---
type: research
title: Dankclaw IOMMU translation off (amd_iommu=off) — decision record
description: Why Dankclaw boots with amd_iommu=off since 2026-09-11, what it buys (13-16% prefill for frank-dragoon), what it costs (the XDNA2 NPU endpoint, machine-wide DMA translation), what was deliberately not copied from the reference machine, and how to revert.
tags:
  - dankclaw
  - iommu
  - kernel
  - grub
  - prefill
  - frank-dragoon
  - halogen
  - frank-npu
timestamp: 2026-09-11
---

# Dankclaw IOMMU translation off — decision record

**Decision (2026-09-11):** Dankclaw boots with **`amd_iommu=off`** instead of
`iommu=pt`, to recover the prefill throughput the large-model runtime loses to
IOMMU translation. The XDNA2 NPU endpoint was accepted as the cost and is
retired ([[frank-npu]]).

## What changed

`GRUB_CMDLINE_LINUX_DEFAULT` in `/etc/default/grub` on Dankclaw:

```
# before
ro quiet splash iommu=pt crashkernel=…

# after
ro quiet splash amd_iommu=off crashkernel=…
```

Then `sudo update-grub` and a reboot. Nothing else on the command line was
touched.

## Evidence

The upstream project published the measurement, and it is the only A/B they
ran: **460 → 385 tok/s prefill at a 2,048-token prompt, 13 to 16 percent**,
taken on a Ryzen AI Max+ 395 with 128 GB — the same SKU as Dankclaw. Their
published serving profile boots this way, so every number in their README is a
number with translation off. Decode is unaffected; prefill is where the loss is.

The mechanism offered is DMA translation on the path every GPU allocation
takes. The project states plainly that this is *their* configuration and not a
tuning guide, that the flag turns off DMA translation machine-wide, and that it
takes the NPU with it.

## What it costs

1. **The NPU endpoint dies.** With IOMMU off there is no PASID/SVA state and
   `/dev/accel/accel0` cannot be opened — `err=-19`, the exact failure
   documented in [[frank-npu]]. Audio input left the fleet with it; image input
   survives on `frank-dragoon` through halogen's vision tower.
2. **Posture change machine-wide.** Dankclaw also serves Caddy (the TLS edge
   and the Anubis/Authentik guard chain), Authentik, Pi-hole, Gitea, and the
   observability stack. DMA translation is now off for all of them. This was
   accepted explicitly as the price of the prefill figure.

## What was deliberately NOT copied

The reference machine's command line has six settings. Only one was measured,
so only that one was taken:

| setting | reference value | why not here |
|---|---|---|
| `amd_iommu=off` | set | **taken** — the measured 13–16 % |
| `amdgpu.gttsize` | `126976` MiB | a size, not a constant: it must equal installed RAM (their table: 128 GB → 126976, 96 GB → 95232, 64 GB → 63488). Dankclaw's GTT already reports **114 GiB ≈ 94 % of its 121 GiB** without the flag, so it would buy nothing |
| `ttm.pages_limit` | `32505856` | the same number in 4 KiB pages (their 128 GB row is 99.3 % of RAM). Pasting it onto a different machine asks the driver for more GTT than exists |
| `amdgpu.vm_update_mode=0` | set | upstream has never run without it, makes no claim about what it buys, and lists it "for completeness rather than recommended" |
| `amdgpu.noretry=0` | set | as above |
| `amdgpu.sg_display=0` | set | as above |

## Verification and revert

```bash
cat /proc/cmdline                        # expect amd_iommu=off, no iommu=pt
ls /sys/kernel/iommu_groups/ | wc -l     # expect 0 groups (was 30)
systemctl is-active frank-npu            # expect inactive (masked)
curl -s http://127.0.0.1:7770/health     # frank-dragoon must still answer 200
```

**To revert:** put `iommu=pt` back in `GRUB_CMDLINE_LINUX_DEFAULT` in place of
`amd_iommu=off`, run `sudo update-grub`, reboot, and unmask/start `frank-npu`
with the archived unit (`.AGENTTRASH/frank-npu.service.flm-npu`) if the NPU
endpoint is wanted back. The IOMMU change is independent of the halogen
deployment — reverting one does not require reverting the other.

## Related

- [[frank-npu]] — the endpoint this retired, including the err=-19 root cause
- [[halogen-flash-server]] — the runtime bought by the prefill figure
- [[kingjones-strix-rocmfp4]] — the runtime it replaced (rollback target)

## Citations

- [peonist-ai/halogen-flash-server](https://github.com/peonist-ai/halogen-flash-server) — README "The host settings these numbers were measured on", CHANGELOG 0.5.5–0.5.8