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fix: add head_dim=256 to fused SDPA full attention kernel #3293

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fix: add head_dim=256 to fused SDPA full attention kernel #3293

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726c9a0
fix: add head_dim=256 to fused SDPA full attention kernel
Thump604 Mar 22, 2026
7397435
perf: route head_dim=256 to unfused SDPA for short sequences
Thump604 Mar 24, 2026
d08d2da
feat: add head_dim=192 to fused SDPA kernel support
Thump604 Mar 25, 2026
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2 changes: 2 additions & 0 deletions mlx/backend/metal/kernels/scaled_dot_product_attention.metal
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Original file line number Diff line number Diff line change
Expand Up @@ -32,10 +32,12 @@ using namespace metal;
instantiate_sdpa_vector(type, 64, 64) \
instantiate_sdpa_vector(type, 96, 96) \
instantiate_sdpa_vector(type, 128, 128) \
instantiate_sdpa_vector(type, 192, 192) \
instantiate_sdpa_vector(type, 256, 256) \
instantiate_sdpa_vector_aggregation(type, 64) \
instantiate_sdpa_vector_aggregation(type, 96) \
instantiate_sdpa_vector_aggregation(type, 128) \
instantiate_sdpa_vector_aggregation(type, 192) \
instantiate_sdpa_vector_aggregation(type, 256)

instantiate_sdpa_vector_heads(float)
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1 change: 1 addition & 0 deletions mlx/backend/metal/kernels/steel/attn/kernels/steel_attention.metal
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Expand Up @@ -12,6 +12,7 @@
attention, dtype, bq, bk, bd, wm, wn, mtype, float)

#define instantiate_attn_shapes_helper(iname, itype, mname, mtype) \
instantiate_attn(iname, itype, 32, 16, 256, 4, 1, mname, mtype) \
instantiate_attn(iname, itype, 32, 16, 128, 4, 1, mname, mtype) \
instantiate_attn(iname, itype, 32, 32, 80, 4, 1, mname, mtype) \
instantiate_attn(iname, itype, 32, 32, 64, 4, 1, mname, mtype)
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12 changes: 10 additions & 2 deletions mlx/backend/metal/scaled_dot_product_attention.cpp
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Expand Up @@ -618,9 +618,17 @@ bool ScaledDotProductAttention::use_fallback(
const bool sdpa_vector_supported_head_dim =
query_head_dim == value_head_dim &&
(query_head_dim == 64 || query_head_dim == 96 || query_head_dim == 128 ||
query_head_dim == 256);
query_head_dim == 192 || query_head_dim == 256);
// For head_dim >= 192, the fused full-attention kernel is slower than
// unfused for short sequences. Only route to fused when kL is large enough
// that the unfused path would exceed Metal buffer limits (the fused kernel
// tiles K/V so it scales to arbitrary sequence lengths).
const bool sdpa_full_large_hd_ok =
(query_head_dim == 192 || query_head_dim == 256) &&
key_sequence_length > 16384;
const bool sdpa_full_supported_head_dim = query_head_dim == value_head_dim &&
(query_head_dim == 64 || query_head_dim == 80 || query_head_dim == 128);
(query_head_dim == 64 || query_head_dim == 80 || query_head_dim == 128 ||
sdpa_full_large_hd_ok);

const bool sdpa_full_supported_mask = !has_mask || has_arr_mask ||
(query_sequence_length <= key_sequence_length && do_causal);
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