Chunk 72.1

This chunk culminated in the definitive root-causing and fixing of the persistent bf16 high-concurrency tool-call corruption. The assistant systematically refuted the remaining hypotheses: a refined canary proved the index-K buffer was pristine (zero live-page overwrites), and a graph-vs-eager differential (GE_DIFF) revealed the bug was a transient Heisenbug suppressed by instrumentation. The decisive evidence came from disabling `SGLANG_OPT_USE_MULTI_STREAM_OVERLAP`, which completely eliminated the corruption (0% across 80-session stress tests vs a 15-18% baseline). The root cause was identified as a multi-stream-overlap race: the C4 sparse indexer runs on an alternate CUDA stream under capture, and its bf16 read-path transient intermediates alias/race with main-stream tensors in the shared captured-graph memory pool. The fix was a single environment variable, requiring no code changes, and was confirmed on a clean git build. The findings were thoroughly documented in the project report. With the corruption fixed, the assistant pivoted to the user's next priority: improving decode throughput scaling from C60 to C90. A comprehensive project plan was created based on live bottleneck analysis (decode is latency/occupancy-bound, step time = 18ms + 1.05ms/req). The assistant rigorously investigated Two-Batch Overlap (TBO) using parallel subagents, concluding it was a definitive no-go due to hard architectural blockers (requires Expert Parallelism, DSV4 decoder layers are not wired for it, and it targets EP all-to-all communication rather than the TP all-reduce which is only ~4% of step time). This saved significant effort that would have been wasted on an infeasible approach. The assistant proceeded to empirically A/B test the top-ranked lever: re-enabling the overlap scheduler (which was disabled to prevent a TP-collective desync wedge). Enabling it on the decode worker showed a modest but real throughput improvement (~5-7% at high concurrency) and passed basic serial wedge tests. However, an aggressive stress test involving abort cascades triggered the structural desync hazard, confirming the underlying bug is still present. The assistant committed to implementing the correct fix—an unconditional "agree-or-defer" all_reduce barrier on the TP CPU group—to safely ship the throughput gain, prioritizing correctness above all else. The entire process was characterized by heavy use of subagents for evidence gathering, meticulous documentation, and a correctness-first approach.

From Phantom to Fix: The Definitive Root-Causing of a CUDA-Graph Corruption Bug in DeepSeek-V4 2944 words

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