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(测试版) 使用缩放点积注意力机制(SDPA)实现高性能Transformer¶
创建于：2023年3月15日 | 最后更新：2024年10月9日 | 最后验证：2024年11月5日
作者: Driss Guessous
总结¶
在本教程中，我们想强调一个新的torch.nn.functional函数，该函数可能对实现transformer架构有所帮助。该函数名为torch.nn.functional.scaled_dot_product_attention。有关该函数的详细描述，请参阅PyTorch文档。该函数已被整合到torch.nn.MultiheadAttention和torch.nn.TransformerEncoderLayer中。
概述¶
在高层次上，这个PyTorch函数根据论文Attention is all you need中的定义计算查询、键和值之间的缩放点积注意力（SDPA）。虽然这个函数可以使用现有的PyTorch函数编写，但融合实现可以提供比简单实现更大的性能优势。
融合实现¶
对于CUDA张量输入，该函数将分派到以下实现之一：
一个用C++定义的PyTorch实现
注意
本教程需要 PyTorch 2.0.0 或更高版本。
import torch
import torch.nn as nn
import torch.nn.functional as F
device = "cuda" if torch.cuda.is_available() else "cpu"
# Example Usage:
query, key, value = torch.randn(2, 3, 8, device=device), torch.randn(2, 3, 8, device=device), torch.randn(2, 3, 8, device=device)
F.scaled_dot_product_attention(query, key, value)
tensor([[[-1.3321, -0.3489, 0.3015, -0.3912, 0.9867, 0.3137, -0.0691,
-1.2593],
[-1.0882, 0.2506, 0.6491, 0.1360, 0.5238, -0.2448, -0.0820,
-0.6171],
[-1.0012, 0.3990, 0.6441, -0.0277, 0.5325, -0.2564, -0.0607,
-0.6404]],
[[ 0.6091, 0.0708, 0.6188, 0.3252, -0.1598, 0.4197, -0.2335,
0.0630],
[ 0.5285, 0.3890, -0.2649, 0.3706, -0.3839, 0.1963, -0.6242,
0.2312],
[ 0.4048, 0.0762, 0.3777, 0.4689, -0.2978, 0.2754, -0.6429,
0.1037]]], device='cuda:0')
显式调度器控制¶
虽然该函数会隐式地分派到三种实现之一，但用户也可以通过使用上下文管理器显式控制分派。此上下文管理器允许用户显式禁用某些实现。如果用户希望确保该函数确实针对其特定输入使用最快的实现，则可以使用上下文管理器来扫描测量性能。
# Lets define a helpful benchmarking function:
import torch.utils.benchmark as benchmark
def benchmark_torch_function_in_microseconds(f, *args, **kwargs):
t0 = benchmark.Timer(
stmt="f(*args, **kwargs)", globals={"args": args, "kwargs": kwargs, "f": f}
)
return t0.blocked_autorange().mean * 1e6
# Lets define the hyper-parameters of our input
batch_size = 32
max_sequence_len = 1024
num_heads = 32
embed_dimension = 32
dtype = torch.float16
query = torch.rand(batch_size, num_heads, max_sequence_len, embed_dimension, device=device, dtype=dtype)
key = torch.rand(batch_size, num_heads, max_sequence_len, embed_dimension, device=device, dtype=dtype)
value = torch.rand(batch_size, num_heads, max_sequence_len, embed_dimension, device=device, dtype=dtype)
print(f"The default implementation runs in {benchmark_torch_function_in_microseconds(F.scaled_dot_product_attention, query, key, value):.3f} microseconds")
# Lets explore the speed of each of the 3 implementations
from torch.nn.attention import SDPBackend, sdpa_kernel
with sdpa_kernel(SDPBackend.MATH):
math_time=benchmark_torch_function_in_microseconds(F.scaled_dot_product_attention, query, key, value)
print(f"The math implementation runs in {math_time:.3f} microseconds")
with sdpa_kernel(SDPBackend.FLASH_ATTENTION):
try:
flash_time=benchmark_torch_function_in_microseconds(F.scaled_dot_product_attention, query, key, value)
print(f"The flash attention implementation runs in {flash_time:.3f} microseconds")
except RuntimeError:
print("FlashAttention is not supported. See warnings for reasons.")
with sdpa_kernel(SDPBackend.EFFICIENT_ATTENTION):
try:
efficient_time=benchmark_torch_function_in_microseconds(F.scaled_dot_product_attention, query, key, value)
print(f"The memory efficient implementation runs in {efficient_time:.3f} microseconds")
except RuntimeError:
print("EfficientAttention is not supported. See warnings for reasons.")
The default implementation runs in 2326.418 microseconds
The math implementation runs in 87382.506 microseconds
The flash attention implementation runs in 2328.379 microseconds
The memory efficient implementation runs in 4305.558 microseconds
硬件依赖性¶
根据您运行上述单元格的机器和可用的硬件，您的结果可能会有所不同。 - 如果您没有GPU并且在CPU上运行，那么使用FP32时上下文管理器将不会产生任何效果，所有三次运行应该返回相似的时间。 - 根据您的显卡支持的计算能力，flash attention或memory efficient可能已经失败。
因果自注意力¶
下面是一个受Andrej Karpathy NanoGPT仓库启发的多头因果自注意力块的示例实现。
class CausalSelfAttention(nn.Module):
def __init__(self, num_heads: int, embed_dimension: int, bias: bool=False, is_causal: bool=False, dropout:float=0.0):
super().__init__()
assert embed_dimension % num_heads == 0
# key, query, value projections for all heads, but in a batch
self.c_attn = nn.Linear(embed_dimension, 3 * embed_dimension, bias=bias)
# output projection
self.c_proj = nn.Linear(embed_dimension, embed_dimension, bias=bias)
# regularization
self.dropout = dropout
self.resid_dropout = nn.Dropout(dropout)
self.num_heads = num_heads
self.embed_dimension = embed_dimension
# Perform causal masking
self.is_causal = is_causal
def forward(self, x):
# calculate query, key, values for all heads in batch and move head forward to be the batch dim
query_projected = self.c_attn(x)
batch_size = query_projected.size(0)
embed_dim = query_projected.size(2)
head_dim = embed_dim // (self.num_heads * 3)
query, key, value = query_projected.chunk(3, -1)
query = query.view(batch_size, -1, self.num_heads, head_dim).transpose(1, 2)
key = key.view(batch_size, -1, self.num_heads, head_dim).transpose(1, 2)
value = value.view(batch_size, -1, self.num_heads, head_dim).transpose(1, 2)
if self.training:
dropout = self.dropout
is_causal = self.is_causal
else:
dropout = 0.0
is_causal = False
y = F.scaled_dot_product_attention(query, key, value, attn_mask=None, dropout_p=dropout, is_causal=is_causal)
y = y.transpose(1, 2).view(batch_size, -1, self.num_heads * head_dim)
y = self.resid_dropout(self.c_proj(y))
return y
num_heads = 8
heads_per_dim = 64
embed_dimension = num_heads * heads_per_dim
dtype = torch.float16
model = CausalSelfAttention(num_heads=num_heads, embed_dimension=embed_dimension, bias=False, is_causal=True, dropout=0.1).to("cuda").to(dtype).eval()
print(model)
CausalSelfAttention(
(c_attn): Linear(in_features=512, out_features=1536, bias=False)
(c_proj): Linear(in_features=512, out_features=512, bias=False)
(resid_dropout): Dropout(p=0.1, inplace=False)
)
NestedTensor 和密集张量支持¶
SDPA 支持 NestedTensor 和 Dense tensor 输入。NestedTensors 处理输入为一批可变长度序列的情况，而无需将每个序列填充到批次中的最大长度。有关 NestedTensors 的更多信息，请参阅 torch.nested 和 NestedTensors Tutorial。
import random
def generate_rand_batch(
batch_size,
max_sequence_len,
embed_dimension,
pad_percentage=None,
dtype=torch.float16,
device="cuda",
):
if not pad_percentage:
return (
torch.randn(
batch_size,
max_sequence_len,
embed_dimension,
dtype=dtype,
device=device,
),
None,
)
# Random sequence lengths
seq_len_list = [
int(max_sequence_len * (1 - random.gauss(pad_percentage, 0.01)))
for _ in range(batch_size)
]
# Make random entry in the batch have max sequence length
seq_len_list[random.randint(0, batch_size - 1)] = max_sequence_len
return (
torch.nested.nested_tensor(
[
torch.randn(seq_len, embed_dimension,
dtype=dtype, device=device)
for seq_len in seq_len_list
]
),
seq_len_list,
)
random_nt, _ = generate_rand_batch(32, 512, embed_dimension, pad_percentage=0.5, dtype=dtype, device=device)
random_dense, _ = generate_rand_batch(32, 512, embed_dimension, pad_percentage=None, dtype=dtype, device=device)
# Currently the fused implementations don't support ``NestedTensor`` for training
model.eval()
with sdpa_kernel(SDPBackend.FLASH_ATTENTION):
try:
print(f"Random NT runs in {benchmark_torch_function_in_microseconds(model, random_nt):.3f} microseconds")
print(f"Random Dense runs in {benchmark_torch_function_in_microseconds(model, random_dense):.3f} microseconds")
except RuntimeError:
print("FlashAttention is not supported. See warnings for reasons.")
/usr/local/lib/python3.10/dist-packages/torch/nested/__init__.py:226: UserWarning:
The PyTorch API of nested tensors is in prototype stage and will change in the near future. (Triggered internally at ../aten/src/ATen/NestedTensorImpl.cpp:178.)
Random NT runs in 561.846 microseconds
Random Dense runs in 948.365 microseconds
使用SDPA与torch.compile¶
随着PyTorch 2.0的发布，引入了一个名为torch.compile()的新功能，它可以提供比eager模式显著的性能提升。缩放点积注意力与torch.compile()完全兼容。为了展示这一点，让我们使用torch.compile()编译CausalSelfAttention模块，并观察由此带来的性能提升。
batch_size = 32
max_sequence_len = 256
x = torch.rand(batch_size, max_sequence_len,
embed_dimension, device=device, dtype=dtype)
print(
f"The non compiled module runs in {benchmark_torch_function_in_microseconds(model, x):.3f} microseconds")
compiled_model = torch.compile(model)
# Let's compile it
compiled_model(x)
print(
f"The compiled module runs in {benchmark_torch_function_in_microseconds(compiled_model, x):.3f} microseconds")
The non compiled module runs in 415.514 microseconds
The compiled module runs in 513.798 microseconds
确切的执行时间取决于机器，然而我的结果如下： 未编译的模块运行时间为 166.616 微秒 编译后的模块运行时间为 166.726 微秒 这并不是我们所期望的。让我们深入挖掘一下。 PyTorch 带有一个令人惊叹的内置分析器，您可以使用它来 检查代码的性能特征。
from torch.profiler import profile, record_function, ProfilerActivity
activities = [ProfilerActivity.CPU]
if device == 'cuda':
activities.append(ProfilerActivity.CUDA)
with profile(activities=activities, record_shapes=False) as prof:
with record_function(" Non-Compilied Causal Attention"):
for _ in range(25):
model(x)
print(prof.key_averages().table(sort_by="cuda_time_total", row_limit=10))
with profile(activities=activities, record_shapes=False) as prof:
with record_function("Compiled Causal Attention"):
for _ in range(25):
compiled_model(x)
print(prof.key_averages().table(sort_by="cuda_time_total", row_limit=10))
# For even more insights, you can export the trace and use ``chrome://tracing`` to view the results
#
# .. code-block:: python
#
# prof.export_chrome_trace("compiled_causal_attention_trace.json").
------------------------------------------------------- ------------ ------------ ------------ ------------ ------------ ------------ ------------ ------------ ------------ ------------
Name Self CPU % Self CPU CPU total % CPU total CPU time avg Self CUDA Self CUDA % CUDA total CUDA time avg # of Calls
------------------------------------------------------- ------------ ------------ ------------ ------------ ------------ ------------ ------------ ------------ ------------ ------------
Non-Compilied Causal Attention 0.00% 0.000us 0.00% 0.000us 0.000us 10.515ms 101.39% 10.515ms 10.515ms 1
Non-Compilied Causal Attention 20.72% 2.282ms 75.23% 8.284ms 8.284ms 0.000us 0.00% 10.371ms 10.371ms 1
aten::linear 1.14% 126.000us 28.07% 3.091ms 61.823us 0.000us 0.00% 7.749ms 154.980us 50
aten::matmul 2.18% 239.531us 24.11% 2.655ms 53.096us 0.000us 0.00% 7.749ms 154.980us 50
aten::mm 15.35% 1.690ms 19.65% 2.164ms 43.274us 7.749ms 74.72% 7.749ms 154.980us 50
ampere_fp16_s1688gemm_fp16_128x128_ldg8_f2f_tn 0.00% 0.000us 0.00% 0.000us 0.000us 5.544ms 53.46% 5.544ms 221.767us 25
aten::scaled_dot_product_attention 1.91% 210.851us 17.21% 1.896ms 75.823us 0.000us 0.00% 2.622ms 104.893us 25
aten::_scaled_dot_product_flash_attention 2.87% 316.371us 15.30% 1.685ms 67.389us 0.000us 0.00% 2.622ms 104.893us 25
aten::_flash_attention_forward 3.42% 377.071us 10.69% 1.177ms 47.081us 2.622ms 25.28% 2.622ms 104.893us 25
void pytorch_flash::flash_fwd_kernel<pytorch_flash::... 0.00% 0.000us 0.00% 0.000us 0.000us 2.622ms 25.28% 2.622ms 104.893us 25
------------------------------------------------------- ------------ ------------ ------------ ------------ ------------ ------------ ------------ ------------ ------------ ------------
Self CPU time total: 11.012ms
Self CUDA time total: 10.371ms
------------------------------------------------------- ------------ ------------ ------------ ------------ ------------ ------------ ------------ ------------ ------------ ------------
Name Self CPU % Self CPU CPU total % CPU total CPU time avg Self CUDA Self CUDA % CUDA total CUDA time avg # of Calls
------------------------------------------------------- ------------ ------------ ------------ ------------ ------------ ------------ ------------ ------------ ------------ ------------
Compiled Causal Attention 0.00% 0.000us 0.00% 0.000us 0.000us 10.569ms 101.84% 10.569ms 10.569ms 1
Compiled Causal Attention 8.70% 984.016us 73.88% 8.360ms 8.360ms 0.000us 0.00% 10.378ms 10.378ms 1
Torch-Compiled Region 8.42% 952.756us 62.97% 7.126ms 285.053us 0.000us 0.00% 10.378ms 415.117us 25
CompiledFunction 26.38% 2.985ms 54.55% 6.174ms 246.943us 0.000us 0.00% 10.378ms 415.117us 25
aten::mm 9.38% 1.061ms 14.07% 1.592ms 31.842us 7.758ms 74.75% 7.758ms 155.157us 50
ampere_fp16_s1688gemm_fp16_128x128_ldg8_f2f_tn 0.00% 0.000us 0.00% 0.000us 0.000us 5.552ms 53.50% 5.552ms 222.092us 25
aten::_scaled_dot_product_flash_attention 2.12% 239.381us 14.10% 1.596ms 63.844us 0.000us 0.00% 2.620ms 104.803us 25
aten::_flash_attention_forward 3.46% 391.220us 10.29% 1.165ms 46.582us 2.620ms 25.25% 2.620ms 104.803us 25
void pytorch_flash::flash_fwd_kernel<pytorch_flash::... 0.00% 0.000us 0.00% 0.000us 0.000us 2.620ms 25.25% 2.620ms 104.803us 25
ampere_fp16_s1688gemm_fp16_128x128_ldg8_f2f_stages_3... 0.00% 0.000us 0.00% 0.000us 0.000us 2.206ms 21.25% 2.206ms 88.222us 25
------------------------------------------------------- ------------ ------------ ------------ ------------ ------------ ------------ ------------ ------------ ------------ ------------
Self CPU time total: 11.316ms
Self CUDA time total: 10.378ms
之前的代码片段生成了一个报告，显示了在编译和非编译模块中消耗最多GPU执行时间的前10个PyTorch函数。分析表明，大多数GPU时间都集中在两个模块的同一组函数上。这里的原因是torch.compile非常擅长消除与PyTorch相关的框架开销。如果你的模型启动了大型、高效的CUDA内核，在这种情况下CausalSelfAttention就是如此，那么PyTorch的开销可以被隐藏。
实际上，你的模块通常不包含单一的CausalSelfAttention块。在实验Andrej Karpathy NanoGPT仓库时，编译模块将每个训练步骤的时间从6090.49ms减少到3273.17ms！这是在NanoGPT的ae3a8d5提交上，使用莎士比亚数据集进行训练时完成的。
使用SDPA与attn_bias子类¶
# As of PyTorch 2.3, we have added a new submodule that contains tensor subclasses.
# Designed to be used with ``torch.nn.functional.scaled_dot_product_attention``.
# The module is named ``torch.nn.attention.bias`` and contains the following two
# utilities for generating causal attention variants:
#
# - ``torch.nn.attention.bias.causal_upper_left``
# - ``torch.nn.attention.bias.causal_lower_right``
#
# .. note::
# The current argument ``is_causal`` in ``torch.nn.functional.scaled_dot_product_attention``
# is the same as using ``torch.nn.attention.bias.causal_upper_left``.
#
from torch.nn.attention.bias import causal_lower_right, causal_upper_left
batch_size = 32
sequence_length_q = 2
sequence_length_kv = 10
num_heads = 16
embed_dimension = 32
dtype = torch.float16
query = torch.rand(batch_size, num_heads, sequence_length_q, embed_dimension, device=device, dtype=dtype)
key = torch.rand(batch_size, num_heads, sequence_length_kv, embed_dimension, device=device, dtype=dtype)
value = torch.rand(batch_size, num_heads, sequence_length_kv, embed_dimension, device=device, dtype=dtype)
upper_left_bias = causal_upper_left(sequence_length_q, sequence_length_kv)
lower_right_bias = causal_lower_right(sequence_length_q, sequence_length_kv)
print(type(upper_left_bias))
print(type(lower_right_bias))
assert type(upper_left_bias) == type(lower_right_bias)
assert issubclass(type(upper_left_bias), torch.Tensor)
# As you can see from the previous output, are the same type ``torch.nn.attention.bias.CausalBias``
# and subclass ``torch.Tensor``
# Lets see what these tensors look like
print(upper_left_bias)
print(lower_right_bias)
# Upper Left Bias aligns the causal attention mask to the upper left corner of the attention scores matrix.
# This only has an impact when the attention scores matrix is not square, which is common for decoding use cases.
# Another way of thinking about this concept is that when you use upper left bias,
# the 0th token in the query is aligned to the 0th token in the key, while for lower right bias,
# Assuming the attention score matrix is two dimensional, ``attn_score[0][0]`` is the attention score
# between the 0th token in the query and the 0th token in the key.
# For lower right bias, the sequence of q is aligned so that the last token in q is aligned to the last token in k
# (for example, ``attn_score[-1][-1])`` is all True since the last token in q is at the same position as the last token in k
# even if the sequence length of q and k are different.
# These objects are intended to be used with sdpa
out_upper_left = F.scaled_dot_product_attention(query, key, value, upper_left_bias)
out_lower_right = F.scaled_dot_product_attention(query, key, value, lower_right_bias)
out_is_causal = F.scaled_dot_product_attention(query, key, value, is_causal=True)
assert torch.allclose(out_upper_left, out_is_causal)
assert not torch.allclose(out_upper_left, out_lower_right)
# These attention biases should also be compatible with torch.compile
compiled_sdpa = torch.compile(F.scaled_dot_product_attention, fullgraph=True)
out_upper_left = compiled_sdpa(query, key, value, upper_left_bias)
<class 'torch.nn.attention.bias.CausalBias'>
<class 'torch.nn.attention.bias.CausalBias'>
tensor([[ True, False, False, False, False, False, False, False, False, False],
[ True, True, False, False, False, False, False, False, False, False]])
tensor([[ True, True, True, True, True, True, True, True, True, False],
[ True, True, True, True, True, True, True, True, True, True]])
结论¶
在本教程中，我们展示了torch.nn.functional.scaled_dot_product_attention的基本用法。我们展示了如何使用sdpa_kernel上下文管理器来确保在GPU上使用特定的实现。此外，我们构建了一个简单的CausalSelfAttention模块，该模块与NestedTensor兼容并且可被torch编译。在此过程中，我们展示了如何使用分析工具来探索用户定义模块的性能特征。
脚本总运行时间： ( 0 分钟 7.698 秒)
