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Add MLX_SDPA_BLOCKS env var for 2-pass vector kernel block-count override #3455

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Add MLX_SDPA_BLOCKS env var for 2-pass vector kernel block-count override #3455

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0b47c46
feat: MLX_SDPA_BLOCKS env var to override 2-pass vector kernel block …
adurham Apr 26, 2026
4b87e47
sdpa: address review feedback on MLX_SDPA_BLOCKS
Apr 27, 2026
a7a77ab
sdpa: inline MLX_SDPA_BLOCKS env read, drop helper
Apr 27, 2026
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3 changes: 3 additions & 0 deletions mlx/backend/metal/scaled_dot_product_attention.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -474,6 +474,9 @@ void sdpa_vector_2pass(
blocks = 32;
}
}
if (int blocks_env = env::get_var("MLX_SDPA_BLOCKS", 0); blocks_env > 0) {
blocks = blocks_env;
}
size_t k_head_stride = k.shape(1) == 1 ? k.strides(0) : k.strides(1);
size_t k_seq_stride = k.strides()[2];
size_t v_head_stride = v.shape(1) == 1 ? v.strides(0) : v.strides(1);
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