Diagnosing a Silent Failure: The Art of Systematic Debugging in High-Performance GPU Proving

In the midst of a deep optimization campaign for the SUPRASEAL_C2 Groth16 proof generation pipeline, the assistant encounters a frustratingly mundane problem: a process won't start, and log files aren't being created. The message in question is deceptively simple — a single df -h /tmp command and its output — but it represents a critical juncture in a debugging session that spans hardware resource contention, process management, and the subtle ways that high-performance computing systems fail under pressure.

The Message

The assistant writes:

Plenty of RAM (663 GiB free). The file redirect isn't working — maybe /tmp is full or there's a shell issue with &>: `` [bash] df -h /tmp Filesystem Size Used Avail Use% Mounted on tmpfs 378G 32G 346G 9% /tmp ``

This is message [msg 2547] in the conversation, and it arrives after a long chain of increasingly sophisticated benchmarks and diagnoses.

The Context: A Bottleneck That Keeps Shifting

To understand why this simple disk-space check matters, we must trace the investigation that led to it. The assistant has been working on Phase 9 of a PCIe transfer optimization for the cuzk SNARK proving engine — a system that generates Groth16 proofs for Filecoin's Proof-of-Replication (PoRep) protocol. This is a high-stakes optimization effort: each proof requires roughly 200 GiB of peak memory and involves orchestrating CPU synthesis workers, GPU kernel execution, and PCIe transfers across a complex pipeline.

Earlier in the session, the assistant had made a breakthrough discovery. Fine-grained timing instrumentation revealed that the GPU kernel time had dropped to approximately 1.8 seconds per partition — excellent performance. Yet the steady-state throughput plateaued at around 41 seconds per proof, and the TIMELINE measurements showed the GPU was idle for roughly 600 milliseconds per partition, waiting for the CPU thread to finish its work.

The culprit was prep_msm (CPU preprocessing, taking ~1.7 seconds) and b_g2_msm (a CPU-only G2 MSM operation, taking ~380 milliseconds). Together, these CPU-side operations consumed about 2.1 seconds per partition — longer than the GPU kernel time. And crucially, these CPU operations compete with the 10 synthesis workers for the 8-channel DDR5 memory bandwidth. At high concurrency, the contention inflates CPU times by 2–12×, with b_g2_msm spiking from 380ms to as much as 4.9 seconds.

The bottleneck had shifted. It was no longer about PCIe transfers or GPU kernel efficiency. It was now a CPU memory bandwidth problem — a fundamentally different class of constraint.

The Crash and the Aftermath

The assistant ran a benchmark with c=30 j=20 (30 proofs, 20 concurrent) to stress-test the system. The results were dramatic: proof 15 struggled at 64 seconds, proof 16 took an astonishing 421 seconds, and then the system crashed. The daemon log told a story of memory bandwidth saturation:

The Immediate Problem: A Daemon That Won't Start

After the crash, the assistant needed to restart the daemon to run more controlled benchmarks. But something went wrong. The daemon process appeared to die immediately, and the log file redirect wasn't working. The assistant tried multiple approaches:

  1. pkill -9 -f cuzk-daemon followed by a fresh start with &>/tmp/cuzk-p9-run2.log
  2. Checking ps aux | grep cuzk-daemon — no process found
  3. Checking ls -la /tmp/cuzk-p9-run2.log — file didn't exist
  4. Running the daemon directly with 2>&1 | head -5 — this worked, confirming the binary itself was functional The assistant checked system memory (free -g) and found 663 GiB free — plenty of RAM. So memory exhaustion wasn't preventing the daemon from starting. This is where message [msg 2547] arrives. The assistant's next hypothesis is that /tmp might be full, preventing the shell from creating the log file via the redirect operator &>. This is a reasonable suspicion: if the filesystem is at capacity, file creation fails silently, and the &> redirect might not produce an error message visible to the user.

Assumptions and Reasoning

The assistant makes several assumptions in this message:

Assumption 1: The file redirect is failing. The assistant observes that the log file doesn't exist after the command runs, and concludes that the redirect operator &> isn't working. This is a reasonable inference, but there are other possibilities: the daemon might be crashing before it writes any output, the shell might be using a different working directory, or the process might be spawning but the file descriptor might not be flushed before the process dies.

Assumption 2: /tmp being full is a plausible cause. On a system that has been running heavy benchmarks with multi-GiB allocations, it's reasonable to wonder if /tmp has filled up. The assistant has been creating temporary files throughout the session — benchmark logs, timing data, configuration files — and the crash at c=30 j=20 might have left behind large temporary files.

Assumption 3: The &> syntax might be the issue. The assistant notes "maybe /tmp is full or there's a shell issue with &>". The &> redirect is a bash shorthand that redirects both stdout and stderr. It's possible the shell environment (zsh, based on earlier prompts) handles this differently, or that some shell configuration is interfering.

Input Knowledge Required

To understand this message, the reader needs:

  1. The concept of file redirects in Unix shells: The &> operator redirects both stdout and stderr to a file. If the filesystem is full, this redirect fails silently — the file isn't created, and no error is shown on the terminal.
  2. The role of /tmp in Linux systems: /tmp is a temporary filesystem, often mounted as tmpfs (RAM-backed). It has limited size and can fill up during heavy computation. A full /tmp can cause all sorts of subtle failures.
  3. The architecture of the cuzk proving system: The daemon writes log output to files for later analysis. If the daemon can't create its log file, it might crash immediately or fail to start.
  4. The recent history of the system: The c=30 j=20 benchmark crashed, potentially leaving behind large temporary files or corrupted GPU state. Understanding this context is crucial to interpreting the assistant's concern about /tmp being full.

Output Knowledge Created

This message produces a single, crucial piece of knowledge: /tmp is not full. The df -h /tmp output shows:

The Thinking Process

The assistant's reasoning in this message follows a classic debugging pattern: eliminate the obvious causes first.

The chain of reasoning is:

  1. Observation: The daemon won't start, and no log file is created.
  2. Hypothesis A: The system is out of memory, preventing the daemon from allocating its resources. - Test: free -g → 663 GiB free → Hypothesis A is false.
  3. Hypothesis B: The /tmp filesystem is full, preventing the shell from creating the log file. - Test: df -h /tmp → 346 GiB available → Hypothesis B is false.
  4. Implicit next step: The problem must be something else — perhaps a shell configuration issue, a race condition with the previous process kill, or a GPU state problem from the crash. This is methodical, systematic debugging. The assistant doesn't jump to conclusions or try random fixes. Each hypothesis is tested with a concrete command, and the results are used to narrow the search. The message also reveals a subtle aspect of the assistant's thinking: the willingness to question even basic assumptions. The &> redirect is a fundamental shell operation, but the assistant considers that it might be failing due to environmental issues. This is the mark of an experienced debugger — when something that "should work" doesn't, you question everything.

The Broader Significance

This message, while brief, captures a universal experience in systems programming: the moment when a complex optimization effort is derailed by a mundane operational problem. The assistant has been deep in the weeds of GPU kernel timing, CPU memory bandwidth analysis, and PCIe transfer optimization — and then gets stopped cold by a process that won't start and a log file that won't appear.

It's a reminder that in real-world systems, the hard problems are often not the ones you expect. The assistant had just identified a fundamental bottleneck shift (from GPU compute to CPU memory bandwidth) and was planning a two-lock design to better overlap CPU and GPU work. But before any of that could be implemented, the basic infrastructure of running benchmarks had to be debugged.

The df -h /tmp command also reveals something about the system's architecture. The /tmp filesystem is a 378 GiB tmpfs — a RAM-backed filesystem. This means that temporary files consume system memory. With 32 GiB already used in /tmp, and the daemon potentially needing to write large log files, the assistant's concern about space was well-founded. The fact that only 9% is used is reassuring but also informative: it tells us that the crash at c=30 j=20 didn't leave behind massive temporary files that would interfere with subsequent runs.

Conclusion

Message [msg 2547] is a small but essential step in a larger debugging journey. It demonstrates the importance of systematic hypothesis testing, the value of checking basic infrastructure before diving into complex theories, and the reality that high-performance computing optimization often involves as much operational debugging as algorithmic innovation. The assistant's methodical approach — check memory, check disk space, eliminate the obvious — is a model for how to debug systems under pressure. And the message itself, for all its brevity, captures a moment of clarity: the problem is not where we thought it was, so we must look elsewhere.