AD-4.27bpw on 6GB VRAM (GTX 1660 Ti): numbers, 32k ladder, hiccups + fixes

#20
by TitanMamo10 - opened

Running AD-4.27bpw on a 6GB card β€” numbers + hiccups (ik_llama.cpp)

Field report on Qwen3.8-Flash-Next-AD-4.27bpw-Q4_K_M-M64 (33 shards,
SHA-verified). Box: GTX 1660 Ti 6GB (TU116) / i5-9400F / 32GB DDR4 / NVMe.
Runtime: ik_llama.cpp upstream-main + GGML_CUDA_FORCE_MMQ=ON,
GGML_CUDA_NO_PINNED=1, --fit, prefetch-experts, --defer-ple.

Best configs measured with a prefill ladder (accumulating history,
server-reported prompt_n as ground truth β€” 2k-prompt benches miss the
transient OOM class entirely):

  • 16k ctx (q8 KV, ub1024, m384): tg 7.31, PP 40.63, gen-after 2.84,
    quality probes pass. Sweet spot on 6GB.
  • 48k ctx (q4+had KV, ub128): PP 6.31, sanity/gen fine, quality pass.
    Slow but real 48k window.
  • 32k experiments: q6/ub512 β†’ 19k ceiling; q4/ub512 β†’ 23k; q6/ub384 β†’ 27k;
    q4/ub384 β†’ 30.8k proven (PP ~17). ub1024 does not fit at 32k.

Hiccups, all diagnosed:

  • The Q8_0 dense stack (4.6GB on GPU) leaves almost no transient
    headroom: long re-prefills OOM from ~6.5k tokens up on the 16k config,
    and the fit refuses any margin above 512 at 48k (dense alone β‰ˆ 5.1GB).
    Unlike small-dense quants, headroom cannot be bought with fit margin
    here β€” only smaller ub chunks and lighter KV move the ceiling (
    +4k per
    KV step: q8 β†’ q6 β†’ q4).
  • Same ggml_abort fork wedge after fatals as above β€” fixed upstream in
    ikawrakow/ik_llama.cpp PR #2416 (clean deaths since, verified live).
  • Same hybrid checkpoint illegal-access (ik #1762 class) β€” checkpoints off.
  • q6 KV decode tax ~40% on TU116 (prefill unaffected).

Functional quality probes (math/reasoning/explanation checks) pass at every
rung; the 89.5% top-1 claim feels plausible from the seat, though we did
not re-run your harness. Thanks for the M64 shard layout β€” PLE isolation
streams cleanly from NVMe.

Packaged the AD-4.27 numbers from this thread (16k/48k configs, 32k ladder to 30.8k, dense-headroom analysis) into a public repo:

https://github.com/TitanMamo/q38-6gb

Complete ladder tables with per-rung data, exact flags, harness + guard scripts. MIT licensed, corrections welcome.

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