use the discrete GPU -->
    <friendlyname>NVIDIA GeForce RTX 4060 Laptop GPU</friendlyname>
</gpu>

Hybrid AMD + NVIDIA System

<!-- System: AMD Radeon + NVIDIA GeForce -->
<gpu>
    <!-- Use NVIDIA GPU for NVENC encoding support -->
    <friendlyname>NVIDIA GeForce RTX 4070</friendlyname>
</gpu>

GPU Enumeration in Driver Logs

The driver logs all available GPUs on startup:

[INFO] Available GPUs:
[INFO]   0: NVIDIA GeForce RTX 4090 (24576 MB) - LUID: 12345-67890
[INFO]   1: Intel(R) UHD Graphics 770 (128 MB) - LUID: 54321-9876
[INFO] Selected GPU: NVIDIA GeForce RTX 4090

Use these logs to verify GPU selection and find exact GPU names.

Runtime GPU Detection

The driver can detect and report GPU information at runtime:

// From Driver.cpp
vector<GPUInfo> getAvailableGPUs() {
    vector<GPUInfo> gpus;
    ComPtr<IDXGIFactory1> factory;
    CreateDXGIFactory1(IID_PPV_ARGS(&factory));
    
    for (UINT i = 0;; i++) {
        ComPtr<IDXGIAdapter> adapter;
        if (!SUCCEEDED(factory->EnumAdapters(i, &adapter))) {
            break;
        }
        
        DXGI_ADAPTER_DESC desc;
        adapter->GetDesc(&desc);
        
        GPUInfo info{
            desc.Description,
            adapter,
            desc
        };
        gpus.push_back(info);
    }
    
    return gpus;
}

Advanced: Direct LUID Configuration

For advanced scenarios, you can directly query and use LUIDs:

Query GPU LUID

Use DXGI to enumerate adapters and retrieve their LUIDs:

DXGI_ADAPTER_DESC desc;
adapter->GetDesc(&desc);
LUID luid = desc.AdapterLuid;

Apply LUID to adapter

The driver uses the LUID internally:

IDARG_IN_ADAPTERSETRENDERADAPTER arg{};
arg.PreferredRenderAdapter = luid;
IddCxAdapterSetRenderAdapter(adapter, &arg);

Direct LUID manipulation requires driver code modifications. Most users should use name or bus-based selection.

Command-Line GPU Configuration

You can change the GPU at runtime using the command-line interface:

# Set GPU by name
vddutil --gpu "NVIDIA GeForce RTX 4090"

# Set GPU by name and bus
vddutil --gpu "NVIDIA GeForce RTX 4090,1"

See the PowerShell scripts reference for automated GPU configuration tools.

Troubleshooting

Driver uses wrong GPU

Possible causes:

GPU name mismatch
friendlyname still set to default
Typo in GPU name

Solutions:

Check driver logs for available GPUs
Verify exact GPU name from Device Manager
Use bus number if you have identical GPUs

GPU not found error

Possible causes:

GPU name doesn’t match exactly
GPU disabled or not present
Driver hasn’t detected GPU yet

Solutions:

Verify GPU is enabled in BIOS
Check Device Manager for GPU status
Try default to use automatic selection

Can't distinguish between identical GPUs

Cause: Using name-only selection with multiple identical GPUsSolution: Use bus-based selection: GPU_NAME,BUS_NUMBER

GPU selection ignored

Possible causes:

Configuration not applied
Driver not restarted
Syntax error in XML

Solutions:

Verify XML syntax is correct
Restart driver after configuration change
Check driver logs for errors

Best Practices

Use default for single-GPU systems

If you only have one GPU, keep <friendlyname>default</friendlyname>.

Use names for multi-GPU (different models)

When GPUs are different, use the exact GPU name.

Use bus numbers for identical GPUs

For systems with multiple identical GPUs, always include the bus number.

Verify in logs

Always check driver logs to confirm the correct GPU was selected.

Consider power and thermals

Choose the GPU that can handle the additional virtual display workload.

Performance Considerations

Use discrete GPU

Dedicated GPUs provide better performance than integrated graphics.

Consider encoding workload

If using hardware encoding, select GPU with best encoder (NVENC, VCE).

Balance workloads

In multi-GPU systems, distribute workloads across GPUs.

Monitor GPU usage

Use GPU monitoring tools to verify the selected GPU is being used.

Settings Overview

Complete configuration file reference

PowerShell Scripts

Automate GPU configuration with scripts

Getting Started

Core Concepts

Configuration

Management

Advanced Features

Use Cases

Troubleshooting

Overview

Basic GPU Configuration

Automatic GPU Selection

Name-Based GPU Selection

Finding GPU Names

PCI Bus-Based Selection

Finding PCI Bus Numbers

Why Use Bus Numbers?

LUID-Based Selection

LUID Structure

LUID Resolution Process

Alternative: adapter.txt File

Multi-GPU Configuration Examples

Dual GPU System (Different GPUs)

Dual GPU System (Identical GPUs)

Laptop with iGPU + dGPU

Hybrid AMD + NVIDIA System

GPU Enumeration in Driver Logs

Runtime GPU Detection

Advanced: Direct LUID Configuration

Command-Line GPU Configuration

Troubleshooting

Best Practices

Performance Considerations

Use discrete GPU

Consider encoding workload

Balance workloads

Monitor GPU usage

Settings Overview

PowerShell Scripts

Build docs developers (and LLMs) love

Getting Started

Core Concepts

Configuration

Management

Advanced Features

Use Cases

Troubleshooting

​Overview

​Basic GPU Configuration

​Automatic GPU Selection

​Name-Based GPU Selection

​Finding GPU Names

​PCI Bus-Based Selection

​Finding PCI Bus Numbers

​Why Use Bus Numbers?

​LUID-Based Selection

​LUID Structure

​LUID Resolution Process

​Alternative: adapter.txt File

​Multi-GPU Configuration Examples

​Dual GPU System (Different GPUs)

​Dual GPU System (Identical GPUs)

​Laptop with iGPU + dGPU

​Hybrid AMD + NVIDIA System

​GPU Enumeration in Driver Logs

​Runtime GPU Detection

​Advanced: Direct LUID Configuration

​Command-Line GPU Configuration

​Troubleshooting

​Best Practices

​Performance Considerations

Use discrete GPU

Consider encoding workload

Balance workloads

Monitor GPU usage

​Related Topics

Settings Overview

PowerShell Scripts

Build docs developers (and LLMs) love

Overview

Basic GPU Configuration

Automatic GPU Selection

Name-Based GPU Selection

Finding GPU Names

PCI Bus-Based Selection

Finding PCI Bus Numbers

Why Use Bus Numbers?

LUID-Based Selection

LUID Structure

LUID Resolution Process

Alternative: adapter.txt File

Multi-GPU Configuration Examples

Dual GPU System (Different GPUs)

Dual GPU System (Identical GPUs)

Laptop with iGPU + dGPU

Hybrid AMD + NVIDIA System

GPU Enumeration in Driver Logs

Runtime GPU Detection

Advanced: Direct LUID Configuration

Command-Line GPU Configuration

Troubleshooting

Best Practices

Performance Considerations

Related Topics