PCIe Bifurcation Impact on Multi-GPU and NVMe Benchmarks

This article explores how PCIe bifurcation affects computer hardware benchmarking performance when running multiple GPUs or NVMe SSDs. By logically splitting a single high-bandwidth PCIe slot into multiple smaller lane configurations, users can connect more high-speed devices directly to the CPU. Below, we examine the exact performance trade-offs this lane splitting has on synthetic gaming tests, 3D rendering, and high-speed storage benchmarks, detailing where bottlenecks occur and where performance scales linearly.

What is PCIe Bifurcation?

PCIe bifurcation is a motherboard feature that divides the electrical lanes of a single PCIe slot into multiple smaller groups. Typically, a single x16 slot is split into two x8 slots (x8/x8), one x8 and two x4 slots (x8/x4/x4), or four x4 slots (x4/x4/x4/x4). This allows users to install riser cards or expansion adapters to run multiple devices from one physical motherboard slot without using expensive PLX switch chips.

Impact on Multi-GPU Benchmarking

When using PCIe bifurcation for multiple graphics cards, an x16 slot is usually split into an x8/x8 configuration. Halving the available bandwidth per GPU impacts different benchmarks in specific ways.

Synthetic Gaming Benchmarks (e.g., 3DMark Time Spy, Port Royal)

  • PCIe 4.0 and 5.0: There is virtually no performance loss. Modern GPUs rarely saturate a full x16 PCIe 4.0 or 5.0 connection. Dropping to x8 results in a benchmark score difference of 1% to 2%, which is within the margin of error.
  • PCIe 3.0: Modern high-end GPUs will experience a slight bottleneck on PCIe 3.0 x8, resulting in a 3% to 8% drop in synthetic graphics scores due to restricted bandwidth during heavy texture streaming.

3D Rendering Benchmarks (e.g., Blender, V-Ray, OctaneBench)

  • During the Render: Bifurcation has near-zero impact on the actual rendering phase. Once the scene geometry and textures are loaded into the GPU's VRAM, the PCIe bus is barely utilized. Two GPUs running at x8/x8 will nearly double the benchmark score of a single GPU at x16.
  • Scene Loading: Because bandwidth is halved, the initial loading of large scenes into the VRAM takes milliseconds longer, marginally affecting total benchmark completion times.

AI and Machine Learning Benchmarks (e.g., MLPerf)

  • High Impact: Multi-GPU AI training relies heavily on peer-to-peer communication across the PCIe bus if direct bridges (like NVLink) are absent. Splitting an x16 slot into x8/x8 halves the communication speed between the cards, which can reduce multi-GPU scaling efficiency by 10% to 20% in bandwidth-heavy models.

Impact on Multi-NVMe Benchmarking

PCIe bifurcation provides massive performance gains for storage benchmarking. Most consumer M.2 NVMe SSDs natively use four PCIe lanes (x4). By bifurcating an x16 slot into an x4/x4/x4/x4 configuration using a quad-M.2 adapter card, you give four individual drives their maximum native bandwidth.

Individual Drive Benchmarks (e.g., CrystalDiskMark, ATTO)

  • Zero Bottlenecks: Because each drive gets its required four lanes, benchmarking a single drive on a bifurcated card yields the exact same sequential and random read/write speeds as installing it directly into a motherboard M.2 slot.

RAID 0 Array Benchmarks

  • Linear Scaling: Bifurcation shines when benchmarking four NVMe drives in a software RAID 0 array. Because the x16 slot usually connects directly to the CPU, all four drives bypass the motherboard chipset (DMI) bottleneck.
  • Maximum Throughput: In sequential read/write benchmarks, four PCIe 4.0 NVMe drives in a bifurcated x4/x4/x4/x4 slot can achieve upwards of 25,000 MB/s to 28,000 MB/s—nearly quadrupling single-drive scores.

Latency and Thermal Considerations in Benchmarking

While lane allocation dictates bandwidth, two secondary factors of PCIe bifurcation directly affect benchmark scores:

  • Lower Latency: Unlike expansion cards that use multiplexer (PLX) chips to split lanes, native motherboard bifurcation connects devices directly to the CPU. This eliminates switch latency, resulting in slightly higher 4K random read/write storage scores and better 1% low frame rates in GPU tests.
  • Thermal Throttling: Bifurcation often requires placing GPUs or NVMe drives very close together on adapter cards. During sustained benchmarks (like 3DMark Stress Tests or long file transfers), the restricted airflow can cause components to overheat and thermally throttle, artificially lowering your final benchmark scores. Active cooling is mandatory to maintain peak performance.