The Zombie Daemon and the Phantom Download: Debugging a Crashed GPU Proving Node

Introduction

In the high-stakes world of GPU-accelerated zero-knowledge proving, a daemon crash on a remote instance can mean hours of lost computation, wasted GPU rental costs, and a frustrating debugging session conducted entirely through SSH. Message 4033 captures one such moment: a developer, having just discovered that the cuzk proving daemon on a remote RTX 5090 instance has become a zombie process, digs deeper to understand what went wrong. The message is a single SSH command — a multi-line diagnostic probe — and its output reveals a surprising and contradictory picture that challenges the developer's assumptions about the system's state.

Context: The State of Play

To understand why this message was written, we must first understand what preceded it. The developer had been engaged in a long-running optimization and stabilization effort for cuzk, a CUDA-based zero-knowledge proving daemon deployed on vast.ai GPU rental instances. The work spanned memory management, pinned memory pools, PI-controlled dispatch pacing, Docker deployment infrastructure, and OOM (out-of-memory) crash recovery. Two live instances were being monitored: an RTX 4090 instance in Norway that had completed its benchmark successfully, and an RTX 5090 instance in Canada (C.32897009) that was the current focus of concern.

In the immediately preceding message ([msg 4031]), the developer had SSHed into the RTX 5090 instance and discovered something alarming: the cuzk daemon was a zombie process (<defunct>). The benchmark wrapper script (benchmark.sh) was still running, but the actual proving daemon was dead. The system showed ample free memory (approximately 220 GiB free out of 504 GiB total host RAM), which was puzzling — if the daemon had been OOM-killed, one would expect memory pressure to have eased. The absence of OOM messages in dmesg (checked in [msg 4032]) further complicated the picture.

Message 4033 is the natural next step in this investigation: the developer needs to understand why the daemon crashed and whether the benchmark script has detected the crash and taken recovery action.

The Diagnostic Probe: What Was Asked and Why

The SSH command in message 4033 is a carefully constructed diagnostic bundle, designed to answer several questions simultaneously:

ssh -o ConnectTimeout=10 -o StrictHostKeyChecking=no -p 17008 root@ssh6.vast.ai '
echo "=== benchmark-full.log ==="
cat /tmp/benchmark-full.log
echo "=== any other logs ==="
ls -la /tmp/*.log 2>/dev/null
echo "=== benchmark.sh process tree ==="
pstree -p 1603 2>/dev/null || ps --forest -g 1603 2>/dev/null
echo "=== cgroup memory ==="
cat /sys/fs/cgroup/memory/memory.usage_in_bytes 2>/dev/null || cat /sys/fs/cgroup/memory.current 2>/dev/null
cat /sys/fs/cgroup/memory/memory.limit_in_bytes 2>/dev/null || cat /sys/fs/cgroup/memory.max 2>/dev/null
' 2>&1

Each command targets a specific piece of the puzzle:

1. cat /tmp/benchmark-full.log — The full benchmark log should reveal the sequence of events leading up to the crash. Did the benchmark complete any proofs? Did it detect the OOM? Did the OOM recovery loop kick in?
2. ls -la /tmp/*.log — A search for any other log files that might contain error messages, stack traces, or diagnostic output from the daemon or the benchmark script.
3. pstree -p 1603 — The process tree of the benchmark script (PID 1603) would show whether child processes are still running, whether the script is waiting for something, or whether it has spawned a new daemon instance as part of OOM recovery.
4. cat /sys/fs/cgroup/memory/... — The cgroup memory metrics would reveal the current memory pressure from the container's perspective. This is critical because vast.ai runs workloads inside Docker containers with cgroup memory limits, and the developer had recently made detect_system_memory() cgroup-aware precisely to handle this scenario. The cgroup usage and limit would show whether the system was still near its memory ceiling. The design of this command reveals the developer's mental model: they suspect the benchmark script's OOM recovery logic may have triggered, or alternatively, that the crash was not OOM-related at all (given the ample free memory and absent OOM messages). The process tree check would distinguish between "script waiting for dead child" and "script has moved on."

The Surprising Output: A Download in Progress

The output defied expectations:

=== benchmark-full.log ===
C1 output not found at /data/32gbench/c1.json
Downloading from https://pub-08ae819c828244bdbe5f615fd8c5e144.r2.dev/c1.json ...
#=#=#                                                                          
##O#-#                                                                         
                                                                            0.0%
#####                                                                      7.0%
##########               ...

The benchmark log showed that the script was downloading the test data file (c1.json) from a remote URL. The progress indicators (#=#=#, ##O#-#, 0.0%, 7.0%) show a download in its early stages. This is deeply puzzling for several reasons:

• The test data should already exist at /data/32gbench/c1.json — this is a well-established test instance.
• If the daemon had crashed and the benchmark script had entered its OOM recovery loop, one would expect it to wait, then restart the daemon, not re-download test data.
• The download progress suggests the benchmark script may have been restarted from scratch, or that the log file was truncated/overwritten. The output is also truncated — the log continues with download progress bars but the SSH session captured only the beginning. This truncation is itself informative: the download was still in progress when the SSH command ran, meaning the benchmark script was actively doing network I/O, not waiting idly for a dead child.

Assumptions and Reasoning

The developer made several implicit assumptions when constructing this diagnostic command:

1. The benchmark script is still the original process (PID 1603) — The process tree check assumes PID 1603 is still the benchmark wrapper and that its child processes would reveal the recovery state.
2. The log file contains the full history — The assumption that /tmp/benchmark-full.log would contain the complete sequence of events, including any OOM recovery attempts.
3. Cgroup metrics are available — The command uses || fallbacks between cgroup v1 and v2 paths, reflecting the developer's knowledge that vast.ai instances may use either version.
4. The crash was not a simple OOM kill — Given the absence of OOM messages in dmesg and the ample free host memory, the developer is already questioning the OOM hypothesis. The cgroup memory check would confirm or refute this. The output challenges these assumptions in interesting ways. The download log suggests the benchmark script may have been restarted (perhaps by a supervisor process), or that the log file is from a fresh run after the OOM recovery loop completed a full restart. The process tree output is notably absent from the captured result — it may have been cut off, or the command may have failed silently.

Input Knowledge Required

To fully understand this message, a reader needs:

1. Knowledge of the system architecture: cuzk is a CUDA ZK proving daemon that runs GPU-based zero-knowledge proofs. It uses a pinned memory pool for GPU data transfers and a memory budget system to avoid OOM kills.
2. Knowledge of the deployment context: The daemon runs inside a Docker container on vast.ai, a GPU rental platform. Containers have cgroup memory limits that may differ from host RAM. The benchmark.sh script orchestrates proving benchmarks and has an OOM recovery loop.
3. Knowledge of the debugging history: The developer had previously discovered the daemon was a zombie process ([msg 4031]) and checked for OOM messages ([msg 4032]). This message is the third in a sequence of diagnostic SSH commands.
4. Knowledge of Linux process states: A "zombie" process (<defunct>) is one that has terminated but whose parent has not yet called wait() to reap its exit status. This indicates the benchmark script (the parent) is still running but hasn't processed the daemon's termination.
5. Knowledge of cgroup memory accounting: The cgroup v1 paths (memory.usage_in_bytes, memory.limit_in_bytes) and v2 paths (memory.current, memory.max) are standard Linux cgroup interfaces for tracking container memory.

Output Knowledge Created

This message produces several valuable pieces of information:

1. The benchmark log shows a fresh download of test data, which is inconsistent with a simple daemon crash. This suggests either the benchmark script was restarted (perhaps by a supervisor or by the OOM recovery loop's full restart logic) or the log file was truncated.
2. The download was in progress at the time of the SSH check, meaning the benchmark script was actively executing, not blocked waiting for a zombie child.
3. The cgroup memory metrics were not captured in the output (the output ends with download progress bars), which is itself informative — the SSH session may have timed out or the output was too large.
4. The absence of any error messages or crash logs in the benchmark log suggests the daemon's crash was not detected by the benchmark script's error handling, or that the log file was overwritten by a new run.

The Thinking Process Revealed

The structure of the SSH command reveals the developer's reasoning process in real-time. They are systematically testing hypotheses:

• Hypothesis 1: The benchmark script detected the crash and entered OOM recovery. → Check the log for recovery messages.
• Hypothesis 2: The crash generated error output. → Search for other log files.
• Hypothesis 3: The benchmark script is stuck waiting for the zombie. → Check the process tree.
• Hypothesis 4: The crash was cgroup-level OOM. → Check cgroup memory metrics. The use of fallback commands (||) for both the process tree and cgroup paths shows an awareness of environmental variability — different vast.ai instances may have different tools and cgroup versions available. The truncation of the output is a reminder of the practical constraints of remote debugging over SSH: large log files, slow network connections, and command timeouts can all interfere with diagnosis. The developer would need to follow up with a more targeted command (perhaps tail -n 50 instead of cat) to get the complete picture.

Conclusion

Message 4033 is a snapshot of a developer in the thick of a challenging remote debugging session. The discovery that the cuzk daemon is a zombie process has opened a new line of investigation, and this SSH command is the first probe into that mystery. The output — showing a test data download in progress — raises more questions than it answers, setting the stage for further investigation. Was the benchmark script restarted? Did the OOM recovery logic trigger a full re-download? Or is there a deeper issue with the daemon's lifecycle management? The answers to these questions would determine the next steps in what would become a multi-layered debugging effort spanning bash scripting bugs, memory accounting mismatches, and the fundamental challenge of operating GPU proving workloads within tight container memory budgets.