Is using MTP actually slower?

#4
by jian2023 - opened

Why does using MTP actually make things much slower than not using it?

when i test this model in latest lm studio on my mac, it is much slower than https://huggingface.co/mlx-community/Qwen3.5-9B-MLX-4bit.
what is the problem, lm studio or llama.cpp not support it?

in my tests MTP is much slower in long context (128K) !
MTP -> 3 t/s
non MTP -> 15 t/s

in 8K Context MTP is doing well

Have you checked that the mtp fits into your vram together with the large context?

Have you checked that the mtp fits into your vram together with the large context?

Yes , I also tried qwen 3.5 4B and got same problem with MTP (it completely fits in my GPU VRAM)

I think I found the cause, as I ran into the same issue. I recommend lowering --spec-draft-n-max to 2 or 3 (you'll need to test what works best—in my case, 2 yielded the best performance). The speed is significantly better than running without MTP speculative decoding.
MTP off: 14~16 t/s
--spec-draft-n-max 6 (default from model card readme): 2~3 t/s
--spec-draft-n-max 2: 24~31 t/s

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