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@@ -13,6 +13,10 @@ Intel Arc GPUs look great on paper for LLM inference: ample VRAM, wide memory bu
 
 The root causes are multi-layered: missing kernel optimizations in `llama.cpp`, a fragmented Intel software stack (five semi-independent efforts that don't interoperate), quantization-specific dispatch path bugs, and an overall underinvestment in open-source kernel development for Intel GPU architectures.
 
+## Empirical Findings
+
+- **[Empirical Findings](empirical_findings.md)** — Real-world benchmarks and configurations from an Arc A770 + RX 580 system running llama.cpp with Qwen3.5-35B-A3B MoE. Includes driver setup (xe vs i915), SYCL/Vulkan status, performance tables, and working/broken configuration matrix.
+
 ## Overviews
 
 Each overview was independently produced by a different LLM, analyzing community issues, kernel source code, driver stacks, and benchmark data:
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+# Empirical Findings: Arc A770 + RX 580 on llama.cpp
+
+**Date:** April 2026
+**Hardware:** AMD Ryzen 7 5800X, 32GB DDR4, Intel Arc A770 16GB, AMD RX 580 4GB
+**OS:** CachyOS (Arch-based), kernel 6.19.10-1-cachyos
+**Model:** Qwen3.5-35B-A3B (MoE, 35B total / 3B active) — UD-IQ4_XS (17.5 GB)
+
+---
+
+## 1. Hardware & Driver Setup
+
+### Intel Arc A770
+
+- **PCIe**: 0b:00.0 (DG2, device 56a0)
+- **Driver**: `xe` kernel module — **not i915**
+  - i915 causes GPU hangs on compute workloads (ecode `12:1:85def5fb`)
+  - Switched via modprobe: `options i915 force_probe=!56a0` + `options xe force_probe=56a0`
+- **Compute path**: SYCL/Level Zero is the **only reliable compute path**
+  - Vulkan compute: **BROKEN** — `DeviceLostError` on all compute workloads, even on xe driver
+  - Vulkan display: works fine (desktop environment runs on this GPU)
+- **oneAPI**: basekit 2025.3.1-6, DPC++ 2025.3.2
+  - 2025.0.4 is too old (missing `sycl/ext/oneapi/work_group_static.hpp`)
+  - `source /opt/intel/oneapi/setvars.sh` required before cmake AND at runtime (SYCL backend crashes without it)
+
+### AMD RX 580
+
+- **PCIe**: 04:00.0 (Polaris10)
+- **Driver**: amdgpu + RADV (Mesa Vulkan)
+- **Vulkan**: works perfectly out of the box (needs `vulkan-radeon` package)
+- **OpenCL**: via rusticl (ROCm broken on kernel 6.19.x)
+
+### Multi-GPU OpenCL
+
+```
+Platform #0: Intel(R) OpenCL Graphics → Arc A770
+Platform #1: rusticl → Mesa Arc A770 (DG2) + AMD RX 580 (polaris10)
+```
+
+DRI devices: `card0` = RX 580 (04:00.0), `card1` = Arc A770 (0b:00.0)
+Render nodes: `renderD128` = Arc A770, `renderD129` = RX 580
+
+---
+
+## 2. llama.cpp Build
+
+Built with oneAPI SYCL + Vulkan support:
+
+```bash
+source /opt/intel/oneapi/setvars.sh
+cmake -B build -DGGML_SYCL=ON -DGGML_VULKAN=ON \
+  -DCMAKE_C_COMPILER=icx -DCMAKE_CXX_COMPILER=icpx \
+  -DCMAKE_BUILD_TYPE=Release -DVulkan_INCLUDE_DIR=/usr/include \
+  -DGGML_AVX2=ON -DGGML_AVX=ON -DGGML_FMA=ON -DGGML_F16C=ON -DGGML_SSE42=ON
+cmake --build build --config Release -j$(nproc)
+```
+
+**Critical**: `icx`/`icpx` does NOT auto-detect SIMD with `GGML_NATIVE` — you must explicitly set AVX2/FMA/F16C/SSE42 flags.
+
+---
+
+## 3. Performance Benchmarks
+
+### Token generation (t/s) — Qwen3.5-35B-A3B UD-IQ4_XS
+
+| Config | Device | Prompt t/s | Gen t/s | Notes |
+|--------|--------|------------|---------|-------|
+| `-ngl 38 --flash-attn on` | SYCL0 (Arc A770) | 18.8 | **21.3** | Best speed, fragile — crashes if ngl too high |
+| `--cpu-moe -ngl 99` | Vulkan1 (RX 580) | 42.6 | **20.3** | Best reliable config |
+| `--cpu-moe -ngl 99` | SYCL0 (Arc A770) | 36.0 | 18.0 | Intel SYCL slower than AMD Vulkan |
+| `--cpu-moe -ngl 99` (no AVX2) | SYCL0 (Arc A770) | 19.5 | 17.5 | AVX2 only helps prompt, not gen |
+| `-ngl 0 -ot ".*_exps=SYCL0"` | SYCL0 | 15.5 | 10.3 | CPU-driven graph = slow |
+| `--cpu-dense -ngl 99` | SYCL0 (Arc A770) | 27.9 | 9.0 | Inverse of cpu-moe, bad |
+| Cross-GPU SYCL+Vulkan | Mixed | — | 5.5 | Garbage output, data corruption |
+| Cross-GPU Vulkan+Vulkan | Mixed | — | — | DeviceLostError crash |
+
+### Key observations
+
+1. **Generation is memory bandwidth bound** — DDR4 ~50 GB/s is the wall for MoE experts on CPU, not GPU compute.
+2. **AVX2 gives 3x prompt speedup** (19 → 57 t/s) but **zero gen improvement** — confirms bandwidth bottleneck.
+3. **Q4_K_XL vs IQ4_XS = identical gen speed** — same bandwidth, different compute, same result.
+4. **Q8 KV cache (`-ctk q8_0 -ctv q8_0`) = no speed difference** on RX 580.
+5. **RX 580 ($40 used) beats Arc A770 ($350) on SYCL with --cpu-moe** — Intel's software stack is the bottleneck, not the hardware.
+6. **Cross-GPU tensor splitting between different backends is broken** — corruption or crashes.
+
+---
+
+## 4. What Works, What Doesn't
+
+### Working configurations
+
+| Feature | Status |
+|---------|--------|
+| Arc A770 SYCL/Level Zero compute | Works |
+| Arc A770 partial layer offload (`-ngl 38`) | Works |
+| RX 580 Vulkan compute | Works perfectly |
+| `--cpu-moe` with GPU for dense layers | Works, best approach for MoE |
+| Flash attention on SYCL (no --cpu-moe) | Works |
+| Multi-GPU OpenCL enumeration | Works |
+
+### Broken
+
+| Feature | Failure mode |
+|---------|-------------|
+| Arc A770 Vulkan compute | `DeviceLostError` GPU hangs (both i915 and xe) |
+| i915 driver on Arc A770 | GPU hangs on compute (ecode `12:1:85def5fb`) |
+| Flash attention on SYCL + `--cpu-moe` | Crashes on 2nd prompt (`fattn-tile.hpp:1255 fatal error`) |
+| Cross-GPU SYCL+Vulkan tensor split | Data corruption, garbage output |
+| Cross-GPU Vulkan+Vulkan | `DeviceLostError` crash |
+| `-ngl 99` on SYCL without --cpu-moe | OOM (model 16.3GB > 15.5GB usable VRAM) |
+| SYCL without setvars.sh | Crashes at startup |
+| ROCm on kernel 6.19.x | Broken, use rusticl instead |
+
+---
+
+## 5. Qwen3.5 Model Quirks
+
+- **`--reasoning off` required** — otherwise generates infinite empty thinking tokens (500MB+ of newlines)
+- **`--flash-attn off` needed** when using `--cpu-moe` on SYCL (crashes on multi-turn)
+- **`--flash-attn on` works** on SYCL with partial layer offload (`-ngl 38`)
+
+---
+
+## 6. Recommended Configs
+
+### Reliable daily driver (RX 580 Vulkan)
+
+```bash
+ZES_ENABLE_SYSMAN=1 ./llama.cpp/build/bin/llama-cli \
+  -m Qwen3.5-35B-A3B-UD-IQ4_XS.gguf \
+  -ngl 99 --device Vulkan1 --cpu-moe \
+  -c 2048 --reasoning off
+```
+
+20.3 t/s generation. Just works.
+
+### Maximum speed (Arc A770 SYCL, fragile)
+
+```bash
+source /opt/intel/oneapi/setvars.sh && ZES_ENABLE_SYSMAN=1 \
+  ./llama.cpp/build/bin/llama-cli \
+  -m Qwen3.5-35B-A3B-UD-IQ4_XS.gguf \
+  -ngl 38 --device SYCL0 \
+  -c 4000 --reasoning off --flash-attn on
+```
+
+21.3 t/s generation. Crashes if you push ngl too high.
+
+---
+
+## 7. Conclusions
+
+The Arc A770 is not fundamentally slow hardware — 512 GB/s memory bandwidth should destroy a $40 RX 580 on bandwidth-bound workloads. The fact that it doesn't tells you everything about Intel's software stack maturity:
+
+- **SYCL/Level Zero is the only working compute path** on Linux. Vulkan compute is broken.
+- **The xe driver is mandatory** — i915 GPU hangs on compute workloads.
+- **oneAPI version matters enormously** — too old and the SYCL backend won't compile.
+- **For MoE models, `--cpu-moe` is the only sane strategy** — it keeps the massive expert tensors on CPU (bandwidth-bound anyway) and uses GPU for the smaller dense layers.
+- **Intel needs to fix Vulkan compute on Arc** — this is the biggest single blocker for mainstream llama.cpp users who don't want to deal with oneAPI.