---
layout: docs
page_title: device block in the job specification
description: |-
  Configure task access to devices in the `device` block of the Nomad job specification.
---

# `device` block in the job specification

<Placement groups={['job', 'group', 'task', 'resources', 'device']} />

The `device` block is used to create both a scheduling and runtime requirement
that the given task has access to the specified devices. A device is a hardware
device that is attached to the node and may be made available to the task.
Examples are GPUs, FPGAs, and TPUs.

When a `device` block is added, Nomad will schedule the task onto a node that
contains the set of device(s) that meet the specified requirements. The `device` block
allows the operator to specify as little as just the type of device required,
such as `gpu`, all the way to specifying arbitrary constraints and affinities.
Once the scheduler has placed the allocation on a suitable node, the Nomad
Client will invoke the device plugin to retrieve information on how to mount the
device and what environment variables to expose. For more information on the
runtime environment, please consult the individual device plugin's documentation.

See the [device plugin's documentation][devices] for a list of supported devices.

```hcl
job "docs" {
  group "example" {
    task "server" {
      resources {
        device "nvidia/gpu" {
          count = 2

          constraint {
            attribute = "${device.attr.memory}"
            operator  = ">="
            value     = "2 GiB"
          }

          affinity {
            attribute = "${device.attr.memory}"
            operator  = ">="
            value     = "4 GiB"
            weight    = 75
          }
        }
      }
    }
  }
}
```

In the above example, the task is requesting two GPUs, from the Nvidia vendor,
but is not specifying the specific model required. Instead it is placing a hard
constraint that the device has at least 2 GiB of memory and that it would prefer
to use GPUs that have at least 4 GiB. This examples shows how expressive the
`device` block can be.

~> Device support is currently limited to Linux, and container based drivers
due to the ability to isolate devices to specific tasks.

## `device` Parameters

- `name` `(string: "")` - Specifies the device required. The following inputs
  are valid:

  - `<device_type>`: If a single value is given, it is assumed to be the device
    type, such as "gpu", or "fpga".

  - `<vendor>/<device_type>`: If two values are given separated by a `/`, the
    given device type will be selected, constraining on the provided vendor.
    Examples include "nvidia/gpu" or "amd/gpu".

  - `<vendor>/<device_type>/<model>`: If three values are given separated by a `/`, the
    given device type will be selected, constraining on the provided vendor, and
    model name. Examples include "nvidia/gpu/1080ti" or "nvidia/gpu/2080ti".

- `count` `(int: 1)` - Specifies the number of instances of the given device
  that are required.

- `constraint` <code>([Constraint][]: nil)</code> - Constraints to restrict
  which devices are eligible. This can be provided multiple times to define
  additional constraints. See below for available attributes.

- `affinity` <code>([Affinity][]: nil)</code> - Affinity to specify a preference
  for which devices get selected. This can be provided multiple times to define
  additional affinities. See below for available attributes.

## `device` Constraint and Affinity Attributes

The set of attributes available for use in a `constraint` or `affinity` are as
follows:

<table>
  <thead>
    <tr>
      <th>Variable</th>
      <th>Description</th>
      <th>Example Value</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td>
        <code>{'${device.ids}'}</code>
      </td>
      <td>Comma separated list of device IDs in the group</td>
      <td>
        <code>9afa5da1-8f39-25a2-48dc-ba31fd7c0023,c248b547-fed7-4d67-ade5-73a27d280ac4</code>
      </td>
    </tr>
    <tr>
      <td>
        <code>{'${device.type}'}</code>
      </td>
      <td>The type of device</td>
      <td>
        <code>"gpu", "tpu", "fpga"</code>
      </td>
    </tr>
    <tr>
      <td>
        <code>{'${device.vendor}'}</code>
      </td>
      <td>The device's vendor</td>
      <td>
        <code>"amd", "nvidia", "intel"</code>
      </td>
    </tr>
    <tr>
      <td>
        <code>{'${device.model}'}</code>
      </td>
      <td>The device's model</td>
      <td>
        <code>"1080ti"</code>
      </td>
    </tr>
    <tr>
      <td>
        <code>
          ${'{'}device.attr.&lt;property&gt;{'}'}
        </code>
      </td>
      <td>Property of the device</td>
      <td>
        <code>{'${device.attr.memory} => 8 GiB'}</code>
      </td>
    </tr>
  </tbody>
</table>

For the set of attributes available, please see the individual [device plugin's
documentation][devices].

### Attribute Units and Conversions

Devices report their attributes with strict types and can also provide unit
information. For example, when a GPU is reporting its memory, it can report that
it is "4096 MiB". Since Nomad has the associated unit information, a constraint
that requires greater than "3.5 GiB" can match since Nomad can convert between
these units.

The units Nomad supports are as follows:

<table>
  <thead>
    <tr>
      <th>Base Unit</th>
      <th>Values</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td>
        <code>Byte</code>
      </td>
      <td>
        <code>
          <strong>Base 2</strong>: KiB, MiB, GiB, TiB, PiB, EiB
        </code>
        <br />
        <code>
          <strong>Base 10</strong>: kB, KB (equivalent to kB), MB, GB, TB, PB,
          EB
        </code>
      </td>
    </tr>
    <tr>
      <td>
        <code>Byte Rates</code>
      </td>
      <td>
        <code>
          <strong>Base 2</strong>: KiB/s, MiB/s, GiB/s, TiB/s, PiB/s, EiB/s
        </code>
        <br />
        <code>
          <strong>Base 10</strong>: kB/s, KB/s (equivalent to kB/s), MB/s, GB/s,
          TB/s, PB/s,EB/s
        </code>
      </td>
    </tr>
    <tr>
      <td>
        <code>Hertz</code>
      </td>
      <td>
        <code>MHz, GHz</code>
      </td>
    </tr>
    <tr>
      <td>
        <code>Watts</code>
      </td>
      <td>
        <code>mW, W, kW, MW, GW</code>
      </td>
    </tr>
  </tbody>
</table>

Conversion is only possible within the same base unit.

## `device` Examples

The following examples only show the `device` blocks. Remember that the
`device` block is only valid in the placements listed above.

### Single Nvidia GPU

This example schedules a task with a single Nvidia GPU made available.

```hcl
device "nvidia/gpu" {}
```

### Multiple Nvidia GPU

This example schedules a task with a two Nvidia GPU made available.

```hcl
device "nvidia/gpu" {
  count = 2
}
```

### Single Nvidia GPU with Specific Model

This example schedules a task with a single Nvidia GPU made available and uses
the name to specify the exact model to be used.

```hcl
device "nvidia/gpu/1080ti" {}
```

This is a simplification of the following:

```hcl
device "gpu" {
  count = 1

  constraint {
    attribute = "${device.vendor}"
    value     = "nvidia"
  }

  constraint {
    attribute = "${device.model}"
    value     = "1080ti"
  }
}
```

### Affinity with Unit Conversion

This example uses an affinity to tell the scheduler it would prefer if the GPU
had at least 1.5 GiB of memory. The following are both equivalent as Nomad can
do unit conversions.

Specified in `GiB`:

```hcl
device "nvidia/gpu" {
  affinity {
    attribute = "${device.attr.memory}"
    operator  = ">="
    value     = "1.5 GiB"
    weight    = 75
  }
}
```

Specified in `MiB`:

```hcl
device "nvidia/gpu" {
  affinity {
    attribute = "${device.attr.memory}"
    operator  = ">="
    value     = "1500 MiB"
    weight    = 75
  }
}
```

### Affinity Towards Specific GPU Devices

This example uses affinity to indicate scheduling preference towards specific
GPU devices, using their UUID as selection criteria. Since devices are
fingerprinted as a group, you may specify multiple IDs as a comma separated
list.

```hcl
device "nvidia/gpu" {
  affinity {
    attribute = "${device.ids}"
    operator  = "set_contains"
    value     = "9afa5da1-8f39-25a2-48dc-ba31fd7c0023,c248b547-fed7-4d67-ade5-73a27d280ac4"
  }
}
```

[affinity]: /nomad/docs/job-specification/affinity 'Nomad affinity Job Specification'
[constraint]: /nomad/docs/job-specification/constraint 'Nomad constraint Job Specification'
[devices]: /nomad/plugins/devices 'Nomad Device Plugins'
