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quantize.py
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quantize.py
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# Copyright (c) Meta Platforms, Inc. and affiliates.
# All rights reserved.
# This source code is licensed under the license found in the
# LICENSE file in the root directory of this source tree.
import time
from pathlib import Path
import torch
import torch.nn as nn
import torch.nn.functional as F
from model import Transformer, ConditionalFeedForward
##### Quantization Primitives ######
def dynamically_quantize_per_channel(x, quant_min, quant_max, target_dtype):
# assumes symmetric quantization
# assumes axis == 0
# assumes dense memory format
# TODO(future): relax ^ as needed
# default setup for affine quantization of activations
eps = torch.finfo(torch.float32).eps
# get min and max
min_val, max_val = torch.aminmax(x, dim=1)
# calculate scales and zero_points based on min and max
# reference: https://fburl.com/code/srbiybme
min_val_neg = torch.min(min_val, torch.zeros_like(min_val))
max_val_pos = torch.max(max_val, torch.zeros_like(max_val))
device = min_val_neg.device
# reference: https://fburl.com/code/4wll53rk
max_val_pos = torch.max(-min_val_neg, max_val_pos)
scales = max_val_pos / (float(quant_max - quant_min) / 2)
# ensure scales is the same dtype as the original tensor
scales = torch.clamp(scales, min=eps).to(x.dtype)
zero_points = torch.zeros(min_val_neg.size(), dtype=torch.int64, device=device)
# quantize based on qmin/qmax/scales/zp
# reference: https://www.internalfb.com/code/fbsource/[8edc275012b1]/fbcode/caffe2/torch/ao/quantization/fx/_decomposed.py?lines=63
x_div = x / scales.unsqueeze(-1)
x_round = torch.round(x_div)
x_zp = x_round + zero_points.unsqueeze(-1)
quant = torch.clamp(x_zp, quant_min, quant_max).to(target_dtype)
return quant, scales, zero_points
##### Weight-only int8 per-channel quantized code ######
def replace_linear_weight_only_bit8_per_channel(module, target_dtype):
for name, child in module.named_children():
if isinstance(child, nn.Linear) and name != "gate":
setattr(module, name, WeightOnlyBit8Linear(child.in_features, child.out_features, target_dtype=target_dtype))
elif isinstance(child, ConditionalFeedForward):
num_experts, intermediate_size, dim = child.w1.shape
setattr(module, name, ConditionalFeedForwardBit8(num_experts, intermediate_size, dim, target_dtype=target_dtype))
else:
replace_linear_weight_only_bit8_per_channel(child, target_dtype)
class WeightOnlyBit8QuantHandler:
def __init__(self, mod, target_dtype):
self.mod = mod
self.target_dtype = target_dtype
@torch.no_grad()
def create_quantized_state_dict(self):
cur_state_dict = self.mod.state_dict()
for fqn, mod in self.mod.named_modules():
if isinstance(mod, torch.nn.Linear) and not fqn.endswith(".gate"):
int8_weight, scales, _ = dynamically_quantize_per_channel(mod.weight.float(), -128, 127, self.target_dtype)
cur_state_dict[f"{fqn}.weight"] = int8_weight
cur_state_dict[f"{fqn}.scales"] = scales.to(mod.weight.dtype)
elif isinstance(mod, ConditionalFeedForward):
for weight_idx in range(0, 3):
weight_name = f"w{weight_idx + 1}"
scales_name = f"scales{weight_idx + 1}"
weight = getattr(mod, weight_name)
num_experts, intermediate_size, dim = weight.shape
bit8_weight_list = []
scales_list = []
for expert_idx in range(num_experts):
bit8_weight, scales, _ = dynamically_quantize_per_channel(weight[expert_idx].float(), -128, 127, self.target_dtype)
bit8_weight_list.append(bit8_weight.reshape(1, intermediate_size, dim))
scales_list.append(scales.reshape(1, intermediate_size))
cur_state_dict[f"{fqn}.{weight_name}"] = torch.cat(bit8_weight_list, dim=0)
cur_state_dict[f"{fqn}.{scales_name}"] = torch.cat(scales_list, dim=0)
return cur_state_dict
def convert_for_runtime(self):
replace_linear_weight_only_bit8_per_channel(self.mod, self.target_dtype)
return self.mod
class WeightOnlyBit8Linear(torch.nn.Module):
__constants__ = ['in_features', 'out_features']
in_features: int
out_features: int
weight: torch.Tensor
def __init__(self, in_features: int, out_features: int, bias: bool = True,
device=None, dtype=None, target_dtype=None) -> None:
assert target_dtype is not None
factory_kwargs = {'device': device, 'dtype': dtype}
super().__init__()
self.in_features = in_features
self.out_features = out_features
self.register_buffer("weight", torch.empty((out_features, in_features), dtype=target_dtype))
self.register_buffer("scales", torch.ones(out_features, dtype=torch.bfloat16))
def forward(self, input: torch.Tensor) -> torch.Tensor:
return F.linear(input, self.weight.to(dtype=input.dtype)) * self.scales
class ConditionalFeedForwardBit8(nn.Module):
def __init__(self, num_experts, intermediate_size, dim, target_dtype):
super().__init__()
self.target_dtype = target_dtype
self.register_buffer("w1", torch.empty(num_experts, intermediate_size, dim, dtype=target_dtype))
self.register_buffer("w2", torch.empty(num_experts, dim, intermediate_size, dtype=target_dtype))
self.register_buffer("w3", torch.empty(num_experts, intermediate_size, dim, dtype=target_dtype))
self.register_buffer("scales1", torch.empty(num_experts, intermediate_size, dtype=torch.bfloat16))
self.register_buffer("scales2", torch.empty(num_experts, dim, dtype=torch.bfloat16))
self.register_buffer("scales3", torch.empty(num_experts, intermediate_size, dtype=torch.bfloat16))
def forward(self, x, expert_indices):
w1_weights = self.w1.to(x.dtype)[expert_indices] # [T, A, D, D]
w3_weights = self.w3.to(x.dtype)[expert_indices] # [T, A, D, D]
w2_weights = self.w2.to(x.dtype)[expert_indices]
x1 = F.silu(torch.einsum('ti,taoi -> tao', x, w1_weights) * self.scales1[expert_indices].to(x.dtype))
x3 = torch.einsum('ti, taoi -> tao', x, w3_weights) * self.scales3[expert_indices].to(x.dtype)
expert_outs = torch.einsum('tao, taio -> tai', (x1 * x3), w2_weights) * self.scales2[expert_indices].to(x.dtype) # [T, A, D, D]
return expert_outs
def quantize(
checkpoint_path: Path = Path("checkpoints/mistralai/Mixtral-8x7B-v0.1/model.pth"),
mode: str = 'int8',
label: str = '',
) -> None:
assert checkpoint_path.is_file(), checkpoint_path
device = 'cpu'
precision = torch.bfloat16
print("Loading model ...")
t0 = time.time()
with torch.device('meta'):
model = Transformer.from_name(checkpoint_path.parent.name)
checkpoint = torch.load(str(checkpoint_path), mmap=True, weights_only=True)
model.load_state_dict(checkpoint, assign=True)
model = model.to(dtype=precision, device=device)
if mode == 'int8':
print("Quantizing model weights for int8 weight-only symmetric per-channel quantization")
quant_handler = WeightOnlyBit8QuantHandler(model, target_dtype=torch.int8)
quantized_state_dict = quant_handler.create_quantized_state_dict()
dir_name = checkpoint_path.parent
base_name = checkpoint_path.name
new_base_name = base_name.replace('.pth', f'{label}int8.pth')
else:
raise ValueError(f"Invalid quantization mode {mode} needs to be one of [int8,]")
quantize_path = dir_name / new_base_name
print(f"Writing quantized weights to {quantize_path}")
quantize_path.unlink(missing_ok=True) # remove existing file if one already there
torch.save(quantized_state_dict, quantize_path)
print(f"Quantization complete took {time.time() - t0:.02f} seconds")
return
if __name__ == '__main__':
import argparse
parser = argparse.ArgumentParser(description='Quantize a model.')
parser.add_argument('--checkpoint_path', type=Path, default=Path("checkpoints/meta-llama/Llama-2-7b-chat-hf/model.pth"), help='Path to the model checkpoint to be quantized.')
parser.add_argument('--mode', '-q', type=str, default='int8', choices=['int8', 'int4', 'int4-gptq'], help='type of quantization to perform')
parser.add_argument('--label', type=str, default='_', help='label to add to output filename')
args = parser.parse_args()
quantize(args.checkpoint_path, args.mode, args.label)