[None][feat] DSA: adaptive indexer prefill chunk size for long sequences

[lfr-0531]

Background / Motivation

The DSA indexer's fp8_fp4_mqa_logits / fp8_mqa_logits activation memory
scales with indexer_max_chunk_size * K_compressed (the compressed KV length
of the current request). For very long sequences this can OOM — e.g. a
~500K-token request with the default 32K chunk size needs ~16GB of activation
memory.

The current workaround is to uniformly lower indexer_max_chunk_size (e.g. to
8K). That avoids the OOM but costs prefill throughput for the common case,
where the vast majority of requests are far below these lengths — a blunt
one-size-fits-all setting for what is really a long-tail problem.

Summary

Select the indexer prefill chunk size per-batch based on the largest
compressed KV length (K_compressed) among the context requests, instead of
always using the statically-configured value:

max K_compressed in batch	effective chunk size
> 512K	8K
[256K, 512K]	16K
< 256K	configured (default 32K, unchanged)

Implementation:

• New helper select_indexer_chunk_size(configured_chunk_size, max_k_compressed)
  in dsa.py. It only ever reduces the configured chunk size (never
  increases it), so any explicitly-configured value still acts as an upper
  bound.
• Wired into Indexer.prepare_for_chunked_prefill, on the indexer's own
  chunking path only. The MLA chunked-prefill path is untouched (it already
  bounds the chunk to the MLA chunk).
• max K_compressed is read from indexer_params.kv_lens (already host-side
  during prefill prepare).

Impact

• Long requests (>256K): smaller chunk → lower indexer activation memory,
  avoids the OOM without manual tuning.
• Common case (<256K): behavior unchanged — keeps the larger,
  higher-throughput chunk size.
• Safe to vary per-batch because the prefill path does not use CUDA graphs.
• No API/config changes; existing indexer_max_chunk_size continues to act as
  the upper bound.

Notes

• Draft for early review / discussion. Thresholds (256K / 512K) and chunk
  sizes (8K / 16K / 32K) are centralized in _INDEXER_CHUNK_SIZE_HEURISTIC for
  easy tuning.
• TODO before un-drafting: add a unit test for select_indexer_chunk_size
  boundaries and validate end-to-end memory/perf on a long-sequence workload.

🤖 Generated with Claude Code

[lfr-0531]

Superseded by #15459 (branch renamed to user/fanrongl/dsv4-indexer-chunk-heuristic). Closing this one.