unsloth/studio/backend/core/inference/diffusion_cache.py
2026-07-10 10:48:53 +00:00

539 lines
22 KiB
Python

# SPDX-License-Identifier: AGPL-3.0-only
# Copyright 2026-present the Unsloth AI Inc. team. All rights reserved. See /studio/LICENSE.AGPL-3.0
"""Opt-in step caching for the diffusion transformer (First-Block-Cache).
Across denoising steps a DiT's output changes little once the trajectory settles, so most of
the transformer can be reused. First-Block-Cache (FBCache) computes the first block, and if
its residual barely changed from the previous step (within ``threshold``) it skips the
remaining blocks and reuses their cached output. diffusers ships it natively
(``transformer.enable_cache(FirstBlockCacheConfig(...))`` for CacheMixin models, or the
standalone ``apply_first_block_cache`` hook).
Measured on Flux.1-dev (28 steps, 1024px, B200): ~1.4x on top of torch.compile (2.83 ->
2.03 s) at LPIPS ~0.08 vs the no-cache output -- deep inside the speed-for-quality bar.
OFF by default and a deliberate per-load opt-in, because the win scales with step count: a
few-step distilled model (e.g. Z-Image-Turbo at ~8 steps) has almost no headroom and a
single skipped step is a large fraction of the trajectory, so caching is for many-step
models (Flux / Qwen-Image). It composes with torch.compile only with ``fullgraph=False``
(the cache's compiler-disabled decision is a graph break), which the speed layer switches to
automatically when a cache is engaged. Best-effort: an incompatible model (e.g. a transformer
whose block signature the hook does not recognise) is caught and the load proceeds uncached.
torch / diffusers imported lazily.
"""
from __future__ import annotations
from typing import Any, Optional
TC_OFF = "off"
TC_AUTO = "auto"
TC_FBCACHE = "fbcache"
TC_MAGCACHE = "magcache"
TC_MODES = (TC_FBCACHE, TC_MAGCACHE)
# FBCache residual thresholds: higher skips more steps (faster, lower quality). The dense
# bf16 default; a quantised transformer shifts the residual distribution, so it needs a
# higher threshold for the cache to trigger at all (per ParaAttention's fp8 guidance).
DEFAULT_FBCACHE_THRESHOLD = 0.08
QUANT_FBCACHE_THRESHOLD = 0.12
# MagCache (diffusers >= 0.39): skips whole steps from a PRE-CALIBRATED residual-magnitude
# curve with an accumulated-error budget, a consecutive-skip cap, and a no-skip retention
# window over the early steps -- so unlike FBCache the divergence from the uncached
# trajectory is bounded. Measured on HunyuanVideo-1.5-720p (B200, 50 steps, 720p clip):
# threshold 0.12 = 1.5x end-to-end at LPIPS 0.147 vs the same uncached stack with the SAME
# composition (FBCache at its 0.08 default reached 2.4x but LPIPS 0.54: a brighter,
# visibly different clip -- why the fbcache auto policy excludes this family).
DEFAULT_MAGCACHE_THRESHOLD = 0.12
MAGCACHE_MAX_SKIP_STEPS = 3
MAGCACHE_RETENTION_RATIO = 0.2
# The auto policy's step-count bar: FBCache's win scales with step count (each skipped
# step is a larger quality hit on a short trajectory), so auto engages it only at 20+
# steps -- full "dev"-style schedules (28+) qualify, distilled turbo models (4-9) never do.
FBCACHE_MIN_STEPS = 20
# Per-family MagCache magnitude-ratio curves (MagCacheConfig.mag_ratios), calibrated with
# diffusers' calibrate mode on the family base checkpoints at the default 50-step schedule
# (720p clip, B200). The curve is checkpoint-dependent but highly stable where it matters:
# the CFG cond/uncond branches differ by <= 0.014 and a 30-step calibration matches the
# 50-step curve within 0.027 after nearest-interpolation, so ONE curve per family is
# enough -- diffusers interpolates it to the actual step count. Conditional-branch curve
# per the MagCache calibration guidance.
_MAGCACHE_720P_RATIOS = (
1.0,
1.0226,
1.0093,
1.001,
1.0008,
1.0001,
0.9995,
1.0003,
0.9998,
0.9993,
0.9994,
0.9993,
0.9997,
1.0002,
0.9994,
0.9985,
0.9987,
0.9997,
0.9979,
0.9987,
0.9985,
0.9982,
0.9977,
0.998,
0.9979,
0.9971,
0.9968,
0.9967,
0.9964,
0.9965,
0.9959,
0.9954,
0.995,
0.9938,
0.9942,
0.9924,
0.9924,
0.9907,
0.9905,
0.9878,
0.9867,
0.9845,
0.9808,
0.9773,
0.9715,
0.9652,
0.9529,
0.9347,
0.9011,
0.83,
)
_MAGCACHE_480P_RATIOS = (
1.0,
1.0077,
1.0138,
1.0043,
1.0029,
0.9986,
0.9966,
1.0,
1.0006,
0.9996,
0.9993,
0.9986,
1.0,
0.9993,
0.9966,
0.9986,
0.9988,
0.9991,
0.998,
0.9977,
0.9976,
0.9971,
0.9973,
0.9969,
0.996,
0.9961,
0.9949,
0.9958,
0.9933,
0.9942,
0.9941,
0.9926,
0.9929,
0.9916,
0.9923,
0.9887,
0.99,
0.9882,
0.9865,
0.9833,
0.9827,
0.9791,
0.9763,
0.9718,
0.9657,
0.9563,
0.9454,
0.9264,
0.8967,
0.8382,
)
_MAGCACHE_FAMILY_RATIOS: dict[str, tuple[float, ...]] = {
"hunyuanvideo-1.5": _MAGCACHE_480P_RATIOS,
"hunyuanvideo-1.5-720p": _MAGCACHE_720P_RATIOS,
}
# Families whose AUTO step-cache decision engages MagCache instead of FBCache. On
# HunyuanVideo-1.5 FBCache free-runs (no skip cap, no error budget) and derails the
# trajectory (LPIPS 0.54 + a luma shift at its default threshold), while MagCache holds
# the same composition at 1.5x -- see the constants above. Every other family keeps the
# measured FBCache default. An EXPLICIT "fbcache"/"magcache" request always wins.
_FAMILY_AUTO_CACHE_MODE: dict[str, str] = {
"hunyuanvideo-1.5": TC_MAGCACHE,
"hunyuanvideo-1.5-720p": TC_MAGCACHE,
}
def auto_cache_mode(family: Optional[str]) -> str:
"""The cache mode the AUTO policy engages for ``family`` (mode only; the step-count
bar and the engage call are the caller's job). MagCache additionally needs a
calibrated ratio curve: a family routed here without one runs uncached (the
apply_step_cache magcache branch checks), never silently falls back to FBCache."""
return _FAMILY_AUTO_CACHE_MODE.get(str(family or "").strip().lower(), TC_FBCACHE)
def normalize_transformer_cache(value: Optional[str]) -> Optional[str]:
"""Lower/strip a requested cache mode; None / "" / "none" / "off" -> None (disabled),
"auto" -> TC_AUTO (the loader decides from the step count).
Raises ValueError for an unsupported value so a bad request is rejected cheaply."""
if value is None:
return None
normalized = str(value).strip().lower().replace("-", "_")
if not normalized or normalized in ("none", "off"):
return None
if normalized == TC_AUTO:
return TC_AUTO
if normalized not in TC_MODES:
raise ValueError(
f"Unsupported transformer_cache '{value}'. Use one of: off, auto, "
f"{', '.join(TC_MODES)}."
)
return normalized
# Transformer block classes whose FBCache metadata is missing from the installed
# diffusers. The First-Block-Cache hook reads each block's (hidden_states,
# encoder_hidden_states) return layout from TransformerBlockRegistry; diffusers 0.39
# registers the HunyuanVideo 1.0 blocks but not the 1.5 ones, so enable_cache raises
# "Model class HunyuanVideo15TransformerBlock not registered" on a DiT that is
# otherwise fully cache-compatible: CacheMixin, one homogeneous ``transformer_blocks``
# list of residual-additive dual-stream blocks returning (hidden_states,
# encoder_hidden_states) -- the exact layout of the registered 1.0 block. Keyed by the
# TRANSFORMER class name so only a family that needs the patch pays for it, and probed
# via TransformerBlockRegistry.get first so a diffusers release that ships the
# registration natively makes this a no-op.
# transformer class -> ((block module, block class, hs index, ehs index), ...)
_EXTRA_BLOCK_METADATA: dict[str, tuple[tuple[str, str, int, Optional[int]], ...]] = {
"HunyuanVideo15Transformer3DModel": (
(
"diffusers.models.transformers.transformer_hunyuan_video15",
"HunyuanVideo15TransformerBlock",
0,
1,
),
),
}
def _ensure_block_metadata_registered(transformer: Any, logger: Any = None) -> None:
"""Register the missing FBCache block metadata for ``transformer``'s family (see
``_EXTRA_BLOCK_METADATA``). Best-effort: a failure just leaves enable_cache to raise
its own error and the load runs uncached, exactly as before this patch."""
specs = _EXTRA_BLOCK_METADATA.get(type(transformer).__name__)
if not specs:
return
try:
import importlib
from diffusers.hooks._helpers import TransformerBlockMetadata, TransformerBlockRegistry
for module_name, cls_name, hs_index, ehs_index in specs:
block_cls = getattr(importlib.import_module(module_name), cls_name)
try:
TransformerBlockRegistry.get(block_cls)
continue # a newer diffusers registers it natively
except ValueError:
pass
TransformerBlockRegistry.register(
block_cls,
TransformerBlockMetadata(
return_hidden_states_index = hs_index,
return_encoder_hidden_states_index = ehs_index,
),
)
if logger is not None:
logger.info("diffusion.cache: registered %s block metadata for fbcache", cls_name)
except Exception as exc: # noqa: BLE001 -- best-effort; enable_cache surfaces the real error
_warn(logger, "block metadata registration", exc)
def _invalidate_child_registry_cache(transformer: Any) -> None:
"""Drop the HookRegistry's cached child-registry list after (un)installing hooks.
``cache_context`` propagates the state context through ``_get_child_registries``,
which diffusers 0.39 caches on first use. An UNCACHED generation already calls
``cache_context`` (the pipeline wraps every denoise call), creating the
transformer-level registry with an EMPTY cached child list -- so a later
``enable_cache`` (the auto step-count toggle engaging FBCache mid-session) installs
block hooks that ``_set_context`` never reaches, and the first cached forward dies
with "No context is set". Invalidate the stale cache so the next ``cache_context``
rebuilds it over the freshly hooked blocks. Best-effort and cheap (one attribute)."""
registry = getattr(transformer, "_diffusers_hook", None)
if registry is not None and getattr(registry, "_child_registries_cache", None) is not None:
try:
registry._child_registries_cache = None
except Exception: # noqa: BLE001 -- diffusers internals moved; leave as-is
pass
def _pipeline_opens_cache_context(pipe: Any) -> bool:
"""Whether the pipeline enters ``transformer.cache_context(...)`` in its denoise loop.
The First-Block-Cache hook requires it at run time, and a CacheMixin transformer alone
does NOT guarantee it: Flux Kontext / img2img / inpaint / controlnet reuse the CacheMixin
FluxTransformer2DModel but never open a cache_context. Read from the pipeline ``__call__``
source, resolved off the instance so a per-expert proxy view (``_SecondDiTView``)
delegates to the real pipe; if it cannot be read, report False so the cache stays off."""
import inspect
call = getattr(pipe, "__call__", None)
if call is None:
return False
try:
src = inspect.getsource(call)
except (OSError, TypeError):
return False
# Match the actual call `cache_context(` -- a bare mention in a comment/docstring lacks
# the paren, so this does not false-positive on prose.
return "cache_context(" in src
def apply_step_cache(
pipe: Any,
*,
mode: Optional[str],
threshold: Optional[float] = None,
quant_active: bool = False,
family: Optional[str] = None,
steps: Optional[int] = None,
logger: Any = None,
) -> Optional[str]:
"""Engage step caching on ``pipe.transformer``. Returns the mode actually engaged, or
None when disabled / unsupported (the load then runs uncached). ``threshold`` overrides
the default; ``quant_active`` raises the FBCache default so the cache still triggers on
a quantised transformer. The magcache mode additionally needs ``family`` (to look up the
calibrated ratio curve) and ``steps`` (MagCache interpolates that curve over the
configured step count and sizes its no-skip retention window from it). Best-effort:
never raises for an incompatible model."""
mode = normalize_transformer_cache(mode)
if mode is None or mode == TC_AUTO:
# AUTO must be resolved by the loader (step-count policy) before reaching the
# engage call; treat a stray auto as off rather than crashing the load.
return None
transformer = getattr(pipe, "transformer", None)
if transformer is None:
return None
if mode == TC_MAGCACHE:
thr = threshold if threshold is not None else DEFAULT_MAGCACHE_THRESHOLD
else:
thr = (
threshold
if threshold is not None
else (QUANT_FBCACHE_THRESHOLD if quant_active else DEFAULT_FBCACHE_THRESHOLD)
)
# Engage only via the transformer's native enable_cache (the diffusers CacheMixin path):
# the lower-level apply_first_block_cache hook would install on a non-CacheMixin
# transformer too (e.g. Z-Image), whose pipeline opens no cache_context and would crash
# the first generation -- so a model without enable_cache runs uncached per the
# best-effort contract instead of being reported as cached and then failing.
enable_cache = getattr(transformer, "enable_cache", None)
if not callable(enable_cache):
_warn(logger, mode, RuntimeError("transformer has no cache_context (not a CacheMixin)"))
return None
# A CacheMixin transformer is necessary but NOT sufficient: the First-Block-Cache hook
# raises "No context is set" on the first forward unless the PIPELINE wraps its denoise
# loop in transformer.cache_context(...). Flux Kontext / img2img / inpaint / controlnet
# reuse the CacheMixin FluxTransformer2DModel yet their __call__ opens no cache_context,
# so engaging FBCache there would crash every default generation -- run uncached instead.
if not _pipeline_opens_cache_context(pipe):
_warn(
logger, mode, RuntimeError("pipeline __call__ opens no cache_context; running uncached")
)
return None
# Some cache-compatible block classes are missing from the installed diffusers'
# FBCache metadata registry (HunyuanVideo-1.5); register them before enable_cache.
# Both hook families (FBCache / MagCache) read the same block metadata.
_ensure_block_metadata_registered(transformer, logger)
try:
if mode == TC_MAGCACHE:
ratios = _MAGCACHE_FAMILY_RATIOS.get(str(family or "").strip().lower())
if ratios is None:
# No silent FBCache fallback: the family was routed to magcache exactly
# because FBCache derails it, so an uncalibrated checkpoint runs uncached.
_warn(
logger,
mode,
RuntimeError(f"no calibrated mag_ratios for family '{family}'"),
)
return None
if not steps or int(steps) <= 0:
_warn(logger, mode, RuntimeError("magcache needs the step count to engage"))
return None
from diffusers.hooks import MagCacheConfig
config: Any = MagCacheConfig(
threshold = thr,
max_skip_steps = MAGCACHE_MAX_SKIP_STEPS,
retention_ratio = MAGCACHE_RETENTION_RATIO,
num_inference_steps = int(steps),
mag_ratios = list(ratios),
)
# The curve is interpolated over the CONFIGURED step count, so the marker
# carries it: the auto toggle re-engages on a step-count change.
marker = f"{mode}@{thr}#s{int(steps)}"
else:
try:
from diffusers import FirstBlockCacheConfig
except ImportError: # older diffusers exports it only from diffusers.hooks
from diffusers.hooks import FirstBlockCacheConfig
config = FirstBlockCacheConfig(threshold = thr)
marker = f"{mode}@{thr}"
enable_cache(config)
# A prior uncached generation may have frozen an empty child-registry list on
# the transformer's HookRegistry; the block hooks just installed would then
# never receive the cache context. Must follow every enable_cache.
_invalidate_child_registry_cache(transformer)
try:
transformer._unsloth_step_cache = marker
except Exception: # noqa: BLE001 — marker is best-effort
pass
if logger is not None:
logger.info("diffusion.cache: %s engaged (threshold=%s)", mode, thr)
return mode
except Exception as exc: # noqa: BLE001 — incompatible model -> run uncached
# enable_cache can fail after hooking some blocks; drop any partial hooks so
# the reported-uncached model doesn't actually run half-cached.
try:
transformer.disable_cache()
except Exception: # noqa: BLE001
pass
_warn(logger, mode, exc)
return None
def effective_denoise_steps(steps: int, strength: Optional[float]) -> int:
"""The number of steps diffusers ACTUALLY denoises for a request.
An image-conditioned workflow with ``strength`` < 1 (img2img / upscale / inpaint) runs
only a fraction of ``num_inference_steps``: diffusers' ``get_timesteps`` computes
``init_timestep = min(int(num_inference_steps * strength), num_inference_steps)`` and
denoises exactly ``init_timestep`` steps -- the product is FLOORED, not rounded. The auto
step-cache policy must key on THIS count -- e.g. a 28-step upscale at strength 0.35 runs
``int(9.8) = 9`` real steps, exactly the short trajectory FBCache should stay off (each
skipped step is a large quality hit). ``strength`` None (txt2img / reference) or >= 1 -> the
full count.
"""
s = int(steps)
if strength is None or float(strength) >= 1.0:
return s
return max(1, min(int(s * float(strength)), s))
def effective_request_strength(
request_strength: Optional[float],
has_init_image: bool,
pipe_accepts_strength: bool,
pipe_default_strength: Any,
) -> Optional[float]:
"""The strength the pipe will ACTUALLY apply, for keying the auto step-cache policy.
Only image-conditioned pipelines that take ``strength`` apply it (txt2img / a pipe without
the kwarg run the full trajectory -> None). When the request omits ``strength`` the loader
does NOT pass the kwarg, so the pipe runs its OWN signature default (< 1 for every img2img /
inpaint pipeline here, e.g. 0.6); the policy must key on that default, not the full step
count, or FBCache engages on a fraction of the advertised steps. A non-numeric default
(``inspect.Parameter.empty``) falls back to the full count (None).
"""
if not (has_init_image and pipe_accepts_strength):
return None
if request_strength is not None:
return request_strength
return pipe_default_strength if isinstance(pipe_default_strength, (int, float)) else None
def _disengage_step_cache(
transformer: Any,
*,
reason: str,
logger: Any = None,
) -> bool:
"""disable_cache + clear the marker; True when the transformer is now uncached."""
disable_cache = getattr(transformer, "disable_cache", None)
if not callable(disable_cache):
return False
try:
disable_cache()
transformer._unsloth_step_cache = None
if logger is not None:
logger.info("diffusion.cache: step cache disengaged (%s)", reason)
return True
except Exception as exc: # noqa: BLE001 -- keep the cache rather than crash
_warn(logger, "step cache disable", exc)
return False
def maybe_toggle_step_cache(
pipe: Any,
*,
steps: int,
quant_active: bool = False,
threshold: Optional[float] = None,
mode: str = TC_FBCACHE,
family: Optional[str] = None,
logger: Any = None,
) -> Optional[str]:
"""Generation-time enable/disable for an AUTO cache decision, keyed on the actual
step count: engage ``mode`` (the family's auto cache mode) at ``FBCACHE_MIN_STEPS``
or more, run uncached below it. Idempotent (the ``_unsloth_step_cache`` marker tracks
the engaged state), so calling it on every generation is cheap -- except a magcache
step-count change, which re-engages so the ratio curve is re-interpolated over the
actual schedule. Only the loader's auto path calls this; an explicit user choice is
never toggled. Returns the mode now active (or None when uncached)."""
transformer = getattr(pipe, "transformer", None)
if transformer is None:
return None
engaged = getattr(transformer, "_unsloth_step_cache", None)
want = int(steps) >= FBCACHE_MIN_STEPS
if (
want
and engaged
and mode == TC_MAGCACHE
and f"#s{int(steps)}" not in str(engaged)
and _disengage_step_cache(
transformer, reason = f"magcache re-interpolating for {steps} steps", logger = logger
)
):
engaged = None
if want and not engaged:
return apply_step_cache(
pipe,
mode = mode,
threshold = threshold,
quant_active = quant_active,
family = family,
steps = steps,
logger = logger,
)
if not want and engaged:
if _disengage_step_cache(
transformer,
reason = f"auto: {steps} steps < {FBCACHE_MIN_STEPS}",
logger = logger,
):
return None
return mode
return mode if engaged else None
def _warn(logger: Any, what: str, exc: Exception) -> None:
if logger is not None:
logger.warning("diffusion.cache: %s unavailable (%s); running uncached", what, exc)