benchmark_flash_attention.py

from functools import partial
import math
import torch
import torch.nn as nn
import torch.nn.functional as F

from einops import rearrange, repeat

from benchmarks.utils import benchmark_all, benchmark_forward, benchmark_backward, benchmark_combined
from bert_padding import unpad_input, pad_input
from flash_attn_interface import flash_attn_func


def attention_ref(qkv, attn_mask, dropout_p, upcast=False, causal=False):
    """
    Arguments:
        qkv: (batch_size, seqlen, 3, nheads, head_dim)
        attn_mask: (batch_size, seqlen)
        dropout_p: float
    Output:
        output: (batch_size, seqlen, nheads, head_dim)
        attention: softmax after dropout
    """
    q, k, v = (qkv.float() if upcast else qkv).unbind(dim=2)
    seqlen = qkv.shape[1]
    d = qkv.shape[-1]
    scores = torch.einsum('bthd,bshd->bhts', q, k / math.sqrt(d))
    scores.masked_fill_(rearrange(~attn_mask, 'b s -> b 1 1 s'), float('-inf'))
    if causal:
        causal_mask = torch.triu(torch.ones(seqlen, seqlen, dtype=torch.bool, device=qkv.device), 1)
        scores.masked_fill_(causal_mask, float('-inf'))
    attention = torch.softmax(scores, dim=-1)
    attention_drop = F.dropout(attention, dropout_p)
    output = torch.einsum('bhts,bshd->bthd', attention_drop , v)
    # return output.to(dtype=qkv.dtype), attention.to(dtype=qkv.dtype)
    return output.to(dtype=qkv.dtype)


torch.manual_seed(0)
repeats = 30
batch_size = 64
nheads = 16
seqlen = 1024
n = 1024
d = n // nheads
dropout_p = 0.1
causal = False
dtype = torch.float16
device = 'cuda'

x = torch.randn(batch_size, seqlen, n, device='cuda', dtype=dtype, requires_grad=True)
Wqkv = torch.nn.Linear(nheads * d, 3 * nheads * d, device=device, dtype=dtype)

lengths = torch.randint(seqlen - 20, seqlen, (batch_size, 1), device='cuda')
attention_mask_bool = repeat(torch.arange(seqlen, device='cuda'), 's -> b s', b=batch_size) < lengths
attention_mask = torch.zeros(batch_size, seqlen, device='cuda', dtype=dtype)
attention_mask[~attention_mask_bool] = -10000.0
attention_mask = rearrange(attention_mask, 'b s -> b 1 1 s')

x_unpad, indices, cu_seqlens, max_seqlen_in_batch = unpad_input(x, attention_mask_bool)
qkv_unpad = rearrange(Wqkv(x_unpad), 'nnz (t h d) -> nnz t h d', t=3,
                      h=nheads).detach().requires_grad_()
qkv = rearrange(Wqkv(x), 'b s (t h d) -> b s t h d', t=3, h=nheads).detach().requires_grad_()

fn = lambda qkv_unpad: flash_attn_func(qkv_unpad, cu_seqlens, dropout_p, max_seqlen_in_batch, causal=causal)
benchmark_all(fn, qkv_unpad, repeats=repeats, desc='FlashAttention')
fn = lambda qkv: attention_ref(qkv, attention_mask_bool, dropout_p, causal=causal)
benchmark_all(fn, qkv, repeats=repeats, desc='PyTorch Standard Attention')