example_triton_nsa.py

# Copyright (c) Microsoft Corporation.
# Licensed under the MIT License.
# ruff: noqa
import torch
from typing import Optional

import torch
import triton
import triton.language as tl
from einops import rearrange

from fla.ops.common.utils import (prepare_chunk_indices, prepare_lens, prepare_token_indices)
from fla.utils import autocast_custom_bwd, autocast_custom_fwd, contiguous

from reference import naive_nsa


@triton.autotune(
    configs=[triton.Config({}, num_warps=num_warps) for num_warps in [1, 2, 4, 8, 16]],
    key=['BS', 'BK', 'BV'],
)
@triton.jit
def parallel_nsa_fwd_kernel(
    q,
    k,
    v,
    o,
    lse,
    scale,
    block_indices,
    T,
    H: tl.constexpr,
    HQ: tl.constexpr,
    G: tl.constexpr,
    K: tl.constexpr,
    V: tl.constexpr,
    S: tl.constexpr,
    BS: tl.constexpr,
    BK: tl.constexpr,
    BV: tl.constexpr,
):
    i_v, i_t, i_bh = tl.program_id(0), tl.program_id(1), tl.program_id(2)
    i_b, i_h = i_bh // H, i_bh % H

    bos, eos = i_b * T, i_b * T + T

    k += (bos * H + i_h) * K
    v += (bos * H + i_h) * V
    block_indices += (bos + i_t) * H * S + i_h * S

    NS = S

    p_q = tl.make_block_ptr(q + (bos + i_t) * HQ * K, (HQ, K), (K, 1), (i_h * G, 0), (G, BK),
                            (1, 0))
    p_o = tl.make_block_ptr(o + (bos + i_t) * HQ * V, (HQ, V), (V, 1), (i_h * G, i_v * BV), (G, BV),
                            (1, 0))
    p_lse = lse + (bos + i_t) * HQ + i_h * G + tl.arange(0, G)

    # the Q block is kept in the shared memory throughout the whole kernel
    # [G, BK]
    b_q = tl.load(p_q, boundary_check=(0, 1))
    b_q = (b_q * scale).to(b_q.dtype)
    # [G, BV]
    b_o = tl.zeros([G, BV], dtype=tl.float32)

    b_m = tl.full([G], float('-inf'), dtype=tl.float32)
    b_acc = tl.zeros([G], dtype=tl.float32)
    for i in range(NS):
        i_s = tl.load(block_indices + i).to(tl.int32) * BS
        if i_s <= i_t:
            p_k = tl.make_block_ptr(k, (K, T), (1, H * K), (0, i_s), (BK, BS), (0, 1))
            p_v = tl.make_block_ptr(v, (T, V), (H * V, 1), (i_s, i_v * BV), (BS, BV), (1, 0))
            # [BK, BS]
            b_k = tl.load(p_k, boundary_check=(0, 1))
            # [BS, BV]
            b_v = tl.load(p_v, boundary_check=(0, 1))
            # [G, BS]
            b_s = tl.dot(b_q, b_k)
            b_s = tl.where((i_t >= (i_s + tl.arange(0, BS)))[None, :], b_s, float('-inf'))

            # [G]
            b_m, b_mp = tl.maximum(b_m, tl.max(b_s, 1)), b_m
            b_r = tl.exp(b_mp - b_m)
            # [G, BS]
            b_p = tl.exp(b_s - b_m[:, None])
            # [G]
            b_acc = b_acc * b_r + tl.sum(b_p, 1)
            # [G, BV]
            b_o = b_o * b_r[:, None] + tl.dot(b_p.to(b_q.dtype), b_v)

            b_mp = b_m
    b_o = b_o / b_acc[:, None]
    b_m += tl.log(b_acc)
    tl.store(p_o, b_o.to(p_o.dtype.element_ty), boundary_check=(0, 1))
    tl.store(p_lse, b_m.to(p_lse.dtype.element_ty))


def parallel_nsa_fwd(
    q: torch.Tensor,
    k: torch.Tensor,
    v: torch.Tensor,
    block_indices: torch.Tensor,
    block_size: int,
    scale: float,
):
    B, T, H, K, V, S = *k.shape, v.shape[-1], block_indices.shape[-1]
    HQ = q.shape[2]
    G = HQ // H
    BS = block_size
    if torch.cuda.get_device_capability()[0] >= 9:
        BK = min(256, triton.next_power_of_2(K))
        BV = min(256, triton.next_power_of_2(V))
    else:
        BK = min(128, triton.next_power_of_2(K))
        BV = min(128, triton.next_power_of_2(V))
    NK = triton.cdiv(K, BK)
    NV = triton.cdiv(V, BV)
    assert NK == 1, "The key dimension can not be larger than 256"

    grid = (NV, T, B * H)
    o = torch.empty(B, T, HQ, V, dtype=v.dtype, device=q.device)
    lse = torch.empty(B, T, HQ, dtype=torch.float32, device=q.device)
    print("grid", grid)
    parallel_nsa_fwd_kernel[grid](
        q=q,
        k=k,
        v=v,
        o=o,
        lse=lse,
        scale=scale,
        block_indices=block_indices,
        H=H,
        HQ=HQ,
        G=G,
        T=T,
        K=K,
        V=V,
        S=S,
        BS=BS,
        BK=BK,
        BV=BV,
    )
    return o, lse


class ParallelNSAFunction(torch.autograd.Function):

    @staticmethod
    @contiguous
    @autocast_custom_fwd
    def forward(ctx, q, k, v, block_indices, block_size, scale, offsets):
        ctx.dtype = q.dtype

        # 2-d sequence indices denoting the offsets of tokens in each sequence
        # for example, if the passed `offsets` is [0, 2, 6],
        # then there are 2 and 4 tokens in the 1st and 2nd sequences respectively, and `token_indices` will be
        # [[0, 0], [0, 1], [1, 0], [1, 1], [1, 2], [1, 3]]
        token_indices = prepare_token_indices(offsets) if offsets is not None else None

        o, lse = parallel_nsa_fwd(
            q=q, k=k, v=v, block_indices=block_indices, block_size=block_size, scale=scale)
        ctx.save_for_backward(q, k, v, o, lse)
        ctx.block_indices = block_indices
        ctx.block_size = block_size
        ctx.scale = scale
        return o.to(q.dtype)


def parallel_nsa(q: torch.Tensor,
                 k: torch.Tensor,
                 v: torch.Tensor,
                 block_indices: torch.LongTensor,
                 block_size: int = 64,
                 scale: Optional[float] = None,
                 cu_seqlens: Optional[torch.LongTensor] = None,
                 head_first: bool = False) -> torch.Tensor:
    r"""
    Args:
        q (torch.Tensor):
            queries of shape `[B, T, HQ, K]` if `head_first=False` else `[B, HQ, T, K]`.
        k (torch.Tensor):
            keys of shape `[B, T, H, K]` if `head_first=False` else `[B, H, T, K]`.
            GQA is enforced here. The ratio of query heads (HQ) to key/value heads (H) must be a power of 2 and >=16.
        v (torch.Tensor):
            values of shape `[B, T, H, V]` if `head_first=False` else `[B, H, T, V]`.
        block_indices (torch.LongTensor):
            Block indices of shape `[B, T, H, S]` if `head_first=False` else `[B, H, T, S]`.
            `S` is the number of selected blocks for each query token, which is set to 16 in the paper.
        block_size (int):
            Selected block size. Default: 64.
        scale (Optional[int]):
            Scale factor for attention scores.
            If not provided, it will default to `1 / sqrt(K)`. Default: `None`.
        head_first (Optional[bool]):
            Whether the inputs are in the head-first format. Default: `False`.
        cu_seqlens (torch.LongTensor):
            Cumulative sequence lengths of shape `[N+1]` used for variable-length training,
            consistent with the FlashAttention API.

    Returns:
        o (torch.Tensor):
            Outputs of shape `[B, T, HQ, V]` if `head_first=False` else `[B, HQ, T, V]`.
    """
    if scale is None:
        scale = k.shape[-1]**-0.5
    if cu_seqlens is not None:
        assert q.shape[0] == 1, "batch size must be 1 when cu_seqlens are provided"
    if head_first:
        q, k, v, block_indices = map(lambda x: rearrange(x, 'b h t d -> b t h d'),
                                     (q, k, v, block_indices))
    o = ParallelNSAFunction.apply(q, k, v, block_indices, block_size, scale, cu_seqlens)
    if head_first:
        o = rearrange(o, 'b t h d -> b h t d')
    return o


if __name__ == "__main__":
    B, T, H, HQ, D, S, block_size, dtype, scale = 1, 64, 1, 16, 32, 1, 64, torch.float16, 0.1

    q = torch.randn((B, T, HQ, D), dtype=dtype, device='cuda').requires_grad_(True)
    k = torch.randn((B, T, H, D), dtype=dtype, device='cuda').requires_grad_(True)
    v = torch.randn((B, T, H, D), dtype=dtype, device='cuda').requires_grad_(True)
    do = torch.randn((B, T, HQ, D), dtype=dtype, device='cuda')

    block_indices = torch.full((B, T, H, S), T, dtype=torch.long, device='cuda')
    for b in range(B):
        for t in range(T):
            for h in range(H):
                i_i = torch.randperm(max(1, (t // block_size)))[:S]
                block_indices[b, t, h, :len(i_i)] = i_i
    block_indices = block_indices.sort(-1)[0]

    block_counts = torch.randint(1, S + 1, (B, T, H), device='cuda')

    ref = naive_nsa(
        q=q,
        k=k,
        v=v,
        block_indices=block_indices,
        block_counts=block_counts,
        block_size=block_size,
        scale=scale)

    # print(ref)

    tri = parallel_nsa(
        q=q, k=k, v=v, block_indices=block_indices, block_size=block_size, scale=scale)

    # print(tri)

    torch.testing.assert_close(ref, tri, atol=1e-2, rtol=1e-2)

    # import flash_attn
    # # gqa
    # o_gqa = flash_attn.flash_attn_func(
    #     q,
    #     k,
    #     v,
    #     softmax_scale=scale,
    # )
    # print(o_gqa)

    # torch.testing.assert_close(o_gqa, tri, atol=1e-2, rtol=1e-2)