The Pragmatic Pivot: Choosing Software Topology Over Hardware Perfection in a Proxmox GPU VM

Introduction

In the intricate dance between hardware idealism and software pragmatism, few moments are as telling as the one captured in message 388 of this opencode session. The assistant, after an extensive multi-message investigation into the PCIe topology of a Proxmox virtual machine hosting eight NVIDIA RTX PRO 6000 Blackwell GPUs, arrives at a critical decision point. The message represents a strategic pivot—abandoning the pursuit of a perfect hardware-level NUMA topology fix in favor of a software workaround that is safer, more reliable, and achievable within the constraints of the Proxmox virtualization platform.

This article examines message 388 in depth: its reasoning, its decisions, its assumptions, and the knowledge it both consumes and produces. The message is deceptively short—a few paragraphs of reasoning followed by a single bash command—but it encapsulates the culmination of a lengthy debugging journey and the birth of a new tactical direction.

The Message

The assistant writes:

OK. The approach using raw QEMU args is risky with Proxmox because Proxmox auto-generates the QEMU command line for hostpci devices. If we add -device vfio-pci manually via args while also having hostpci entries, we'd get duplicate device passthrough.

The safest approach that works with Proxmox is actually simpler than I thought. Proxmox already creates the virtual PCIe root ports for each hostpci device. The issue is just that those root ports don't have NUMA affinity. We can add pxb-pcie devices via args that sit between the root bus and provide NUMA context, but we'd need Proxmox to attach the hostpci devices to those buses instead of auto-generated ones.

Let me take the pragmatic path — NCCL topology XML — while also trying something that could work at the Proxmox level. The NCCL topo file is the most reliable way to fix NCCL's topology awareness:

[bash] ssh [REDACTED] "nvidia-smi --query-gpu=index,pci.bus_id,gpu_bus_id --format=csv,noheader" 2>&1 0, 00000000:01:00.0, 00000000:01:00.0 1, 00000000:02:00.0, 00000000:02:00.0 2, 00000000:03:00.0, 00000000:03:00.0 3, 00000000:04:00.0, 00000000:04:00.0 4, 00000000:05:00.0, 00000000:05:00.0 5, 00000000:06:00.0, 00000000:06:00.0 6, 00000000:07:00.0, 00000000:07:00.0 7, 00000000:08:00.0, 00000000:08:00.0

WHY This Message Was Written: The Reasoning and Motivation

To understand why this message exists, one must appreciate the journey that led to it. The broader segment (Segment 3) is titled "Investigated and attempted to enable P2P DMA for GPUs in a Proxmox VM by modifying host kernel parameters, migrating to Q35 chipset, fixing BAR allocation, and disabling ACS, but ultimately found that the hardware topology (each GPU on its own PCIe root complex) fundamentally prevents P2P."

The assistant had been on a multi-message quest spanning from message 370 onward. The user had provided a bare-metal nvidia-smi topo matrix showing a clean NUMA topology where GPUs 0-3 were on NUMA node 0 with NODE interconnects, GPUs 4-7 were on NUMA node 1 with NODE interconnects, and cross-socket communication was marked SYS. In the VM, however, all GPU pairs showed PHB (PCIe Host Bridge) with no NUMA distinction—a significantly degraded topology that would cripple NCCL's ability to optimize communication patterns for large-scale model inference.

The assistant's motivation in message 388 is to resolve this degradation. The user is running a production-like ML inference workload (GLM-5-NVFP4 on 8 GPUs with SGLang), and the cross-GPU communication bottleneck directly impacts inference throughput. Every millisecond of NCCL communication delay translates to slower token generation. The assistant is acutely aware of this stakes, having previously benchmarked the system and identified PCIe P2P latency as the primary bottleneck.

The immediate trigger for message 388 is the assistant's realization, articulated in the preceding messages, that the Proxmox VM's virtual PCIe topology lacks NUMA affinity. The GPUs are visible in the VM, they have full 96GB VRAM, and they operate correctly—but nvidia-smi topo -m shows PHB for all GPU pairs, and cat /sys/bus/pci/devices/*/numa_node returns -1 for all GPUs. The virtual PCIe root ports created by Proxmox for each hostpci device simply don't carry NUMA node information.

The assistant had explored several avenues before this message. They had:

Investigated the host's PCIe topology and IOMMU groups
Modified the Proxmox host kernel command line to enable IOMMU passthrough
Migrated the VM from the legacy i440FX chipset to Q35 with proper PCIe passthrough
Fixed BAR allocation failures by adding pci=realloc to the guest kernel
Attempted ACS (Access Control Services) disable to merge IOMMU groups
Discovered that each GPU sits on its own dedicated PCIe root complex on the ASUS ESC8000A-E13 motherboard, a hardware topology that fundamentally prevents IOMMU group merging Now, in message 388, the assistant is facing the next logical question: given that the hardware topology cannot be changed, can the virtual topology be improved?## The Decision Process: Three Approaches Considered and One Chosen The core of message 388 is a decision-making process. The assistant evaluates three approaches to fix the NUMA topology problem and chooses one. Let's examine each.

Approach 1: Raw QEMU `-device vfio-pci` Args (Rejected)

The most thorough fix would be to bypass Proxmox's hostpci mechanism entirely and pass GPUs through using raw QEMU -device vfio-pci arguments. This would give complete control over the virtual PCIe topology, allowing the assistant to create PCIe expander buses (pxb-pcie) with explicit NUMA node associations. The GPUs could be placed on virtual PCIe switches that mirror the physical NUMA domains.

The assistant correctly identifies the fatal flaw: Proxmox auto-generates the QEMU command line from its configuration. If hostpci entries exist in the VM config, Proxmox will generate -device vfio-pci lines for them. Adding duplicate -device vfio-pci lines via the -args mechanism would result in double passthrough of the same devices, which would either fail or cause undefined behavior. The two systems would conflict.

This is a crucial insight about Proxmox's architecture. Proxmox is not a thin wrapper around QEMU—it is an opinionated management layer that owns the QEMU command-line generation. Custom args are appended to the generated command line, not merged into it. This means you cannot override or replace the device creation that Proxmox handles; you can only add supplementary devices. For the NUMA topology fix, the assistant would need to replace the auto-generated PCIe topology, not supplement it.

The assistant's reasoning is sound: this approach is "risky with Proxmox" and would create "duplicate device passthrough." The decision to reject it is correct.

Approach 2: `pxb-pcie` Devices via Args (Considered but Deferred)

The assistant briefly considers a middle ground: adding pxb-pcie (PCIe expander bus) devices via custom args that sit between the root bus and provide NUMA context. The idea is that Proxmox creates the virtual PCIe root ports for each hostpci device, and pxb-pcie devices could be inserted to provide NUMA affinity to those ports.

However, the assistant identifies the problem: "we'd need Proxmox to attach the hostpci devices to those buses instead of auto-generated ones." The pxb-pcie devices would create new PCIe buses, but the hostpci devices would still be attached to the auto-generated root ports. The NUMA affinity would apply to the expander bus, not to the GPUs themselves. This approach might improve the situation somewhat, but it's unclear whether NCCL would see the benefit, since nvidia-smi topo -m derives its topology from the actual PCIe hierarchy visible to the GPUs.

The assistant doesn't fully reject this approach—they say "while also trying something that could work at the Proxmox level"—but they deprioritize it in favor of the more reliable software workaround. This is a pragmatic trade-off: the pxb-pcie approach would require experimentation, potential VM crashes, and uncertain results, while the NCCL XML approach is well-documented and guaranteed to work.

Approach 3: NCCL Topology XML (Chosen)

The chosen approach is to create an NCCL topology XML file. NCCL (NVIDIA Collective Communications Library) has a mechanism to accept an XML file describing the GPU interconnect topology. When this file is present, NCCL uses it instead of the runtime-detected topology from nvidia-smi topo -m. This means the assistant can fabricate a topology that mirrors the bare-metal layout, telling NCCL that GPUs 0-3 are connected via NODE to each other and SYS to GPUs 4-7, and vice versa.

The assistant calls this "the most reliable way to fix NCCL's topology awareness," and this assessment is accurate. The NCCL topology XML is a supported, documented feature. It doesn't require any changes to the VM configuration, no host kernel modifications, no VM reboots, and no risk of breaking the GPU passthrough. It is a pure software intervention that operates at the library level, where the actual communication decisions are made.

The bash command that follows the reasoning—querying nvidia-smi for GPU index and PCI bus ID mapping—is the first step in constructing this XML file. The assistant needs to know which VM PCI bus corresponds to which GPU index to write accurate topology entries.## Assumptions Embedded in the Message

Every decision rests on assumptions, and message 388 contains several that deserve examination.

Assumption 1: NCCL topology XML is sufficient. The assistant assumes that fixing NCCL's topology awareness will translate into measurable performance improvement. This is a reasonable assumption—NCCL uses topology information to choose communication algorithms (ring, tree, NVLink-based) and to optimize buffer registration and data transfer paths. However, it's worth noting that the NCCL XML only affects NCCL's internal decisions. It does not change the actual PCIe paths the data travels. If the underlying hardware limitation is PCIe bandwidth or latency (rather than NCCL's algorithm choice), the XML may not help. The assistant seems aware of this, framing it as a "software workaround" rather than a true fix.

Assumption 2: Proxmox cannot be made to generate NUMA-aware PCIe topologies. The assistant assumes that Proxmox's auto-generated PCIe root ports cannot be given NUMA affinity through standard configuration. This is likely correct—Proxmox's hostpci syntax is relatively simple (mapping=...,pcie=1,rombar=0,etc.) and does not include NUMA node parameters. However, the assistant does not exhaustively verify this by checking Proxmox documentation or source code. The assumption is based on practical experience with the platform.

Assumption 3: The pxb-pcie approach would conflict with Proxmox's device management. The assistant assumes that adding pxb-pcie devices via -args while keeping hostpci entries would not work because the GPUs would remain attached to the auto-generated root ports. This is technically correct—pxb-pcie creates a new bus, but the existing hostpci devices are already attached to their respective root ports. You cannot retroactively move a PCIe device to a different bus in QEMU without recreating the device on that bus.

Assumption 4: The bare-metal topology is the ideal target. The assistant assumes that matching the bare-metal NUMA topology (NODE within socket, SYS across sockets) will restore bare-metal performance. This is reasonable, but it's worth questioning whether the virtualized PCIe paths have the same latency characteristics as bare metal even when the topology labels match. A NODE label in the NCCL XML does not create a NODE-quality interconnect—it just tells NCCL to treat the link as if it were one.

Input Knowledge Required to Understand This Message

To fully grasp message 388, a reader needs knowledge spanning several domains:

Proxmox VE architecture: Understanding that Proxmox auto-generates QEMU command lines from its configuration files, that hostpci entries create virtual PCIe root ports, and that custom -args are appended rather than merged. Without this knowledge, the assistant's rejection of the raw QEMU args approach seems arbitrary.

QEMU PCIe topology: Knowledge of pxb-pcie (PCIe expander bus), how NUMA affinity is assigned to PCIe buses in QEMU, and the relationship between virtual PCIe root ports and physical device passthrough. The assistant's brief consideration of pxb-pcie assumes the reader understands what this device does.

NCCL internals: Understanding that NCCL uses topology information from nvidia-smi topo -m to optimize collective communication, and that NCCL supports an XML-based topology override. The assistant's choice of the NCCL XML approach relies on this feature's existence and reliability.

The session history: The reader must know about the bare-metal topology matrix (message 370), the physical-to-virtual GPU mapping (messages 372-384), the NUMA node discovery (message 385), and the VM's Q35 chipset configuration (message 387). The assistant's reasoning in message 388 builds directly on these findings.

The user's goal: The user is running GLM-5-NVFP4 inference on 8 GPUs with SGLang. Cross-GPU communication is a bottleneck. The assistant is trying to improve inference throughput. Without this context, the NUMA topology fix seems like an academic exercise rather than a performance-critical intervention.

Output Knowledge Created by This Message

Message 388 produces several forms of knowledge:

The PCI bus-to-GPU index mapping: The bash command output shows that VM GPU indices 0-7 map to PCI buses 01:00.0 through 08:00.0 sequentially. This is the raw material needed to construct the NCCL topology XML. The assistant now knows that GPU 0 is at PCI 01:00.0, GPU 1 at 02:00.0, etc.

The decision to use NCCL XML: The message establishes the NCCL topology XML approach as the chosen path forward. This decision shapes all subsequent work in the session. It represents a shift from hardware-level fixes (kernel parameters, chipset migration, ACS disable) to software-level fixes (library configuration).

The rejection of raw QEMU args: By explicitly stating why the raw QEMU approach is risky, the message creates a documented rationale for not pursuing that path. This prevents future backtracking or wasted effort.

The deferred consideration of pxb-pcie: The assistant leaves the door open for a Proxmox-level fix, creating a potential future work item. This is important because the NCCL XML approach, while practical, is a workaround rather than a true fix. If the user later needs genuine NUMA affinity (e.g., for memory bandwidth-sensitive workloads that don't go through NCCL), the pxb-pcie approach may need revisiting.

The Thinking Process: A Window Into Debugging Methodology

The assistant's reasoning in message 388 reveals a disciplined debugging methodology. Let's trace the thought process:

Problem identification: The VM's GPUs show PHB topology with no NUMA affinity, unlike the bare-metal NODE/SYS topology.
Solution space enumeration: The assistant identifies three approaches: raw QEMU args, pxb-pcie via args, and NCCL topology XML.
Constraint analysis: For each approach, the assistant evaluates constraints. Raw QEMU args conflict with Proxmox's auto-generation. pxb-pcie requires Proxmox cooperation that doesn't exist. NCCL XML has no constraints—it's a standalone file.
Risk assessment: The assistant explicitly labels the raw QEMU approach as "risky." The pxb-pcie approach is uncertain. The NCCL XML approach is "the most reliable."
Pragmatic selection: The assistant chooses the approach that works within constraints, has minimal risk, and addresses the core problem (NCCL's topology awareness). This is classic engineering trade-off reasoning.
Immediate action: The assistant doesn't just decide—they take the first concrete step (querying the PCI bus mapping) to begin implementing the chosen approach. This thinking process is notable for its clarity and honesty. The assistant doesn't pretend the NCCL XML approach is a perfect solution. They call it a "pragmatic path" and acknowledge that they're "also trying something that could work at the Proxmox level." This dual-track thinking—pursuing the reliable fix while keeping the ideal fix as a future possibility—is characteristic of experienced system engineers.

Mistakes and Incorrect Assumptions

While the message is well-reasoned, there are potential issues worth examining:

The NCCL XML approach may not address the root cause. The actual performance bottleneck might be PCIe bandwidth or latency, not NCCL's algorithm choice. If the GPUs are communicating over virtualized PCIe paths that traverse the Proxmox host's IOMMU and hypervisor, the latency may be dominated by virtualization overhead rather than topology. In that case, making NCCL think the topology is better won't improve actual throughput. The assistant seems to implicitly acknowledge this by calling it a "workaround" rather than a fix.

The assumption that pxb-pcie cannot work may be premature. While the assistant correctly identifies that hostpci devices are attached to auto-generated root ports, there might be a way to influence which root port they attach to. Proxmox's hostpci entries are assigned to virtual PCIe slots in order (hostpci0 → slot 0, hostpci1 → slot 1, etc.), and QEMU's Q35 chipset has multiple PCIe root ports. If the assistant could map hostpci0-3 to one NUMA node's root port and hostpci4-7 to another's, the GPUs might inherit NUMA affinity. This would require modifying the VM config to use specific PCIe slot assignments rather than the default sequential assignment. The assistant doesn't explore this possibility.

The message doesn't validate that NCCL XML works in this environment. The assistant assumes NCCL will respect the XML override, but this depends on the NCCL version, the environment variables set, and whether the XML file is placed in the correct location. In practice, NCCL's XML override can be finicky—some versions require specific XML schemas, and the file must be readable at the time NCCL initializes. The assistant doesn't mention these potential pitfalls.

Conclusion

Message 388 is a masterclass in pragmatic engineering decision-making under constraints. The assistant faces a complex problem—degraded GPU interconnect topology in a virtualized environment—and evaluates multiple solutions against the constraints of the Proxmox platform. The chosen approach (NCCL topology XML) is not the most elegant or thorough, but it is the most reliable and least risky. It represents the wisdom of choosing the fix that works over the fix that is ideal.

The message also reveals something important about the nature of virtualization for GPU workloads: no matter how carefully you configure the hypervisor, there are aspects of the hardware topology that cannot be perfectly virtualized. The assistant's journey through kernel parameters, chipset migration, BAR allocation, ACS disable, and now NCCL XML is a testament to the layered complexity of modern ML infrastructure. Each layer—hardware, hypervisor, guest OS, driver, library—introduces its own constraints and workarounds.

For the reader, message 388 offers a template for thinking about similar problems: enumerate the solution space, evaluate against real constraints, assess risk honestly, and choose the pragmatic path. The NCCL topology XML may not be a perfect fix, but it is a fix that can be implemented now, without rebooting the VM, without touching the host, and without risking the stability of a production inference server. Sometimes that is the best kind of fix there is.