# NVLink

### About NVLink

NVLink is supported **only** on instance flavors that include **8× NVIDIA H100 or H200 GPUs**.\
In this configuration, NVLink provides high-bandwidth, low-latency GPU-to-GPU communication, enabling:

* **Faster model training**, especially for large models that require frequent inter-GPU data exchange
* **Improved scaling efficiency** when using distributed training frameworks (e.g., Megatron-LM, DeepSpeed, PyTorch FSDP)
* **Reduced communication bottlenecks** compared to PCIe-only GPU connectivity
* **Higher overall compute throughput** for workloads that depend on multi-GPU synchronization

Instances with fewer than 8 GPUs do **not** support NVLink.

### Enable NVLink for 8x GPUs Flavors

> **Note:** NVLink is **not enabled by default** in FPT-provided images.\
> Users must manually enable NVLink if required.

**To enable NVLink support**, follow the steps below:

* Open the file:\
  \&#xNAN;**`/etc/default/grub.d/00-fci-grub.cfg`**
* Locate the following line and **remove or comment it out**:

```
GRUB_CMDLINE_LINUX_DEFAULT="nvidia.NVreg_NvLinkDisable=1" 
```

* Update the GRUB configuration:

```
sudo update-grub 
```

Here is a clean, well-structured **user-guide rewrite**:

***

### Verification

To verify that NVLink has been enabled successfully, run the following commands.

#### 1. Check NVLink status

```bash
nvidia-smi nvlink --status
```

The output should show **active NVLink connections** with their link speeds (e.g., **25 GB/s per lane**).

#### 2. Check GPU topology

```bash
nvidia-smi topo -m
```

The output should display **NVLink (NV##)** connections between all GPUs, similar to the example below:

```
        GPU0   GPU1   GPU2   GPU3   GPU4   GPU5   GPU6   GPU7   CPU Affinity   NUMA Affinity
GPU0     X    NV18   NV18   NV18   NV18   NV18   NV18   NV18   0-127          0-1
GPU1   NV18     X    NV18   NV18   NV18   NV18   NV18   NV18   0-127          0-1
GPU2   NV18   NV18     X    NV18   NV18   NV18   NV18   NV18   0-127          0-1
GPU3   NV18   NV18   NV18     X    NV18   NV18   NV18   NV18   0-127          0-1
GPU4   NV18   NV18   NV18   NV18     X    NV18   NV18   NV18   0-127          0-1
GPU5   NV18   NV18   NV18   NV18   NV18     X    NV18   NV18   0-127          0-1
GPU6   NV18   NV18   NV18   NV18   NV18   NV18     X    NV18   0-127          0-1
GPU7   NV18   NV18   NV18   NV18   NV18   NV18   NV18     X    0-127          0-1
```

#### Legend

* **X** : Same GPU
* **SYS** : PCIe + inter-NUMA interconnect
* **NODE**: PCIe + interconnect within one NUMA node
* **PHB** : PCIe Host Bridge
* **PXB** : Multiple PCIe bridges
* **PIX** : Single PCIe bridge
* **NV#** : NVLink connection with # bonded links

***

### Troubleshooting

If you encounter errors such as:

* `system not yet initialized`
* Issues when calling `torch.cuda.device_count()` or `torch.cuda.get_device_name(i)`

Restart the NVIDIA Fabric Manager:

```bash
sudo systemctl restart nvidia-fabricmanager
```

Then retry your application or verification steps.


---

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