The Silent Benchmark: A Diagnostic Probe into a Missing GPU Process

At the heart of a complex debugging session spanning cloud infrastructure, container orchestration, and GPU proving pipelines, a single bash command reveals the essence of systematic troubleshooting. Message [msg 981] appears, at first glance, to be unremarkable — just another SSH command checking a log file on a remote instance. But this message is a critical diagnostic pivot point in a chain of investigation that began when a freshly deployed Vast.ai instance completed its parameter download, signaled readiness, and then... went silent.

Context: The Mystery of the Missing Benchmark

The story leading up to this message is one of careful infrastructure construction. The assistant had been building a sophisticated system: a Docker container for Filecoin proof generation using the CuZK proving engine, a vast-manager service to orchestrate instances on the Vast.ai marketplace, and a lifecycle entrypoint.sh script that automates the sequence from parameter download through benchmark to production proving. After resolving a critical bug in the vast-manager's instance matching logic — where it relied on Vast.ai's API label field that remained perpetually null — and deploying a fix using ID-based fallback matching, the system was finally ready for its first end-to-end test.

Instance 32710471 (a 2x RTX 3090 machine in BC Canada) had been created with the --ssh --onstart-cmd workaround, since Vast.ai's SSH launch mode replaces the Docker ENTRYPOINT with its own init script. The entrypoint ran in the background, downloaded 56GB of Filecoin proof parameters, and signaled completion. Then, at [msg 976], the user reported a troubling observation: the entrypoint log showed "Starting benchmark: 12 proofs, concurrency 5, partition-workers 10," but there were no benchmark logs appearing and no GPU activity visible in nvidia-smi. The benchmark had seemingly evaporated into thin air.

The assistant's first response, at [msg 977], was to check running processes on the instance. The result was puzzling: only the parent entrypoint.sh (PID 369) was alive, with no child processes for benchmark.sh, cuzk-daemon, or cuzk-bench. The entrypoint was running but appeared to be doing nothing. A check of the entrypoint log at [msg 978] showed only the tail end of the paramfetch download progress — the last lines were still showing the file download at 99%, not the benchmark execution that the setup log claimed had started.

This is where the assistant faced a diagnostic fork. The primary log (/var/log/entrypoint.log) showed only download progress. The process list showed a parent shell waiting. The user's report indicated the benchmark had been launched. Something was happening that the primary logs weren't capturing.

The Diagnostic Probe: Reading the Setup Log

Message [msg 981] is the assistant's decision to check a secondary log source: /tmp/setup.log. This file is written by the entrypoint.sh script during its initialization phase, before the main entrypoint log takes over. The command is:

ssh -p [REDACTED] root@[REDACTED] "tail -50 /tmp/setup.log 2>/dev/null" 2>&1

This is a deliberate investigative choice. The assistant is reasoning: "The entrypoint log shows paramfetch output, but the setup log says benchmark started. Let me check the setup log to see if there's more detail about what happened after the benchmark launch." The 2>/dev/null is a defensive measure — if the file doesn't exist, the command returns cleanly rather than erroring. The tail -50 ensures enough context is captured to see both the end of the paramfetch phase and the beginning of the benchmark phase.

The output reveals something important but incomplete:

FILE: /var/tmp/filecoin-proof-parameters/v28-proof-of-spacetime-fallback-merkletree-poseidon_hasher-8-8-0-0377ded656c6f524f1618760bffe4e0a1c51d5a70c4509eedae8a27555733edc.params
-------------------------------------------------------------------------------

2026-03-12T00:54:01.704Z	INFO	paramfetch	fastparamfetch/paramfetch.go:227	Parameter file /var/tmp/filecoin-proof-parameters/v28-stacked-proof-of-replication-merkletree-poseidon_hasher-8-8-0-sha256_hasher-82a357d2f2ca81dc61bb45f4a762807aedee1...

The output is truncated — it shows the paramfetch completion log but cuts off before the benchmark section. This truncation is itself informative. The tail -50 command returned only the paramfetch portion of the log, suggesting either that the benchmark section is very short (perhaps just the launch command with no subsequent output) or that the setup.log doesn't contain the benchmark output the assistant was hoping to find.

The Thinking Process Revealed

What makes this message interesting is what it reveals about the assistant's mental model and diagnostic strategy. The assistant is working through a layered investigation:

  1. Process-level check ([msg 977]): "Is the benchmark running?" → No child processes found.
  2. Primary log check ([msg 978]): "What does the entrypoint log say?" → Shows only download progress, not benchmark execution.
  3. Secondary log check ([msg 981]): "What does the setup log say?" → Shows paramfetch completed, but benchmark section is missing or truncated. The assistant is systematically eliminating log sources, building a picture of where the benchmark process disappeared. The assumption is that the benchmark did start (the setup log says so) but then failed silently. The question is: what kind of failure would cause a complete absence of both log output and GPU activity? The assistant's next steps, visible in [msg 982] through [msg 985], build on this probe. After seeing the truncated setup.log output, the assistant checks the process's file descriptors and finds the entrypoint is in do_wait — a kernel wait state indicating it's waiting for a child process to complete. Then the assistant discovers the benchmark output files (/tmp/benchmark-full.log, /tmp/cuzk-bench-daemon.log, /tmp/cuzk-bench-config.toml) and reads them to find the root cause: a broken pipe / transport error during the warmup proof's gRPC communication between the cuzk-bench client and the cuzk-daemon.

Assumptions and Knowledge Requirements

This message assumes significant domain knowledge from the reader. To understand why the assistant is checking /tmp/setup.log, one must know:

Output Knowledge Created

The output of this message is partial but valuable. It confirms that paramfetch completed successfully at 00:54:01 UTC, with both the PoRep and PoSt parameter files verified. This eliminates paramfetch failure as a possible cause of the benchmark silence. The truncation of the output (the log line cuts off mid-path) is a side effect of the terminal output capture, but the key information — paramfetch succeeded — is clearly visible.

More importantly, this message sets up the next diagnostic step. By showing that the setup.log doesn't contain the benchmark output, the assistant is led to check other log files (/tmp/benchmark-full.log, /tmp/cuzk-bench-daemon.log), which ultimately reveal the broken pipe error. The message is a stepping stone in a causal chain: without this probe, the assistant might have continued investigating the wrong log source or assumed the benchmark never launched.

Broader Significance

This message exemplifies a pattern that recurs throughout the session: the assistant's methodical approach to diagnosing silent failures in a distributed system. The benchmark didn't crash with an error message visible in the primary log — it simply stopped producing output. The assistant had to reconstruct the failure from secondary indicators: process states, file descriptors, log files in non-standard locations. This is the reality of debugging complex pipelines where failures propagate across process boundaries and log streams.

The message also highlights the importance of log architecture. The system had multiple log files (/var/log/entrypoint.log, /tmp/setup.log, /tmp/benchmark-full.log, /tmp/cuzk-bench-daemon.log), each capturing a different phase of the lifecycle. The assistant's ability to navigate between these logs — knowing which file to check at each stage — reflects a deep understanding of the system's design. This is knowledge built through the iterative development of the entrypoint and benchmark scripts over the course of the session.

In the end, the broken pipe error traced back to the gRPC connection between cuzk-bench and cuzk-daemon timing out during the long first synthesis. The fix involved hardening benchmark.sh to handle warmup proof failures gracefully rather than aborting with set -e, and the assistant would go on to deploy this fix to the running instance. But that resolution was only possible because of diagnostic steps like this one — a simple SSH command to tail a log file, asking the question: "What happened after the benchmark started?"