The Moment of Truth: Running the First Instrumented E2E Test to Diagnose a 20% Performance Regression

In the disciplined practice of performance engineering, there comes a pivotal moment when all the preparatory work—the code changes, the instrumentation, the build-system fixes—converges into a single decisive measurement. Message [msg 962] captures exactly such a moment in the cuzk project's Phase 4 optimization effort. After implementing five optimizations (A1 SmallVec, A2 Pre-sizing, A4 Parallel B_G2, B1 cudaHostRegister, D4 Per-MSM window tuning) and discovering that the combined changes had regressed proof time from an 88.9-second baseline to 106 seconds, the assistant had spent the preceding messages systematically eliminating suspects. The A2 pre-sizing optimization had been partially reverted. CUDA timing instrumentation (CUZK_TIMING printf's) had been added to the GPU host code. And crucially, in the messages immediately before [msg 962], the assistant had discovered that the printf output was being lost to full buffering when stdout was redirected to a file—a subtle but critical bug that would have rendered the entire instrumentation effort invisible.

The Buffering Bug and Its Fix

The story of [msg 962] begins with a classic systems programming gotcha. The CUDA host code in groth16_cuda.cu contained printf statements emitting CUZK_TIMING: lines at each phase of GPU proving—pinning, prep MSM, NTT/MSM, batch addition, tail MSM, and B_G2 MSM. These were intended to provide the fine-grained phase-level breakdown needed to pinpoint which optimization was causing the regression. But when the daemon's stdout was redirected to a log file, the C runtime's default buffering behavior kicked in: instead of line-buffering (the default for terminals), printf to a file uses full buffering, meaning output accumulated in a 4 KB+ buffer and was never flushed before the process exited or the relevant section completed.

The assistant diagnosed this in [msg 937] by checking the log file and finding zero CUZK_TIMING lines despite confirming the strings were compiled into the binary. The fix was surgical: replace each printf(&#34;CUZK_TIMING: ...&#34;) with fprintf(stderr, &#34;CUZK_TIMING: ...&#34;); fflush(stderr);, ensuring immediate flush to the log file. This was applied across six locations in the CUDA source (<msg id=938–944>), followed by a forced rebuild of the supraseal-c2 static library by deleting the cached build artifacts ([msg 948]).

Executing the Instrumented Benchmark

Message [msg 962] is the first run of the benchmark after this fix. The command is straightforward:

time /home/theuser/curio/extern/cuzk/target/release/cuzk-bench \
  --addr http://127.0.0.1:9821 \
  single \
  --type porep \
  --c1 /data/32gbench/c1.json 2>&1 | tee /tmp/cuzk-phase4-bench2.log

This invokes the cuzk-bench utility to submit a single PoRep (Proof-of-Replication) C2 proof job to the running daemon. The --addr flag points to the local daemon listening on port 9821. The --c1 flag provides the path to a pre-computed C1 output JSON file—the intermediate representation from the first phase of Filecoin proof generation. The 2&gt;&amp;1 | tee arrangement ensures both stdout and stderr are captured to a log file while also displayed on the terminal.

The result arrives 101 seconds later:

=== Proof Result ===
status:    COMPLETED
job_id:    fdb4e3c6-2a25-4a21-aa86-727fff509923
timings:   total=101331 ms (queue=240 ms, srs=0 ms, synth=61002 ms, gpu=40087 ms)
wall time: 101443 ms
proof:     1920 bytes (hex: 8ba52e18...)

The proof completed successfully. Total time: 101.3 seconds. This is still 12.4 seconds above the 88.9-second baseline, but it's 4.7 seconds better than the 106-second regression seen before the A2 revert. The phase-level breakdown shows synthesis at 61.0 seconds and GPU proving at 40.1 seconds, with negligible queue time (240 ms) and zero SRS loading time (the SRS was preloaded at daemon startup).

What This Message Reveals—And What It Doesn't

On its own, [msg 962] provides the top-level timing breakdown but not the GPU-internal phase timings. The real payoff comes in the immediately following message, [msg 963], where the assistant greps the daemon log for CUZK_TIMING and gets:

CUZK_TIMING: pin_abc_ms=5733 num_circuits=10 abc_bytes_each=4168923808
CUZK_TIMING: prep_msm_ms=1722
CUZK_TIMING: gpu_tid=0 ntt_msm_h_ms=21417
CUZK_TIMING: gpu_tid=0 batch_add_ms=1446
CUZK_TIMING: gpu_tid=0 tail_msm_ms=1259 gpu_total_ms=24123
CUZK_TIMING: b_g2_msm_ms=22849 num_circuits=10

This is the data that makes the entire exercise worthwhile. The pin_abc_ms=5733 line is the bombshell: the B1 optimization (cudaHostRegister to pin host memory for the a/b/c vectors) is taking 5.7 seconds, not the estimated 150–300 milliseconds. The assumption that memory pinning would be cheap was catastrophically wrong. Pinning ~125 GiB of host memory (10 circuits × ~3.9 GiB each) requires touching every page to wire it for DMA, and on this AMD Zen4 Threadripper PRO 7995WX system, that page-turning overhead dominates.

The GPU-internal breakdown is equally illuminating: NTT/MSM on G1 takes 21.4 seconds, B_G2 MSM takes 22.8 seconds, and the remaining phases (prep, batch add, tail MSM) account for about 4.4 seconds. The GPU total of 24.1 seconds (from the tail MSM line) plus the 5.7 seconds of pinning overhead plus 1.7 seconds of prep MSM gives approximately 31.5 seconds of GPU-side work, which aligns with the 40.1-second gpu timing when accounting for PCIe transfers and other orchestration overhead.

The Broader Diagnostic Strategy

Message [msg 962] sits at a critical juncture in the regression diagnosis. The assistant is executing a systematic isolation strategy:

  1. Revert the most suspicious change first (A2 pre-sizing, already partially done).
  2. Add instrumentation to measure what's actually happening (CUZK_TIMING printf's).
  3. Fix the instrumentation to ensure data is captured (the fflush fix).
  4. Run the instrumented test to collect data (this message).
  5. Analyze the data to identify the next culprit (B1's 5.7s overhead).
  6. Revert the identified culprit and re-test (B1 reverted in subsequent messages).
  7. If regression persists, drill deeper with microbenchmarks (the synth-only benchmark built later). This is textbook performance debugging: never guess, always measure. The assistant resists the temptation to speculate about which optimization is causing the problem and instead invests effort in making the invisible visible. The CUDA timing instrumentation, the buffering fix, and the careful build management are all investments in observability.

Assumptions and Their Consequences

Several assumptions underpin this message. The assistant assumes that the daemon is correctly configured with the Phase 4 changes compiled in, that the SRS has been preloaded (confirmed by the srs=0 ms timing), and that the CUDA timing instrumentation will now produce visible output thanks to the fflush(stderr) fix. These assumptions are validated by the results.

A more subtle assumption is that the 101.3-second result is representative and not an outlier. Single-run benchmarks on GPU systems can be noisy due to GPU clock scaling, thermal throttling, or memory controller initialization. The assistant does not run multiple iterations here—that would come later in the synth-only microbenchmark phase, where three iterations per configuration are used. For this initial diagnostic pass, a single run is sufficient to identify the B1 pinning overhead, which is so large (5.7 seconds) that it dwarfs any measurement noise.

Input Knowledge Required

To fully understand [msg 962], one needs knowledge of the cuzk project's architecture: that it implements a pipelined Groth16 proving engine for Filecoin PoRep, that it uses a daemon-worker model with gRPC communication, and that the cuzk-bench tool is a test harness. One also needs to know the Phase 4 optimization taxonomy (A1–A5, B1–B3, D1–D4) and the established baseline of 88.9 seconds for a single 32 GiB PoRep proof. The preceding messages document the buffering bug and its fix, the A2 revert, and the build-system nuances of forcing CUDA recompilation.

Output Knowledge Created

This message produces the first reliable timing data after the Phase 4 changes. It establishes that:

The Thinking Process

The assistant's reasoning in this message is execution-focused: the diagnostic plan has been formulated in previous messages, the instrumentation has been fixed, and now it's time to gather data. The choice of time prefixing the command is deliberate—it provides wall-clock timing as a cross-check against the daemon's internal timing. The use of tee ensures the output is both visible in real-time and captured for later analysis. The assistant is thinking ahead: "I need this data in a file so I can grep it, correlate it with the daemon log, and build a precise picture of where time is going."

The message also reflects an understanding of the system's end-to-end flow. The assistant knows that the daemon was started with SRS preloading, that the GPU is idle and ready, and that the C1 input file is available. The 240 ms queue time confirms that the scheduler is not a bottleneck. The zero SRS time confirms that preloading works correctly. Every number in the output is checked against expectations.

Conclusion

Message [msg 962] is a textbook example of instrumented performance diagnosis in a complex distributed system. It represents the payoff of careful preparatory work—adding instrumentation, fixing build-system issues, and systematically reverting suspicious changes. The data it produces directly identifies the B1 cudaHostRegister optimization as adding 5.7 seconds of overhead, far exceeding its estimated benefit. This single measurement saves the team from pursuing a dead-end optimization path and redirects attention to the real remaining regression in synthesis time. In the broader narrative of the cuzk Phase 4 effort, this message is the turning point where guesswork ends and data-driven optimization begins.