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[ET-VK][ez] Ensure that attn_weight buffers do not exceed GPU buffer numel limit #15651

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Nov 7, 2025
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[ET-VK][ez] Ensure that attn_weight buffers do not exceed GPU buffer numel limit #15651

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[ET-VK][ez] Ensure that attn_weight buffers do not exceed GPU buffer …
Nov 6, 2025
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Update on "[ET-VK][ez] Ensure that attn_weight buffers do not exceed …
Nov 7, 2025
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4 changes: 4 additions & 0 deletions backends/vulkan/runtime/graph/ComputeGraph.h
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Original file line number Diff line number Diff line change
Expand Up @@ -639,6 +639,10 @@ class ComputeGraph final {

bool device_name_contains(const char* substr);

int64_t max_buffer_numel() {
return static_cast<int64_t>(context_->adapter_ptr()->max_buffer_numel());
}

//
// Graph Building
//
Expand Down
35 changes: 28 additions & 7 deletions backends/vulkan/runtime/graph/ops/impl/SDPA.cpp
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Expand Up @@ -471,10 +471,31 @@ void sdpa_impl(ComputeGraph& graph, const std::vector<ValueRef>& args) {
VK_CHECK_COND(graph.val_is_none(attn_mask));

const int64_t num_q_heads = graph.size_at<int64_t>(-2, q_projected);
const int64_t max_seq_len = graph.size_at<int64_t>(-3, q_projected);

int64_t max_seq_len = graph.size_at<int64_t>(-3, q_projected);
const int64_t max_context_len = graph.size_at<int32_t>(-3, k_cache);

const utils::StorageType attn_weights_storage =
graph.storage_type_of(q_projected);

// If using buffer storage for attn weights, we need to ensure that the buffer
// numel limit is not exceeded. If needed, manually adjust max_seq_len based
// on the buffer numel limit.
if (attn_weights_storage == utils::kBuffer) {
const int64_t max_buffer_numel = graph.max_buffer_numel();
if (num_q_heads * max_seq_len * max_context_len >= max_buffer_numel) {
// Compute the maximum possible value for max_seq_len that will hit
// the buffer numel limit.
max_seq_len = max_buffer_numel / (num_q_heads * max_context_len);
// Adjust down to the nearest multiple of 4 to make sure the limit is
// not hit.
if (max_seq_len % 4 != 0) {
max_seq_len = (max_seq_len / 4) * 4;
} else {
max_seq_len -= 4;
}
}
}

std::vector<int64_t> attn_weight_full_sizes = {
1, // batch
num_q_heads,
Expand All @@ -485,14 +506,14 @@ void sdpa_impl(ComputeGraph& graph, const std::vector<ValueRef>& args) {
&graph,
attn_weight_full_sizes,
graph.dtype_of(q_projected),
graph.storage_type_of(q_projected),
attn_weights_storage,
utils::kWidthPacked);

TmpTensor attn_weights_softmax(
&graph,
attn_weight_full_sizes,
graph.dtype_of(q_projected),
graph.storage_type_of(q_projected),
attn_weights_storage,
utils::kWidthPacked);

add_sdpa_compute_attn_weights_node(
Expand Down Expand Up @@ -528,9 +549,9 @@ void sdpa_with_kv_cache_impl(

utils::StorageType cache_storage = graph.storage_type_of(q_projected);
const ValueRef k_cache =
prepack_standard(graph, k_cache_data, cache_storage, utils::kWidthPacked);
graph.add_tensor_like(k_cache_data, cache_storage, utils::kWidthPacked);
const ValueRef v_cache =
prepack_standard(graph, v_cache_data, cache_storage, utils::kWidthPacked);
graph.add_tensor_like(v_cache_data, cache_storage, utils::kWidthPacked);

update_cache_impl(graph, {k_projected, k_cache, input_pos_symint, -1});
update_cache_impl(graph, {v_projected, v_cache, input_pos_symint, -1});
Expand Down Expand Up @@ -573,7 +594,7 @@ void compute_attn_weight_with_kv_cache_impl(

(void)sequence_len;

utils::StorageType cache_storage = graph.storage_type_of(q_projected);
const utils::StorageType cache_storage = graph.storage_type_of(q_projected);
const ValueRef k_cache =
graph.add_tensor_like(k_cache_data, cache_storage, utils::kWidthPacked);
const ValueRef v_cache =
Expand Down
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