GPU Fabric Physics 2026: Why 800G Isn’t Enough for 100k-GPU Training
The NCCL Timeout Nightmare You dropped $50 million on H200s. Wired them up with 800G OSFP optics. Fired up your 100,000-GPU cluster for the “Big Run.” Six hours in, everything’s humming—until the loss curve just flatlines. Logs start screaming: NCCL_WATCHDOG_TIMEOUT. It’s not a bad GPU. It’s not a driver crash. Honestly, it’s just physics. Once…