vllm.model_executor.layers.mamba.ops.ssd_combined ¶

def _mamba_chunk_scan_combined_fwd(
    x,
    dt,
    A,
    B,
    C,
    chunk_size,
    out,
    D=None,
    z=None,
    dt_bias=None,
    initial_states=None,
    return_intermediate_states=False,
    seq_idx=None,
    cu_seqlens=None,
    cu_chunk_seqlens=None,
    last_chunk_indices=None,
    dt_softplus=False,
    dt_limit=(0.0, float("inf")),
    state_dtype=None,
):
    assert is_int_pow_2(chunk_size), "chunk_size must be integer power of 2"
    seqlen, nheads, headdim = x.shape
    _, ngroups, dstate = B.shape
    assert nheads % ngroups == 0
    assert B.shape == (seqlen, ngroups, dstate)
    assert dt.shape == (seqlen, nheads)
    assert A.shape == (nheads,)
    assert C.shape == B.shape
    if z is not None:
        assert z.shape == x.shape
    if D is not None:
        assert D.shape == (nheads, headdim) or D.shape == (nheads,)
    if seq_idx is not None:
        assert seq_idx.shape == (cu_chunk_seqlens.shape[0] - 1,)
    if B.stride(-1) != 1:
        B = B.contiguous()
    if C.stride(-1) != 1:
        C = C.contiguous()
    if (
        x.stride(-1) != 1 and x.stride(0) != 1
    ):  # Either M or K dimension should be contiguous
        x = x.contiguous()
    if (
        z is not None and z.stride(-1) != 1 and z.stride(0) != 1
    ):  # Either M or K dimension should be contiguous
        z = z.contiguous()
    if D is not None and D.stride(-1) != 1:
        D = D.contiguous()
    assert cu_seqlens is not None, "Assuming varlen input - must supply cu_seqlens"

    if initial_states is not None:
        assert initial_states.shape == (len(cu_seqlens) - 1, nheads, headdim, dstate)

    # This function executes 5 sub-functions for computing mamba
    # - a good resource is the blog https://goombalab.github.io/blog/2024/mamba2-part3-algorithm/
    #   which has a minimal implementation to understand the below operations
    # - as explained by the blog, mamba is a special case of causal attention
    # - the idea is to chunk the attention matrix and compute each
    #   submatrix separately using different optimizations.
    # - see the blog and paper for a visualization of the submatrices
    #   which we refer to in the comments below

    # 1. Compute chunked cumsum of A * dt
    # - here dt may go through a softplus activation
    dA_cumsum, dt = _chunk_cumsum_fwd(
        dt,
        A,
        chunk_size,
        cu_chunk_seqlens,
        dt_bias=dt_bias,
        dt_softplus=dt_softplus,
        dt_limit=dt_limit,
    )

    # 2. Compute the state for each intra-chunk
    # (right term of low-rank factorization of off-diagonal blocks; B terms)
    states = _chunk_state_fwd(
        B, x, dt, dA_cumsum, cu_chunk_seqlens, states_in_fp32=True
    )

    # 3. Compute the inter-chunk SSM recurrence; produces correct SSM states at chunk boundaries
    # (middle term of factorization of off-diag blocks; A terms)
    # - for handling chunked prefill, this requires i) initial_states and
    #   ii) seq_idx to be all specified.
    # - When a new seq_idx is detected, we will stop passing the prev_state
    #   and switch accordingly to the init_state corresponding to the new seq_idx.
    states = _state_passing_fwd(
        rearrange(states, "... p n -> ... (p n)"),
        dA_cumsum,  # (nheads, nchunks, chunk_size)
        cu_chunk_seqlens,
        initial_states=rearrange(initial_states, "... p n -> ... (p n)")
        if initial_states is not None
        else None,  # (batch, nheads, headdim*dstate)
        seq_idx=seq_idx,
        out_dtype=state_dtype if state_dtype is not None else C.dtype,
    )
    states = rearrange(states, "... (p n) -> ... p n", n=dstate)

    # 4. Compute batched matrix multiply for C_j^T B_i terms
    CB = _bmm_chunk_fwd(C, B, chunk_size, cu_chunk_seqlens, output_dtype=torch.float32)

    # 5. Scan and compute the diagonal blocks, taking into
    #    account past causal states.
    # - if initial states are provided, then states information will be
    #   augmented with initial_states.
    # - to do this properly, we need to account for example changes in
    #   the continuous batch, therefore we introduce pseudo chunks, which is
    #   a chunk that is split up each time an example changes.
    # - in each (pseudo) chunk, we detect if the previous (pseudo) chunk had
    #   a seq_idx change, in which case we take states information from
    #   init_states.
    _chunk_scan_fwd(
        CB,
        x,
        dt,
        dA_cumsum,
        C,
        states,
        cu_chunk_seqlens,
        out,  # in-place update
        seq_idx,
        D=D,
        z=z,
        initial_states=initial_states,
    )

    if return_intermediate_states:
        return states
    else:
        return states[last_chunk_indices]

def is_int_pow_2(n):
    return isinstance(n, int) and n > 0 and (n & (n - 1)) == 0

def mamba_chunk_scan_combined_varlen(
    x,
    dt,
    A,
    B,
    C,
    chunk_size,
    cu_seqlens,
    cu_chunk_seqlens,
    last_chunk_indices,
    seq_idx,
    out,
    D=None,
    z=None,
    dt_bias=None,
    initial_states=None,
    dt_softplus=False,
    dt_limit=(0.0, float("inf")),
    return_intermediate_states=False,
    state_dtype=None,
):
    """
    Argument:
        x: (seqlen, nheads, headdim)
        dt: (seqlen, nheads)
        A: (nheads)
        B: (seqlen, ngroups, dstate)
        C: (seqlen, ngroups, dstate)
        chunk_size: int
        cu_seqlens: (batch + 1,)
        cu_chunk_seqlens: (nchunks + 1,)
        last_chunk_indices: (batch,)
        seq_idx: (nchunks,)
        out: (seqlen, nheads, headdim) preallocated output tensor
        D: (nheads, headdim) or (nheads,)
        z: (seqlen, nheads, headdim)
        dt_bias: (nheads,)
        initial_states: (batch, nheads, headdim, dstate)
        dt_softplus: Whether to apply softplus to dt
        out: (seqlen, nheads, headdim) preallocated output tensor
        state_dtype: The data type of the ssm state
    Return:
        varlen_states: (batch, nheads, headdim, dstate)
    """

    assert cu_seqlens is not None, "cu_seqlens must be provided assuming varlen input"
    assert seq_idx is not None

    varlen_states = _mamba_chunk_scan_combined_fwd(
        x,
        dt,
        A,
        B,
        C,
        chunk_size,
        out,
        D=D,
        z=z,
        dt_bias=dt_bias,
        initial_states=initial_states,
        return_intermediate_states=return_intermediate_states,
        seq_idx=seq_idx,
        cu_seqlens=cu_seqlens,
        cu_chunk_seqlens=cu_chunk_seqlens,
        last_chunk_indices=last_chunk_indices,
        dt_softplus=dt_softplus,
        dt_limit=dt_limit,
        state_dtype=state_dtype,
    )

    return varlen_states