# Elastic Fabric Adapter


Elastic Fabric Adapter (EFA) is a network device that has OS-bypass capabilities for low-latency network communications with other instances on the same subnet. EFA is exposed by using Libfabric, and can be used by applications using the Messaging Passing Interface (MPI).

To use EFA with AWS ParallelCluster and a Slurm scheduler, set [`SlurmQueues`](Scheduling-v3.md#Scheduling-v3-SlurmQueues) / [`ComputeResources`](Scheduling-v3.md#Scheduling-v3-SlurmQueues-ComputeResources) / [`Efa`](Scheduling-v3.md#yaml-Scheduling-SlurmQueues-ComputeResources-Efa) / [`Enabled`](Scheduling-v3.md#yaml-Scheduling-SlurmQueues-ComputeResources-Efa-Enabled) to `true`.

To view the list of Amazon EC2 instances that support EFA, see [Supported instance types](https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/efa.html#efa-instance-types) in the *Amazon EC2 User Guide for Linux Instances*.

We recommend that you run your EFA-enabled instances in a placement group. This way the instances are launched into a low-latency group in a single Availability Zone. For more information on how to configure placement groups with AWS ParallelCluster, see [`SlurmQueues`](Scheduling-v3.md#Scheduling-v3-SlurmQueues) / [`Networking`](Scheduling-v3.md#Scheduling-v3-SlurmQueues-Networking) / [`PlacementGroup`](Scheduling-v3.md#yaml-Scheduling-SlurmQueues-Networking-PlacementGroup).

**Note**  
Elastic Fabric Adapter (EFA) isn't supported over different availability zones. For more information, see [Scheduling](Scheduling-v3.md) / [SlurmQueues](Scheduling-v3.md#Scheduling-v3-SlurmQueues) / [Networking](Scheduling-v3.md#Scheduling-v3-SlurmQueues-Networking) / [SubnetIds](Scheduling-v3.md#yaml-Scheduling-SlurmQueues-Networking-SubnetIds).

**Note**  
By default, Ubuntu distributions enable ptrace (process trace) protection. ptrace protection is disabled so that Libfabric works properly. For more information, see [Disable ptrace protection](https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/efa-start.html#efa-start-ptrace) in the *Amazon EC2 User Guide*.

## Default EFA network configuration


Starting in AWS ParallelCluster 3.15.0, when EFA is enabled, AWS ParallelCluster automatically configures EFA-only network interfaces to separate EFA traffic from IP traffic. This maximizes EFA bandwidth while minimizing IP address consumption. AWS ParallelCluster determines the optimal configuration based on the capabilities of the instance type.

This default configuration is recommended for most workloads, including tightly-coupled HPC and distributed AI/ML training.

## Customizing EFA network interfaces


If your workload requires a different network configuration, such as maximizing ENA bandwidth on secondary network cards or configuring a subset of available network cards, you can override the default settings using the [`SlurmQueues`](Scheduling-v3.md#Scheduling-v3-SlurmQueues) / [`ComputeResources`](Scheduling-v3.md#Scheduling-v3-SlurmQueues-ComputeResources) / [`LaunchTemplateOverrides`](Scheduling-v3.md#yaml-Scheduling-SlurmQueues-ComputeResources-LaunchTemplateOverrides) parameter. This replaces the entire network interface configuration of the compute nodes with the configuration defined in your launch template.

For a step-by-step walkthrough, see [Customize compute node network interfaces with launch template overrides](tutorial-network-customization-v3.md).

**Warning**  
If you configure network interfaces in a way that is not supported by the instance type, instances will fail to launch. To verify the supported network configurations for your instance type, see [DescribeInstanceTypes](https://docs.aws.amazon.com/AWSEC2/latest/APIReference/API_DescribeInstanceTypes.html) in the *Amazon EC2 API Reference*.

For more information, see [Elastic Fabric Adapter](https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/efa.html) in the *Amazon EC2 User Guide* and [Scale HPC workloads with elastic fabric adapter and AWS ParallelCluster](https://aws.amazon.com/blogs/opensource/scale-hpc-workloads-elastic-fabric-adapter-and-aws-parallelcluster/) in the *AWS Open Source Blog*.