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Add num_splits for mha_varlen_fwd FA2, support batch invariant #110

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Add num_splits for mha_varlen_fwd FA2, support batch invariant #110

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3 changes: 2 additions & 1 deletion csrc/flash_attn/flash_api.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -533,6 +533,7 @@ mha_varlen_fwd(at::Tensor &q, // total_q x num_heads x head_size, total_q := \s
int window_size_right,
const float softcap,
const bool return_softmax,
int num_splits,
std::optional<at::Generator> gen_) {

// Otherwise the kernel will be launched from cuda:0 device
Expand Down Expand Up @@ -706,7 +707,7 @@ mha_varlen_fwd(at::Tensor &q, // total_q x num_heads x head_size, total_q := \s
std::tie(softmax_lse_accum, out_accum) =
set_params_splitkv(params, batch_size, num_heads, head_size,
max_seqlen_k, max_seqlen_q, head_size_rounded,
p_dropout, /*num_splits*/ 0, get_num_sm(get_current_device()), opts);
p_dropout, num_splits, get_num_sm(get_current_device()), opts);
}

if (leftpad_k_.has_value()) {
Expand Down
3 changes: 2 additions & 1 deletion csrc/flash_attn/flash_api_torch_lib.cpp
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Expand Up @@ -35,6 +35,7 @@ mha_varlen_fwd(at::Tensor &q, // total_q x num_heads x head_size, total_q := \s
int window_size_right,
const float softcap,
const bool return_softmax,
int num_splits,
std::optional<at::Generator> gen_);

std::vector<at::Tensor>
Expand Down Expand Up @@ -109,7 +110,7 @@ TORCH_LIBRARY_EXPAND(TORCH_EXTENSION_NAME, ops) {
"Tensor cu_seqlens_k, Tensor? seqused_k, Tensor? leftpad_k, Tensor? block_table, Tensor? alibi_slopes, "
"int max_seqlen_q, int max_seqlen_k, float p_dropout, float softmax_scale, bool zero_tensors, "
"bool is_causal, int window_size_left, int window_size_right, float softcap, bool return_softmax, "
"Generator? gen) -> Tensor[]");
"int num_splits, Generator? gen) -> Tensor[]");
ops.impl("varlen_fwd", torch::kCUDA, make_pytorch_shim(&mha_varlen_fwd));

ops.def("fwd_kvcache(Tensor! q, Tensor kcache, Tensor vcache, Tensor? k, Tensor? v, Tensor? seqlens_k, "
Expand Down
1 change: 1 addition & 0 deletions vllm_flash_attn/flash_attn_interface.py
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Expand Up @@ -271,6 +271,7 @@ def flash_attn_varlen_func(
real_window_size[1],
softcap,
return_softmax_lse and dropout_p > 0,
num_splits,
None,
)
elif fa_version == 3:
Expand Down