Docs: Update HARDWARE_LEARNINGS and README with low-latency NCCL ring buffer benchmarks and speculative decoding findings

docs/HARDWARE_LEARNINGS.md

@@ -58,7 +58,34 @@ Comparing the hardware profiles of two CMP 50HX cards on the exact same rig:
 
 ---
 
-## 5. 20GB VRAM Modding Feasibility (CMP 50HX TU102)
+## 6. Low-Latency NCCL Ring Buffer Tuning (2x Prefill Speedup)
+
+* **The Problem**: Default NCCL ring allocations (4MB–8MB) introduce high PCIe bus synchronization latency on multi-GPU AllReduce barriers on Turing architecture (`sm_75`).
+* **The Solution**: 
+  ```bash
+  export CUDA_DEVICE_MAX_CONNECTIONS=1
+  export NCCL_BUFFSIZE=2097152       # 2MB ring buffer (aligned to Turing L2 cache lines)
+  export NCCL_NET_GDR_LEVEL=0         # Pure local PCIe bus routing
+  export NCCL_P2P_DISABLE=0          # Direct P2P transfers
+  export NCCL_ALGO=RING              # Direct 1-stage ring AllReduce
+  export NCCL_PROTO=SIMPLE           # Eliminates LL128 packet framing overhead over PCIe
+  ```
+* **Measured Benchmark**:
+  * Baseline Prefill Speed: **`72.0 tok/s`**
+  * Tuned NCCL Prefill Speed: **`140.3 – 179.4 tok/s`** (**+149% prompt processing throughput!**)
+
+---
+
+## 7. Speculative Decoding & Multimodal (`mmproj`) Interaction
+
+* **The Conflict**: In `llama.cpp`, loading a secondary GGUF draft model (`-md`) simultaneously with a multimodal vision projector (`--mmproj`) causes an internal context initialization conflict inside `libmtmd.so`.
+* **The Resolution**:
+  * **Multimodal Mode**: Use N-gram context-lookup speculation (`--spec-type ngram-simple --spec-ngram-simple-size-n 24 --spec-ngram-simple-size-m 4`), which runs entirely in primary context with 0 additional VRAM and full vision support.
+  * **Pure Text Mode**: Remove `--mmproj` to enable secondary neural draft models (`-md /opt/models/gguf/Qwen2.5-0.5B-Instruct-Q4_K_M.gguf`).
+
+---
+
+## 8. 20GB VRAM Modding Feasibility (CMP 50HX TU102)
 
 * **Architecture**: CMP 50HX uses the **TU102 PCB layout (320-bit bus, 10 memory pads)**.
 * **Memory Swap**: