unsloth/studio/backend/core/inference/gpu_arbiter.py

# SPDX-License-Identifier: AGPL-3.0-only
# Copyright 2026-present the Unsloth AI Inc. team. All rights reserved. See /studio/LICENSE.AGPL-3.0

"""Single-GPU arbiter for Studio's heavy GPU consumers.

The chat backends, diffusion, and video share one GPU. Before taking it each calls
``acquire_for(owner)``, which evicts the current other owner so two large models never sit in VRAM
at once. The arbiter only sequences ownership (freeing is each backend's teardown); eviction runs
under the lock, so a transfer is atomic vs other acquires.
"""

from __future__ import annotations

import threading
from typing import Any, Callable, Optional

from loggers import get_logger

logger = get_logger(__name__)

CHAT = "chat"
DIFFUSION = "diffusion"
VIDEO = "video"

_lock = threading.Lock()
_owner: Optional[str] = None


def _evict_chat() -> None:
    import time

    from core.inference import get_inference_backend
    from routes.inference import get_llama_cpp_backend

    llama = get_llama_cpp_backend()
    # is_active (process exists), not is_loaded (exists AND healthy): a chat model still starting
    # up holds VRAM but isn't healthy, so is_loaded would skip it and let the load race diffusion.
    if llama.is_active:
        llama.unload_model()
    orchestrator = get_inference_backend()
    if orchestrator.active_model_name:
        orchestrator.unload_model(orchestrator.active_model_name)
    # Kill the subprocess too: its base CUDA context holds VRAM diffusion needs.
    orchestrator._shutdown_subprocess(timeout = 5.0)
    # The driver reclaims the killed VRAM asynchronously; wait for it to settle before diffusion
    # allocates, else a warm chat->diffusion handoff can transiently OOM.
    llama._wait_for_vram_settle(since_kill = time.monotonic())


def _evict_diffusion() -> None:
    # Unload whichever engine the router has active (diffusers or native sd.cpp), so a
    # chat acquire frees the right one.
    from core.inference.diffusion_engine_router import get_active_diffusion_engine
    get_active_diffusion_engine().unload()


def _evict_video() -> None:
    from core.inference.video import get_video_backend
    get_video_backend().unload()


# Patchable in tests via monkeypatch.setitem. Ownership is exclusive, so acquire_for's
# evict-the-current-owner generalises to any number of registered owners.
_EVICTORS = {CHAT: _evict_chat, DIFFUSION: _evict_diffusion, VIDEO: _evict_video}


def acquire_for(owner: str, register: Optional[Callable[[], Any]] = None) -> Any:
    """Make ``owner`` the sole GPU owner, evicting the other if it holds it.

    ``register``, if given, runs under the arbiter lock right after ownership transfers,
    and its return value is returned. Registering the in-flight load HERE -- not after
    ``acquire_for`` returns -- closes the window where a competing acquire could evict this
    owner before its load is marked in-flight: eviction would then find nothing to cancel
    and both loaders would allocate VRAM at once. ``register`` must be quick (it holds the
    lock) and must not re-enter the arbiter. If it raises, ownership stays with ``owner`` --
    matching the pre-register behaviour where a failed load left the handoff in place.
    """
    global _owner
    if owner not in _EVICTORS:
        raise ValueError(f"unknown GPU owner: {owner!r}")
    with _lock:
        if _owner is not None and _owner != owner:
            logger.info("gpu_arbiter: evicting %s for %s", _owner, owner)
            _EVICTORS[_owner]()
        _owner = owner
        return register() if register is not None else None


def release(owner: str) -> None:
    """Drop ``owner``'s claim (no-op if it isn't the current owner)."""
    global _owner
    with _lock:
        if _owner == owner:
            _owner = None


def release_if(owner: str, predicate: Callable[[], bool]) -> bool:
    """Drop ``owner``'s claim only if it still holds it AND ``predicate()`` is true, atomically.

    A slow unload's "nothing resident / no load in flight" check and the ``release`` must not
    straddle a concurrent same-owner load: that load's ``acquire_for(register=...)`` re-registers
    ownership UNDER this lock, so a plain check-then-``release`` could pass the stale check and then
    clear the newer claim (``release`` is owner-guarded but identity-less). Evaluating the predicate
    under the lock closes that window -- the load's register runs either fully before or fully after.
    ``predicate`` must be quick (it holds the lock) and must not re-enter the arbiter. Returns True
    iff ownership was dropped."""
    global _owner
    with _lock:
        if _owner != owner or not predicate():
            return False
        _owner = None
        return True


def current_owner() -> Optional[str]:
    return _owner