More changes to matmul blog

* Add header for dispatch
* Move to 16,16 workgroups everywhere 2D
* Make it clear dispatch count it running on CPU not dispatches are for CPU

Author: LegNeato

blog/2024-11-21-optimizing-matrix-mul/code/crates/cpu/matmul/src/variants.rs

@@ -81,7 +81,7 @@ impl Gpu for Workgroup2d {
 
 impl GridComputation for Workgroup2d {
     fn workgroup(&self) -> UVec3 {
-        UVec3::new(8, 8, 1)
+        UVec3::new(16, 16, 1)
     }
 
     fn dispatch_count(&self, m: u32, n: u32) -> UVec3 {

blog/2024-11-21-optimizing-matrix-mul/code/crates/gpu/workgroup_2d/src/lib.rs

@@ -4,7 +4,7 @@ use settings::Dimensions;
 use spirv_std::glam::UVec3;
 use spirv_std::spirv;
 
-#[spirv(compute(threads(8, 8)))]
+#[spirv(compute(threads(16, 16)))]
 pub fn matmul(
     #[spirv(global_invocation_id)] global_id: UVec3,
     #[spirv(uniform, descriptor_set = 0, binding = 0)] dimensions: &Dimensions,

blog/2024-11-21-optimizing-matrix-mul/index.md

@@ -175,7 +175,10 @@ examples.
 
 :::
 
-Each workgroup, since it’s only one thread, processes one `result[i, j]`.
+#### Dispatching workgroups
+
+Each workgroup, since it’s only one thread (`#[spirv(compute(threads(1)))]`), processes
+one `result[i, j]`.
 
 To calculate the full matrix, we need to launch as many entries as there are in the
 matrix. Here we specify that (`Uvec3::new(m * n, 1, 1`) on the CPU:
@@ -241,7 +244,7 @@ Although we don't change much about our code, if we distribute our work in 2 dim
 we're able to bypass these limits and launch more workgroups that are larger. This
 allows us to calculate a 4096x4096 matmul.
 
-We update our `compute(threads(256)))` to `compute(threads((8, 8)))`, and make the small
+We update our `compute(threads(256)))` to `compute(threads((16, 16)))`, and make the small
 change to `row` and `col` from Zach's post to increase speed:
 
 import { RustWorkgroup2d } from './snippets/workgroup_2d.tsx';

blog/2024-11-21-optimizing-matrix-mul/snippets/naive.tsx

@@ -59,7 +59,7 @@ export const RustNaiveWorkgroupCount: React.FC = () => (
     language="rust"
     className="text-xs"
     lines="26-34"
-    title="Calculating how many workgroup dispatches are needed on the CPU"
+    title="Calculating on the CPU how many workgroup dispatches are needed"
   >
     {RustWorkgroupCount}
   </Snippet>
@@ -71,7 +71,7 @@ export const RustNaiveDispatch: React.FC = () => (
     className="text-xs"
     lines="145,147"
     strip_leading_spaces
-    title="Using wgpu on the CPU to dispatch to the GPU"
+    title="Using wgpu on the CPU to dispatch workgroups to the GPU"
   >
     {RustWgpuBackend}
   </Snippet>