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phase 10: GPU add/mul kernels + BF16 precision analysis

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Kernel additions:
- add_f32/bf16, mul_f32/bf16 CUDA kernels (element-wise, on GPU)
- Refactored activation.rs with dispatch_unary/dispatch_binary helpers
- Qwen3 and GPT-2 now use GPU add/mul instead of CPU round-trips

GPT-2 add_bias also moved to GPU (broadcast via tile + GPU add)

BF16 precision analysis (docs/benchmarks/phase10-qwen3.md):
- Root cause: separate attention kernels materialize BF16 intermediates
  (QK^T→BF16→scale→BF16→mask→BF16→softmax→BF16 vs HF's fused FP32 path)
- HF itself SDPA vs Eager also differs by ~0.125 logit
- xserv vs HF: ~1-2 logit systematic offset, but same top-1 in 84% cases
- Industry standard for BF16: top-5 overlap (we achieve 100%)
- Fix path: Flash Attention (Phase 14) to fuse attention in FP32

Performance: TTFT 138→119ms, TBT 144→137ms (GPU ops faster than CPU)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
This commit is contained in:
Gahow Wang
2026-05-22 11:35:26 +08:00
parent 268e40d764
commit be5c64ea8a
6 changed files with 143 additions and 51 deletions
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47
crates/xserv-kernels/src/activation.rs
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@@ -8,43 +8,53 @@ unsafe extern "C" {
fn launch_silu_bf16(x: *const c_void, out: *mut c_void, n: i32, stream: *mut c_void);
fn launch_scale_f32(x: *const c_void, out: *mut c_void, scale: f32, n: i32, stream: *mut c_void);
fn launch_scale_bf16(x: *const c_void, out: *mut c_void, scale: f32, n: i32, stream: *mut c_void);
fn launch_add_f32(a: *const c_void, b: *const c_void, out: *mut c_void, n: i32, stream: *mut c_void);
fn launch_add_bf16(a: *const c_void, b: *const c_void, out: *mut c_void, n: i32, stream: *mut c_void);
fn launch_mul_f32(a: *const c_void, b: *const c_void, out: *mut c_void, n: i32, stream: *mut c_void);
fn launch_mul_bf16(a: *const c_void, b: *const c_void, out: *mut c_void, n: i32, stream: *mut c_void);
}
pub fn gelu(x: &Tensor) -> Tensor {
assert!(x.is_contiguous());
assert!(matches!(x.device(), Device::Cuda(_)));
fn dispatch_unary(x: &Tensor, f32_fn: unsafe extern "C" fn(*const c_void, *mut c_void, i32, *mut c_void),
bf16_fn: unsafe extern "C" fn(*const c_void, *mut c_void, i32, *mut c_void)) -> Tensor {
assert!(x.is_contiguous() && matches!(x.device(), Device::Cuda(_)));
let out = Tensor::zeros(x.shape(), x.dtype(), x.device());
let n = x.numel() as i32;
unsafe {
match x.dtype() {
DType::F32 => launch_gelu_f32(x.data_ptr() as _, out.data_ptr() as *mut c_void, n, std::ptr::null_mut()),
DType::BF16 => launch_gelu_bf16(x.data_ptr() as _, out.data_ptr() as *mut c_void, n, std::ptr::null_mut()),
_ => panic!("unsupported dtype for gelu"),
DType::F32 => f32_fn(x.data_ptr() as _, out.data_ptr() as *mut c_void, n, std::ptr::null_mut()),
DType::BF16 => bf16_fn(x.data_ptr() as _, out.data_ptr() as *mut c_void, n, std::ptr::null_mut()),
_ => panic!("unsupported dtype"),
}
}
xserv_cuda::device::synchronize().unwrap();
out
}
pub fn silu(x: &Tensor) -> Tensor {
assert!(x.is_contiguous());
assert!(matches!(x.device(), Device::Cuda(_)));
let out = Tensor::zeros(x.shape(), x.dtype(), x.device());
let n = x.numel() as i32;
fn dispatch_binary(a: &Tensor, b: &Tensor,
f32_fn: unsafe extern "C" fn(*const c_void, *const c_void, *mut c_void, i32, *mut c_void),
bf16_fn: unsafe extern "C" fn(*const c_void, *const c_void, *mut c_void, i32, *mut c_void)) -> Tensor {
assert_eq!(a.shape(), b.shape());
assert!(a.is_contiguous() && b.is_contiguous());
assert!(matches!(a.device(), Device::Cuda(_)));
assert_eq!(a.dtype(), b.dtype());
let out = Tensor::zeros(a.shape(), a.dtype(), a.device());
let n = a.numel() as i32;
unsafe {
match x.dtype() {
DType::F32 => launch_silu_f32(x.data_ptr() as _, out.data_ptr() as *mut c_void, n, std::ptr::null_mut()),
DType::BF16 => launch_silu_bf16(x.data_ptr() as _, out.data_ptr() as *mut c_void, n, std::ptr::null_mut()),
_ => panic!("unsupported dtype for silu"),
match a.dtype() {
DType::F32 => f32_fn(a.data_ptr() as _, b.data_ptr() as _, out.data_ptr() as *mut c_void, n, std::ptr::null_mut()),
DType::BF16 => bf16_fn(a.data_ptr() as _, b.data_ptr() as _, out.data_ptr() as *mut c_void, n, std::ptr::null_mut()),
_ => panic!("unsupported dtype"),
}
}
xserv_cuda::device::synchronize().unwrap();
out
}
pub fn gelu(x: &Tensor) -> Tensor { dispatch_unary(x, launch_gelu_f32, launch_gelu_bf16) }
pub fn silu(x: &Tensor) -> Tensor { dispatch_unary(x, launch_silu_f32, launch_silu_bf16) }
pub fn scale(x: &Tensor, scale_val: f32) -> Tensor {
assert!(x.is_contiguous());
assert!(matches!(x.device(), Device::Cuda(_)));
assert!(x.is_contiguous() && matches!(x.device(), Device::Cuda(_)));
let out = Tensor::zeros(x.shape(), x.dtype(), x.device());
let n = x.numel() as i32;
unsafe {
@@ -57,3 +67,6 @@ pub fn scale(x: &Tensor, scale_val: f32) -> Tensor {
xserv_cuda::device::synchronize().unwrap();
out
}
pub fn add(a: &Tensor, b: &Tensor) -> Tensor { dispatch_binary(a, b, launch_add_f32, launch_add_bf16) }
pub fn mul(a: &Tensor, b: &Tensor) -> Tensor { dispatch_binary(a, b, launch_mul_f32, launch_mul_bf16) }