base.py

import inspect
from itertools import product
from typing import Iterator, Literal

import torch
from compressed_tensors.offload.dist_utils import as_broadcastable, is_distributed
from compressed_tensors.quantization import (
    QuantizationStrategy,
    disable_quantization,
    forward_quantize,
)
from compressed_tensors.utils import (
    align_modules,
    get_execution_device,
    get_lowest_common_ancestor_name,
    getattr_chain,
    match_modules_set,
    match_named_modules,
    patch_attrs,
    update_offload_parameter,
)
from loguru import logger
from pydantic import ConfigDict, PrivateAttr, field_validator
from torch import distributed as dist
from torch.nn import Module
from torch.utils._pytree import tree_leaves
from tqdm import tqdm

from llmcompressor.core import Event, EventType, State, active_session
from llmcompressor.modifiers import Modifier
from llmcompressor.modifiers.awq.mappings import (
    AWQMapping,
    ResolvedMapping,
    get_layer_mappings_from_architecture,
)
from llmcompressor.modifiers.quantization.calibration import (
    call_observer,
    update_weight_global_scale,
    update_weight_zp_scale,
)
from llmcompressor.modifiers.quantization.quantization import QuantizationMixin
from llmcompressor.modifiers.utils import update_fused_layer_weight_global_scales
from llmcompressor.modifiers.utils.hooks import HooksMixin
from llmcompressor.modifiers.utils.pytorch_helpers import is_moe_model
from llmcompressor.observers.base import Observer
from llmcompressor.pipelines.cache import IntermediatesCache
from llmcompressor.sentinel import Sentinel
from llmcompressor.utils import wait_for_comms
from llmcompressor.utils.helpers import calibration_forward_context
from llmcompressor.utils.pytorch.module import (
    get_module_to_name_dict,
)

__all__ = ["AWQModifier"]


class AWQModifier(Modifier, QuantizationMixin):
    """
    Implements the AWQ (Activation-Weighted Quantization) algorithm,
    as described in https://arxiv.org/pdf/2306.00978. The algorithm
    significantly reduces quantization error by protecting only 1%
    of the most salient weight channels.

    Instead of relying on raw weight values, AWQ identifies important channels by
    analyzing activation patterns, focusing on the channels in the weight tensor that
    are most responsive to the input. To reduce quantization error, it scales these
    channels in a way that preserves the model's original behavior, using scaling
    factors computed offline from activation statistics.

    Because this modifier manipulates the weights of the model, it can only be used in
    in one-shot and not during training. Activation ranges are determined by running a
    small set of calibration data through the model.

    example recipe:
    ```yaml
    AWQModifier:
      mappings:
        - smooth_layer: "re:.*self_attn_layer_norm"
          balance_layers: ["re:.*q_proj", "re:.*k_proj", "re:.*v_proj"]
        - smooth_layer: "re:.*final_layer_norm"
          balance_layers: ["re:.*fc1"]
        # activation_hook_target specifies which submodule of the parent to hook
        # for activation caching.
        # This change is only useful for MoE models with parallel transformer blocks,
        # and one should use the default value (None) in most cases.
      ignore: ["lm_head"]
      config_groups:
        group_0:
          targets:
            - "Linear"
          input_activations: null
          output_activations: null
          weights:
            num_bits: 4
            type: int
            symmetric: false
            strategy: group
            group_size: 128
    ```

    Lifecycle:

    - on_initialize
        - resolve mappings
        - capture kwargs needed for forward passes into modules
    - on_start
        - set up activation cache hooks to capture input activations
            to balance layers
    - on sequential epoch end
        - apply smoothing to each smoothing layer
            - consume cached activations across all batches
                - clear cached activations as they are used
            - find best smoothing scale for each smoothing layer via grid search
            - apply best scales to model weights
            - raise error if any unused activations remain
    - on_end
        - re-run logic of sequential epoch end (in case of basic pipeline)
        - set scales and zero points
        - remove activation hooks
    - on_finalize
        - clear resolved mappings and captured activations

    :param sequential_targets: list of module names to compress in
        the same calibration pass
    :param mappings: list activation layers to smooth, and which layers to
        scale the output such that activations are smoothed.
        Each entry of the mapping list should be a list itself, in which the first
        entry is a list of layers who share the same input activation (the one to be
        to smoothed) and the second entry is the layer whose output is scaled to
        achieve the smoothing.
        If regex is used, it matches layers with the largest overlap in module name.
        Each mapping may also include an ``activation_hook_target``: a dotted
        attribute path relative to the parent module (lowest common ancestor)
        specifying which submodule to hook for activation caching. This is useful
        for parallel transformer blocks where the default (hooking
        ``balance_layers[0]``) would capture the wrong activations.
    :param ignore: list of layers to ignore during quantization (not smoothed).
        It should match the name of layers whose outputs are scaled to achieve
        smoothing (the second entry of the mappings list).
    :param offload_device: offload cached args to this device, which reduces memory
        requirements but requires more time to move data between cpu and execution
        device. Defaults to None, so cached args are not offloaded. Consider setting
        to torch.device("cpu") if you are encountering OOM errors
    :param duo_scaling: whether to use duo scaling, which uses both input activations
        and weights to determine the scaling factor. Defaults to True
        If True, both activations and weights are used.
        If False, only activations are used.
        If "both", half the grid search is performed with duo_scaling=False and the
        other half is performed with duo_scaling=True.
    :param n_grid: when performing the best scales grid search for each mapping,
        this specifies how many grid points should be used. To decrease the runtime,
        at the possible cost of slightly worse scales, this can be decreased.
        Defaults to 20
    """

    # Allow arbitrary types because AWQMapping has fields of type torch.nn.Module
    model_config: ConfigDict = ConfigDict(arbitrary_types_allowed=True)

    # User-provided vars (in addition to QuantizationMixin args)
    sequential_targets: str | list[str] | None = None
    mappings: list[AWQMapping] | None = None
    offload_device: torch.device | None | Sentinel = Sentinel("not_provided")
    duo_scaling: bool | Literal["both"] = True
    n_grid: int = 20

    # Private vars set during initialization, cleared during finalization
    _resolved_mappings: list[ResolvedMapping] = PrivateAttr(default_factory=list)
    # Cache list of forward input args for each parent module, one dict for each batch
    _parent_args_cache: dict[Module, IntermediatesCache] = PrivateAttr(
        default_factory=dict
    )
    # Dict[smooth layer name, [activation sums, activation counts]]
    _smooth_activation_stats: dict[str, list[torch.Tensor]] = PrivateAttr(
        default_factory=dict
    )
    # List to store error metrics for each layer
    _error_metrics: list[dict] = PrivateAttr(default_factory=list)

    def on_initialize(self, state: State, **kwargs) -> bool:
        """
        Initialize AWQ on the given state
        Initialize quantization, resolve mappings, cache module kwargs

        :param state: state to run AWQ on
        :return: True on a successful run, False otherwise
        """

        # apply config to model and prepare calibration hooks
        if QuantizationMixin.has_config(self):
            QuantizationMixin.initialize_quantization(self, state.model)

        # Validate that duo_scaling is only used with per-channel quantization
        if self.duo_scaling is not False:
            for _, module in match_named_modules(
                state.model, self.resolved_targets, self.ignore
            ):
                if (
                    hasattr(module, "quantization_scheme")
                    and hasattr(module.quantization_scheme, "weights")
                    and module.quantization_scheme.weights.strategy
                    == QuantizationStrategy.TENSOR
                ):
                    raise ValueError(
                        "duo_scaling is only supported with per-channel quantization "
                        "strategies (group or channel), but found TENSOR strategy. "
                        "Please set duo_scaling=False or use a per-channel "
                        "quantization strategy."
                    )

        if self.mappings is None:
            logger.info("No AWQModifier.mappings provided, inferring from model...")
            self.mappings = get_layer_mappings_from_architecture(
                architecture=state.model.__class__.__name__
            )

        # Set default offload_device
        if self.offload_device == Sentinel("not_provided"):
            # Check if we have a MoE model
            if is_moe_model(state.model):
                self.offload_device = torch.device("cpu")
                logger.info(
                    "MoE model detected: setting offload_device to 'cpu' by default "
                    "to reduce memory usage. You can override this by explicitly "
                    "setting offload_device in your recipe."
                )
            else:
                # For non-MoE models, convert sentinel to None
                # (no offloading by default)
                self.offload_device = None

        self._set_resolved_mappings(state.model)

        return True

    def on_start(self, state: State, event: Event, **kwargs):
        self.started_ = True

        # Check for unsupported token masking with MoE up_proj -> down_proj mappings
        if state.loss_masks is not None and self._has_moe_up_down_proj_mapping():
            raise ValueError(
                "Token masking (use_loss_mask=True) is not supported with "
                "up_proj -> down_proj mappings in MoE models. The MoE routing "
                "mechanism dispatches tokens to different experts, and the loss mask "
                "cannot be properly aligned with this dispatch. Please either "
                "disable token masking or exclude the up_proj -> down_proj mapping "
                "for MoE layers from the AWQ configuration."
            )

        # register quantization calibration hooks
        # assume quantization has been initialized by this modifier or one before it
        QuantizationMixin.start_calibration(self, state.model)
        # AWQ performs forward passes during _apply_smoothing
        # before any scales or zero points are updated
        # Quantization must be disabled, otherwise NaNs will
        # appear in quantized forward method
        state.model.apply(disable_quantization)

        self._setup_activation_cache_hooks()

    def on_event(self, state: State, event: Event, **kwargs):
        if event.type_ == EventType.CALIBRATION_EPOCH_START:
            if not self.started_:
                self.on_start(state, None)

        elif event.type_ == EventType.SEQUENTIAL_EPOCH_END:
            # Run smoothing in case of sequential pipeline
            QuantizationMixin.sync_activation_observers(self, state.model)
            self._apply_smoothing(state.model)

        elif event.type_ == EventType.CALIBRATION_EPOCH_END:
            # Run smoothing in case of basic pipeline
            QuantizationMixin.sync_activation_observers(self, state.model)
            self._apply_smoothing(state.model)

            if not self.ended_:
                self.on_end(state, None)

    def on_end(self, state: State, event: Event, **kwargs):
        """
        Finish calibrating by setting scales and zero-points,
         removing observers and calibration hooks
        """
        self._assert_all_activations_consumed()

        self.ended_ = True

        named_modules = list(
            match_named_modules(state.model, self.resolved_targets, self.ignore)
        )

        # For TENSOR_GROUP (nvfp4), calculate global scales after smoothing
        for _, module in tqdm(named_modules, desc="Updating global scales"):
            update_weight_global_scale(module)

        # For TENSOR_GROUP (nvfp4), fuse global scales for attention and MLP layers
        # This is a requirement for vLLM inference.
        for module in tqdm(state.model.modules(), desc="Fusing global scales"):
            update_fused_layer_weight_global_scales(module)

        # Calculate scales and zero points using the fused global scales
        for _, module in tqdm(named_modules, desc="Calibrating weights"):
            update_weight_zp_scale(module)

        QuantizationMixin.end_calibration(self, state.model)

        # remove activation hooks
        self.remove_hooks()

    def on_finalize(self, state: State, **kwargs) -> bool:
        """
        Clean up by clearing the activations and mapping data

        :param state: unused
        :return: True
        """
        if not self.ended_:
            self.on_end(state, None)

        self._log_error_metrics()

        self._parent_args_cache.clear()
        self._smooth_activation_stats.clear()
        self._resolved_mappings.clear()
        self._error_metrics.clear()

        return True

    def _set_resolved_mappings(self, model: Module) -> None:
        """
        Transforms the list of activations to smooth and their corresponding weights
        into ResolvedMapping objects, resolving regular expressions.
        Result is stored in _resolved_mappings.

        For each activation in the mapping list, we find the corresponding weight to
        balance by searching for the longest substring. For instance, if our balance
        weight is ".*re:.*q_proj" and the activation is "re:.*self_attn_layer_norm" we
        would match model.layer.0.p_proj to model.layer.0.self_attn_layer_norm and
        repeat for model.layer.1 and so on
        """
        resolved_mappings: list[ResolvedMapping] = []
        module_to_name = get_module_to_name_dict(model)
        # Get names of modules targeted for quantization (excludes ignored)
        targeted_names = set(
            name
            for name, _ in match_named_modules(
                model, self.resolved_targets, self.ignore
            )
        )
        for mapping in self.mappings:
            # we deliberately don't use the ignore list when matching mappings,
            # so that we can handle layers that need smoothing but not quantization
            # we only skip if no layers in mapping are targeted for quantization.
            for smooth_layers, *nested_balance_layers in match_modules_set(
                model, (mapping.smooth_layer, *mapping.balance_layers)
            ):
                if len(smooth_layers) > 1:
                    raise ValueError(
                        "AWQ needs to match a single smoothlayer for each mapping but "
                        f"got {[module_to_name.get(s) for s in smooth_layers]}"
                        f" for mapping: {mapping}"
                    )
                smooth_layer = smooth_layers[0]
                smooth_name = module_to_name.get(smooth_layer)

                # [[b00, b01, b02...], [b10, b11, b12,...], ...] ↓
                #                             [b00, b01, b02, ..., b10, b11, b12, ...]
                balance_layers = tree_leaves(nested_balance_layers)
                balance_names = [
                    module_to_name.get(balance_layer)
                    for balance_layer in balance_layers
                ]

                # Check if at least one layer is targeted for quantization
                any_targeted = smooth_name in targeted_names or any(
                    bn in targeted_names for bn in balance_names
                )

                all_compatible = _check_layers_are_compatible(
                    smooth_layer, smooth_name, balance_layers, balance_names
                )

                skip_message: str | None = None
                if not all_compatible:
                    skip_message = " because found incompatible balance layers"
                elif not any_targeted:
                    skip_message = " because no layers are targeted for quantization"
                elif len(balance_layers) == 0:
                    skip_message = " because no balance layers were found"

                if skip_message:
                    logger.warning(
                        f"skipping AWQ for {smooth_name} for mapping {mapping}"
                        + skip_message
                    )

                    continue

                ancestor_name, ancestor = get_lowest_common_ancestor_with_avoid(
                    balance_names, model, torch.nn.ModuleList
                )

                activation_hook_target = None
                if mapping.activation_hook_target:
                    activation_hook_target = getattr_chain(
                        ancestor, mapping.activation_hook_target
                    )
                    if activation_hook_target is None:
                        raise ValueError(
                            f"activation_hook_target '{mapping.activation_hook_target}'"
                            f" not found on parent module '{ancestor_name}'"
                        )

                resolved_mappings.append(
                    ResolvedMapping(
                        smooth_name,
                        smooth_layer,
                        balance_layers,
                        balance_names=balance_names,
                        parent=ancestor,
                        parent_name=ancestor_name,
                        activation_hook_target=activation_hook_target,
                    )
                )
        self._resolved_mappings = resolved_mappings
        return

    def _setup_activation_cache_hooks(self) -> None:
        """
        Attach a forward hook to each activation we want to smooth. This allows us to
        calculate the dynamic range during calibration
        """

        def cache_parent_kwargs_hook(
            module: Module,
            args: tuple[torch.Tensor, ...],
            kwargs,
        ):
            values = inspect.signature(module.forward).bind(*args, **kwargs)
            self._parent_args_cache[module].append(values.arguments)

        def create_cache_smooth_activations_hook_fn(smooth_name):
            def cache_smooth_activations_hook(
                _module: Module,
                args: tuple[torch.Tensor, ...],
                _output: torch.Tensor,
            ):
                activations = args[0].abs().detach()

                # Get loss mask for current batch from state
                session = active_session()
                state = session.state
                loss_masks = state.loss_masks if state else None
                batch_idx = state.current_batch_idx if state else -1
                loss_mask = (
                    loss_masks[batch_idx] if loss_masks and batch_idx >= 0 else None
                )

                if loss_mask is not None:
                    # Mask: [batch, seq] -> [batch, seq, 1]
                    mask = loss_mask.to(activations.device).unsqueeze(-1)
                    flat_activations = activations.flatten(0, -2)  # [batch*seq, hidden]
                    flat_mask = mask.flatten(0, -2).squeeze(-1)
                    masked_activations = flat_activations[flat_mask.bool()]
                else:
                    masked_activations = activations.flatten(0, -2)

                # accumulate activation sum&count
                new_sum = masked_activations.float().sum(dim=0).cpu()
                new_count = torch.tensor(masked_activations.size(0)).cpu()
                if smooth_name not in self._smooth_activation_stats:
                    self._smooth_activation_stats[smooth_name] = [
                        torch.zeros_like(new_sum),
                        torch.zeros_like(new_count),
                    ]
                self._smooth_activation_stats[smooth_name][0] += new_sum
                self._smooth_activation_stats[smooth_name][1] += new_count

            return cache_smooth_activations_hook

        for mapping in self._resolved_mappings:
            # parent kwargs needed for future forward passes
            # same parent may appear multiple times in resolved mappings
            if mapping.parent not in self._parent_args_cache:
                self._parent_args_cache[mapping.parent] = IntermediatesCache(
                    None,
                    self.offload_device,
                )
                self.register_hook(
                    mapping.parent,
                    cache_parent_kwargs_hook,
                    "forward_pre",
                    with_kwargs=True,
                )

            # input activations to balance layers needed for loss function
            # storing inputs to first balance layer is sufficient
            # other balance layers get the same input
            #
            # For parallel transformer blocks (e.g. Command A, Gemma 3) the first
            # balance layer may not receive the right activations.  When
            # activation_hook_target is set on the mapping, hook that module
            # instead of balance_layers[0].
            layer_to_hook = mapping.activation_hook_target or mapping.balance_layers[0]
            self.register_hook(
                layer_to_hook,
                create_cache_smooth_activations_hook_fn(mapping.smooth_name),
                "forward",
            )

    @torch.no_grad()
    def _apply_smoothing(self, model: Module) -> None:
        """
        Calculate the best scaling factors for each layer to smooth activations and
        apply the scaling factors to the weights of the next layer to offset the
        smoothing

        :param model: model to apply smoothing to
        """
        # NOTE: When using SequentialPipeline, not all the mappings
        # will have cached activations in the segment being updated
        mappings_to_smooth = [
            mapping
            for mapping in self._resolved_mappings
            if mapping.smooth_name in self._smooth_activation_stats
        ]
        for mapping in tqdm(mappings_to_smooth, desc="Smoothing"):
            smooth_layer = mapping.smooth_layer
            balance_layers = mapping.balance_layers
            parent_module = mapping.parent

            with (
                align_modules([parent_module, smooth_layer, *balance_layers]),
                calibration_forward_context(model),
                HooksMixin.disable_hooks(),
            ):
                # Compute output of unquantized module
                fp16_outputs = self._run_samples(parent_module)
                if len(fp16_outputs) == 0 or all(f.numel() == 0 for f in fp16_outputs):
                    logger.info(
                        f"Skipping smooth_layer {mapping.smooth_name}, no activations "
                        "found to scale. This can occasionally occur in MoE models "
                        "when certain experts are not activated by calibration samples."
                    )
                    del self._smooth_activation_stats[mapping.smooth_name]
                    continue
                if not all(
                    [fp16_output.isfinite().all() for fp16_output in fp16_outputs]
                ):
                    logger.warning(
                        f"Skipping smooth_layer {mapping.smooth_name}, NaN or inf "
                        "outputs found during forward pass of the parent module "
                        f"{mapping.parent_name}. The model is either generating NaN "
                        "output with provided calibration data set, or the mappings "
                        "are incorrectly set and modifying the model in undesired "
                        "ways. If you encounter this consistently, raise an issue at "
                        "https://github.com/vllm-project/llm-compressor/issues"
                    )
                    del self._smooth_activation_stats[mapping.smooth_name]
                    continue

                orig_layer_weights = {
                    balance_layer: balance_layer.weight.clone()
                    for balance_layer in mapping.balance_layers
                }

                best_scales = self._compute_best_scale(
                    mapping, fp16_outputs, orig_layer_weights
                )

                @torch.no_grad()
                def _smooth(
                    module: Module, orig_layer_weights: dict[Module, torch.Tensor]
                ):
                    scales = best_scales.to(module.weight.device)
                    if module in balance_layers:
                        update_offload_parameter(
                            module,
                            "weight",
                            orig_layer_weights[module].to(module.weight.device)
                            * scales.view(1, -1),
                        )
                    elif module == smooth_layer:
                        if module.weight.ndim == 1:
                            update_offload_parameter(
                                module,
                                "weight",
                                module.weight.div_(scales),
                            )
                        else:
                            # NOTE: edge case when smooth layer number of out_features
                            # is not equal to balance layer number of in_features
                            # e.g. when fused qkv_proj is used to smooth o_proj
                            # in this case, default to scaling the last output features
                            # because the desired smooth layer is v_proj
                            # https://github.com/casper-hansen/AutoAWQ/blob/main/awq/quantize/scale.py#L123
                            weight = module.weight
                            weight[-scales.size(0) :].div_(scales.view(-1, 1))
                            update_offload_parameter(module, "weight", weight)
                        if hasattr(module, "bias") and module.bias is not None:
                            update_offload_parameter(
                                module,
                                "bias",
                                module.bias.div_(scales),
                            )

                for layer in balance_layers:
                    _smooth(layer, orig_layer_weights)
                _smooth(smooth_layer, orig_layer_weights)

                # remove caches needed to smooth this mapping
                del self._smooth_activation_stats[mapping.smooth_name]
                del orig_layer_weights

        for v in self._parent_args_cache.values():
            v.batch_intermediates.clear()
        self._assert_all_activations_consumed()

    @torch.no_grad()
    def _run_samples(self, module: Module) -> list[torch.Tensor]:
        cache = self._parent_args_cache[module]
        use_prefetch = active_session().state.sequential_prefetch
        batch_iter = cache.iter_prefetch() if use_prefetch else cache
        outputs = [module(**batch_kwargs) for batch_kwargs in batch_iter]
        return [
            # If tuple, assume that first argument is the input
            output[0] if isinstance(output, tuple) else output
            for output in outputs
        ]

    def _compute_best_scale(
        self,
        mapping: ResolvedMapping,
        fp16_outputs: list[torch.Tensor],
        orig_layer_weights: dict[torch.nn.Module, torch.Tensor],
    ) -> torch.Tensor:
        """
        Select best scales for a given mapping in a grid search
        Best scales are those that minimize MSE loss of quantized weight
            outputs compared to fp16_outputs

        L(s) = || Q(W * s) (s^-1 * X) - W * X ||
        Q: weight quantization function | _pseudo_quantize_tensor(W * s)
        X: inputs from calib dataset    | X
        W: original weights in FP16     | layer
        s: per channel scaling factor   | s^-1 * X

        :param mapping: best scales will be found for the ResolvedMapping.
        :param fp16_outputs: output of mapping.parent in unquantized case,
            one tensor for each batch.
        :return: tensor of best scales, one for each channel
        """
        history = []
        best_ratio = -1
        best_scales = None
        best_error = float("inf")
        initial_error = None

        device = get_execution_device(mapping.parent)

        x_sum, count = self._smooth_activation_stats[mapping.smooth_name]
        if is_distributed():
            x_sum, count = _allreduce_data_sum([x_sum, count])
        x_mean = x_sum.to(device) / count.to(device)

        if self.duo_scaling:
            w_mean = self._compute_layer_means(mapping.balance_layers).to(device)

        match self.duo_scaling:
            # if self.duo_scaling is "both", perform half the grid search with
            # duo_scaling off and half with duo_scaling on
            case "both":
                n_grid = int(self.n_grid / 2)
                duo_scalings = [False, True]
            case _:
                n_grid = self.n_grid
                duo_scalings = [self.duo_scaling]

        # Where appropriate, replace observers with memoryless_minmax
        # for duration of grid search
        balance_layers_to_patch = [
            balance_layer
            for balance_layer in mapping.balance_layers
            if hasattr(balance_layer, "quantization_scheme")
            and hasattr(balance_layer.quantization_scheme, "weights")
        ]
        with patch_attrs(
            balance_layers_to_patch,
            "weight_observer",
            [
                Observer.load_from_registry(
                    "memoryless_minmax",
                    base_name="weight",
                    args=balance_layer.quantization_scheme.weights,
                    module=balance_layer,
                )
                for balance_layer in balance_layers_to_patch
            ],
        ):
            total_iterations = n_grid * len(duo_scalings)
            pbar = tqdm(
                product(range(n_grid), duo_scalings),
                total=total_iterations,
                desc=f"Grid search for {mapping.smooth_name}",
                leave=False,
            )
            for grid_idx, use_duo_scaling in pbar:
                # create new scales
                ratio = grid_idx / n_grid

                # NOTE: s^-1 * x is fused here, according to paper
                if use_duo_scaling:
                    scales = (x_mean.pow(ratio) / (w_mean.pow(1 - ratio) + 1e-4)).clamp(
                        min=1e-4
                    )
                else:
                    scales = x_mean.pow(ratio).clamp(min=1e-4).view(-1)
                scales = scales / (scales.max() * scales.min()).sqrt()
                scales[torch.isinf(scales)] = 1
                scales[torch.isnan(scales)] = 1
                _scalesview = scales.view(1, -1).to(device)

                # Q(W * s)
                for balance_layer in balance_layers_to_patch:
                    if not hasattr(balance_layer, "quantization_scheme") or not hasattr(
                        balance_layer.quantization_scheme, "weights"
                    ):
                        continue

                    w_qscheme = balance_layer.quantization_scheme.weights
                    balance_layer.weight.data.copy_(
                        orig_layer_weights[balance_layer].to(_scalesview.device)
                        * _scalesview
                    )

                    should_calculate_gparam = (
                        w_qscheme.strategy == QuantizationStrategy.TENSOR_GROUP
                    )
                    call_observer(
                        balance_layer,
                        "weight",
                        balance_layer.weight,
                        should_calculate_gparam=should_calculate_gparam,
                    )
                    balance_layer.weight.data = (
                        forward_quantize(
                            balance_layer,
                            balance_layer.weight,
                            "weight",
                            w_qscheme,
                        )
                        / _scalesview
                    ).to(balance_layer.weight.dtype)

                # Apply fused global scales for TENSOR_GROUP during grid search
                # to match inference behavior
                if balance_layers_to_patch and all(
                    getattr(layer.quantization_scheme.weights, "strategy", None)
                    == QuantizationStrategy.TENSOR_GROUP
                    for layer in balance_layers_to_patch
                ):
                    update_fused_layer_weight_global_scales(mapping.parent)

                # W * X
                int_w_outputs = self._run_samples(mapping.parent)

                # compute mean squared error (L2 norm)
                loss = self._compute_loss(fp16_outputs, int_w_outputs)
                del int_w_outputs

                if initial_error is None:
                    initial_error = loss

                history.append(
                    {"ratio": ratio, "duo_scaling": use_duo_scaling, "error": loss}
                )
                if loss < best_error:
                    best_error = loss
                    best_ratio = ratio
                    best_scales = scales.clone()
                pbar.set_postfix({"best_error": f"{best_error:.3e}"})

        if best_ratio == -1:
            logger.debug(history)
            raise Exception(
                "No finite loss was found in best scalesgrid search. This typically "
                "means NaN values are appearing in the forward pass of the parent "
                "module. If you encounter this error, raise an issue at "
                "https://github.com/vllm-project/llm-compressor/issues"
            )

        err_reduction = best_error / initial_error if initial_error > 0 else 1.0
        logger.debug(
            f"AWQ grid search for {mapping.smooth_name}: "
            f"initial error = {initial_error:.3e}, "
            f"best error = {best_error:.3e}, "
            f"error reduction rate (best/initial) = {err_reduction * 100:.3f}%"
        )

        # Store error metrics for this layer
        self._error_metrics.append(
            {
                "layer_name": mapping.smooth_name,
                "parent_name": mapping.parent_name,
                "initial_error": initial_error,
                "best_error": best_error,
                "reduction": err_reduction,
            }
        )

        assert (
            torch.isnan(best_scales).sum() == 0
        ), f"Nan found in scales: {best_scales}"

        return best_scales.detach().cpu()

    @torch.no_grad()
    def _compute_loss(
        self,
        fp16_outputs: list[torch.Tensor],
        int_w_outputs: list[torch.Tensor],
    ) -> float:
        session = active_session()
        loss_masks = session.state.loss_masks if session.state else None

        device = fp16_outputs[0].device
        loss = torch.tensor(0.0, device=device)
        num_elements = torch.tensor(0, device=device)

        # Compute the MSE loss for each batch
        for batch_idx, (fp16_batch, int_w_batch) in enumerate(
            zip(fp16_outputs, int_w_outputs)
        ):
            loss_mask = loss_masks[batch_idx] if loss_masks else None

            if loss_mask is not None:
                token_mask = loss_mask.to(fp16_batch.device) == 1  # (batch, seq)
                fp16_masked = fp16_batch[token_mask]  # (num_masked_tokens, hidden)
                int_w_masked = int_w_batch.to(fp16_batch.device)[token_mask]
                loss += torch.nn.functional.mse_loss(
                    fp16_masked, int_w_masked, reduction="sum"
                )
                num_elements += fp16_masked.numel()
            else:
                loss += torch.nn.functional.mse_loss(
                    fp16_batch, int_w_batch.to(fp16_batch.device), reduction="sum"
                )
                num_elements += fp16_batch.numel()

        if is_distributed():
            loss, num_elements = _allreduce_data_sum([loss, num_elements])
        # Normalize the loss by the total number of elements
        return (loss / num_elements).item()

    def _log_error_metrics(self):
        """
        Log the error metrics (initial error, best error, reduction).
        """

        # Prepare data for saving
        metrics_data = {
            "quantization_config": {
                "duo_scaling": self.duo_scaling,
                "n_grid": self.n_grid,
            },
            "total_layers": len(self._error_metrics),
            "metrics": self._error_metrics,
        }

        # Save to disk
        logger.debug(f"AWQ per-mapping error metrics: {metrics_data}")

        # Also print summary statistics
        reductions = [m["reduction"] for m in self._error_metrics]
        avg_reduction = sum(reductions) / len(reductions)
        min_reduction = min(reductions)
        max_reduction = max(reductions)
        sorted_reductions = sorted(reductions)
        median_reduction = sorted_reductions[len(sorted_reductions) // 2]
        logger.debug(
            f"Error reduction statistics: "
            f"avg={avg_reduction:.4f}, median={median_reduction:.4f}, "
            f"min={min_reduction:.4f}, max={max_reduction:.4f}"
        )

    def _assert_all_activations_consumed(self):
        """
        Confirm all activations have been consumed
        If not, something has gone wrong
        """
        if len(self._smooth_activation_stats) != 0:
            raise RuntimeError("Some cached activations were not used")

    def _has_moe_up_down_proj_mapping(self) -> bool:
        """
        Check if any resolved mapping is an up_proj -> down_proj mapping
        where the balance layers are MoE experts (indicated by '.experts.'
        in the name).

        Token masking is not supported for such mappings because the MoE
        routing mechanism dispatches tokens to different experts, and the
        loss mask cannot be properly aligned with this dispatch.
        """
        for mapping in self._resolved_mappings:
            # Check if this is an up_proj -> down_proj mapping
            if mapping.smooth_name.endswith("up_proj"):
                for balance_name in mapping.balance_names:
                    if (
                        balance_name.endswith("down_proj")
                        and ".experts." in balance_name
                    ):
                        return True
        return False

    @staticmethod
    def _compute_layer_means(layers: list[Module]) -> torch.Tensor:
        """
        Compute per-channel/group/block/tensor mean of normalised weights
        for all passed in layers taking into account the quantization_scheme.

        To minimize memory requirements, layers are reduced to a running total
            of sums and counts when calculating mean
        """
        # to calculate mean without having to carry full population
        weight_total_count = 0
        weight_total_sum = 0

        for layer in layers:
            if not hasattr(layer, "weight"):
                logger.warning(
                    "Unable to find weight param for targeted"
                    f" layer {type(layer)}, skipping"
                )
                continue
            weight = layer.weight.clone()
            orig_shape = weight.shape

            q_args = getattr_chain(layer, "quantization_scheme.weights", None)
            if not q_args:
                logger.warning(
                    "Unable to find quantization scheme for "
                    f"targeted layer {type(layer)}, skipping"
                )
                continue

            match q_args.strategy:
                # chunk size is the size of the size of the
                # set of elements that get quantized together
                case QuantizationStrategy.TENSOR:
                    chunk_size = weight.numel()
                case QuantizationStrategy.CHANNEL:
                    chunk_size = weight.size(1)
                case QuantizationStrategy.GROUP | QuantizationStrategy.TENSOR_GROUP:
                    chunk_size = q_args.group_size
                case QuantizationStrategy.BLOCK:
                    block_height, block_width = q_args.block_structure
                    weight = (  # (row, col) = (num_H*block_H, num_W*block_W)
                        weight.unflatten(0, (-1, block_height))
                        .unflatten(-1, (-1, block_width))
                        .transpose(1, 2)  # ↳ (num_H, num_W, block_H, block_W)
                    )
                    intermediate_shape = weight.shape
                    chunk_size = block_height * block_width

            # need to get to shape (num_chunks x chunk_size)
            weight = weight.reshape(-1, chunk_size)
            # normalize
            weight.abs_()
            weight.div_(weight.amax(dim=1, keepdim=True) + 1e-6)
            # Reshape back to original dimensions
            if q_args.strategy == QuantizationStrategy.BLOCK:
                weight = weight.view(intermediate_shape).transpose(1, 2)

            # back to (rows, cols)
            weight = weight.reshape(orig_shape)
            # Gets the average rescaled magnitude for each output channel
            weight_total_count += weight.size(0)
            weight_sum = weight.sum(0, dtype=torch.float64)
            weight_total_sum += weight_sum

        return weight_total_sum / weight_total_count

    @field_validator("duo_scaling")
    @classmethod
    def validate_duo_scaling(cls, v):
        """Validate that duo_scaling is either True, False, or 'both' (lowercase)"""
        if v not in (True, False, "both"):
            raise ValueError(f"duo_scaling must be True, False, or 'both', got {v!r}")
        return v


def _check_layers_are_compatible(
    smooth_layer, smooth_name, balance_layers, balance_names
):
    """
    returns True if they are all compatible
    returns False if any smooth & balance layers are incompatible
    """
    for balance_layer, balance_name in zip(balance_layers, balance_names):
        # exclude v_proj->o_proj mappings whose shapes are incompatible
        # https://github.com/mit-han-lab/llm-awq/pull/67#issuecomment-1681632777
        if (
            isinstance(smooth_layer, torch.nn.Linear)
            and isinstance(balance_layer, torch.nn.Linear)
            and balance_name.endswith(".o_proj")
            and (
                (
                    smooth_name.endswith(".v_proj")
                    and smooth_layer.out_features != balance_layer.in_features
                )
                or (
                    smooth_name.endswith(".qkv_proj")
                    and smooth_layer.out_features != 3 * balance_layer.in_features
                )
            )
        ):
            return False
    return True


def get_lowest_common_ancestor_with_avoid(
    balance_names: Iterator[str], model: Module, avoid=torch.nn.ModuleList
):
    """
    Get the lowest ancestor that is not the avoided class/type.
    see compressed_tensors.utils.get_lowest_common_ancestor_name
    for detail on case handling.

    NOTE: primarily used to exclude parents of type ModuleList, which don't play
    nicely with hooks because their forward method is never directly
    called for MoE models. See Qwen3MoeSparseMoeBlock for example, experts
    are selected based on router output and their forward method is called.
    https://github.com/huggingface/transformers/blob/v4.52.4/src/transformers/models/qwen3_moe/modeling_qwen3_moe.py#L233
    """
    ancestor_name = get_lowest_common_ancestor_name(balance_names)

    while True:
        if ancestor_name == "":
            return "", model
        ancestor = model.get_submodule(ancestor_name)
        if not isinstance(ancestor, avoid):
            return ancestor_name, ancestor
        ancestor_name = ".".join(ancestor_name.split(".")[:-1])


def _allreduce_data_sum(data: list[torch.Tensor]) -> list[torch.Tensor]:
    # needs to be on device to broadcast
    device = torch.device(f"cuda:{torch.cuda.current_device()}")
    data = [datum.to(device) for datum in data]

    pending_comms = []
    for datum in data:
        pending_comms.append(
            dist.all_reduce(
                as_broadcastable(datum), op=dist.ReduceOp.SUM, async_op=True
            )
        )
    wait_for_comms(pending_comms)
    return data