Diagnosing Pinned Memory Pool Convergence: A Surgical Log Query in the cuzk GPU Pipeline

In the iterative development of a high-performance GPU proving pipeline for the cuzk zero-knowledge proving system, few operations are as deceptively simple as a remote log query. Yet message [msg 3464] — a single ssh command piped through grep — represents a critical diagnostic pivot in the refinement of the pinned memory pool. The message, reproduced in full, reads:

[assistant] [bash] ssh -p 40612 root@141.0.85.211 'grep "pinned pool" /data/cuzk-pctrl2.log | grep -v "checkout (reuse)" | grep -v checkin | head -20'

This is not merely a developer poking at logs. It is the culmination of a multi-round investigation into whether the zero-copy pinned memory pool — a core optimization that eliminates GPU H2D transfer bottlenecks — is converging to a steady state of buffer reuse, or whether it continues to allocate new buffers throughout a proving run, consuming memory without bound.

The Context: A Pool That Should Converge

The pinned memory pool (PinnedPool) was the subject of extensive prior work in this session ([chunk 0.0]). It replaces per-allocation cudaHostAlloc/cudaFree calls with a pre-allocated pool of pinned (page-locked) host memory buffers that are checked out by synthesis workers, filled with circuit data, and handed to the GPU. After GPU processing, buffers are checked back into the pool for reuse. The critical metric is the reuse ratio: if the pool converges, almost all checkouts should be reuses rather than fresh allocations, and the total buffer count should plateau early in the run.

The conversation leading up to [msg 3464] reveals growing concern about this convergence. At [msg 3459], the user instructed: "Before restarting look at pinned buffer behavior of current deployment" — a prudent step before deploying the new pacer1 binary with the PI-controlled dispatch pacer. The assistant's initial query at [msg 3460] grepped broadly for pinned pool events, returning a wall of checkin events. A follow-up at [msg 3461] counted 1,530 checkout (reuse) events. But the user pushed back at [msg 3462]: "Seems like we're allocating still even very late" — a concern that the pool might not be converging.

The assistant's immediate response at [msg 3463] was to count checkout (alloc) events — fresh allocations from the pool's fallback path. The result was zero. This seemed to contradict the user's intuition. But the user's concern was about raw allocation events, not checkouts. The assistant needed to look at the actual allocated new buffer log lines — events that occur when the pool grows its buffer count, not when a worker checks out a buffer.

The Surgical Query

Message [msg 3464] is the assistant's refined diagnostic. The command structure reveals careful thinking about log noise:

  1. grep "pinned pool" — selects all log lines from the pinned_pool module, which emits structured log events for every pool operation: allocations, checkouts (reuse and alloc), and checkins.
  2. grep -v "checkout (reuse)" — removes the dominant noise. With 1,530 reuse events, these lines would drown out the signal. The -v flag inverts the match, excluding lines containing that string.
  3. grep -v checkin — further removes the checkin events, which are also high-frequency. Checkins happen every time a GPU worker finishes with a buffer, returning it to the free list.
  4. head -20 — limits output to the first 20 matching lines, providing a sample rather than overwhelming the terminal. The result is a clean view of only the allocated new buffer events — the pool growth events that the user was worried about. The output shows lines like:
2026-03-13T21:21:44.255338Z INFO synthesize_snap_deals_partition{job_id="ps-snap-3644166-35164-1232009" partition=3}: cuzk_core::pinned_pool: pinned pool: allocated new buffer size_gib=2.415463447570801 total_buffers=1 total_gib=2.415463447570801

This single allocation event, timestamped at 21:21:44, shows the pool growing from 0 to 1 buffer — the very first allocation. But the user's concern was about late allocations. The head -20 truncation means the output shows only the earliest events. The assistant immediately recognized this limitation and followed up at [msg 3465] with a more targeted query: counting total allocations (wc -l) and showing the last 5 (tail -5). That query revealed 348 total allocations, with the latest at 21:45:14 showing total_buffers=344 and total_gib=280.19 — confirming the user's suspicion that allocations were still happening deep into the run.

Input Knowledge Required

To understand this message, one must grasp several layers of the system architecture:

Assumptions and Their Validity

The assistant made several implicit assumptions in crafting this query:

Assumption 1: The log file exists and is accessible. This was validated by the prior successful queries at [msg 3460] and [msg 3461]. The SSH session returned data, confirming connectivity.

Assumption 2: The grep patterns correctly isolate allocation events. The pinned pool: prefix is emitted by all pool operations, but the allocated new buffer substring is unique to growth events. The -v exclusions for checkout (reuse) and checkin are correct — but note that checkout (alloc) events (which would also contain "alloc") are not excluded, so they would appear in the output. However, the prior query at [msg 3463] confirmed zero checkout (alloc) events, so this is not a concern.

Assumption 3: The head -20 truncation provides a representative sample. This assumption was partially invalid — it showed only the earliest events, not the late allocations the user was concerned about. The assistant recognized this and immediately issued a follow-up query at [msg 3465] with wc -l and tail -5 to get the full picture.

Assumption 4: The user's concern about "allocating still even very late" referred to pool growth events rather than checkout-alloc events. This was the critical interpretive leap. The user's phrasing was ambiguous — "allocating" could mean any allocation-related activity. The assistant initially interpreted it as checkout-alloc events (finding zero), but the refined query correctly targeted pool growth events, revealing 348 allocations with the latest at 21:45:14.

Output Knowledge Created

This message, combined with its follow-up at [msg 3465], produced several actionable insights:

  1. The pool does not converge to a stable size. With 344 buffers allocated totaling 280 GiB, and allocations still occurring at 21:45:14 (near the end of a run that started at ~21:21), the pool grows monotonically throughout the workload. This contradicts the design goal of a fixed-size pool that reaches steady state early.
  2. The reuse path works (1,530 reuses), but it does not prevent pool growth. The pool is growing because the peak concurrent demand exceeds the current buffer count, triggering fallback allocations. Each allocation adds a permanent buffer to the pool — they are never freed.
  3. The dispatch pacer may be causing the growth. The pctrl2 P-controller dispatches synthesis jobs in bursts, which can create transient spikes in concurrent buffer demand. Each spike forces a new allocation if the free list is empty. Over time, this ratchets the pool size upward.
  4. Memory pressure is a real concern. 280 GiB of pinned host memory is substantial. Pinned memory cannot be swapped and competes with GPU memory for physical RAM. If the pool grows unbounded, it could exhaust host memory or trigger OOM kills.

The Broader Significance

This diagnostic exchange exemplifies the engineering discipline required to build high-performance GPU pipelines. The pinned memory pool is a textbook optimization — eliminate per-operation allocations by reusing buffers — but its effectiveness depends entirely on the dispatch scheduler's ability to maintain a stable pipeline depth. A bursty dispatcher (like the P-controller) creates demand spikes that defeat the pool's reuse logic, forcing growth. The PI-controlled pacer being deployed as pacer1 aims to smooth dispatch precisely to avoid these spikes.

The assistant's methodical log analysis — from broad grep to targeted exclusion to count-and-tail — demonstrates how to isolate signal from noise in high-volume structured logs. The key insight is that "zero checkout-alloc events" and "many allocated-new-buffer events" are not contradictory: they describe different pool operations. The pool's fallback path (checkout-alloc) was never triggered because the pool always had some free buffers, but it kept growing because the peak demand exceeded the pool size, forcing new buffer creation rather than reuse.

This finding would directly inform the design of the synthesis throughput cap added later in the session ([msg 3466] and beyond), which limits the dispatch rate to prevent synthesis contention and, indirectly, to reduce the buffer demand spikes that drive pool growth.