mamba_ssm.py

# Copyright (c) 2024, Tri Dao, Albert Gu.
# Adapted from https://github.com/state-spaces/mamba/blob/main/mamba_ssm/ops/triton/selective_state_update.py

import torch
import triton
import triton.language as tl
from packaging import version

from vllm import _custom_ops as ops

TRITON3 = version.parse(triton.__version__) >= version.parse("3.0.0")

if TRITON3:

    @triton.jit
    def softplus(dt):
        dt = tl.where(dt <= 20.0, tl.math.log(tl.math.exp(dt) + 1), dt)
        return dt
else:

    @triton.jit
    def softplus(dt):
        dt = tl.where(dt <= 20.0, tl.math.log1p(tl.exp(dt)), dt)
        return dt


@triton.heuristics(
    {"HAS_DT_BIAS": lambda args: args["dt_bias_ptr"] is not None})
@triton.heuristics({"HAS_D": lambda args: args["D_ptr"] is not None})
@triton.heuristics({"HAS_Z": lambda args: args["z_ptr"] is not None})
@triton.heuristics({
    "HAS_STATE_BATCH_INDICES":
    lambda args: args["state_batch_indices_ptr"] is not None
})
@triton.heuristics(
    {"BLOCK_SIZE_DSTATE": lambda args: triton.next_power_of_2(args["dstate"])})
@triton.jit
def _selective_scan_update_kernel(
    # Pointers to matrices
    state_ptr,
    x_ptr,
    dt_ptr,
    dt_bias_ptr,
    A_ptr,
    B_ptr,
    C_ptr,
    D_ptr,
    z_ptr,
    out_ptr,
    state_batch_indices_ptr,
    # Matrix dimensions
    batch,
    nheads,
    dim,
    dstate,
    nheads_ngroups_ratio,
    # Strides
    stride_state_batch,
    stride_state_head,
    stride_state_dim,
    stride_state_dstate,
    stride_x_batch,
    stride_x_head,
    stride_x_dim,
    stride_dt_batch,
    stride_dt_head,
    stride_dt_dim,
    stride_dt_bias_head,
    stride_dt_bias_dim,
    stride_A_head,
    stride_A_dim,
    stride_A_dstate,
    stride_B_batch,
    stride_B_group,
    stride_B_dstate,
    stride_C_batch,
    stride_C_group,
    stride_C_dstate,
    stride_D_head,
    stride_D_dim,
    stride_z_batch,
    stride_z_head,
    stride_z_dim,
    stride_out_batch,
    stride_out_head,
    stride_out_dim,
    # Meta-parameters
    DT_SOFTPLUS: tl.constexpr,
    TIE_HDIM: tl.constexpr,
    BLOCK_SIZE_M: tl.constexpr,
    HAS_DT_BIAS: tl.constexpr,
    HAS_D: tl.constexpr,
    HAS_Z: tl.constexpr,
    HAS_STATE_BATCH_INDICES: tl.constexpr,
    BLOCK_SIZE_DSTATE: tl.constexpr,
):
    pid_m = tl.program_id(axis=0)
    pid_b = tl.program_id(axis=1)
    pid_h = tl.program_id(axis=2)

    # If HAS_STATE_BATCH_INDICES is true, then the ssm state's batch coordinate
    # is taken from the state_batch_indices_ptr Otherwise, the state coordinate
    # is the same as the batch id.
    if HAS_STATE_BATCH_INDICES:
        state_batch_indices_ptr += pid_b
        state_batch_idx = tl.load(state_batch_indices_ptr)
        state_ptr += (state_batch_idx * stride_state_batch +
                      pid_h * stride_state_head)
    else:
        state_ptr += pid_b * stride_state_batch + pid_h * stride_state_head

    x_ptr += pid_b * stride_x_batch + pid_h * stride_x_head
    dt_ptr += pid_b * stride_dt_batch + pid_h * stride_dt_head
    if HAS_DT_BIAS:
        dt_bias_ptr += pid_h * stride_dt_bias_head
    A_ptr += pid_h * stride_A_head
    B_ptr += pid_b * stride_B_batch + (pid_h //
                                       nheads_ngroups_ratio) * stride_B_group
    C_ptr += pid_b * stride_C_batch + (pid_h //
                                       nheads_ngroups_ratio) * stride_C_group
    if HAS_Z:
        z_ptr += pid_b * stride_z_batch + pid_h * stride_z_head
    out_ptr += pid_b * stride_out_batch + pid_h * stride_out_head

    offs_m = pid_m * BLOCK_SIZE_M + tl.arange(0, BLOCK_SIZE_M)
    offs_n = tl.arange(0, BLOCK_SIZE_DSTATE)
    state_ptrs = state_ptr + (offs_m[:, None] * stride_state_dim +
                              offs_n[None, :] * stride_state_dstate)
    x_ptrs = x_ptr + offs_m * stride_x_dim
    dt_ptrs = dt_ptr + offs_m * stride_dt_dim
    if HAS_DT_BIAS:
        dt_bias_ptrs = dt_bias_ptr + offs_m * stride_dt_bias_dim
    if HAS_D:
        D_ptr += pid_h * stride_D_head
    A_ptrs = A_ptr + (offs_m[:, None] * stride_A_dim +
                      offs_n[None, :] * stride_A_dstate)
    B_ptrs = B_ptr + offs_n * stride_B_dstate
    C_ptrs = C_ptr + offs_n * stride_C_dstate
    if HAS_D:
        D_ptrs = D_ptr + offs_m * stride_D_dim
    if HAS_Z:
        z_ptrs = z_ptr + offs_m * stride_z_dim
    out_ptrs = out_ptr + offs_m * stride_out_dim

    state = tl.load(state_ptrs,
                    mask=(offs_m[:, None] < dim) & (offs_n[None, :] < dstate),
                    other=0.0)
    x = tl.load(x_ptrs, mask=offs_m < dim, other=0.0).to(tl.float32)
    if not TIE_HDIM:
        dt = tl.load(dt_ptrs, mask=offs_m < dim, other=0.0).to(tl.float32)
        if HAS_DT_BIAS:
            dt += tl.load(dt_bias_ptrs, mask=offs_m < dim,
                          other=0.0).to(tl.float32)
        if DT_SOFTPLUS:
            dt = softplus(dt)
        A = tl.load(A_ptrs,
                    mask=(offs_m[:, None] < dim) & (offs_n[None, :] < dstate),
                    other=0.0).to(tl.float32)
        dA = tl.exp(A * dt[:, None])
    else:
        dt = tl.load(dt_ptr).to(tl.float32)
        if HAS_DT_BIAS:
            dt += tl.load(dt_bias_ptr).to(tl.float32)
        if DT_SOFTPLUS:
            dt = softplus(dt)
        A = tl.load(A_ptr).to(tl.float32)
        dA = tl.exp(A * dt)  # scalar, not a matrix

    B = tl.load(B_ptrs, mask=offs_n < dstate, other=0.0).to(tl.float32)
    C = tl.load(C_ptrs, mask=offs_n < dstate, other=0.0).to(tl.float32)
    if HAS_D:
        D = tl.load(D_ptrs, mask=offs_m < dim, other=0.0).to(tl.float32)
    if HAS_Z:
        z = tl.load(z_ptrs, mask=offs_m < dim, other=0.0).to(tl.float32)

    dB = B[None, :] * dt[:, None] if not TIE_HDIM else B * dt
    state = state * dA + dB * x[:, None]
    tl.store(state_ptrs,
             state,
             mask=(offs_m[:, None] < dim) & (offs_n[None, :] < dstate))
    out = tl.sum(state * C[None, :], axis=1)
    if HAS_D:
        out += x * D
    if HAS_Z:
        out *= z * tl.sigmoid(z)
    tl.store(out_ptrs, out, mask=offs_m < dim)


def selective_state_update(state,
                           x,
                           dt,
                           A,
                           B,
                           C,
                           D=None,
                           z=None,
                           dt_bias=None,
                           dt_softplus=False,
                           state_batch_indices=None):
    """
    Argument:
        state: (batch, dim, dstate) or (batch, nheads, dim, dstate)
        x: (batch, dim) or (batch, nheads, dim)
        dt: (batch, dim) or (batch, nheads, dim)
        A: (dim, dstate) or (nheads, dim, dstate)
        B: (batch, dstate) or (batch, ngroups, dstate)
        C: (batch, dstate) or (batch, ngroups, dstate)
        D: (dim,) or (nheads, dim)
        z: (batch, dim) or (batch, nheads, dim)
        dt_bias: (dim,) or (nheads, dim)
    Return:
        out: (batch, dim) or (batch, nheads, dim)
    """
    has_heads = state.dim() > 3
    if state.dim() == 3:
        state = state.unsqueeze(1)
    if x.dim() == 2:
        x = x.unsqueeze(1)
    if dt.dim() == 2:
        dt = dt.unsqueeze(1)
    if A.dim() == 2:
        A = A.unsqueeze(0)
    if B.dim() == 2:
        B = B.unsqueeze(1)
    if C.dim() == 2:
        C = C.unsqueeze(1)
    if D is not None and D.dim() == 1:
        D = D.unsqueeze(0)
    if z is not None and z.dim() == 2:
        z = z.unsqueeze(1)
    if dt_bias is not None and dt_bias.dim() == 1:
        dt_bias = dt_bias.unsqueeze(0)

    _, nheads, dim, dstate = state.shape
    batch = x.shape[0]

    assert x.shape == (batch, nheads, dim)
    assert dt.shape == x.shape
    assert A.shape == (nheads, dim, dstate)
    ngroups = B.shape[1]
    assert nheads % ngroups == 0, "nheads must be divisible by ngroups"
    assert B.shape == (batch, ngroups, dstate)
    assert C.shape == B.shape
    if D is not None:
        assert D.shape == (nheads, dim)
    if z is not None:
        assert z.shape == x.shape
    if dt_bias is not None:
        assert dt_bias.shape == (nheads, dim)
    if state_batch_indices is not None:
        assert state_batch_indices.shape == (batch, )
    out = torch.empty_like(x)
    grid = lambda META: (triton.cdiv(dim, META['BLOCK_SIZE_M']), batch, nheads)
    z_strides = ((z.stride(0), z.stride(1), z.stride(2)) if z is not None else
                 (0, 0, 0))
    # We don't want autotune since it will overwrite the state
    # We instead tune by hand.
    BLOCK_SIZE_M, num_warps = ((32, 4) if dstate <= 16 else
                               ((16, 4) if dstate <= 32 else
                                ((8, 4) if dstate <= 64 else
                                 ((4, 4) if dstate <= 128 else ((4, 8))))))
    tie_hdim = A.stride(-1) == 0 and A.stride(-2) == 0 and dt.stride(
        -1) == 0 and dt_bias.stride(-1) == 0
    with torch.cuda.device(x.device.index):
        _selective_scan_update_kernel[grid](
            state,
            x,
            dt,
            dt_bias,
            A,
            B,
            C,
            D,
            z,
            out,
            state_batch_indices,
            batch,
            nheads,
            dim,
            dstate,
            nheads // ngroups,
            state.stride(0),
            state.stride(1),
            state.stride(2),
            state.stride(3),
            x.stride(0),
            x.stride(1),
            x.stride(2),
            dt.stride(0),
            dt.stride(1),
            dt.stride(2),
            *(dt_bias.stride(0),
              dt_bias.stride(1)) if dt_bias is not None else 0,
            A.stride(0),
            A.stride(1),
            A.stride(2),
            B.stride(0),
            B.stride(1),
            B.stride(2),
            C.stride(0),
            C.stride(1),
            C.stride(2),
            *(D.stride(0), D.stride(1)) if D is not None else 0,
            z_strides[0],
            z_strides[1],
            z_strides[2],
            out.stride(0),
            out.stride(1),
            out.stride(2),
            dt_softplus,
            tie_hdim,
            BLOCK_SIZE_M,
            num_warps=num_warps,
        )
    if not has_heads:
        out = out.squeeze(1)
    return out


def selective_scan_fn(u,
                      delta,
                      A,
                      B,
                      C,
                      D=None,
                      z=None,
                      delta_bias=None,
                      delta_softplus=False,
                      return_last_state=False,
                      position_indices=None,
                      prev_state=None):
    """if return_last_state is True, returns (out, last_state)
    last_state has shape (batch, dim, dstate). 
    """
    if u.stride(-1) != 1:
        u = u.contiguous()
    if delta.stride(-1) != 1:
        delta = delta.contiguous()
    if D is not None:
        D = D.contiguous()
    if B.stride(-1) != 1:
        B = B.contiguous()
    if C.stride(-1) != 1:
        C = C.contiguous()
    if z is not None and z.stride(-1) != 1:
        z = z.contiguous()
    if B.dim() == 3:
        B = B.unsqueeze(1)
    if C.dim() == 3:
        C = C.unsqueeze(1)
    n_chunks = int((u.shape[-1] + 2048 - 1) / 2048)
    x = torch.zeros((
        u.shape[0],
        u.shape[1],
        n_chunks,
        int(A.shape[1] * 2),
    ),
                    device=u.device,
                    dtype=torch.float32,
                    requires_grad=False)
    x[:, :, 0, 0::2] = 1
    if prev_state is not None:
        x[:, :, 0, 1::2].copy_(prev_state)
    out, x, *rest = ops.selective_scan_fwd(u, delta, A, B, C, D, z, delta_bias,
                                           delta_softplus, position_indices, x)
    last_state = x[:, :, -1, 1::2]  # (batch, dim, dstate)
    if z is None:
        return out if not return_last_state else (out, last_state)
    else:
        out_z = rest[0]
        return out_z if not return_last_state else (out_z, last_state)