Hardware Synchronization: Swap Groups and Barriers

In a networked cluster driving a CAVE, "close enough" is not good enough for synchronization. If Node A (the left wall) swaps its buffers even one millisecond before Node B (the front wall), the user will see a distracting horizontal line or a "misaligned" object across the corner where the two walls meet. This is known as Cluster Tearing.

Swap Groups: Intra-Machine Sync

A Swap Group synchronizes multiple GPUs or multiple displays connected to a single workstation.

When one GPU in the group is ready to present, the hardware holds the "Swap" signal until all other GPUs in that same machine are also ready. This ensures that the two outputs of a single dual-GPU workstation always update their pixels at the exact same refresh cycle.

Swap Barriers: Inter-Machine Sync

A Swap Barrier is the network extension of a swap group. This works across multiple workstations.

  1. Hardware Connection: A physical cable (often a BNC or RJ45 sync cable) connects the GPUs across the cluster.

  2. The Handshake: When Node A finishes its frame, it sends a "Ready" signal over the cable.

  3. The Wait: Node A blocks its execution.

  4. The Release: Only when every node in the cluster has sent its "Ready" signal does the hardware release the barrier, allowing every GPU to swap their buffers simultaneously.

Implementing the Barrier in Vulkan

We use the VK_NV_present_barrier extension to join these hardware groups.

// Checking for Present Barrier support and joining a group
auto features2 = physicalDevice.getFeatures2<vk::PhysicalDeviceFeatures2, vk::PhysicalDevicePresentBarrierFeaturesNV>();
const auto& barrierFeatures = features2.get<vk::PhysicalDevicePresentBarrierFeaturesNV>();

if (barrierFeatures.presentBarrier) {
    // When creating the swapchain, we join Barrier 1
    // This barrier is managed by the physical sync cable between machines
    vk::SwapchainPresentBarrierCreateInfoNV barrierInfo{};
    barrierInfo.presentBarrierEnable = VK_TRUE;
    barrierInfo.barrierID = 1;

    vk::SwapchainCreateInfoKHR createInfo{};
    createInfo.pNext = &barrierInfo;
    // ...
}

Genlock vs. Swaplock

  • Genlock (Generator Lock): Syncs the beginning of the video signal (the start of the scanline). This prevents "rolling" artifacts.

  • Swaplock (Swap Barrier): Syncs the moment the front and back buffers are swapped. This prevents "stale frame" artifacts.

By combining Genlock (signal timing) with Swap Barriers (frame timing), we achieve the "Gold Standard" of spatial synchronization: Frame-Perfect Cluster Rendering. The entire room updates as if it were a single piece of digital paper.

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