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docs: update recommended boot parameters to use amd_iommu=off for improved performance

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Donato Capitella
2026-05-15 19:08:21 +01:00
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README.md
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@@ -146,11 +146,14 @@ This should work on any Strix Halo. For a complete list of available hardware, s
Add these boot parameters to enable unified memory while reserving a minimum of 4 GiB for the OS (max 124 GiB for iGPU):
`iommu=pt amdgpu.gttsize=126976 ttm.pages_limit=32505856`
> [!WARNING]
> Based on [benchmarking by Lars Urban (@urbanswelt)](https://github.com/urbanswelt), there is definitive indication that setting `amd_iommu=off` performs better than the previously recommended `iommu=pt`. Key result: `amd_iommu=off` is 5-12% faster than either IOMMU-enabled mode. See [Issue #66](https://github.com/kyuz0/amd-strix-halo-toolboxes/issues/66#issuecomment-4460612951) for details.
`amd_iommu=off amdgpu.gttsize=126976 ttm.pages_limit=32505856`
| Parameter | Purpose |
|-----------------------------|--------------------------------------------------------------------------------------------|
| `iommu=pt` | Sets IOMMU to "Pass-Through" mode. This helps performance, reducing overhead for the iGPU unified memory access. |
| `amd_iommu=off` | Disables the AMD IOMMU. This improves performance and stability over `iommu=pt`. |
| `amdgpu.gttsize=126976` | Caps GPU unified memory to 124 GiB; 126976 MiB ÷ 1024 = 124 GiB |
| `ttm.pages_limit=32505856` | Caps pinned memory to 124 GiB; 32505856 × 4 KiB = 126976 MiB = 124 GiB |