Skip to content

flash attention: correct head dim check and L_k padding #736

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Merged
merged 2 commits into from
Jul 23, 2025
Merged

flash attention: correct head dim check and L_k padding #736

Changes from all commits
Commits
Show all changes
2 commits
Select commit Hold shift + click to select a range
4ba622b
flash attention: correct head dim check and L_k padding
Green-Sky Jul 21, 2025
0dd58bf
disable debug logging again
Green-Sky Jul 21, 2025
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
39 changes: 31 additions & 8 deletions ggml_extend.hpp
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -840,18 +840,34 @@ __STATIC_INLINE__ struct ggml_tensor* ggml_nn_attention_ext(struct ggml_context*

float scale = (1.0f / sqrt((float)d_head));

// if (flash_attn) {
// LOG_DEBUG("attention_ext L_q:%d L_k:%d n_head:%d C:%d d_head:%d N:%d", L_q, L_k, n_head, C, d_head, N);
// }
int kv_pad = 0;
//if (flash_attn) {
// LOG_DEBUG("attention_ext L_q:%d L_k:%d n_head:%d C:%d d_head:%d N:%d", L_q, L_k, n_head, C, d_head, N);
//}
// is there anything oddly shaped?? ping Green-Sky if you can trip this assert
GGML_ASSERT(((L_k % 256 == 0) && L_q == L_k) || !(L_k % 256 == 0));

bool can_use_flash_attn = true;
can_use_flash_attn = can_use_flash_attn && (
d_head == 64 ||
d_head == 80 ||
d_head == 96 ||
d_head == 112 ||
d_head == 128 ||
d_head == 256
);
#if 0
can_use_flash_attn = can_use_flash_attn && L_k % 256 == 0;
can_use_flash_attn = can_use_flash_attn && d_head % 64 == 0; // double check

// cuda max d_head seems to be 256, cpu does seem to work with 512
can_use_flash_attn = can_use_flash_attn && d_head <= 256; // double check
#else
if (can_use_flash_attn && L_k % 256 != 0) {
// TODO(Green-Sky): might be worth just padding by default
if (L_k == 77 || L_k == 4208 || L_k == 3952) {
kv_pad = GGML_PAD(L_k, 256) - L_k;
} else {
can_use_flash_attn = false;
}
}
#endif

if (mask != nullptr) {
// TODO(Green-Sky): figure out if we can bend t5 to work too
Expand All @@ -864,11 +880,18 @@ __STATIC_INLINE__ struct ggml_tensor* ggml_nn_attention_ext(struct ggml_context*
ggml_tensor* kqv = nullptr;
// GGML_ASSERT((flash_attn && can_use_flash_attn) || !flash_attn);
if (can_use_flash_attn && flash_attn) {
// LOG_DEBUG("using flash attention");
//LOG_DEBUG(" uses flash attention");
if (kv_pad != 0) {
//LOG_DEBUG(" padding k and v dim1 by %d", kv_pad);
k = ggml_pad(ctx, k, 0, kv_pad, 0, 0);
}
k = ggml_cast(ctx, k, GGML_TYPE_F16);

v = ggml_cont(ctx, ggml_permute(ctx, v, 0, 2, 1, 3)); // [N, n_head, L_k, d_head]
v = ggml_reshape_3d(ctx, v, d_head, L_k, n_head * N); // [N * n_head, L_k, d_head]
if (kv_pad != 0) {
v = ggml_pad(ctx, v, 0, kv_pad, 0, 0);
}
v = ggml_cast(ctx, v, GGML_TYPE_F16);

if (mask != nullptr) {
Expand Down
Loading