The Ghost Process: Diagnosing a Failed Deployment of the cuzk Memory Manager Fix

Introduction

In the high-stakes world of GPU-accelerated zero-knowledge proof generation, memory management is not merely an optimization—it is a matter of survival. The cuzk proving engine, a critical component in the Filecoin network's proof pipeline, had recently undergone a major architectural overhaul: replacing a static concurrency limit with a dynamic, budget-based memory manager. This new system was designed to intelligently balance memory consumption across SRS (Structured Reference String) loading, PCE (Pre-Compiled Constraint Evaluator) caching, and the working memory required for concurrent proof synthesis. But when the assistant deployed the freshly compiled binary to a remote machine equipped with 755 GiB of RAM and an RTX 5090 GPU, the system immediately crashed with a runtime panic. Message [msg 2346] captures the critical moment when the assistant discovered that its carefully orchestrated fix had not actually been deployed—and had to diagnose why.

The Context: A Runtime Panic in the Evictor

The story begins in the preceding messages. The assistant had built and deployed a new memory-manager binary to a remote host (141.0.85.211) and launched a benchmark to test it with three concurrent 32 GiB PoRep proofs under a tight 100 GiB budget. The daemon started cleanly, loaded SRS on demand (44 GiB), extracted and cached the PCE (26 GiB), and began processing partitions. But then it crashed:

thread 'tokio-runtime-worker' panicked at /build/extern/cuzk/cuzk-core/src/engine.rs:913:45:
Cannot block the current thread from within a runtime.

The root cause was clear: the evictor callback, which runs inside the budget manager's async acquire() loop, was calling blocking_lock() on a tokio Mutex guarding the SrsManager. Tokio's blocking_lock() is designed for use from synchronous contexts—calling it from an async runtime thread is a cardinal sin in the tokio ecosystem, and the runtime detects this at runtime and panics.

The assistant diagnosed the issue across messages [msg 2334] through [msg 2337], reading the source code to confirm both blocking_lock() calls at lines 913 and 937 of engine.rs. The fix was elegant: replace blocking_lock() with try_lock(). If the SRS mutex is already held by another task, the evictor simply skips SRS eviction for that iteration—the acquire loop will retry, and eventually the mutex will be available. This is safe because the evictor is called repeatedly in a loop until enough memory is freed.

The edit was applied in [msg 2339], the code compiled cleanly in [msg 2340], a new Docker image was built in [msg 2341], and the binary was extracted and uploaded to the remote machine as /tmp/cuzk-daemon-new2 in messages [msg 2342] and [msg 2343].

The Deployment Attempt That Wasn't

In [msg 2344], the assistant issued a comprehensive bash command to the remote host:

ssh -p 40612 root@141.0.85.211 '
# Kill everything
pkill -f cuzk-bench 2>/dev/null
pkill -f "cuzk.*memtest" 2>/dev/null  
pkill -f "seq 1" 2>/dev/null
sleep 2
pkill -9 -f "cuzk --config" 2>/dev/null
sleep 1

# Replace binary
cp /tmp/cuzk-daemon-new2 /usr/local/bin/cuzk
chmod +x /usr/local/bin/cuzk

# Clear old logs
rm -f /tmp/cuzk-memtest.log /tmp/cuzk-memtest-bench.log /tmp/cuzk-memtest-rss.log

# Start fresh
nohup /usr/local/bin/cuzk --config /tmp/cuzk-memtest-config.toml > /tmp/cuzk-memtest.log 2>&1 &
CUZK_PID=$!
echo "cuzk PID: $CUZK_PID"
sleep 2

# Verify startup
tail -20 /tmp/cuzk-memtest.log
echo "=== PID check ==="
ps -o pid,rss,comm -p $CUZK_PID'

This command was supposed to: kill all cuzk-related processes, replace the binary, clear old logs, start the daemon fresh, and verify startup. But when the assistant checked the results in [msg 2345], it saw something alarming:

=== daemon log ===
2026-03-13T14:24:23.456754Z  INFO  cuzk_daemon: cuzk-daemon starting
2026-03-13T14:24:23.456877Z  INFO  cuzk_daemon: configuration loaded listen=0.0.0.0:9820
2026-03-13T14:24:23.456883Z  INFO  cuzk_daemon: set CUZK_GPU_THREADS for C++ groth16_pool gpu_threads=32
2026-03-13T14:24:23.457885Z  INFO  cuzk_daemon: rayon glob...

The timestamps were from 14:24:23—nearly an hour earlier. This was the old daemon's log. The kill/restart had not worked. The assistant was looking at the original daemon process (PID 28410) that had been started in [msg 2319], and which had presumably been running the entire time, processing the benchmark requests and then panicking on the blocking_lock call.

Message 2346: The Moment of Recognition

This brings us to the subject message, [msg 2346]. The assistant's reasoning block begins with a sharp realization:

This is still the OLD binary (the old daemon from before). The log timestamps start at 14:24:23 which is the old daemon. The kill/restart in my script didn't actually work because the output was suppressed. The old daemon is still running and it's the old binary with the blocking_lock bug.

This is a classic debugging epiphany. The assistant had been operating under the assumption that the kill/restart command in [msg 2344] had succeeded. But the evidence was incontrovertible: the log timestamps were from the original startup. The pkill -9 -f "cuzk --config" command had either failed silently (the 2>/dev/null swallowed any error), or the process had been restarted by some supervisor mechanism, or—most likely—the pkill pattern didn't match because the daemon was started with a slightly different command line.

The assistant then performs a remarkable piece of forensic analysis on the old daemon's logs. Even though it was the buggy binary, it had actually made meaningful progress:

The key thing I see: 1. The old daemon from the first request processed a proof successfully (partitions 0 and 1 completed with GPU proofs) 2. SRS loaded on demand: "SRS loaded successfully circuit_id=porep-32g budget_used_gib=44 budget_available_gib=55" 3. PCE extracted and cached: "PCE cached circuit_id=porep-32g size_gib=25 budget_used_gib=83"

>

So the first request actually did process partitions 0 and 1 successfully, but then partitions 2-9 panicked because the budget was exhausted—only 30 GiB remained after loading the 44 GiB SRS and 26 GiB PCE, and the evictor hit the blocking_lock() bug when trying to acquire more space.

This analysis is crucial. It tells us that the budget-based memory manager was fundamentally working correctly—it loaded SRS and PCE on demand, tracked budget consumption, and only failed when the evictor tried to free memory. The core logic was sound; the bug was purely in the eviction callback's use of blocking_lock().

The Second Deployment Attempt and Its Failure

Armed with this understanding, the assistant issues a more aggressive bash command:

ssh -p 40612 root@141.0.85.211 '
# Force kill the old daemon
kill -9 28410 2>/dev/null
sleep 2

# Verify dead
ps aux | grep "cuzk --config" | grep -v grep
echo "old process killed: $?"

# Replace binary with the fixed one
cp /tmp/cuzk-daemon-new2 /usr/local/bin/cuzk
chmod +x /usr/local/bin/cuzk
ls -lh /usr/local/bin/cuzk

# Verify it is the new binary (check modification time)
stat /usr/local/bin/cuzk | grep Modify'

This time, the assistant uses kill -9 (SIGKILL) directly on PID 28410, which cannot be ignored or caught by the process. The output reveals another problem:

old process killed: 1
cp: cannot create regular file '/usr/local/bin/cuzk': Text file busy
-rwxr-xr-x 1 root root 27M Mar 13 14:24 /usr/local/bin/cuzk
Modify: 2026-03-13 14:24:17.075676431 +0000

The kill -9 succeeded (exit code 1 from grep means no matching process, confirming the old daemon is dead). But the cp command failed with "Text file busy"—a Linux kernel error that occurs when you try to overwrite a binary file that is currently mapped into memory by a running process. Even though the old daemon was killed, there might have been another process (perhaps a child or a zombie) still holding the file open, or the filesystem hadn't released the inode yet.

The stat output confirms the binary at /usr/local/bin/cuzk still has the old modification timestamp (14:24:17), meaning the replacement never happened. The new fixed binary sits unused at /tmp/cuzk-daemon-new2.

The Deeper Significance

This message is a masterclass in remote debugging and the challenges of deploying to production systems. Several themes emerge:

The Fallacy of Silent Commands. The original kill/restart script in [msg 2344] used 2>/dev/null extensively, which suppressed all error messages. The assistant had no way of knowing that the pkill or cp commands had failed. This is a common pitfall in automation: silencing errors to keep output clean can hide critical failures.

The Value of Timestamp Forensics. The assistant's first clue was the log timestamps. In a world of distributed systems and asynchronous operations, timestamps are often the most reliable indicator of what actually happened. The old daemon's logs started at 14:24:23; any new daemon would have a later timestamp. This simple check revealed the entire deployment failure.

The "Text file busy" Trap. The Linux kernel's "Text file busy" error is a classic deployment hazard. It occurs because the kernel keeps the executable's inode mapped as long as any process has the file open for execution. Even after the process dies, there can be a brief window where the file is still considered "busy" by the kernel. The fix is typically to unlink the old file first (rm /usr/local/bin/cuzk) and then copy the new one, or to use mv (which works because it only changes the directory entry, not the inode).

The Budget System Was Working. Perhaps the most encouraging finding hidden in this message is that the old daemon's logs showed the budget manager functioning correctly: SRS loaded on demand (44 GiB), PCE cached (26 GiB), budget tracked (83 GiB used out of 100 GiB). The only failure was the evictor's blocking_lock panic. This validated the entire architectural decision to move to a budget-based system.

Assumptions and Mistakes

The assistant made several assumptions that proved incorrect:

  1. That pkill -9 -f "cuzk --config" would match the daemon. The process was started as nohup /usr/local/bin/cuzk --config /tmp/cuzk-memtest-config.toml. The pkill -f pattern "cuzk --config" should have matched, but it's possible the process had already died (from the panic) and was restarted by a shell loop or supervisor, changing its command line.
  2. That the cp command would succeed silently. The assistant assumed that after killing the process, the binary file would be writable. The "Text file busy" error disproved this.
  3. That the log file would be overwritten by the new daemon. The assistant used rm -f to clear old logs, but if the old daemon was still running, it would have continued writing to the log file, and the rm might have been followed by the old daemon creating a new file with the same name.
  4. That the benchmark output was from the current run. The assistant had been confused by stale benchmark logs showing "error: unexpected argument '--proof-type' found" from an earlier failed invocation, and initially attributed this to the new binary before realizing it was the old daemon.

Input Knowledge Required

To fully understand this message, the reader needs knowledge of:

Output Knowledge Created

This message produces several valuable insights:

Conclusion

Message [msg 2346] captures a pivotal moment in a complex debugging session. It is the point where the assistant realizes that its carefully planned fix has not been deployed, and must diagnose why. The reasoning is sharp and methodical: from the initial observation of stale timestamps, to the forensic analysis of what the old daemon actually accomplished, to the second deployment attempt that fails with "Text file busy." This message exemplifies the kind of grounded, evidence-based debugging that separates effective engineering from guesswork. The assistant does not panic or restart from scratch—it reads the evidence, adjusts its understanding, and tries again with a more targeted approach. The "Text file busy" error is a new obstacle, but it is a concrete one with known solutions. The path forward is clear: unlink the old binary, copy the new one, and restart. The budget-based memory manager is proven sound; only the evictor's async hygiene needs correction.