# Device selection

*Estimated read time: 3 minutes*

Pick where your model runs by selecting a **device type** (runtime + hardware) and optionally a **device index**. This guide focuses on device selection. For performance tuning (e.g., latency, throughput, etc.), see the advanced guides.

## Model JSON info

Each model package defines a default and supported device list in its JSON file:

{% code overflow="wrap" %}

```json
"DEVICE": [
  {
    "DeviceType": "HAILO8L",
    "RuntimeAgent": "HAILORT",
    "SupportedDeviceTypes": "HAILORT/HAILO8L, HAILORT/HAILO8"
  }
]
```

{% endcode %}

**Interpretation**:

* **Default**: `HAILO8L` and `HAILORT` — derived from `DeviceType` and `RuntimeAgent`
* **Supported list**: `HAILORT/HAILO8L`, `HAILORT/HAILO8`.

At runtime, this shows up as `model.supported_device_types` (read‑only).

## Device discovery

{% stepper %}
{% step %}
**Load the model**

{% code overflow="wrap" %}

```python
from degirum_tools import ModelSpec
model_spec = ModelSpec(
    model_name="…",
    zoo_url="degirum/hailo",
    inference_host_address="@local",
)
model = model_spec.load_model()
```

{% endcode %}
{% endstep %}

{% step %}
**Check supported device types**

{% code overflow="wrap" %}

```python
print(
    "supported:", model.supported_device_types
)  # e.g., ['HAILORT/HAILO8L', 'HAILORT/HAILO8']
```

{% endcode %}
{% endstep %}

{% step %}
**Check what's available on this machine**

* CLI: `degirum sys-info` → lists discovered hardware per runtime
* Or, after choosing a type, read `model.devices_available` (read‑only indices for that type)

{% code overflow="wrap" %}

```python
model.device_type = "HAILORT/HAILO8L"  # Pick a type to query
print("available indices:", model.devices_available)  # e.g., [0] or [0,1]
```

{% endcode %}
{% endstep %}
{% endstepper %}

## Choose a device type

You can set `device_type` to pass a string for a fixed type (deterministic) or as a list (to allow fallbacks). If a list is used, PySDK picks the first available.

**Deterministic pick** (fixed device type):

{% code overflow="wrap" %}

```python
# Lock to one runtime/hardware type
model_spec = ModelSpec(
    ..., model_properties={"device_type": "HAILORT/HAILO8L"}
)
model = model_spec.load_model()
```

{% endcode %}

**Graceful fallback** (try multiple types in order):

{% code overflow="wrap" %}

```python
# Prefer HAILO8, then HAILO8L, then CPU as a last resort
model_spec = ModelSpec(
    ...,
    model_properties={
        "device_type": ["HAILORT/HAILO8", "HAILORT/HAILO8L", "CPU"]
    },
)
model = model_spec.load_model()  # First available wins
```

{% endcode %}

{% hint style="info" %}
Use exact strings from `model.supported_device_types` to avoid typos.
{% endhint %}

### (Optional) Pin a specific device index

On multi‑card systems, you can set `devices_selected` to choose which device ID to use:

{% code overflow="wrap" %}

```python
model.device_type = "HAILORT/HAILO8L"
print(model.devices_available)  # e.g., [0, 1]
model.devices_selected = [1]  # Choose card #1
```

{% endcode %}

* `devices_available`: read‑only list of indices for the current `device_type`
* `devices_selected`: writable list of indices to use

## Set‑once vs. tweak‑later

* **Set once in** `ModelSpec`: Use `device_type` (single or list) to ensure consistent, team‑wide defaults.
* **Tweak later on the** `model`: Use `devices_selected` to pin a card, or switch `device_type` interactively while exploring.

## Common pitfalls

* **String mismatch**: Use only values from `model.supported_device_types`.
* **“No device found”**: Check hardware presence using `degirum sys-info`. Consider fallback lists for `device_type`.
* **Wrong device used**: Set `devices_selected = [index]` after choosing `device_type`.