koboldcpp/ggml/src/ggml-vulkan/vulkan-shaders/flash_attn_split_k_reduce.comp
Jeff Bolz 6efcd65945
vulkan: optimize flash attention split_k_reduce (#14554)
* vulkan: allow FA split_k with smaller KV values

* vulkan: spread split_k_reduce work across more threads

k_num can get rather large. Use the whole workgroup to reduce the M/L values.

Launch a thread for each element in the HSV dimension of the output. Helps a
lot for large HSV (like deepseek).
2025-07-08 20:11:42 +02:00

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Text

#version 450
#extension GL_EXT_control_flow_attributes : enable
layout(constant_id = 0) const uint BLOCK_SIZE = 32;
layout(local_size_x_id = 0, local_size_y = 1, local_size_z = 1) in;
layout (binding = 0) readonly buffer A {float data_a[];};
layout (binding = 1) writeonly buffer D {float data_d[];};
layout (push_constant) uniform parameter {
uint D;
uint N;
uint ne3;
uint k_num;
} p;
shared float tmpsh[BLOCK_SIZE];
void main() {
// Each workgroup handles a row
const uint n = gl_WorkGroupID.x;
const uint tid = gl_LocalInvocationID.x;
const uint iq3 = gl_WorkGroupID.z;
uint D = p.D;
uint N = p.N;
uint k_num = p.k_num;
uint l_offset = D * N * p.ne3 * k_num + N * iq3 * k_num * 2 + n;
uint m_offset = D * N * p.ne3 * k_num + N * iq3 * k_num * 2 + N + n;
uint lm_stride = N * 2;
// Compute the max m value for the row
float m_max = -1.0/0.0;
for (uint k = 0; k + tid < k_num; k += BLOCK_SIZE) {
float m = data_a[m_offset + (k + tid) * lm_stride];
m_max = max(m_max, m);
}
// reduce across the workgroup
tmpsh[tid] = m_max;
barrier();
[[unroll]] for (uint s = BLOCK_SIZE/2; s > 0; s >>= 1) {
if (tid < s) {
m_max = max(m_max, tmpsh[tid + s]);
tmpsh[tid] = m_max;
}
barrier();
}
m_max = tmpsh[0];
barrier();
// Compute L based on m_max
float L = 0;
for (uint k = 0; k + tid < k_num; k += BLOCK_SIZE) {
float l = data_a[l_offset + (k + tid) * lm_stride];
float m = data_a[m_offset + (k + tid) * lm_stride];
L += exp(m - m_max) * l;
}
// reduce across the workgroup
tmpsh[tid] = L;
barrier();
[[unroll]] for (uint s = BLOCK_SIZE/2; s > 0; s >>= 1) {
if (tid < s) {
L += tmpsh[tid + s];
tmpsh[tid] = L;
}
barrier();
}
L = tmpsh[0];
L = 1.0 / L;
// D dimension is split across workgroups in the y dimension
uint d = tid + gl_WorkGroupID.y * BLOCK_SIZE;
// Scale and sum the O contributions based on m_max and store the result to memory
if (d < D) {
float O = 0.0;
[[unroll]] for (uint k = 0; k < k_num; ++k) {
uint o_offset = D * N * (k + iq3 * k_num) + D * n + d;
float m = data_a[m_offset + k * lm_stride];
O += exp(m - m_max) * data_a[o_offset];
}
O *= L;
data_d[iq3 * D * N + D * n + d] = O;
}
}