§3.11. Three consecutive "what about X as a base?" questions in one session,
each answerable in minutes, none of which had been asked before a 7-hour
training window was committed. Writing the check down so it runs first.
1. does it fit for TRAINING - BF16 weights against the real measured peak,
not the weight figure (Gemma-4 is 48.1 GiB of weights and peaks at
79.7 GiB at mb2/seq-16k). Model-line names lie: "Mistral Small 4" is 119 B,
238 GB in BF16, more than both cards combined. QLoRA is not an escape
hatch for MoE - bitsandbytes walks nn.Linear and fused 3-D experts are not
that.
2. if MoE - does the serving engine implement get_expert_mapping. Zero means
LoRA cannot be served at all. gemma4*.py -> 0; deepseek_v2, mixtral,
glm4_moe, ernie45_moe -> present.
3. does the model class support LoRA - and GREP THE CLASS, NOT THE FILE.
Point 3 has teeth and I nearly got it wrong twice in one turn. mistral.py greps
as SupportsLoRA=0 and is fully LoRA-capable via LlamaForCausalLM.
mistral_large_3.py greps as 0 for both and inherits get_expert_mapping from
DeepseekV3ForCausalLM. Capability is inherited; a file-level grep misses it and
only MRO resolution answers it. Same class of error as asserting a substring
instead of an effective value.
Worked results recorded for the three candidates evaluated:
Gemma-4 26B-A4B fits, no expert mapping -> trainable, MERGE-ONLY
Mistral Small 4 119B 238 GB, has mapping -> servable, NOT trainable here
Ministral 3 14B ~28 GB, dense, inherited -> passes all three
Adds a fourth glance at architecture shape, since it predicts how much of this
playbook applies at all: uniform head_dim <= 128 with no sliding window keeps
both flash and cuDNN reachable and makes §3.1/§3.3 moot, while mixed head dims
plus a sliding window is exactly what forces dense O(n^2) attention onto
Ampere-generation kernels for 65% of the step.
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