PCIe Retimers and Redrivers in Motherboard Benchmarking

This article explores how PCIe signal conditioning components—specifically retimers and redrivers—impact computer hardware benchmarking on motherboards with long copper traces. As PCIe generations advance to 4.0 and 5.0, high-frequency signals degrade rapidly over distance, necessitating these components to prevent data loss. We will examine the functional differences between redrivers and retimers, how they prevent performance-throttling errors, and the subtle latency trade-offs they introduce during synthetic and real-world benchmarking of GPUs and NVMe SSDs.

The Long Trace Problem

Modern hardware benchmarks push massive amounts of data through a motherboard's copper pathways, known as traces. In PCIe 4.0 (16 GT/s) and PCIe 5.0 (32 GT/s), the frequency of these signals is so high that the motherboard material itself absorbs the signal over short distances—often just a few inches.

On larger motherboards (such as E-ATX or workstation boards) or when routing lanes to secondary PCIe slots and distant M.2 drives, the traces become too long for the signal to survive intact. The signal suffers from attenuation (loss of strength) and jitter (timing errors). Without signal conditioning, the system will either fail to detect the hardware or automatically downgrade the link speed to an older, slower PCIe generation, ruining benchmark results.

Redrivers vs. Retimers

To fix signal degradation on long traces, motherboard manufacturers place signal conditioning chips along the pathway. There are two types:

  • Redrivers: These are analog components. A redriver takes a weakening signal, applies equalization to sharpen the waveform, and amplifies it so it can travel further. Because it is a purely analog boost, a redriver also amplifies any background noise the signal has picked up along the way. Redrivers are inexpensive and used for moderately long traces.
  • Retimers: These are advanced, mixed-signal (analog and digital) devices. A retimer is protocol-aware, meaning it actually understands the PCIe data. It captures the degraded signal, decodes the data, strips away all accumulated noise and jitter, and transmits a brand-new, clean signal using its own internal clock. Retimers are required for extremely long traces and are standard in PCIe 5.0 motherboards.

Preventing Bandwidth Loss in Benchmarks

The primary positive effect of redrivers and retimers in benchmarking is the preservation of maximum sustained bandwidth.

When a high-speed PCIe signal degrades over a long trace without conditioning, the receiving hardware experiences Cyclic Redundancy Check (CRC) errors. When an error occurs, the PCIe controller must pause and request that the data packet be sent again.

In a benchmark environment, constant packet retransmission is devastating to performance:

  • GPU Benchmarks: It causes severe micro-stuttering, drops in 1% low framerates, and lower overall scores in bandwidth-heavy tests like 3DMark PCI Express feature tests.
  • Storage Benchmarks: Sequential read and write speeds in tools like CrystalDiskMark will plummet, as the NVMe drive spends clock cycles resending corrupted data rather than pushing new data.

By maintaining signal integrity, retimers and redrivers ensure zero packet loss, allowing hardware in distant slots to achieve the same peak throughput as hardware located right next to the CPU.

The Latency Trade-off

While signal conditioners save bandwidth, they introduce a microscopic penalty: latency.

Redrivers operate with near-zero latency (typically picoseconds) because they amplify the signal on the fly. Retimers, however, must decode, clock, and re-encode the data. This process takes time, usually adding several nanoseconds of delay (equivalent to a few clock cycles) to the data path.

In GPU gaming benchmarks, a few nanoseconds of latency are entirely imperceptible and fall within the margin of error. However, in synthetic storage benchmarking, this latency is measurable. When testing an ultra-fast PCIe 5.0 NVMe SSD on a long trace routed through a retimer, you will see slightly lower scores in 4K Random Read/Write tests at Queue Depth 1 (QD1). Because QD1 tests rely entirely on how fast a single request can travel back and forth, the retimer’s processing delay slightly bottlenecks the drive's true latency performance.

Thermal Throttling of the Signal Path

Retimers are essentially miniature processors and generate significant heat during heavy workloads. During prolonged stress-testing benchmarks, an uncooled retimer can overheat. If a retimer thermally throttles, it can cause the PCIe link to become unstable, resulting in sudden benchmark crashes or drastic mid-test performance drops. Quality motherboards integrate retimers under the main VRM or chipset heatsinks to ensure consistent benchmark repeatability.

Slot Selection for Accurate Benchmarking

Understanding signal conditioning dictates how hardware should be installed for accurate benchmarking.

Motherboard engineers design the primary PCIe x16 slot and the top M.2 slot with the shortest possible traces to the CPU. This often eliminates the need for retimers entirely, or at least minimizes the work they have to do.

If you are benchmarking a GPU or SSD to find its absolute peak capability, it must be installed in the primary slots closest to the CPU. Moving the hardware to a bottom slot forces the data through longer traces and active retimers. While bandwidth will remain high, the added latency of the retimer and the potential for routing through the motherboard chipset will result in slightly lower synthetic benchmark scores.