Chunk 71.2

This chunk opened by executing the user's three explicit requests: writing the detailed long-form bug report, fixing the mass-abort wedge, and tackling the HiCache+bf16 corruption. The wedge fix was cleanly resolved—three parallel subagents converged on the root cause (the NIXL prefill `bootstrap_thread` dying on unhandled decode-side `ABORT` messages), and the mooncake-style fix was applied, verified across multiple abort cascades with zero throughput regression, and committed. The HiCache+bf16 investigation, however, took a major correctional turn. Despite six subagents and extensive instrumentation, the initial host-mirror sizing hypothesis was definitively refuted: the geometry is correct, and checksum instrumentation proved the prompt-side index-K transfers perfectly intact (111/112 rooms matched). The corruption was narrowed to a load-induced race condition tied to the 2× larger bf16 footprint, and while a real device-side `pool_configurator.py` bf16 sizing fix was landed, HiCache remained disabled as the full race was scoped as a follow-up. The user then reported a critical new observation: even with HiCache **off**, their heavy multi-turn workload (2k→80k context) produces a different corruption signature ("losing the plot"), indicating the root cause is broader than just HiCache. The assistant pivoted into a comprehensive parallel investigation to isolate this. A series of decisive tests ruled out the topk-v2 cluster-sync bug (`SGLANG_OPT_USE_TOPK_V2=0` still corrupted), the eager decode path (peak batch size never exceeded the captured graph limit), and the prompt-side index-K transfer (checksums matched). The definitive A/B test at identical high concurrency (60×4, HiCache on) was conclusive: **fp8 index-K → 0% corruption, bf16 index-K → 17% corruption**. This pins the issue squarely on the bf16 index-K path under heavy load. The key themes of this chunk are **systematic hypothesis falsification through evidence-based testing** and the **tension between correlated symptoms and shared root causes**. The wedge was a clean, isolated fix. The HiCache+bf16 corruption turned out to be a complex load-induced race rather than a simple static bug. The user's new evidence forced a re-evaluation, ruling out several high-profile suspects (topk-v2, eager decode, prompt transfer) and narrowing the focus to the **decode-side bf16 index-K store and read path under concurrent load**, which remains the open problem. The deployed state is bf16 ON, HiCache OFF, with the wedge and pool sizing fixes applied, but the fundamental heavy-prefill incoherence is an active investigation targeting the decode-side index-K handling.

The Art of Systematic Falsification: How a Production Debugging Campaign Isolated a bf16 Index-K Concurrency Bug 2941 words

Message Articles

Subagent Sessions