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import torch
try:
import flash_attn_interface
def is_hopper_gpu():
if not torch.cuda.is_available():
return False
device_name = torch.cuda.get_device_name(0).lower()
return "h100" in device_name or "hopper" in device_name
FLASH_ATTN_3_AVAILABLE = is_hopper_gpu()
except ModuleNotFoundError:
FLASH_ATTN_3_AVAILABLE = False
try:
import flash_attn
FLASH_ATTN_2_AVAILABLE = True
except ModuleNotFoundError:
FLASH_ATTN_2_AVAILABLE = False
# FLASH_ATTN_3_AVAILABLE = False
import warnings
__all__ = [
'flash_attention',
'attention',
]
def flash_attention(
q,
k,
v,
q_lens=None,
k_lens=None,
dropout_p=0.,
softmax_scale=None,
q_scale=None,
causal=False,
window_size=(-1, -1),
deterministic=False,
dtype=torch.bfloat16,
version=None,
):
"""
q: [B, Lq, Nq, C1].
k: [B, Lk, Nk, C1].
v: [B, Lk, Nk, C2]. Nq must be divisible by Nk.
q_lens: [B].
k_lens: [B].
dropout_p: float. Dropout probability.
softmax_scale: float. The scaling of QK^T before applying softmax.
causal: bool. Whether to apply causal attention mask.
window_size: (left right). If not (-1, -1), apply sliding window local attention.
deterministic: bool. If True, slightly slower and uses more memory.
dtype: torch.dtype. Apply when dtype of q/k/v is not float16/bfloat16.
"""
half_dtypes = (torch.float16, torch.bfloat16)
assert dtype in half_dtypes
assert q.device.type == 'cuda' and q.size(-1) <= 256
# params
b, lq, lk, out_dtype = q.size(0), q.size(1), k.size(1), q.dtype
def half(x):
return x if x.dtype in half_dtypes else x.to(dtype)
# preprocess query
if q_lens is None:
q = half(q.flatten(0, 1))
q_lens = torch.tensor(
[lq] * b, dtype=torch.int32).to(
device=q.device, non_blocking=True)
else:
q = half(torch.cat([u[:v] for u, v in zip(q, q_lens)]))
# preprocess key, value
if k_lens is None:
k = half(k.flatten(0, 1))
v = half(v.flatten(0, 1))
k_lens = torch.tensor(
[lk] * b, dtype=torch.int32).to(
device=k.device, non_blocking=True)
else:
k = half(torch.cat([u[:v] for u, v in zip(k, k_lens)]))
v = half(torch.cat([u[:v] for u, v in zip(v, k_lens)]))
q = q.to(v.dtype)
k = k.to(v.dtype)
if q_scale is not None:
q = q * q_scale
if version is not None and version == 3 and not FLASH_ATTN_3_AVAILABLE:
warnings.warn(
'Flash attention 3 is not available, use flash attention 2 instead.'
)
# apply attention
if (version is None or version == 3) and FLASH_ATTN_3_AVAILABLE:
# Note: dropout_p, window_size are not supported in FA3 now.
x = flash_attn_interface.flash_attn_varlen_func(
q=q,
k=k,
v=v,
cu_seqlens_q=torch.cat([q_lens.new_zeros([1]), q_lens]).cumsum(
0, dtype=torch.int32).to(q.device, non_blocking=True),
cu_seqlens_k=torch.cat([k_lens.new_zeros([1]), k_lens]).cumsum(
0, dtype=torch.int32).to(q.device, non_blocking=True),
max_seqlen_q=lq,
max_seqlen_k=lk,
softmax_scale=softmax_scale,
causal=causal,
deterministic=deterministic)[0].unflatten(0, (b, lq))
else:
assert FLASH_ATTN_2_AVAILABLE
x = flash_attn.flash_attn_varlen_func(
q=q,
k=k,
v=v,
cu_seqlens_q=torch.cat([q_lens.new_zeros([1]), q_lens]).cumsum(
0, dtype=torch.int32).to(q.device, non_blocking=True),
cu_seqlens_k=torch.cat([k_lens.new_zeros([1]), k_lens]).cumsum(
0, dtype=torch.int32).to(q.device, non_blocking=True),
max_seqlen_q=lq,
max_seqlen_k=lk,
dropout_p=dropout_p,
softmax_scale=softmax_scale,
causal=causal,
window_size=window_size,
deterministic=deterministic).unflatten(0, (b, lq))
# output
return x.type(out_dtype)
def attention(
q,
k,
v,
q_lens=None,
k_lens=None,
dropout_p=0.,
softmax_scale=None,
q_scale=None,
causal=False,
window_size=(-1, -1),
deterministic=False,
dtype=torch.bfloat16,
fa_version=None,
):
if FLASH_ATTN_2_AVAILABLE or FLASH_ATTN_3_AVAILABLE:
return flash_attention(
q=q,
k=k,
v=v,
q_lens=q_lens,
k_lens=k_lens,
dropout_p=dropout_p,
softmax_scale=softmax_scale,
q_scale=q_scale,
causal=causal,
window_size=window_size,
deterministic=deterministic,
dtype=dtype,
version=fa_version,
)
else:
if window_size != (-1, -1):
raise RuntimeError(
"PyTorch SDPA fallback does not support Wan sliding windows"
)
out_dtype = q.dtype
batch_size, max_q_len = q.shape[:2]
outputs = []
for batch_index in range(batch_size):
query_len = (
max_q_len
if q_lens is None
else int(q_lens[batch_index].item())
)
key_len = (
k.shape[1]
if k_lens is None
else int(k_lens[batch_index].item())
)
query = q[batch_index : batch_index + 1, :query_len]
key = k[batch_index : batch_index + 1, :key_len]
value = v[batch_index : batch_index + 1, :key_len]
query = query.transpose(1, 2).to(dtype)
key = key.transpose(1, 2).to(dtype)
value = value.transpose(1, 2).to(dtype)
if q_scale is not None:
query = query * q_scale
if query.shape[1] != key.shape[1]:
if query.shape[1] % key.shape[1]:
raise ValueError(
"query heads must be divisible by key/value heads"
)
repeats = query.shape[1] // key.shape[1]
key = key.repeat_interleave(repeats, dim=1)
value = value.repeat_interleave(repeats, dim=1)
output = torch.nn.functional.scaled_dot_product_attention(
query,
key,
value,
dropout_p=dropout_p,
is_causal=causal,
scale=softmax_scale,
).transpose(1, 2)
if query_len < max_q_len:
output = torch.nn.functional.pad(
output, (0, 0, 0, 0, 0, max_q_len - query_len)
)
outputs.append(output)
return torch.cat(outputs, dim=0).contiguous().to(out_dtype)
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