Splitting the GPU Topology: A Deliberate Act of Infrastructure Surgery

In the middle of an intensive optimization session for a multi-GPU Blackwell inference deployment, a seemingly mundane message arrives — one that reads the contents of a PCI mapping configuration file on a Proxmox host. But this message, <msg id=6057>, is far from routine. It represents the precise moment where the assistant transitions from knowing the system's topology to acting upon it, executing a deliberate reconfiguration that will split eight NVIDIA RTX PRO 6000 Blackwell GPUs across two separate virtualization domains.

The Context: Why This Message Exists

The message is a direct response to a user instruction at <msg id=6034>: "I now want to reconfigure the proxmox host such that the lxc container only has 4 GPUs and the other 4 are available for another VM / VMs through passthrough (nvidia driver to 4 on one numa, other 4 to vfio)." This instruction came after the assistant had been running high-concurrency benchmarks on a Qwen3.5-397B model served by SGLang across all 8 GPUs. The user was satisfied with the benchmark results and wanted to pivot to a new phase: splitting the hardware.

The motivation behind this request is not stated explicitly, but it is easy to infer. Running a single large model across 8 GPUs with tensor parallelism (TP=8) is one valid configuration, but it dedicates the entire machine to a single workload. By splitting the GPUs, the user gains flexibility: the LXC container keeps 4 GPUs for the inference workload (likely with a smaller model or reduced tensor parallelism), while the remaining 4 GPUs become available for passthrough to a separate VM — perhaps for training, experimentation, or a different model serving stack. This is a classic infrastructure trade-off between peak single-workload performance and multi-tenancy.

The assistant had already done extensive groundwork before this message. It had mapped every GPU to its PCI address and NUMA node (<msg id=6040>, <msg id=6041>), verified IOMMU group isolation (<msg id=6049>), unbounded the NUMA 1 GPUs from the nvidia driver and bound them to vfio-pci (<msg id=6050>), and updated the LXC container config to only mount nvidia0 through nvidia3 (<msg id=6055>). The physical re-binding was done. What remained was the logical mapping — the Proxmox PCI mapping configuration that tells the hypervisor which PCI devices belong to which named mapping, which can then be assigned to VMs.

The Message Itself: A Plan Declared, A State Examined

The message opens with a clear declarative statement of intent:

Now let me split the existing pro6000 mapping into two: one for NUMA0 (LXC, nvidia-bound) and one for NUMA1 (VM passthrough, vfio-bound). The existing pro6000 mapping has all 8 GPUs. I'll keep pro6000 for the NUMA0 set and create pro6000-vm for the NUMA1 set.

This is a textbook example of the assistant's reasoning being made visible. It explicitly states:

  1. What it is about to do: split one mapping into two.
  2. Why the split is needed: the existing mapping contains all 8 GPUs, but the new configuration requires separate mappings for the two NUMA domains.
  3. How it will name them: pro6000 (retained for NUMA 0) and pro6000-vm (new, for NUMA 1). The assistant then executes a bash command to read the current PCI mapping file:
ssh root@10.1.2.6 'cat /etc/pve/mapping/pci.cfg'

This is not an idle check. The assistant needs the exact contents of this file to construct the replacement. The PCI mapping file on a Proxmox host is a critical configuration artifact — it defines named mappings that associate PCI device IDs (vendor:device pairs) with specific IOMMU groups, host nodes, and PCI paths. These mappings are what allow Proxmox to present PCI devices to VMs in a structured, repeatable way. Editing this file incorrectly could render GPUs inaccessible to VMs or, worse, cause boot failures.

The output reveals the existing state of the file. It contains mappings for several GPU types that had been configured in earlier sessions: nv3080, nv3090, nv3090-2, nv3090-3, nva6000, nva6000-2, and crucially pro6000 — which lists all 8 Blackwell GPUs with their IOMMU groups, PCI paths, and subsystem IDs. The assistant now has the raw material it needs to construct the split.

Decisions Made and Assumptions Held

Several decisions and assumptions are embedded in this message:

Decision: Keep the existing pro6000 name for NUMA 0. The assistant could have renamed both mappings, but preserving pro6000 for the LXC-bound GPUs minimizes disruption. Any existing references to pro6000 in VM or container configurations will continue to work.

Decision: Name the new mapping pro6000-vm. This is a clear, convention-following name that signals the mapping's purpose. It follows the pattern of other mappings in the file (e.g., nv3090-2, nv3090-3, nva6000-2).

Assumption: The PCI mapping file is the correct abstraction layer. Proxmox supports two methods for PCI passthrough: direct assignment by PCI address in the VM config, and named mappings defined in /etc/pve/mapping/pci.cfg. The assistant assumes that the user will want to use named mappings for the VM, which is the more maintainable approach.

Assumption: The IOMMU groups are correctly isolated. The assistant had verified earlier (<msg id=6049>) that each GPU is in its own IOMMU group, meaning they can be passed through independently. This is a critical prerequisite — if multiple GPUs shared an IOMMU group, they would have to be passed through together, which would complicate the split.

Assumption: The node name (kpro6) is correct. The mappings reference node=kpro6, which is the Proxmox host's node name. The assistant trusts that this hasn't changed.

Input Knowledge Required

To understand this message, one needs knowledge of:

  1. Proxmox PCI mapping syntax: The file format — map id=<vendor:device>,iommugroup=<N>,node=<name>,path=<PCI address>,subsystem-id=<vendor:device> — is specific to Proxmox VE. Each line defines one PCI device in a named mapping group.
  2. IOMMU groups and PCI passthrough: IOMMU groups are the atomic unit of PCI passthrough — all devices in the same group must be passed through together. The assistant had verified each GPU is in its own group.
  3. NUMA topology: The GPUs are split across NUMA nodes 0 and 1. The assistant is using NUMA affinity as the natural boundary for the split, which is sensible because it keeps each mapping's GPUs on the same NUMA domain, optimizing memory access patterns.
  4. The existing hardware inventory: The pro6000 mapping contains 8 GPUs at PCI addresses 01:00.0, 11:00.0, 61:00.0, 71:00.0 (NUMA 0) and 81:00.0, 91:00.0, e1:00.0, f1:00.0 (NUMA 1). The assistant had painstakingly built this mapping across multiple earlier messages.
  5. The broader session context: This is part of a much longer effort (segment 40 of the overall session) that has involved building SGLang from source, patching Blackwell SM120 support, debugging FP8 KV cache accuracy, and deploying production services. The GPU split is a late-stage infrastructure refinement.

Output Knowledge Created

This message produces a critical piece of output knowledge: the current state of the Proxmox PCI mapping file. The assistant now knows exactly what entries exist, their format, and the specific parameters for each GPU in the pro6000 mapping. This knowledge is immediately actionable — in the very next message (<msg id=6058>), the assistant will use this information to write the updated file with the split mappings.

The message also implicitly documents the assistant's plan. By stating the intent before executing, the assistant creates a record of its reasoning that can be reviewed later. This is especially valuable in a complex infrastructure session where multiple changes are being made in rapid succession — a reader (or the user) can trace back to understand why a particular change was made.

The Thinking Process Visible in the Message

The assistant's reasoning is laid out in a clear, sequential structure:

  1. State the goal: "split the existing pro6000 mapping into two"
  2. Define the boundary: "NUMA0 (LXC, nvidia-bound)" vs "NUMA1 (VM passthrough, vfio-bound)"
  3. Explain the current state: "The existing pro6000 mapping has all 8 GPUs"
  4. Announce the naming convention: "I'll keep pro6000 for the NUMA0 set and create pro6000-vm for the NUMA1 set"
  5. Gather the data: Execute cat /etc/pve/mapping/pci.cfg to read the current file This is a classic "plan-then-execute" pattern. The assistant does not blindly edit the file; it first reads it to ensure it has the correct data. This is especially important because the file contains other mappings (nv3080, nv3090, etc.) that must be preserved unchanged. The assistant needs to see the full file to construct a correct replacement. The thinking also reveals an awareness of the file's sensitivity. The assistant does not use a sed one-liner or an awk script to surgically modify the pro6000 section — it plans to read the entire file and then write a complete replacement. This is the safer approach for structured configuration files where partial edits risk syntax errors.

Potential Mistakes and Incorrect Assumptions

One subtle issue deserves mention: the assistant assumes that the PCI mapping file is the only place where the GPU split needs to be recorded. However, for the LXC container, the GPU access is controlled by the mount entries in /etc/pve/lxc/129.conf (already updated in <msg id=6055>), not by the PCI mapping file. The PCI mapping file is primarily used for VM passthrough, not LXC. So the pro6000 mapping (NUMA 0) may not actually be used by the LXC at all — the LXC accesses GPUs through bind-mounted device files. This means keeping the pro6000 name for NUMA 0 is somewhat cosmetic; the real value is in the new pro6000-vm mapping for VM passthrough.

This is not a mistake, but it reflects an assumption that the mapping should mirror the physical topology. The assistant is keeping the configuration clean and consistent, even if the LXC doesn't strictly need the mapping.

Another potential concern: the assistant does not verify that the vfio-pci binding of the NUMA 1 GPUs survived the container stop/start cycle. In <msg id=6050>, the GPUs were bound to vfio-pci dynamically via sysfs, but this binding may not persist across a reboot. The assistant later creates a systemd service (gpu-vfio-split.service) to persist the binding, but that happens in subsequent messages. At this moment, the assistant is operating under the assumption that the dynamic binding is sufficient for the immediate next step (creating the VM), with persistence to be addressed separately.

Conclusion

Message <msg id=6057> is a planning and reconnaissance message that sits at the inflection point between investigation and action. The assistant has completed its discovery of the GPU topology, verified IOMMU isolation, performed the driver rebinding, and updated the LXC configuration. Now it needs to update the Proxmox PCI mapping to make the new topology official and usable for VM passthrough. By reading the current mapping file, the assistant arms itself with the exact data needed to construct the split. The message is short — a single bash command and a few lines of explanation — but it encapsulates a critical moment of decision-making in a complex infrastructure reconfiguration. The assistant's visible reasoning, clear naming conventions, and methodical approach to gathering state before modifying it exemplify the kind of careful, deliberate engineering that the entire session embodies.