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Python function

flash_attention_ragged

flash_attention_ragged()

max.experimental.nn.common_layers.functional_kernels.flash_attention_ragged(kv_params, input, input_row_offsets, kv_collection, layer_idx, mask_variant, scale, local_window_size=-1, sink_weights=None, rel_logits=None, output_dtype=None)

source

Computes flash (self) attention provided the !mo.opaque KV Cache.

Notably, this materializes the attention mask (dependent on MHAMaskVariant) within the kernel. input and input_row_offsets are used together to implement the ragged tensor. input_row_offsets indicates where each batch starts and ends in input.

Note that this is self attention and the KV sequence length is assumed to be equal to the Q sequence length. For KV sequence length != Q sequence length, use cross_attention_ragged().

When rel_logits is set, the kernel gathers an additive relative-position bias by rel_dist = q_pos - k_pos from a (total_q_tokens, heads, extent) table and adds it on every visible position, selecting mo.mha.ragged.paged.rel_logits. This path only supports mask_variant values CAUSAL_MASK (with local_window_size == -1) and SLIDING_WINDOW_CAUSAL_MASK (with a positive local_window_size); sink_weights and rel_logits are mutually exclusive.

Parameters:

• kv_params (KVCacheParams) – KVCacheParams object containing key-value cache parameters.
• input (TensorValue) – TensorValue representing the input tensor with shape [total_seq_len, num_heads, head_dim].
• input_row_offsets (TensorValue) – TensorValue indicating the start and end of each batch in the input tensor with shape [batch_size + 1].
• kv_collection (PagedCacheValues) – PagedCacheValues object for managing key-value cache.
• layer_idx (TensorValue) – TensorValue representing the layer index, expected to have dtype uint32.
• mask_variant (MHAMaskVariant) – MHAMaskVariant specifying the type of attention mask to use. With rel_logits, only CAUSAL_MASK and SLIDING_WINDOW_CAUSAL_MASK are supported.
• scale (float) – float value used to scale the attention scores.
• local_window_size (int) – int specifying the size of the local attention window, default is -1 for no local window.
• sink_weights (TensorValue | None) – Optional tensor of shape [num_heads] containing learnable sink weights for each attention head.
• rel_logits (TensorValue | None) – Optional relative-position bias table with shape [total_seq_len, num_heads, extent]; row r matches input’s own ragged-flat row convention.
• output_dtype (DType | None) – Dtype for the attention output. Defaults to input.dtype.

Return type:

Tensor