TurboBus: Pooling PCIe Bandwidth for LLM Workloads via Scale-Up Fabrics
GPU memory offloading is widely adopted for LLM workloads but shifts the bottleneck to GPU-CPU transfers, which can take up to 90% of the end-to-end inference/training time! Paradoxically, over 60% of PCIe bandwidth remains idle. The root cause is that PCIe links are individually bottlenecked but collectively underutilized. Bursty, phase-driven transfer patterns leave bandwidth idle both within and across jobs. We present TurboBus, which pools PCIe bandwidth across co-located jobs via emerging scale-up fabrics. TurboBus enables any GPU to borrow idle PCIe links from neighboring GPUs, even those belonging to other jobs, while preserving isolation through a privileged daemon. At the core of TurboBus, it streams data through relay GPUs with bounded memory overhead, keeps all links busy through fine-grained PCIe allocation, enables bidirectional transfers leveraging PCIe/NVLink bandwidth asymmetry, and balances fairness and completion time with a size-aware scheduling. We fully implement TurboBus and our experiments show that it reduces first-token latency by up to 40% for on-demand model loading (within 5% of the analytical optimum), achieves up to 1.6x throughput for KV-cache-offloaded inference, and accelerates training by up to 7%, while imposing less than 1% overhead on co-located workloads.