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vulkan: optimize rms_norm, and allow the work to spread across multiple SMs #15281

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97ec47a
vulkan: optimize rms_norm, and allow the work to spread across multip…
jeffbolznv Aug 13, 2025
075dac2
Change add+rms_norm optimization to write out an array of partial sums
jeffbolznv Aug 13, 2025
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142 changes: 120 additions & 22 deletions ggml/src/ggml-vulkan/ggml-vulkan.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -369,6 +369,10 @@ struct vk_device_struct {
bool subgroup_add;
bool subgroup_shuffle;

bool atomic_float_add;
bool add_rms_fusion;
uint32_t atomic_binding_alignment;

bool integer_dot_product;

bool subgroup_size_control;
Expand Down Expand Up @@ -448,6 +452,8 @@ struct vk_device_struct {
vk_pipeline pipeline_mul_norepeat[2][2][2];
vk_pipeline pipeline_div[2][2][2];
vk_pipeline pipeline_div_norepeat[2][2][2];
vk_pipeline pipeline_add_rms[2][2][2];
vk_pipeline pipeline_add_rms_norepeat[2][2][2];

vk_pipeline pipeline_add_id_f32;

Expand Down Expand Up @@ -1144,6 +1150,12 @@ class vk_perf_logger {
timings[name].push_back(time);
return;
}
if (node->op == GGML_OP_RMS_NORM) {
std::string name = ggml_op_name(node->op);
name += "(" + std::to_string(node->ne[0]) + "," + std::to_string(node->ne[1]) + "," + std::to_string(node->ne[2]) + "," + std::to_string(node->ne[3]) + ")";
timings[name].push_back(time);
return;
}
timings[ggml_op_name(node->op)].push_back(time);
}
private:
Expand All @@ -1158,10 +1170,13 @@ struct ggml_backend_vk_context {

size_t semaphore_idx, event_idx;
ggml_vk_garbage_collector gc;
size_t prealloc_size_x, prealloc_size_y, prealloc_size_split_k;
vk_buffer prealloc_x, prealloc_y, prealloc_split_k;
size_t prealloc_size_x, prealloc_size_y, prealloc_size_split_k, prealloc_size_atomic_add, prealloc_size_atomic_add_offset;
vk_buffer prealloc_x, prealloc_y, prealloc_split_k, prealloc_atomic_add;
vk::Fence fence, almost_ready_fence;
bool almost_ready_fence_pending {};
// Set before op_add and unset after op_rms_norm to indicate that the add should
// use atomics to accumulate the square of the vector components
bool do_add_rms_atomic;

vk_buffer buffer_pool[MAX_VK_BUFFERS];

Expand Down Expand Up @@ -2924,8 +2939,8 @@ static void ggml_vk_load_shaders(vk_device& device) {

ggml_vk_create_pipeline(device, device->pipeline_norm_f32, "norm_f32", norm_f32_len, norm_f32_data, "main", 2, sizeof(vk_op_push_constants), {1, 1, 1}, {}, 1);
ggml_vk_create_pipeline(device, device->pipeline_group_norm_f32, "group_norm_f32", group_norm_f32_len, group_norm_f32_data, "main", 2, sizeof(vk_op_push_constants), {1, 1, 1}, {}, 1);
ggml_vk_create_pipeline(device, device->pipeline_rms_norm_f32, "rms_norm_f32", rms_norm_f32_len, rms_norm_f32_data, "main", 3, sizeof(vk_op_binary_push_constants), {1, 1, 1}, {0, 0}, 1);
ggml_vk_create_pipeline(device, device->pipeline_rms_norm_mul_f32, "rms_norm_mul_f32", rms_norm_f32_len, rms_norm_f32_data, "main", 3, sizeof(vk_op_binary_push_constants), {1, 1, 1}, {0, 1}, 1);
ggml_vk_create_pipeline(device, device->pipeline_rms_norm_f32, "rms_norm_f32", rms_norm_f32_len, rms_norm_f32_data, "main", 4, sizeof(vk_op_binary_push_constants), {1, 1, 1}, {0, 0}, 1, true);
ggml_vk_create_pipeline(device, device->pipeline_rms_norm_mul_f32, "rms_norm_mul_f32", rms_norm_f32_len, rms_norm_f32_data, "main", 4, sizeof(vk_op_binary_push_constants), {1, 1, 1}, {0, 1}, 1, true);
ggml_vk_create_pipeline(device, device->pipeline_rms_norm_back_f32, "rms_norm_back_f32", rms_norm_back_f32_len, rms_norm_back_f32_data, "main", 3, sizeof(vk_op_push_constants), {1, 1, 1}, {}, 1);
ggml_vk_create_pipeline(device, device->pipeline_l2_norm_f32, "l2_norm_f32", l2_norm_f32_len, l2_norm_f32_data, "main", 2, sizeof(vk_op_push_constants), {1, 1, 1}, {}, 1);

Expand Down Expand Up @@ -2995,20 +3010,22 @@ static void ggml_vk_load_shaders(vk_device& device) {
};

bool rte = device->float_controls_rte_fp16;
#define CREATE_BINARY(name, namemod, spec) \
#define CREATE_BINARY(name, namemod, spec, bindings) \
for (int s0 : {0,1}) for (int s1 : {0,1}) for (int d : {0,1}) \
ggml_vk_create_pipeline(device, device->pipeline_ ## name ## namemod[s0][s1][d], \
#name + get_suffix(s0, s1, d) + #namemod, name ## _len[s0][s1][d][rte], name ## _data[s0][s1][d][rte], \
"main", 3, sizeof(vk_op_binary_push_constants), {512, 1, 1}, spec, 1);

CREATE_BINARY(add, , {0})
CREATE_BINARY(add, _norepeat, {1})
CREATE_BINARY(sub, , {0})
CREATE_BINARY(sub, _norepeat, {1})
CREATE_BINARY(mul, , {0})
CREATE_BINARY(mul, _norepeat, {1})
CREATE_BINARY(div, , {0})
CREATE_BINARY(div, _norepeat, {1})
"main", (bindings), sizeof(vk_op_binary_push_constants), {512, 1, 1}, spec, 1);

CREATE_BINARY(add, , {0}, 4)
CREATE_BINARY(add, _norepeat, {1}, 4)
CREATE_BINARY(sub, , {0}, 3)
CREATE_BINARY(sub, _norepeat, {1}, 3)
CREATE_BINARY(mul, , {0}, 3)
CREATE_BINARY(mul, _norepeat, {1}, 3)
CREATE_BINARY(div, , {0}, 3)
CREATE_BINARY(div, _norepeat, {1}, 3)
CREATE_BINARY(add_rms, , {0}, 4)
CREATE_BINARY(add_rms, _norepeat, {1}, 4)
#undef CREATE_BINARY

ggml_vk_create_pipeline(device, device->pipeline_add_id_f32, "add_id_f32", add_id_f32_len, add_id_f32_data, "main", 4, sizeof(vk_op_add_id_push_constants), {1, 1, 1}, {}, 1);
Expand Down Expand Up @@ -3281,6 +3298,7 @@ static vk_device ggml_vk_get_device(size_t idx) {
device->coopmat_support = false;
device->integer_dot_product = false;
bool bfloat16_support = false;
bool atomic_float_support = false;

for (const auto& properties : ext_props) {
if (strcmp("VK_KHR_maintenance4", properties.extensionName) == 0) {
Expand Down Expand Up @@ -3320,6 +3338,8 @@ static vk_device ggml_vk_get_device(size_t idx) {
!getenv("GGML_VK_DISABLE_BFLOAT16")) {
bfloat16_support = true;
#endif
} else if (strcmp("VK_EXT_shader_atomic_float", properties.extensionName) == 0) {
atomic_float_support = true;
}
}

Expand Down Expand Up @@ -3536,6 +3556,14 @@ static vk_device ggml_vk_get_device(size_t idx) {
device_extensions.push_back("VK_KHR_shader_integer_dot_product");
}

VkPhysicalDeviceShaderAtomicFloatFeaturesEXT atomic_float_features {};
atomic_float_features.sType = VK_STRUCTURE_TYPE_PHYSICAL_DEVICE_SHADER_ATOMIC_FLOAT_FEATURES_EXT;
if (atomic_float_support) {
last_struct->pNext = (VkBaseOutStructure *)&atomic_float_features;
last_struct = (VkBaseOutStructure *)&atomic_float_features;
device_extensions.push_back("VK_EXT_shader_atomic_float");
}

vkGetPhysicalDeviceFeatures2(device->physical_device, &device_features2);

device->fp16 = device->fp16 && vk12_features.shaderFloat16;
Expand All @@ -3547,6 +3575,7 @@ static vk_device ggml_vk_get_device(size_t idx) {
#endif

device->pipeline_robustness = pl_robustness_features.pipelineRobustness;
device->atomic_float_add = atomic_float_features.shaderBufferFloat32AtomicAdd;

if (device->subgroup_size_control) {
device->subgroup_min_size = subgroup_size_control_props.minSubgroupSize;
Expand Down Expand Up @@ -3861,6 +3890,12 @@ static vk_device ggml_vk_get_device(size_t idx) {

device->disable_fusion = getenv("GGML_VK_DISABLE_FUSION") != nullptr;

device->add_rms_fusion = !device->disable_fusion &&
device->subgroup_add &&
device->atomic_float_add;
device->atomic_binding_alignment =
std::max(4u, (uint32_t)device->properties.limits.minStorageBufferOffsetAlignment);

return device;
}

Expand Down Expand Up @@ -6892,8 +6927,13 @@ static vk_pipeline ggml_vk_op_get_pipeline(ggml_backend_vk_context * ctx, const
switch (op) {
case GGML_OP_ADD:
{
auto pipelines = ggml_are_same_shape(src0, src1) ? ctx->device->pipeline_add_norepeat : ctx->device->pipeline_add;
return pipelines[src0->type == GGML_TYPE_F16][src1->type == GGML_TYPE_F16][dst->type == GGML_TYPE_F16];
if (ctx->do_add_rms_atomic) {
auto pipelines = ggml_are_same_shape(src0, src1) ? ctx->device->pipeline_add_rms_norepeat : ctx->device->pipeline_add_rms;
return pipelines[src0->type == GGML_TYPE_F16][src1->type == GGML_TYPE_F16][dst->type == GGML_TYPE_F16];
} else {
auto pipelines = ggml_are_same_shape(src0, src1) ? ctx->device->pipeline_add_norepeat : ctx->device->pipeline_add;
return pipelines[src0->type == GGML_TYPE_F16][src1->type == GGML_TYPE_F16][dst->type == GGML_TYPE_F16];
}
}
case GGML_OP_SUB:
{
Expand Down Expand Up @@ -7494,7 +7534,12 @@ static void ggml_vk_op_f32(ggml_backend_vk_context * ctx, vk_context& subctx, co
}
} break;
case GGML_OP_RMS_NORM:
elements = { (uint32_t)ne01, (uint32_t)ne02, (uint32_t)ne03 };
if (ctx->do_add_rms_atomic) {
// Run one element per thread, 128 threads per workgroup
elements = { (uint32_t)CEIL_DIV(ne00, 128), 1, 1 };
} else {
elements = { (uint32_t)ne01, (uint32_t)ne02, (uint32_t)ne03 };
}
break;

case GGML_OP_SUM:
Expand Down Expand Up @@ -7642,7 +7687,17 @@ static void ggml_vk_op_f32(ggml_backend_vk_context * ctx, vk_context& subctx, co
}
}

if (op == GGML_OP_GLU) {
if (op == GGML_OP_ADD || op == GGML_OP_RMS_NORM) {
vk_buffer d_A = ctx->prealloc_atomic_add ? ctx->prealloc_atomic_add : d_X;
size_t a_buf_offset = ctx->prealloc_atomic_add ? ctx->prealloc_size_atomic_add_offset : 0;
ggml_vk_sync_buffers(subctx);
ggml_vk_dispatch_pipeline(ctx, subctx, pipeline,
{ vk_subbuffer{ d_X, x_buf_offset, x_sz },
vk_subbuffer{ d_Y, y_buf_offset, y_sz },
vk_subbuffer{ d_D, d_buf_offset, d_sz },
vk_subbuffer{ d_A, a_buf_offset, VK_WHOLE_SIZE },
}, pc, elements);
} else if (op == GGML_OP_GLU) {
// Empty src1 is possible in glu, but the shader needs a buffer
vk_subbuffer subbuf_y;
if (use_src1) {
Expand Down Expand Up @@ -7750,7 +7805,7 @@ static void ggml_vk_add(ggml_backend_vk_context * ctx, vk_context& subctx, const
(uint32_t)src1->ne[0], (uint32_t)src1->ne[1], (uint32_t)src1->ne[2],(uint32_t)src1->ne[3], (uint32_t)src1->nb[0] / src1_type_size, (uint32_t)src1->nb[1] / src1_type_size, (uint32_t)src1->nb[2] / src1_type_size, (uint32_t)src1->nb[3] / src1_type_size,
(uint32_t) dst->ne[0], (uint32_t) dst->ne[1], (uint32_t) dst->ne[2],(uint32_t) dst->ne[3], (uint32_t) dst->nb[0] / dst_type_size, (uint32_t) dst->nb[1] / dst_type_size, (uint32_t) dst->nb[2] / dst_type_size, (uint32_t) dst->nb[3] / dst_type_size,
0,
0.0f, 0.0f, 0,
0.0f, 0.0f, ctx->do_add_rms_atomic,
}, dryrun);
}

Expand Down Expand Up @@ -8213,8 +8268,13 @@ static void ggml_vk_rms_norm(ggml_backend_vk_context * ctx, vk_context& subctx,
(uint32_t)src1->ne[0], (uint32_t)src1->ne[1], (uint32_t)src1->ne[2],(uint32_t)src1->ne[3], (uint32_t)src1->nb[0] / src1_type_size, (uint32_t)src1->nb[1] / src1_type_size, (uint32_t)src1->nb[2] / src1_type_size, (uint32_t)src1->nb[3] / src1_type_size,
(uint32_t) dst->ne[0], (uint32_t) dst->ne[1], (uint32_t) dst->ne[2],(uint32_t) dst->ne[3], (uint32_t) dst->nb[0] / dst_type_size, (uint32_t) dst->nb[1] / dst_type_size, (uint32_t) dst->nb[2] / dst_type_size, (uint32_t) dst->nb[3] / dst_type_size,
0,
op_params[0], 0.0f, 0,
op_params[0], 0.0f, ctx->do_add_rms_atomic,
}, dryrun);

if (ctx->do_add_rms_atomic) {
ctx->prealloc_size_atomic_add_offset += ctx->device->atomic_binding_alignment;
ctx->do_add_rms_atomic = false;
}
}

static void ggml_vk_rms_norm_back(ggml_backend_vk_context * ctx, vk_context& subctx, const ggml_tensor * src0, const ggml_tensor * src1, ggml_tensor * dst, bool dryrun = false) {
Expand Down Expand Up @@ -9492,6 +9552,14 @@ static void ggml_vk_preallocate_buffers(ggml_backend_vk_context * ctx) {
}
ctx->prealloc_split_k = ggml_vk_create_buffer_device(ctx->device, ctx->prealloc_size_split_k);
}
if (ctx->prealloc_atomic_add == nullptr || (ctx->prealloc_size_atomic_add > 0 && ctx->prealloc_atomic_add->size < ctx->prealloc_size_atomic_add)) {
VK_LOG_MEMORY("ggml_vk_preallocate_buffers(atomic_add_size: " << ctx->prealloc_atomic_add << ")");
// Resize buffer
if (ctx->prealloc_atomic_add != nullptr) {
ggml_vk_destroy_buffer(ctx->prealloc_atomic_add);
}
ctx->prealloc_atomic_add = ggml_vk_create_buffer_device(ctx->device, ctx->prealloc_size_atomic_add);
}
}

static bool ggml_vk_compute_forward(ggml_backend_vk_context* ctx, ggml_cgraph * cgraph, ggml_tensor* tensor, int tensor_idx, bool use_fence, bool almost_ready);
Expand Down Expand Up @@ -9547,10 +9615,21 @@ static bool ggml_vk_build_graph(ggml_backend_vk_context * ctx, ggml_cgraph * cgr
return false;
}
break;
case GGML_OP_ADD:
if (node_idx + 1 < cgraph->n_nodes &&
cgraph->nodes[node_idx + 1]->op == GGML_OP_RMS_NORM &&
cgraph->nodes[node_idx + 1]->src[0] == cgraph->nodes[node_idx] &&
ggml_nrows(cgraph->nodes[node_idx + 1]) == 1 &&
ctx->device->add_rms_fusion) {
if (dryrun) {
ctx->prealloc_size_atomic_add += ctx->device->atomic_binding_alignment;
}
ctx->do_add_rms_atomic = true;
}
break;
case GGML_OP_REPEAT:
case GGML_OP_REPEAT_BACK:
case GGML_OP_GET_ROWS:
case GGML_OP_ADD:
case GGML_OP_ADD_ID:
case GGML_OP_ACC:
case GGML_OP_SUB:
Expand Down Expand Up @@ -9667,6 +9746,9 @@ static bool ggml_vk_build_graph(ggml_backend_vk_context * ctx, ggml_cgraph * cgr
// do the only thing needed for the dryrun.
vk_pipeline pipeline = ggml_vk_op_get_pipeline(ctx, src0, src1, src2, node, node->op);
ggml_pipeline_request_descriptor_sets(ctx, pipeline, 1);
if (node->op == GGML_OP_RMS_NORM) {
ctx->do_add_rms_atomic = false;
}
return false;
}
default:
Expand Down Expand Up @@ -10581,6 +10663,10 @@ static ggml_status ggml_backend_vk_graph_compute(ggml_backend_t backend, ggml_cg
vk_instance.pfn_vkQueueBeginDebugUtilsLabelEXT(ctx->device->compute_queue.queue, reinterpret_cast<VkDebugUtilsLabelEXT*>(&dul));
}

ctx->prealloc_size_atomic_add = 0;
ctx->prealloc_size_atomic_add_offset = 0;
ctx->do_add_rms_atomic = false;

uint64_t total_mat_mul_bytes = 0;
for (int i = 0; i < cgraph->n_nodes; i++) {
if (!ctx->device->disable_fusion && ggml_vk_can_fuse(cgraph, i, { GGML_OP_RMS_NORM, GGML_OP_MUL })) {
Expand Down Expand Up @@ -10641,6 +10727,18 @@ static ggml_status ggml_backend_vk_graph_compute(ggml_backend_t backend, ggml_cg
compute_ctx->s->buffer.writeTimestamp(vk::PipelineStageFlagBits::eAllCommands, ctx->device->query_pool, 0);
}

if (ctx->prealloc_size_atomic_add) {
if (ctx->compute_ctx.expired()) {
compute_ctx = ggml_vk_create_context(ctx, ctx->compute_cmd_pool);
ctx->compute_ctx = compute_ctx;
ggml_vk_ctx_begin(ctx->device, compute_ctx);
} else {
compute_ctx = ctx->compute_ctx.lock();
}
// initialize atomic sums to zero.
ggml_vk_buffer_memset_async(compute_ctx, ctx->prealloc_atomic_add, 0, 0, ctx->prealloc_size_atomic_add);
}

// Submit after enough work has accumulated, to overlap CPU cmdbuffer generation with GPU execution.
// Estimate the amount of matmul work by looking at the weight matrix size, and submit every 100MB
// (and scaled down based on model size, so smaller models submit earlier).
Expand Down
23 changes: 22 additions & 1 deletion ggml/src/ggml-vulkan/vulkan-shaders/add.comp
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Original file line number Diff line number Diff line change
@@ -1,12 +1,19 @@
#version 450

#extension GL_EXT_shader_16bit_storage : require
#if ADD_RMS
#extension GL_EXT_shader_atomic_float : enable
#extension GL_KHR_shader_subgroup_arithmetic : enable
#extension GL_KHR_shader_subgroup_basic : enable
#endif

#include "types.comp"
#include "generic_binary_head.comp"

const uint num_threads = 256;

layout (binding = 3) buffer AtomBuf {float data_atom;};

layout(local_size_x = num_threads, local_size_y = 1, local_size_z = 1) in;

void main() {
Expand All @@ -15,15 +22,29 @@ void main() {
// num_threads * num_iter must equal 512, to match the wg_denoms and get_idx calculation
const uint num_iter = 2;

FLOAT_TYPE sum_sq = 0;

[[unroll]] for (uint i = 0; i < num_iter; ++i) {
if (idx >= p.ne) {
continue;
}
uint i00, i01, i02, i03;
get_indices(idx, i00, i01, i02, i03);

data_d[get_doffset() + dst_idx(i00, i01, i02, i03)] = D_TYPE(FLOAT_TYPE(data_a[get_aoffset() + src0_idx(i00, i01, i02, i03)]) + FLOAT_TYPE(data_b[get_boffset() + src1_idx(i00, i01, i02, i03)]));
FLOAT_TYPE sum = FLOAT_TYPE(data_a[get_aoffset() + src0_idx(i00, i01, i02, i03)]) + FLOAT_TYPE(data_b[get_boffset() + src1_idx(i00, i01, i02, i03)]);
sum_sq += sum*sum;

data_d[get_doffset() + dst_idx(i00, i01, i02, i03)] = D_TYPE(sum);

idx += num_threads;
}

#if ADD_RMS
if (p.param3 != 0) {
sum_sq = subgroupAdd(sum_sq);
if (sum_sq != 0 && gl_SubgroupInvocationID == 0) {
atomicAdd(data_atom, sum_sq);
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Just want to point out that this potentially introduces a bit of nondeterminism due to floating point addition not being associative. I don't expect it to be a problem, just want to mention in case anybody is concerned.

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Hm, it's not a good idea to introduce nondeterminism in the computations. Are there alternatives?

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Second commit changes this to write out a partial sum for each workgroup, and the rms_norm shader adds them up, so it's a deterministic order now.

}
}
#endif
}
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