[AMD][dsv4] fix 2P1D 8k1k straggler (decode/prefill context-length asymmetry)#2257
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[AMD][dsv4] fix 2P1D 8k1k straggler (decode/prefill context-length asymmetry)#2257inkcherry wants to merge 1 commit into
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…aggler) Satisfies MOTIVATION_RULES rule 2 (root cause) + rule 3 (fix). Root cause of the 8k1k conc-500/512 straggler tail (last few requests hang to the 1800s client timeout, p99.9 TTFT ~1802s): a prefill/decode context-length asymmetry. models.yaml sets context_length: 9217 for the DeepSeek-V4-Pro prefill role but not the decode role, and server_sglang.sh only appends --context-length to PREFILL_MODE_FLAGS. So prefill runs with context_length=9217 (max input 9211) while decode falls back to the model's default (~1M; observed max_req_input_len 1048570). A request whose input exceeds 9211 -- e.g. random-dataset prompts that retokenize above the 8192 target -- is ACCEPTED by decode (which pre-allocates KV and waits for the transfer) but REJECTED by prefill with HTTP 400 (so prefill never registers the bootstrap room and never sends KV). The decode request then waits forever for a KV transfer that never comes and hangs to the client timeout. (The permanently-pending decode requests also re-query the prefill bootstrap every scheduler iteration, which is what exhausted ephemeral ports in a separate failure mode addressed by the bootstrap-session-pool patch.) Fix: parse a decode context_length from models.yaml and append --context-length to DECODE_MODE_FLAGS, defaulting to the prefill value when unset so the two roles always agree in PD-disaggregation. Over-length requests now fail fast at decode admission (HTTP 400) instead of hanging. Valid requests are unaffected.
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In the 2P1D 8k1k run at concurrency 500/512 a few requests hang until the client timeout (p99.9 TTFT ~1800s).
models.yamlsetscontext_length: 9217on the prefill role only, andserver_sglang.shonly passes--context-lengthto prefill, so decode falls back to the model default (~1M). A request longer than 9216 is accepted by decode (allocates KV, waits for the transfer) but rejected by prefill with HTTP 400, so prefill never sends the KV and that decode request waits forever.Changed
Also pass
--context-lengthto decode (read frommodels.yaml, defaulting to the prefill value) so both sides use the same limit.Validated
Before: conc-500 stalled with 33/1000 (rr=1.0) and 4/1000 (rr=0.8) requests hung to the timeout. After: conc-500 and conc-512 complete 100% in ~140s with normal p99.9. gsm8k accuracy unchanged (0.958).