Chunk 3.0

In this chunk, the focus shifted to resolving the critical cross-GPU communication bottleneck by attempting to enable Peer-to-Peer (P2P) DMA in the Proxmox VM. The primary tasks involved deeply investigating the host's PCIe topology, modifying the Proxmox host kernel command line to enable IOMMU passthrough (`amd_iommu=on iommu=pt`), and migrating the VM from the legacy i440FX chipset to Q35 with proper PCIe device passthrough (`pcie=1`). This migration initially caused a severe BAR allocation failure that prevented 6 of 8 GPUs from being detected, which was subsequently fixed by adding `pci=realloc` to the guest kernel. Significant achievements were made in stabilizing the VM's hardware foundation. All 8 GPUs are now fully operational with their full 96GB VRAM, and host-level validation confirmed PCIe Gen5 x16 links with ASPM disabled. However, the core goal of enabling P2P remained elusive. An attempt to disable ACS (Access Control Services) in the BIOS to merge IOMMU groups was unsuccessful; the GPUs remained in separate groups because each one is physically attached to its own distinct PCIe root complex on the AMD EPYC platform, with no shared PCIe switch. This hardware topology fundamentally prevents the VFIO/IOMMU from granting direct P2P DMA access, regardless of ACS settings. The overarching theme is the stark reality of virtualization overhead on tightly-coupled GPU inference workloads. The investigation systematically peeled back layers—from the VM's chipset and PCIe configuration to the host's IOMMU and ACS settings—only to reveal a fundamental hardware topology constraint that cannot be overcome by software alone. The chunk concludes with a new operational hurdle: the VM fails to start due to an IOMMU group mismatch error (`'iommugroup' does not match for 'pro6000' (51 != 72)`), indicating that the Proxmox PCI device mapping must be updated to reflect the current hardware state before further progress can be made.

The Virtualization Ceiling: How an 8-GPU LLM Deployment Collided with the Physics of PCIe Topology 2339 words

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