[ET-VK] Better work group sizes for matmul #13378
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Pull Request resolved: #13185 ## Context Currently `default_pick_local_wg_size()` (which internally calls `ComputeGraph::create_local_wg_size`) is used to select the local work group size for matrix multiplication ops. However, these functions currently bias the size of the local work group towards the largest dim of the global work group producing local wg sizes like ``` shader globalwg size localwg size =========== ===================== ==================== ============= linear_qga4w_tiled_texture3d_texture3d_texture2d_float {256, 29, 1} {32, 2, 1} 1487 matmul_naive_texture3d_float {29, 115, 32} {4, 2, 8} 712 ``` for matrix multiplication shaders. This behaviour was introduced in D64418632 / #6409. However, through experimental testing a "square" work group size of `{8, 8, 1}` works a lot better for matrix multiplication shaders. The theoretical analysis for this behaviour is that the local work group size determines the memory locations that need to be loaded to compute the overall work group. For a work group with size `{W, H, 1}` the data required to compute the output would be `W * OUTPUT_TILE_W` columns of the weight tensor and `H * OUTPUT_TILE_H` rows of the input tensor. Note that all work group items in the same W index will be requesting the same columns from the weight tensor, and all work group items in the same H index will be requesting the same rows from the input tensor. If `H==W`, then that "balances" the amount of data needed to loaded from each input tensor and may result in better data sharing behaviour among all work group items. Assuming `OUTPUT_TILE_W == OUTPUT_TILE_H == 1`, a local work group of size `{64, 1, 1}` would require 1 unique row from the input tensor an 64 unique columns to be loaded from the weight tensor, resulting in `(1 + 64) * K = 65K` elements to be loaded in total, where K is the size of the shared reduction dim. Conversely, a local work group of size `{8, 8, 1}` would require 8 unique rows / 8 unique columns resulting in only `(8 + 8) * K = 16K` unique elements to be loaded. This highlights the need to use dedicated logic to compute work group sizes for matrix multiplication shaders. ## Changes * Introduce `pick_hw_square_wg_size` * Use the new local work group size determination function for Quantized Linear, Matmul, and Linear ghstack-source-id: 302703877 Differential Revision: [D79813236](https://our.internmc.facebook.com/intern/diff/D79813236/)
🔗 Helpful Links🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/13378
Note: Links to docs will display an error until the docs builds have been completed. ⏳ No Failures, 87 PendingAs of commit 97c9673 with merge base b36d6b6 ( This comment was automatically generated by Dr. CI and updates every 15 minutes. |
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## Context
Currently `default_pick_local_wg_size()` (which internally calls `ComputeGraph::create_local_wg_size`) is used to select the local work group size for matrix multiplication ops. However, these functions currently bias the size of the local work group towards the largest dim of the global work group producing local wg sizes like
```
shader globalwg size localwg size
=========== ===================== ==================== =============
linear_qga4w_tiled_texture3d_texture3d_texture2d_float {256, 29, 1} {32, 2, 1} 1487
matmul_naive_texture3d_float {29, 115, 32} {4, 2, 8} 712
```
for matrix multiplication shaders. This behaviour was introduced in D64418632 / pytorch#6409.
However, through experimental testing a "square" work group size of `{8, 8, 1}` works a lot better for matrix multiplication shaders. The theoretical analysis for this behaviour is that the local work group size determines the memory locations that need to be loaded to compute the overall work group. For a work group with size `{W, H, 1}` the data required to compute the output would be `W * OUTPUT_TILE_W` columns of the weight tensor and `H * OUTPUT_TILE_H` rows of the input tensor. Note that all work group items in the same W index will be requesting the same columns from the weight tensor, and all work group items in the same H index will be requesting the same rows from the input tensor.
If `H==W`, then that "balances" the amount of data needed to loaded from each input tensor and may result in better data sharing behaviour among all work group items. Assuming `OUTPUT_TILE_W == OUTPUT_TILE_H == 1`, a local work group of size `{64, 1, 1}` would require 1 unique row from the input tensor an 64 unique columns to be loaded from the weight tensor, resulting in `(1 + 64) * K = 65K` elements to be loaded in total, where K is the size of the shared reduction dim. Conversely, a local work group of size `{8, 8, 1}` would require 8 unique rows / 8 unique columns resulting in only `(8 + 8) * K = 16K` unique elements to be loaded.
This highlights the need to use dedicated logic to compute work group sizes for matrix multiplication shaders.
## Changes
* Introduce `pick_hw_square_wg_size`
* Use the new local work group size determination function for Quantized Linear, Matmul, and Linear
Differential Revision: [D79813236](https://our.internmc.facebook.com/intern/diff/D79813236/)
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This PR was created by the merge bot to help merge the original PR into the main branch.
ghstack PR number: #13185 by @SS-JIA
^ Please use this as the source of truth for the PR details, comments, and reviews
ghstack PR base: https://github.com/pytorch/executorch/tree/gh/SS-JIA/272/base
ghstack PR head: https://github.com/pytorch/executorch/tree/gh/SS-JIA/272/head
Merge bot PR base: https://github.com/pytorch/executorch/tree/main
Merge bot PR head: https://github.com/pytorch/executorch/tree/gh/SS-JIA/272/orig
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