The Throttle Takes Hold: A Checkpoint in GPU Pipeline Debugging

On March 13, 2026, at 19:58 UTC, a user in the opencode coding session shared a log excerpt from a remote proving machine running the cuzk GPU proving engine. The message, consisting of seven log lines captured from /data/cuzk-pinned3.log, represents a critical checkpoint in an intensive debugging session aimed at resolving severe GPU underutilization in a Filecoin proof generation pipeline. To understand this message fully, one must appreciate the saga that preceded it: days of investigation into why GPU utilization was dropping to near-zero during proof generation, the discovery of a host-to-device (H2D) transfer bottleneck, the design and deployment of a zero-copy pinned memory pool, and the iterative deployment of fixes labeled pinned1, pinned2, and now pinned3. This message is the first evidence that the latest fix—a GPU queue depth throttle—is operational, and it simultaneously reveals both progress and a lingering filesystem race condition.

The Message in Full

The user pasted the following raw log output:

2026-03-13T19:58:26.951123Z  INFO cuzk_core::engine: partition synthesis complete, pushing to GPU queue job_id=ps-snap-3644168-34434-1202734 partition=0 synth_ms=40914
TIMELINE,113776,GPU_QUEUE,ps-snap-3644168-34434-1202734,partition=0
2026-03-13T19:58:27.249133Z  INFO cuzk_core::engine: background PCE extraction complete for SnapDeals
2026-03-13T19:58:30.397201Z  WARN cuzk_core::pipeline: failed to save PCE to disk (non-fatal) circuit_id=snap-32g error=failed to rename /var/tmp/filecoin-proof-parameters/pce-snap-deals-32g.tmp -> /var/tmp/filecoin-proof-parameters/pce-snap-deals-32g.bin
2026-03-13T19:58:31.548533Z  INFO cuzk_core::engine: background PCE extraction complete for SnapDeals
2026-03-13T19:58:34.557393Z  WARN cuzk_core::pipeline: failed to save PCE to disk (non-fatal) circuit_id=snap-32g error=failed to rename /var/tmp/filecoin-proof-parameters/pce-snap-deals-32g.tmp -> /var/tmp/filecoin-proof-parameters/pce-snap-deals-32g.bin
CUZK_TIMING: prep_msm_ms=8711
2026-03-13T19:58:36.513746Z  INFO cuzk_core::engine: background PCE extraction complete for SnapDeals

Each line carries significant meaning. The first line announces that partition 0 of a SnapDeals proof job has completed synthesis in 40,914 milliseconds and is being pushed to the GPU queue. The TIMELINE line is a structured event marker used for post-hoc analysis. Then follow three cycles of PCE extraction completing successfully but failing to save to disk, interleaved with a timing measurement for the MSM (multi-scalar multiplication) preparation step. A third PCE extraction completes at the end.

The GPU Queue Depth Throttle: Design and Motivation

The most important context for this message is the GPU queue depth throttle that was implemented just minutes earlier, in messages [msg 3273] through [msg 3299]. The user had observed a screenshot of the vast-manager status UI showing dozens of partitions in the "purple" state—meaning they had completed synthesis and were waiting for a GPU worker to become available. With 5 jobs × 16 partitions each, the system was synthesizing 80 partitions worth of data into memory, consuming approximately 362 GiB of budget. This left only 5 GiB free, which was insufficient for the 15.8 GiB Pre-Compiled Constraint Evaluator (PCE) cache. As a result, insert_blocking—the function responsible for caching PCE data—was stuck in an infinite retry loop, try_acquire kept returning None, and every synthesis fell through to the slow enforce() path instead of the fast PCE path.

The user's insight was elegant: if synthesis dispatch were throttled based on how many partitions were already queued for GPU, memory pressure would naturally decrease. With only 2 GPU workers (across 2 devices), a queue depth of 8 would be more than sufficient to keep the GPUs fed while dramatically reducing the number of synthesized partitions held in memory. The assistant implemented this by adding a max_gpu_queue_depth field to the PipelineConfig struct (defaulting to 8), adding a len() method to the PriorityWorkQueue, and inserting a throttle check in the synthesis dispatcher loop. The throttle operates before budget acquisition: the dispatcher pops a work item from the synthesis queue, checks if gpu_work_queue.len() >= max_gpu_queue_depth, and if so, sleeps for 100ms and retries. This ensures that budget is never tied up in partitions that are merely waiting for GPU processing.

What the Logs Reveal About Throttle Behavior

The log message shows that partition 0 of job ps-snap-3644168-34434-1202734 has been pushed to the GPU queue after 40.9 seconds of synthesis. Critically, only one partition appears in the GPU queue at this point. In the previous deployment (pinned2), all 80 partitions would have been dispatched nearly simultaneously, flooding the GPU queue and consuming almost all available memory budget. The throttle is clearly working: the dispatcher is holding back subsequent partitions, allowing budget to remain available for other purposes—most importantly, PCE caching.

The 40.9-second synthesis time for partition 0 is expected. This is the first partition of the first job, and PCE has not yet been cached. Without the PCE fast path, synthesis must use the full constraint evaluation via enforce(), which is significantly slower. Once PCE is cached (which we see happening in the subsequent log lines), subsequent partitions should synthesize much faster.

The PCE Extraction Success and Disk Save Failure

The log reveals a fascinating pattern: PCE extraction completes successfully three times, but each time the disk save fails with a "failed to rename" error. The extraction itself is succeeding—the background PCE extraction task runs to completion and logs "background PCE extraction complete for SnapDeals." However, when it attempts to atomically move the temporary file (pce-snap-deals-32g.tmp) to the final path (pce-snap-deals-32g.bin), the rename fails.

This is a known issue with overlay filesystems, which are commonly used in Docker container deployments. The overlay FS does not support rename() across layer boundaries in the same way as a native filesystem, causing the atomic swap to fail. The error is correctly marked as "non-fatal" because the PCE data is already held in an in-memory cache (pce_cache). The disk save is purely a persistence optimization so that future daemon restarts can load PCE from disk instead of recomputing it. Since the in-memory cache is functional, the pipeline can still use the PCE fast path for subsequent partitions.

But why does PCE extraction run three times? This is likely because multiple synthesis workers independently trigger PCE extraction. With synthesis_concurrency = 4, up to four workers may be running simultaneously. When the first worker detects that PCE is not yet cached, it starts background extraction. Other workers may also detect the same condition before the first extraction completes, starting redundant extraction tasks. Each completes successfully, but they all attempt the same disk rename, and all fail for the same reason. This is harmless but wasteful—a future optimization could add a "PCE extraction in progress" flag to prevent redundant work.

The Timing Measurement: prep_msm_ms=8711

The line CUZK_TIMING: prep_msm_ms=8711 reports that the MSM preparation step took 8,711 milliseconds. This is a custom timing instrumentation added during the GPU utilization investigation (segment 21 of the conversation). The MSM preparation involves setting up the multi-scalar multiplication on the GPU, which includes H2D transfers of the A, B, and C vectors. In the pinned2 deployment, these transfers were taking 1,300–12,000 ms because the pinned memory pool was silently falling back to heap allocations due to budget double-counting. With pinned3, the budget integration has been fixed (the PinnedPool::allocate() no longer calls budget.try_acquire()), so pinned allocations should succeed. However, the PCE fast path—which would eliminate the need for these transfers entirely—is not yet active for this first partition because PCE was still being extracted. The 8,711 ms likely still includes H2D transfer overhead.

Assumptions and Their Validity

Several assumptions underpin the interpretation of this message. First, the assumption that the GPU queue depth throttle is the sole reason only one partition appears in the GPU queue. This is plausible but not proven—it could also be that only one job has been dispatched so far, or that other partitions are still synthesizing. The throttle is designed to block dispatch, not synthesis itself, so other partitions could still be in the synthesis work queue waiting to be dispatched. The log does not show the state of the synthesis work queue, so we cannot definitively attribute the single-partition queue depth to the throttle alone.

Second, the assumption that PCE extraction is completing successfully despite the disk save failure. The log confirms extraction completion, but we do not see the subsequent "PCE cached" message that would confirm the in-memory cache was populated. The chunk summary from the analysis pipeline notes that "the PCE disk save race (rename failures on overlay FS) is benign since the in-memory cache works," but the log excerpt shown does not contain the pce_cache::insert confirmation.

Third, the assumption that the throttle will ultimately improve GPU utilization. The theory is that by reducing memory pressure and allowing PCE to be cached, subsequent partitions will synthesize faster and the GPU will have a steady stream of work. However, the throttle could also introduce a new bottleneck: if synthesis is too aggressive in throttling, the GPU could become idle waiting for the next partition. The configurable max_gpu_queue_depth = 8 is a heuristic—if the queue drains faster than synthesis can refill it, GPU utilization could suffer. The user's later observation (in the chunk summary) that "when 20+ syntheses run simultaneously, GPU activity drops to near zero" suggests that the throttle may need to be more reactive, modulating dispatch smoothly rather than in bursts.

Knowledge Created by This Message

This message creates several pieces of actionable knowledge. First, it confirms that the pinned3 binary is running and processing jobs—the daemon started successfully, loaded the configuration with max_gpu_queue_depth = 8, and is producing output. Second, it reveals that PCE extraction is working correctly for SnapDeals, which was a goal of the earlier PCE implementation work (segment 0 of the conversation). Third, it surfaces the overlay filesystem rename race condition, which was previously unknown or unobserved. Fourth, it provides a baseline timing measurement (40.9 seconds for first partition synthesis, 8.7 seconds for MSM prep) against which future improvements can be measured.

The message also implicitly validates the decision to make the disk save failure non-fatal. The pipeline continues processing despite the rename errors, and subsequent PCE extractions continue to complete. This graceful degradation is essential for production reliability.

The Broader Narrative

This message sits at a pivot point in the debugging session. The team has just deployed the third iteration of their pinned memory pool fix, now augmented with a GPU queue depth throttle. The immediate predecessor (pinned2) had fixed the budget double-counting that caused silent fallback to heap allocations, but PCE caching was still failing because budget was exhausted by too many concurrent partitions. The throttle is designed to break this deadlock.

The subsequent messages in the conversation (chunk 1 of segment 24) show that the throttle was only partially successful. While it allowed PCE to be cached (385 GiB budget used, 15 GiB for PCE), the initial batch of 80 partitions had already been dispatched before PCE was available, so the pipeline was still draining those slow unpinned partitions. The user observed that "when 20+ syntheses run simultaneously, GPU activity drops to near zero," suggesting that the poll-based throttle (sleep 100ms, check again) was causing burst dispatches that overwhelmed the pinned pool allocator. This led to the semaphore-based reactive dispatch in pinned4, which replaced polling with a 1:1 modulation where exactly one synthesis is dispatched per GPU completion.

Conclusion

Message [msg 3305] is a snapshot of a system in transition. It captures the moment when a new throttle mechanism first takes effect, when PCE extraction finally succeeds after days of budget starvation, and when a subtle filesystem incompatibility is exposed. For the reader who has followed the GPU utilization investigation from its origins in H2D transfer measurements through the design of the pinned memory pool and the discovery of budget double-counting, this message is the first sign that the fixes are working—even as it reveals the next set of challenges to overcome. It is a testament to the iterative nature of performance debugging: each fix reveals new behavior, each log line contains multiple signals, and the path to optimal GPU utilization is paved with small, measurable victories.