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# original source:
# https://github.com/AminRezaei0x443/memory-efficient-attention/blob/1bc0d9e6ac5f82ea43a375135c4e1d3896ee1694/memory_efficient_attention/attention_torch.py
# license:
# MIT License (see Memory Efficient Attention under the Licenses section in the web UI interface for the full license)
# credit:
# Amin Rezaei (original author)
# Alex Birch (optimized algorithm for 3D tensors, at the expense of removing bias, masking and callbacks)
# brkirch (modified to use torch.narrow instead of dynamic_slice implementation)
# implementation of:
# Self-attention Does Not Need O(n2) Memory":
# https://arxiv.org/abs/2112.05682v2
from functools import partial
import torch
from torch import Tensor
from torch.utils.checkpoint import checkpoint
import math
from typing import Optional, NamedTuple
def narrow_trunc(
input: Tensor,
dim: int,
start: int,
length: int
) -> Tensor:
return torch.narrow(input, dim, start, length if input.shape[dim] >= start + length else input.shape[dim] - start)
class AttnChunk(NamedTuple):
exp_values: Tensor
exp_weights_sum: Tensor
max_score: Tensor
class SummarizeChunk:
@staticmethod
def __call__(
query: Tensor,
key: Tensor,
value: Tensor,
) -> AttnChunk: ...
class ComputeQueryChunkAttn:
@staticmethod
def __call__(
query: Tensor,
key: Tensor,
value: Tensor,
) -> Tensor: ...
def _summarize_chunk(
query: Tensor,
key: Tensor,
value: Tensor,
scale: float,
) -> AttnChunk:
attn_weights = torch.baddbmm(
torch.zeros(1, 1, 1, device=query.device, dtype=query.dtype),
query,
key.transpose(1,2),
alpha=scale,
beta=0,
)
max_score, _ = torch.max(attn_weights, -1, keepdim=True)
max_score = max_score.detach()
exp_weights = torch.exp(attn_weights - max_score)
exp_values = torch.bmm(exp_weights, value) if query.device.type == 'mps' else torch.bmm(exp_weights, value.to(exp_weights.dtype)).to(value.dtype)
max_score = max_score.squeeze(-1)
return AttnChunk(exp_values, exp_weights.sum(dim=-1), max_score)
def _query_chunk_attention(
query: Tensor,
key: Tensor,
value: Tensor,
summarize_chunk: SummarizeChunk,
kv_chunk_size: int,
) -> Tensor:
batch_x_heads, k_tokens, k_channels_per_head = key.shape
_, _, v_channels_per_head = value.shape
def chunk_scanner(chunk_idx: int) -> AttnChunk:
key_chunk = narrow_trunc(
key,
1,
chunk_idx,
kv_chunk_size
)
value_chunk = narrow_trunc(
value,
1,
chunk_idx,
kv_chunk_size
)
return summarize_chunk(query, key_chunk, value_chunk)
chunks: list[AttnChunk] = [
chunk_scanner(chunk) for chunk in torch.arange(0, k_tokens, kv_chunk_size)
]
acc_chunk = AttnChunk(*map(torch.stack, zip(*chunks)))
chunk_values, chunk_weights, chunk_max = acc_chunk
global_max, _ = torch.max(chunk_max, 0, keepdim=True)
max_diffs = torch.exp(chunk_max - global_max)
chunk_values *= torch.unsqueeze(max_diffs, -1)
chunk_weights *= max_diffs
all_values = chunk_values.sum(dim=0)
all_weights = torch.unsqueeze(chunk_weights, -1).sum(dim=0)
return all_values / all_weights
# TODO: refactor CrossAttention#get_attention_scores to share code with this
def _get_attention_scores_no_kv_chunking(
query: Tensor,
key: Tensor,
value: Tensor,
scale: float,
) -> Tensor:
attn_scores = torch.baddbmm(
torch.zeros(1, 1, 1, device=query.device, dtype=query.dtype),
query,
key.transpose(1,2),
alpha=scale,
beta=0,
)
attn_probs = attn_scores.softmax(dim=-1)
del attn_scores
hidden_states_slice = torch.bmm(attn_probs, value) if query.device.type == 'mps' else torch.bmm(attn_probs, value.to(attn_probs.dtype)).to(value.dtype)
return hidden_states_slice
class ScannedChunk(NamedTuple):
chunk_idx: int
attn_chunk: AttnChunk
def efficient_dot_product_attention(
query: Tensor,
key: Tensor,
value: Tensor,
query_chunk_size=1024,
kv_chunk_size: Optional[int] = None,
kv_chunk_size_min: Optional[int] = None,
use_checkpoint=True,
):
"""Computes efficient dot-product attention given query, key, and value.
This is efficient version of attention presented in
https://arxiv.org/abs/2112.05682v2 which comes with O(sqrt(n)) memory requirements.
Args:
query: queries for calculating attention with shape of
`[batch * num_heads, tokens, channels_per_head]`.
key: keys for calculating attention with shape of
`[batch * num_heads, tokens, channels_per_head]`.
value: values to be used in attention with shape of
`[batch * num_heads, tokens, channels_per_head]`.
query_chunk_size: int: query chunks size
kv_chunk_size: Optional[int]: key/value chunks size. if None: defaults to sqrt(key_tokens)
kv_chunk_size_min: Optional[int]: key/value minimum chunk size. only considered when kv_chunk_size is None. changes `sqrt(key_tokens)` into `max(sqrt(key_tokens), kv_chunk_size_min)`, to ensure our chunk sizes don't get too small (smaller chunks = more chunks = less concurrent work done).
use_checkpoint: bool: whether to use checkpointing (recommended True for training, False for inference)
Returns:
Output of shape `[batch * num_heads, query_tokens, channels_per_head]`.
"""
batch_x_heads, q_tokens, q_channels_per_head = query.shape
_, k_tokens, _ = key.shape
scale = q_channels_per_head ** -0.5
kv_chunk_size = min(kv_chunk_size or int(math.sqrt(k_tokens)), k_tokens)
if kv_chunk_size_min is not None:
kv_chunk_size = max(kv_chunk_size, kv_chunk_size_min)
def get_query_chunk(chunk_idx: int) -> Tensor:
return narrow_trunc(
query,
1,
chunk_idx,
min(query_chunk_size, q_tokens)
)
summarize_chunk: SummarizeChunk = partial(_summarize_chunk, scale=scale)
summarize_chunk: SummarizeChunk = partial(checkpoint, summarize_chunk) if use_checkpoint else summarize_chunk
compute_query_chunk_attn: ComputeQueryChunkAttn = partial(
_get_attention_scores_no_kv_chunking,
scale=scale
) if k_tokens <= kv_chunk_size else (
# fast-path for when there's just 1 key-value chunk per query chunk (this is just sliced attention btw)
partial(
_query_chunk_attention,
kv_chunk_size=kv_chunk_size,
summarize_chunk=summarize_chunk,
)
)
if q_tokens <= query_chunk_size:
# fast-path for when there's just 1 query chunk
return compute_query_chunk_attn(
query=query,
key=key,
value=value,
)
res = torch.zeros_like(query)
for i in range(math.ceil(q_tokens / query_chunk_size)):
attn_scores = compute_query_chunk_attn(
query=get_query_chunk(i * query_chunk_size),
key=key,
value=value,
)
res[:, i * query_chunk_size:i * query_chunk_size + attn_scores.shape[1], :] = attn_scores
return res
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