* Studio: pin GPU at 95% headroom and warn on silent CPU fallback
Two related runtime-side fixes for unslothai/unsloth#5106 ("model
loaded fully on RAM instead of VRAM"):
1. GPU pin threshold bump 0.90 -> 0.95
-------------------------------------
``_select_gpus`` and the auto-ctx pin loop in ``start_llama_server``
used a ``pool * 0.90`` threshold to decide whether the model fits on
GPU. Models that needed 91-94% of free VRAM were classified as "does
not fit", so Studio set ``gpu_indices = None`` and shipped
``--fit on`` to llama-server without ``-ngl``. The unsloth
llama.cpp fork's ``--fit on`` then ran with its default
``--fit-target 1024`` (1 GiB margin per device, an upstream default
inherited from ggml-org#18679). On a tight fit where compute
buffers + CUDA context push the projected free below the 1 GiB
target, the fork's fit logic shaves layer weights off the GPU --
slow inference for users whose models would have loaded comfortably
with ``-ngl -1``.
The classic reproducer from #5106 (noahterbest's log):
GGUF size: 20.8 GB, est. KV cache: 0.1 GB, context: 4096,
GPUs free: [(0, 22805)], selected: None, fit: True
20.8 GiB on a 22.27 GiB free RTX 4090 is 94% utilization. The model
fits (1.4 GiB headroom), but the 0.90 threshold kicks it to fit
mode. Bumping to 0.95 keeps these in the fits-on-GPU branch and
emits ``-ngl -1`` directly. The fork's ``--fit on`` still serves as
the safety net for the genuinely-too-large case.
The auto-ctx fallback also re-checks fit at 4096 before handing off
to ``--fit on``: a 20.8 GiB model with a 131072 native context fails
the auto loop at native ctx, falls back to ``min(4096, ctx)``, but
its weights + 4096 KV pin to the GPU comfortably. Without the
re-check we still emitted ``--fit on``.
``_fit_context_to_vram``'s 0.90 budget for context binary search is
intentionally left tighter than the pin fraction. That routine
chooses the slider value, where over-promising would OOM at runtime.
``_select_gpus`` decides whether to pin at all, where being
conservative pushes layers to CPU.
2. Belt-and-suspenders: warn on silent CPU fallback
---------------------------------------------------
After ``_wait_for_health`` succeeds, scan llama-server's stdout for
``model buffer size`` lines. If Studio detected GPUs and intended
GPU use but only CPU buffers were allocated, log a structured
warning citing #5106. Markers cover CUDA / ROCm / Metal / Vulkan /
OpenCL / SYCL backends. New ``_gpu_offload_active: Optional[bool]``
field surfaces the result for any future API consumer.
This catches runtime-load failures the install-time fix cannot
cover (cudart bundle pairing PR #5322 is the install-side
companion): user overriding ``--fit-target``, uncommon driver +
toolkit configurations, future regressions in the install path.
Tests: 10 new cases in studio/backend/tests/test_llama_cpp_context_fit.py:
* TestTightFitPinsToGPU x3: noahterbest's exact reproducer (auto and
explicit ctx pins to GPU at 94%); guard against threshold over-
broadening (genuine overflow still falls back to ``--fit on``).
* TestClassifyGpuOffload x7: CUDA / ROCm / Metal buffer markers
return True; CPU-only buffer lines return False; absent buffer
lines or no GPUs detected return None (no warning).
25 context-fit tests pass (15 baseline + 10 new). 511 tests total
across the affected test files. No regressions.
Refs #5106
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