The Third Launch: Debugging Multi-Node NCCL Deadlocks Across Two DGX Spark Systems

The Message

scp /home/theuser/glm-kimi-sm120-rtx6000bw/spark-launch-qwen35.sh aurora@10.1.230.180:/home/aurora/spark-launch-qwen35.sh && ssh aurora@10.1.230.180 'chmod +x /home/aurora/spark-launch-qwen35.sh && scp /home/aurora/spark-launch-qwen35.sh aurora@192.168.200.13:/home/aurora/spark-launch-qwen35.sh && ssh aurora@192.168.200.13 "nohup /home/aurora/spark-launch-qwen35.sh worker > /home/aurora/sglang-worker.log 2>&1 &" && sleep 3 && nohup /home/aurora/spark-launch-qwen35.sh head > /home/aurora/sglang-head.log 2>&1 & echo "Both launched"'

At first glance, this appears to be a routine deployment command — copy a script to two machines, launch a worker on one, launch a head node on the other. But this message, indexed as message 6645 in a sprawling coding session spanning dozens of segments and thousands of messages, represents something far more significant. It is the third attempt to launch a multi-node inference server for the Qwen3.5-122B-A10B-FP8 model across two NVIDIA DGX Spark systems, and it carries the weight of a specific hypothesis about why the previous two attempts had deadlocked.

The Context: A Multi-Node Nightmare

To understand why this single bash command matters, one must appreciate the landscape that led to it. The assistant was attempting to deploy a 119-billion-parameter FP8 model across two DGX Spark nodes — compact ARM-based Blackwell GPU systems with 120GB of unified memory each, connected via InfiniBand RoCE. This is an exotic and demanding configuration: ARM CPUs, Blackwell (SM121) GPUs, unified memory where RAM and GPU memory share the same pool, and a multi-node tensor parallelism (TP=2) setup that requires the model to be split across two physically separate machines.

The journey had already been arduous. Earlier in this segment ([msg 6428] onward), the assistant had explored both nodes, freed GPU memory by stopping an existing GLM-4.7-Flash container, downloaded the 119GB model from HuggingFace, and rsynced it to the second Spark at ~640MB/s over the InfiniBand link. The initial plan was to use SGLang's official spark image, but that image lacked Qwen3.5 support, and SGLang's multi-node NCCL initialization hung indefinitely. The assistant pivoted to a dedicated vLLM image (hellohal2064/vllm-qwen3.5-gb10) and built a Ray-based multi-node deployment instead — but that's a different story.

The immediate predecessor to message 6645 was a multi-hour debugging session focused on a single, maddening problem: SGLang's multi-node setup would initialize NCCL channels successfully, then hang forever.

The Debugging Trail

The assistant's reasoning process, visible across messages 6636 through 6644, reveals a methodical diagnostic approach. After the first launch attempt ([msg 6635]), the assistant checked the logs and found that NCCL was correctly using NET/IBext_v11 — the InfiniBand transport. Channels were being created. The tree topology was established. But then nothing. The process sat at "CustomAllreduce is disabled because this process group spans across nodes" and never progressed to model loading.

The assistant waited. And waited. At [msg 6639], after two minutes: still stuck. At [msg 6642], after three more minutes: same log line, same GPU memory usage (416MB — barely anything, confirming no weights had been loaded). At [msg 6643], after seven total minutes of deadlock: "Completely stuck for 7+ minutes now — no progress at all. Something is wrong."

This patience is itself noteworthy. The assistant considered the possibility that ARM-based NCCL initialization might simply be slow — a reasonable hypothesis given the unusual hardware. But the complete absence of any new log output for seven minutes, combined with the fact that GPU memory usage hadn't budged, ruled out slow initialization. This was a deadlock.

The Hypothesis

The assistant's diagnosis at [msg 6643] is sharp: "This is a deadlock after the first NCCL communicator init. SGLang creates a second process group (likely for the Gloo backend or a pipelining group) that's hanging."

This is the critical reasoning step. The assistant recognized that SGLang's initialization sequence creates multiple NCCL communicator groups — one for the main tensor parallelism, and potentially others for custom allreduce fusion or pipelining. The first group (the one whose NCCL channel setup was visible in the logs) had completed successfully. But a second group creation was hanging, likely because it was trying to establish a communicator that required a different configuration or because a synchronization barrier was never reached.

The clue was in the --disable-custom-all-reduce flag. The assistant noticed that disable_custom_all_reduce=False in the server args — meaning custom allreduce was enabled. On a multi-node setup, custom allreduce is typically disabled automatically (as the log message "CustomAllreduce is disabled because this process group spans across nodes" confirms), but the assistant hypothesized that SGLang might still be trying to create the custom allreduce group and hanging during that attempt.

The fix was simple: explicitly pass --disable-custom-all-reduce to prevent SGLang from even attempting to create the second NCCL group. The assistant edited the launch script at [msg 6644], then issued message 6645 to deploy the updated script and launch again.

Assumptions and Risks

The assistant made several assumptions with this launch. First, that the deadlock was indeed caused by a second NCCL group creation rather than some other issue (e.g., a TCP store timeout, a firewall blocking the NCCL bootstrap port 20001, or a fundamental incompatibility between SGLang's distributed initialization and the DGX Spark's ARM+unified memory architecture). Second, that disabling custom allreduce would not degrade performance — a reasonable assumption for a two-node TP=2 setup where inter-node allreduce is handled by NCCL directly. Third, that the worker and head would find each other on the first attempt after the fix, despite the previous launch having the same networking configuration (which had already been fixed in [msg 6622] to use the correct InfiniBand subnet IPs).

The risk of this approach was that if the hypothesis was wrong, another seven minutes would be wasted waiting for a deadlock that would never resolve. The assistant had already spent significant time on two failed launches. A wrong hypothesis would mean starting over from scratch with a different diagnosis.

Input Knowledge Required

Understanding this message requires knowledge of several domains. The reader must understand NCCL (NVIDIA Collective Communications Library) and its role in multi-GPU tensor parallelism — how it creates communicator groups, uses bootstrap ports for rendezvous, and establishes P2P channels over InfiniBand or Ethernet. They must understand SGLang's server architecture: the distinction between the main NCCL communicator for TP and the custom allreduce fusion that SGLang implements for intra-node communication. They must understand the DGX Spark's unique hardware: ARM Cortex-X925 CPUs, Blackwell SM121 GPUs, unified memory, and the InfiniBand RoCE interconnect that requires explicit interface binding via GLOO_SOCKET_IFNAME and NCCL_SOCKET_IFNAME. They must understand the SSH proxy jump pattern (-J aurora@10.1.230.180) used to reach the second Spark which is on a private InfiniBand subnet. And they must understand the deployment lifecycle: copying scripts, setting permissions, launching in background with nohup, and monitoring via log files.

Output Knowledge Created

This message produced a running SGLang multi-node instance — or it produced another deadlock. The outcome is not visible within the message itself (the bash tool timed out after 30 seconds, as the metadata shows). The real output would be discovered in subsequent messages where the assistant checks the logs. But the message itself creates knowledge in a different sense: it tests a specific hypothesis about NCCL deadlock causation. Whether the launch succeeds or fails, the result informs the assistant's understanding of SGLang's distributed initialization behavior on Blackwell ARM hardware.

The Thinking Process

The thinking visible in the preceding messages reveals a diagnostic pattern that any systems engineer would recognize. The assistant moved through escalating levels of investigation: first checking basic connectivity (are the ports open? [msg 6631]), then enabling NCCL debug logging ([msg 6633]), then examining the NCCL channel establishment ([msg 6636]), then waiting extended periods to rule out slow initialization (<msg id=6639, 6642>), and finally forming a hypothesis about the root cause ([msg 6643]). Each step ruled out simpler explanations before arriving at the more subtle deadlock theory.

What makes this thinking particularly impressive is the assistant's ability to reason about SGLang's internal initialization sequence without access to its source code in the conversation. The assistant inferred that SGLang creates multiple NCCL process groups based on the log pattern — one group succeeded (channels established, trees built), but the process never printed "Init COMPLETE" which would indicate all groups were ready. This is a form of "log archaeology" — reconstructing program behavior from its diagnostic output.

The assistant also demonstrated awareness of the ARM platform's特殊性. The comment "on ARM with unified memory, NCCL init can be very slow" at [msg 6642] shows an understanding that this is not a standard x86 server environment and that timing assumptions may not hold. This prevented a premature conclusion of deadlock — the assistant gave the process a full seven minutes before deciding it was truly stuck.

The Broader Significance

Message 6645 sits at a pivot point in the deployment. The assistant had already solved one networking issue (Gloo using localhost instead of the InfiniBand interface) and was now tackling a deeper distributed systems problem. The NCCL deadlock on multi-node SGLang is a known challenge — it appears in forum threads and GitHub issues — and the assistant's hypothesis about custom allreduce group creation aligns with community knowledge about SGLang's initialization sequence.

Whether this third launch succeeded or not, the message captures a moment of technical judgment: the decision to try a specific fix based on incomplete evidence, knowing that each attempt costs time and that the hardware environment is unusual enough that standard debugging heuristics may not apply. It is the kind of calculated gamble that defines complex systems engineering — not guessing, but reasoning from first principles about what could cause a process to hang after successfully establishing NCCL channels, and then testing that reasoning with a minimal change.