The InfiniBand Discovery That Didn't Unstick the Model
In the intricate dance of deploying a 122-billion-parameter language model across two DGX Spark nodes, the assistant reaches a moment of diagnostic clarity that simultaneously confirms a correct hypothesis and reveals the stubborn persistence of a deeper problem. Message [msg 6637] is a brief but revealing checkpoint in a multi-hour debugging session: the assistant has discovered that NCCL (NVIDIA Collective Communications Library) is correctly using the InfiniBand interconnect (NET/IBext_v11) between the two nodes, but the model loading process remains stuck at the same initialization barrier it has been hung at for the past several minutes.
The Message
The assistant writes:
NCCL is using NET/IBext_v11 — InfiniBand! That's the correct transport. Let me check if it got past this point:
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``bash ssh aurora@10.1.230.180 'grep -E "Init.*end|Load weight|weight begin|KV Cache|Memory pool|fired up|ready" /home/aurora/sglang-head.log' 2>&1 ``
>
`` [2026-04-08 22:57:51] server_args=ServerArgs(model_path='/models/Qwen3.5-122B-A10B-FP8', tokenizer_path='/models/Qwen3.5-122B-A10B-FP8', tokenizer_mode='auto', tokenizer_worker_num=1, skip_tokenizer_init=False, load_format='auto', model_loader_extra_config='{}', trust_remote_code=True, context_length=None, is_embedding=False, enable_multimodal=None, revision=None, model_impl='auto', host='0.0.0.0', port=30000, fastapi_root_path='', grpc_mode=False, skip_server_warmup=False, warmups=None, nccl_po... ``
The assistant's observation is correct: NCCL has selected NET/IBext_v11 as its transport, meaning it successfully discovered and bound to the InfiniBand (RoCE v2) interconnect between the two DGX Spark nodes. This is the ideal outcome — InfiniBand offers far lower latency and higher bandwidth than TCP/IP for GPU-to-GPU communication, which is essential for tensor parallelism across nodes. The exclamation mark and "That's the correct transport" reveal a moment of satisfaction: a previous configuration step (setting NCCL_SOCKET_IFNAME and GLOO_SOCKET_IFNAME to the correct RoCE interface in [msg 6622]) has paid off.
The Reasoning and Motivation
Why was this message written? The assistant is in the middle of a multi-round debugging session. In the preceding messages ([msg 6625] through [msg 6636]), the assistant launched the SGLang server across two DGX Spark nodes, waited for it to initialize, and discovered that the process was hanging. The log showed CustomAllreduce is disabled because this process group spans across nodes — confirming the two nodes had connected — but then no further progress for over five minutes. The assistant added NCCL_DEBUG=INFO to the launch script in [msg 6633] and relaunched, then checked the NCCL debug output in [msg 6636], which revealed the InfiniBand transport selection.
Message [msg 6637] is the follow-up diagnostic: having confirmed NCCL is using the right transport, the assistant now needs to determine whether the NCCL initialization actually completed and the process moved on to loading model weights. The grep command is carefully crafted to search for any of the known progress milestones in the SGLang initialization sequence: Init.*end (torch distributed initialization finished), Load weight or weight begin (model weight loading started), KV Cache (KV cache allocation), Memory pool (memory pool setup), fired up or ready (server is serving). The fact that none of these appear — only the initial server_args line from the very first log entry — tells the assistant that the process is still stuck at the same NCCL initialization barrier.
Assumptions and Knowledge
This message rests on several layers of knowledge. First, the assistant understands the SGLang server initialization sequence intimately: it knows that after server_args is logged, the next milestone is Init torch distributed begin/end, followed by weight loading, then KV cache and memory pool setup, and finally the server becoming ready. The grep pattern encodes this mental model of the startup pipeline.
Second, the assistant knows that NCCL's transport selection is critical for multi-node performance. NET/IBext_v11 is the NCCL InfiniBand Verbs transport (version 11 of the extension API). On the DGX Spark nodes, the InfiniBand interconnect is actually implemented as RoCE (RDMA over Converged Ethernet) v2, which uses the same verbs API. The assistant correctly identifies this as the optimal transport — far better than the fallback NET/Socket TCP path.
Third, the assistant assumes that if NCCL initialization completed, the log would show Init torch distributed ends shortly after the NCCL channel setup messages. This is a reasonable assumption based on the SGLang source code structure, where init_torch_distributed is a discrete step that logs both its start and completion.
The key incorrect assumption — or rather, the hypothesis being tested — is that the NCCL transport selection was the cause of the hang. The assistant previously suspected that NCCL might be trying to use the wrong network interface (e.g., the external IP on 10.1.230.180 which was unreachable from the second Spark, as discovered in earlier messages). Setting NCCL_SOCKET_IFNAME and GLOO_SOCKET_IFNAME to the RoCE interface fixed the transport selection, but message [msg 6637] reveals that this fix was insufficient: NCCL is using the right transport but the initialization is still not completing.
The Diagnostic Gap
The grep result is striking: only the server_args line matches. This means the process hasn't even logged Init torch distributed begin — or if it did, it hasn't logged Init torch distributed ends. Looking back at the earlier logs from [msg 6625], we see that Init torch distributed begin was logged in a previous launch attempt. But in this launch (with NCCL_DEBUG enabled), it seems the process is stuck even earlier, or the NCCL debug output is interleaved differently.
This creates an interesting diagnostic puzzle: the NCCL debug output in [msg 6636] showed channel setup completing (all 8 channels configured, trees built, P2P chunksize set), but the process never logged the completion of Init torch distributed. This suggests the hang is after NCCL channel setup but before the distributed initialization completion — possibly in a collective communication operation (like barrier() or an all-reduce) that is the first real NCCL operation after channel setup.
Knowledge Created
This message creates important negative knowledge: the InfiniBand transport is working correctly, but the hang persists. This rules out the network interface configuration as the root cause and forces the assistant to look deeper — at NCCL's actual communication patterns, at potential firewall issues on the NCCL communication ports, at the unified memory pressure on the DGX Spark nodes, or at a fundamental incompatibility between SGLang's NCCL usage and the GB10 platform.
The message also documents the exact state of the server at a specific timestamp (2026-04-08 22:57:51), providing a baseline for comparison with subsequent diagnostic attempts. The assistant will go on to investigate further, eventually discovering that the issue is related to Ray's OOM killer and memory pressure, leading to a pivot from SGLang to a dedicated vLLM image built specifically for Qwen3.5 on GB10.
The Broader Context
This message sits at a critical juncture in the deployment. The assistant has already solved several networking challenges: configuring node IPs to use the InfiniBand subnet, setting up the TCP rendezvous store on port 20000, and configuring NCCL/Gloo socket interface names. The InfiniBand discovery in [msg 6636] felt like a breakthrough — the right transport was finally being used. But message [msg 6637] is the reality check: the breakthrough wasn't enough. The model is still not loading.
This pattern — solving one problem only to reveal a deeper one — is characteristic of complex distributed system debugging. Each layer of abstraction (Docker networking, NCCL transport selection, torch distributed initialization, model weight loading) has its own failure modes, and each fix only exposes the next barrier. The assistant's methodical approach of checking one hypothesis at a time, using targeted grep commands on the logs, is a textbook example of structured remote debugging.
In the end, the assistant will pivot away from SGLang entirely for this multi-node deployment, switching to a custom vLLM image (hellohal2064/vllm-qwen3.5-gb10) that handles the Qwen3.5 model and GB10 platform more reliably. But message [msg 6637] captures the moment when the InfiniBand fix was verified and found insufficient — a small but essential data point in the long chain of reasoning that leads to the final successful deployment.